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A A S H T O T I T L E CHBTW 95
= 0639804
0033532 OLA
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Ob39804 0033533 T 5 4
A A S H T O T I T L E CHBTW 95
American Association of State Highway and Transportation Officials
Executive Committtee
1994-1995
Voting Members
Officers:
President: Wayne Shackelford,Georgia
Vice President: Bill Burnett, Texas
Secretary/Treasurer: Clyde E. Pyers, Maryland
Regional Representatives:
Region I
Patrick Garahan, Vermont
Region II
Ben Watts, Florida
Region III
Darre1 Rensink, Iowa
Region IV
Larry Bonine, Arizona
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
Non-Voting Members
Executive Director: Francis B. Francois, Washington, D.C.
11
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I
Ob37804 0031534 7 7 0
AASHTO T I T L E CHBTW 75
AASHTO Highway Subcommittee on Bridges and Structures 1995
JAMES E. SIEBELS, COLORADO, Chairman
G. CHARLES LEWIS, GEORGIA, Vice Chairman
STANLEY GORDON, FEDERAL HIGHWAY ADMINISTRATION, Secretary
PENNSYLVANIA, Mahendra G. Pate1
PUERTO RICO, Jose L. Melendez, Hector Camacho
RHODE ISLAND, Kazem Farhournand
SOUTH CAROLINA, Rocque L. Kneece
SOUTH DAKOTA, John C. Cole
TENNESSEE, Ed Wasserman
TEXAS, Charles C. Terry
U.S. DOT, Stanley Gordon (FHWA), Nick E. Mpars
(USCG)
UTAH, David L. Christensen
VERMONT, Warren B. Tripp
VIRGINIA, Malcolm T. Kerley
WASHINGTON, M. Myint Lwin
WEST VIRGINIA, James Sothen
WISCONSIN, Stanley W. Woods
WYOMING, David H. Pope
ALBERTA, Dilip K.Dasmohapatra
MANITOBA, W. Saltzberg
NORTHERN MARIANA ISLANDS,
John C. Pangalinan
NEW BRUNSWICK, G.A. Rushton
NEWFOUNDLAND, Peter Lester
NORTHWEST TERRITORIES, Jivko Jivkov
NOVA SCOTIA, C.Y.S. Nguan
ONTARIO, Ranjit S . Reel
SASKATCHEWAN,Lome J. Hamblin
ENGLAND, Philip J. Andrews
MASS. METRO. DIST. COMM., David Lenhardt
N.J. TURNPIKE AUTHORITY, Wallace R. Grant
PORT AUTHORITY OF NY & NJ, Joseph K. Kelly
NY STATE BRIDGE AUTHORITY, William Moreau
BUREAU OF INDIAN AFFAIRS, (vacant)
U.S. DEPARTMENT OF AGRICULTURE-FOREST
SERVICE, Steve L. Bunnell
MILITARY TRAFFIC MANAGEMENT COMMAND,
(vacant)
U.S. ARMY CORPS OF ENGINEER-DEPT. OF THE
ARMY, Paul C. T. Tan
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ALABAMA, William F. Conway
ALASKA, Steve Bradford, Ray Shumway
ARIZONA, William R. Bruesch, F. Daniel Davis
ARKANSAS, Dale F. Loe
CALIFORNIA, James E. Roberts
COLORADO, A.J. Siccardi
CONNECTICUT, Gordon Barton
DELAWARE, Chao H. Hu
D.C., Gary A. Burch, Charles F. Williams, Jacob Patnaik
FLORIDA, Jerry Potter
GEORGIA, Paul Liles
HAWAII, Donald C. Ornellas
IDAHO, L. Scott Stokes
ILLINOIS, Ralph E. Anderson
INDIANA, John J. White
IOWA, William A. Lundquist
KANSAS, Kenneth F. Hurst
KENTUCKY, Richard Sutherland
LOUISIANA, Wayne Aymond
MAINE, Larry L. Roberts, James E. Tuley
MARYLAND, Earle S. Freedman
MASSACHUSETTS, Joseph P.Gill
MICHIGAN, Sudhakar Kulkarni
MINNESOTA, Donald J. Flemming
MISSISSIPPI, Wilbur F. Massey
MISSOURI, Allen F. Laffoon
MONTANA, William S . Fullerton
NEBRASKA, Lyman D. Freemon
NEVADA, Floyd I. Marcucci
NEW HAMPSHIRE, James A. Moore
NEW JERSEY, Robert Pege
NEW MEXICO, Martin A. G a r n i c k
NEW YORK, Michael J. Cuddy, Amn Shirole
NORTH CAROLINA, John L. Smith
NORTH DAKOTA, Steven J. Miller
OHIO, B.David Hanhilammi
OKLAHOMA, Veldo M. Goins
OREGON, Terry J. Shike
...
111
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AASHTO T I T L E
CHBTW 95 œ Ob39804 0033535 827 œ
PREFACE
This construction handbook has been developed for use by contractors and construction engineers
involved in bridge consíruction on Federal-aid highway projects. This document may also be of interest to
faisework design engineers, and supplements information found in the Guide Design Specgcation for Bridge
Temporary Works.") The content is construction-oriented, focusing primarily on standards of material quality
and means and methods of construction. The handbook contains chapters on falsework, formwork, and
temporary retaining structures. For more indepth discussion on a particular topic, related literature and
references are identified.
This study was conducted under FHWA Contract No. DTFH61-91-C-O0088by Wiss, Janney, Elstner
Associates, Inc., Northbrook, Illinois. The project was directed by the Scaffolding, Shoring, and Forming Task
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
Group of the FHWA, whose comments and review were very helpful in the preparation of this document. The
task group consisted of the following Federal, State, and industry representatives:
Sheila Rimai Duwadi, Federal Highway Administration
James R. Hoblitzeii. Federal Highway Administration
Donald W. Miller, Federal Highway Administration
William S. Cross, Federal Highway Administration
Ian M. Friedland, Transportation Research Board
James M. Stout, California Department of Transportation
Donald Flemming, Minnesota Department of Transportation
Nick Yaksich, Associated General Contractors
Kent Starwait, American Road and Transportation Builders Association
Ramon Cook, The Burke Company
Robert Desjardins, Cianbro Corp.
Richard F. Hoffman, McLean Contracting
Robert T. Ratay, Consulting Engineer
Additional information and input was solicited from other individuals and indusuy associations in their
fields of interest. Special recognition is extended to representatives of the Shoring and Forming Engineering
Committee of the Scaffolding, Shoring, and Forming Institute: L.Edwin Dunn, California Department of
Transportation (Retired); Robert G. Lukas, Ground Engineering Consultants, Inc.; Alan D. Fisher, Cianbro
Corporation: Mark K. Kaler, Dayton-Superior Corporation: Harry B. Lancelot, Richmond Screw Anchor
Company; Donald F. Meinheit and Raymond H.R. Tide.
iv
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AASHTO
TITLE CHBTW 95
Ob39804 003L53b 7b3
TABLE OF CONTENTS
Page
Chapter
1.
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . .
...........................
SCOPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DEFINITIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
RELATED PUBLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.
FALSEWORK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
...........................
MATERIALS AND MANUFACTURED COMPONENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Structural Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Timber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Manufactured Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
FOUNDATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Shallow Foundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DeepFoundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Protection of the Foundation Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CONSTRUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
TimberConstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Vertical Shoring Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cable Bracing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bridge Deck Falsework . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Traffic Openings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
LOADING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Loads During Falsework Erection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Concrete Placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Load Redistribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
OtherConditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
INSPECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VerticalTake-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Inspection During Concrete Placement ...............................
Inspection After Concrete Placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.
FORMWORK
.........................................................
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
FORM COMPONENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sheathing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Structural Supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Form Accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
LOADS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
FORMWORKTYPES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Job-Built Formwork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Modular Formwork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Stay-in-Place Formwork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Gang Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Plate Girder Forms ..............................................
CONSTRUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
FORM MAINTENANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
V
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1
1
1
2
3
3
3
7
10
12
12
13
14
19
19
19
21
24
25
27
30
30
30
30
32
32
32
32
33
33
35
35
35
36
40
43
47
49
50
50
51
51
53
54
56
AASHTO T I T L E CHBTW 95 W 0 6 3 9 8 0 4 0033537 LTT
TABLE OF CONTENTS (Continued)
Chapter
4
.
Page
TEMPORARY RETAINING STRUCTURES
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CLASSIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Woodsheeting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SoldierPiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Steel Sheet Piles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Tangent Piles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SELECTION OF COFFERDAM SCHEME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
RELATIVECOSTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SELECTION OF SUPPORT METHOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SEALING AND BUOYANCY CONTROL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SEEPAGE CONTROL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CONSTRUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Timber Sheet Pile Cofferdam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Soldier PileAiVood Lagging Cofferdam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Steel Sheet Pile Cofferdam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Soiland Rock Anchors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Internal Bracing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
59
59
62
62
66
67
68
70
70
71
72
75
76
76
77
78
80
83
APPENDIX A . SECTION PROPERTIES OF STANDARD DRESSED (S4S)
AND ROUGH SAWN LUMBER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
89
APPENDIX B. FALSEWORK AND FORMWORK DESIGN EXAMPLES
91
.
APPENDIX C RECOMMENDED THICKNESSES OF WOOD LAGGING
APPENDIX D. STEEL SHEET PILE DATA
..................
113
........................................
115
.............................................................
121
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REFERENCES
....................
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A A S H T O T I T L E CHETW 95 W Ob39804 0031538 5 3 6
LIST OF FIGURES
Figure No.
Page
1.
Acceptable and unacceptable weld profiles
2.
Shapes in which knots appear in various structurai members and methods of measuremen.t
3.
Determination of combined slope of grain...........................................
4.
Frame and braced tower buckling modes
5.
Adjustable horizontal shoring beams spanning between bridge piers and temporary timber bents
6.
Adjustable overhang bracket for precast concrete stringer ...............................
7.
Analysis of plate bearing tests
..................................................
15
8.
Analysis of pile loading tests
..................................................
16
9.
Washout under sill support
..........................................
7
. . . . . . . .9
..........................................
9
11
.... 11
12
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10.
....................................................
17
Sole plate and bracing details for falsework supported on a sloped surface ...................17
11 .
Timber cross-bracing between longitudinal stringers
12.
Cantilevered ledger beam at temporary pile bent
13.
Examples of plan bracing of modular frames
14.
Bracing detail for screw leg supporting a sloped soffit
15.
Typical installation of wire rope clip..............................................
25
16.
Bridgedeck falsework
.......................................................
26
17.
Traffícopenings
18.
Deformation of spans subject to post-tensioning
.....................................
31
19.
Formworkcomponents .......................................................
35
20.
Plywood sheathing for horizontal formwork
........................................
36
21.
Form ties
................................................................
45
22.
Coil tiesystem
23 .
Exterior and interior formwork hangers
24.
Distribution of concrete pressure with form height
25 .
Lateral pressure of concrete on formwork
..................................
20
.....................................
20
.......................................
22
.................................
...........................................................
23
28
.............................................................
...........................................
47
....................................
48
..........................................
49
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A A S H T O T I T L E CHBTW 95
28.
m
Page
......................................................... 50
Assembled gang form ............................................. ......... 52
Job-buiit formwork
32.
................................................
52
Plate girder form spanning between two supports ....................................
53
Plate girder forms used to fom a bridge pier .......................................
54
Vibration of concrete ........................................................
55
Installation of wedges .......................................................55
33.
Coilboltassembly ..........................................................
34.
Typicalcofferdams
35 .
Internally braced cofferdam systems
36.
Self-supporting and externally anchored cofferdam systems
37 .
Types of timber sheet piling
38 .
Louver effect for woad lagging
39.
Steel soldier piles
40.
Concrete in-fill between soldier piles
41.
Wood lagging to front flange
42.
Typical steel sheet-piling sections
...............................................
66
43 .
Typical pile arrangements .....................................................
67
44.
Peneiration of sheeting required to prevent piping in isotropic sand
........................
73
45 .
Penetration of sheeting required to prevent piping in stratified sand
........................
74
46.
Wood sheeting systems
......................................................
76
47 .
Soldier pile retained with soil anchors
............................................
78
48.
Sheet pile àriving procedure
...................................................
79
49.
Sheet pile installation
........................................................
84
50.
Typical framing arrangements
..................................................
86
51.
Typical connection for inclined brace and horizontal wale
..............................
87
29.
30.
31 .
Gang form for wali construction
.........................................................
.............................................
56
60
60
............................. 61
...................................................
63
.................................................
64
..........................................................
64
.............................................
65
..................................................
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65
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27.
Ob39804 0033539 472
LIST OF FIGURES (Continued)
.
Figure No
26.
m
AASHTO T I T L E CHBTW 75
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
.
= Ob37804
003L540 1 9 4
LIST OF FIGURES (Continued)
Page
Figure No
52.
Typical wale and anchor rod details
..............................................
88
53.
92
54.
............................................
Load-deflection curve for steel overhang bracket ....................................
55 .
Needle beam for slab overhang
56.
Pier cap on friction collar
57 .
Normal interlock swing is at least 10" on arch web and straight web shapes
58.
Steel sheet piling interlocks in the normal position
59.
Slab falsework with overhang bracket
................................................
101
....................................................
105
.................
..................................
Steel sheet piling interlocks in the reverse position (not recommended) ....................
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116
117
117
Ob39404 003l154L O20
A A S H T O T I T L E CHBTW 95
LIST OF TABLES
.
Page
Table No
1.
Early ASTM steel specifications
2.
.................................................
3
Permissible variations in cross section for W and HP shapes
4
3.
Permissible variations in camber and sweep
.............................
.........................................
4
4.
Matching filler metal requirements ................................................
6
5.
Referred analysis of carbon steel for good weldability..................................
6
.
7.
Fahework depth and span relationship
8.
Formulas for stress and deflection calculations for plywood
6
............................................
29
Grade-use guide for Plyform sheathing ............................................
37
9.
............................. 38
Section properties for Plyform Class I and Class II. and Structural I Plyform ................. 39
10.
Design stresses for Plyform
11.
Formulas for safe support spacings of joists and ledgers
12.
Beam formulas
13.
Typical equipment for construction of tiebacks
14.
Section properties of standard dressed (S4S) lumber
15.
Section properties of rough sawn lumber
16.
Recommended thickness of wood lagging for various soil types
17.
Standard sheet piling (cuca 1972)
18.
H-pileproperties
...................................................
39
................................
41
............................................................
42
......................................
81
..................................
89
..........................................
90
.........................
113
..............................................
118
..........................................................
119
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A A S H T O T I T L E CHBTW 95 W Ob39804 0031542 Tb7 D
ABBREVIATIONS
AASHTO
AC1
AISC
AIS1
AITC
ANSI
APA
ASCE
ASTM
AWS
BOCA
FHWA
NAVFAC
NDS
NFPA
OSHA
UBC
American Association of State Highway and Transportation Officials
American Concrete Institute
American Institute of Steel Construction
American Iron and Steel Institute
American Institute of Timber Construction
American National Standards Institute
American Plywood Association
American Society of Civil Engineers
American Society for Testing and Materials
American Welding Society
Building Officials & Code Administrators
Federal Highway Administration
Naval Facilities Engineering Command
National Design Specification for Wood Construction
National Forest Products Association
Occupational Safety and Health Administration
Uniform Building Code
GENERAL NOTATIONS
in
ft
Ibf
m
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N
kg
inches
feet
pounds (force)
meters
newtons
kilograms
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A A S H T Q T I T L E CHBTW 95 M Ob39804 0033543 9T3
cnr5
2u-
E E E c r
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Ë-ËË
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C
6
..
W
.s e ZË
õz
œm%*
82
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AASHTO T I T L E CHBTW 95
0639804 0031544 8 3 T
CHAPTER 1. INTRODUCTION
SCOPE
This construction handbook has been developed for use by contractors and construction engineers
involved in bridge construction on Federal-aid highway projects. This document may also be of interest to
falsework design engineers, and supplements information found in the Guide Design Specification for Bridge
Temporary Works.") The content is construction-oriented, focusing primarily on standards of material quality
and means and methods. This handbook contains chapters on falsework, formwork, and temporary retaining
structures. For more indepth discussion on a particular topic, related literature and references are identified.
Chapter Two. Falsework identifies material standards, the assessment and protection of foundations,
construction-relatedtopics, loading considerations, and inspection guidelines. Methods for in situ testing of
foundations are identified. General guidelines regarding timber construction, proprietary shoring systems, cable
bracing, bridge deck falsework, and traffic openings are also discussed.
Chapter Three. Formwork identifies and describes the various components and formwork types
commonly used in bridge construction. Information on load considerations and design nomogmphs are provided.
General guidelines relating to formwork construction and form maintenance are also discussed.
Chapter Four. Temporary Retaining Structures focuses primarily on cofferdams and their application
to bridge construction. As indicated by the chapter title, however, generai topics relating to a wide range of
temporary retaining structures are also addressed. Specific topics include classification of construction types,
relative costs, sealing and buoyancy control, seepage control, and protection. The construction of timber sheet
pile Cofferdams, soldier pile and wood lagging cofferdams, and steel sheet pile cofferdams is reviewed. Methods
of internal bracing, and soil and rock anchorage are also discussed.
Section properties of standard dressed and rough lumber, bridge deck falsework design examples,
recommended thicknesses for wood lagging, and steel sheet pile data are included as appendixes. Definitions
and related publications are identified below.
DEFINITIONS
For the purpose of this manual, the following definitions apply. These definitions are not intended to be
exclusive, but are generally consistent with the common usage of these terms.
Falsework - Temporary construction work used to support the permanent structilre until it becomes selfsupporting. Falsework would include steel or timber beams, girders, columns, piles and foundations, and any
proprietary equipment, including modular shoring frames, post shores, and adjustable horizontal shoring.
Shoring - A component of falsework such as horizontal. vertical, or inclined support members. For the
purpose of this document, this term is used interchangeably with falsework.
Formwork - A temporary structure or mold used to retain the plastic or fluid concrete in its designated
shape until it hardens. Formwork must have enough strength to resist the fluid pressure exerted by plastic
concrete and any additional fluid pressure effects generated by vibration.
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Cofferdam - A temporary watertight enclosure that allows construction of the permanent structure under
dry conditions.
RELATED PUBLICATIONS
California Falsework Manual, California Department of Transportation, Sacramento, CA, 1988.
Certifcation Program for Bridge Temporary Works (FHWA-RD-93-033), Federal Highway Administration,
Washington, DC,1993.
Formwork for Concrete (SP-4, Fifth Edition, American Concrete institute, Detroit, MI, 1989.
Foundation Construction, A. Brinton Carson, McGraw-Hill, New York, NY, 1965.
Guide Design Specification for Bridge Temporary Works (FHWA-RD-93-032). Federal Highway Administration,
Washington, DC,1993.
Guide Standard Specificationfor Bridge Temporary Works (FHWA-RD-93-031), Federal Highway
Administration, Washington, DC, 1993.
Haridbook of Temporary Structures in Construction, R. T. Ratay, Ed., First Edition, McGraw-Hill Book
Company, New York, 1984.
Lateral Support Systems and Underpinning, Vols. I, II, III (FHWA-RD-75-I28, 129, 130). Federai Highway
Administration, Washington, DC. 1976.
Soil Mechanics, Foundations, and Earth Structures (NAVFAC DM-7), Depanment of the Navy, Alexandria, VA,
May 1982.
Standard Specijications for Highway Bridges, American Association of State Highway and Transportation
Officials, Washington, DC (Readers are cautioned to use latest edition).
Syrühesis of Falsework, Formwork, and ScafJolding for Highway Bridge Srructures (FHWA-RD-91-062). Federal
Highway Administration, Washington, DC, November 1991.
Temporary Works, J.R. Illingworth, Thomas Telford, London, England, 1987.
2
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m
AASHTO T I T L E CHBTW 95
Ob39804 003154b 602
CHAPTER 2. FALSEWORK
MATERIALS AND MANUFACTURED COMPONENTS
Structural Steel
Quality of Steel - Steel grades greater than ASTM A36 are generally not recommended for faisework
construction. The Guide Design Specification for Bridge Temporary Works permits the use of higher working
stresses for other grades of steel, provided the grade of steel can be identified. Identification is the contractor's
be ASTM A36. For reference, some of the more common steel designations predating ASTM A36 are provided
in table 1.
Table 1. Early ASTM steel specifications,'')
ASTM reauirement
Date
Specification
Remark
1924-1931
ASTM A I
Structural steel
55,000 to 65,000
M T.S. or not less than 30,000
Rivet steel
46,000 to 56,000
M T.S. or not less than 25,000
Structural steel
55,000 to 65,000
Yi T.S. or not less than 30,000
Rivet steel
46.000 to 56,000
M T.S. or not less than 25,OOO
ASTM A9
Tensile strength, Ibf/in2
Minimum yield point, lbfhn'
1939- 1948
ASTM A7-A9
Structural steel
60,000 to 72,000
Yi T.S. or not less than 33,000
1939-1949
ASTM A141-39
Rivet steel
52,000 to 62,000
M T.S. or not less than 28,000
Conversion: 1,ûOû ibf/in2 = 6.89 N / m 2
Dimensional Tolerances - Rolling smctural shapes and plates involves such factors as roll wear,
subsequent roll dressing, temperature variations, etc., which cause the finished product to vary from published
profiles. Mill dimensional tolerances are identified in ASTM A6, Standard Specifcation for General
Requirements for Rolled Steel Plates, Shapes, Sheet Piling, and Bars for Structural Use.(') This information is
provided in tables 2 and 3 for general reference.
Conditioning of Salvaged Steel - ASTM A6 also provides guidelines for the conditioning of plates,
structural shapes, and steel sheet piling, as follows:
Plate Conditioning - Plates may be conditioned by the manufacturer or processor for the removal of
imperfections or depressions on the top and bottom surfaces by grinding, provided the area ground is
well faired without abrupt changes in contour and the grinding does not reduce the thickness of the plate
by: (1) more than 7 percent under the normal thickness for plates ordered to weight per square fi, but
in no case more than
1/8
in (3.2 mm); or (2) below the permissible minimum thickness for plates
ordered to thickness in inches or millimeters.
3
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responsibility. if steel properties are unknown and test samples are not taken, steel can generally be assumed to
m
AASHTO T I T L E CHBTW 95
Ob39804 0033547 549
T'
m
T'
T
A, depth, in
Section
nominal
size, in
T + T',
B, flange width, in
E',
flanges, out
over
Under
over
Under
of square,
theoretical theoretical theoretical theoretical max., in
center,
max., in
C, m a , depth at
any c m s scction
over theordical
depth, in
web off
To 12, incl.
1I8
1I8
114
3/16
114
3/16
114
over 12
118
1/8
1I4
3/16
5/16
3/16
114
I
H O I i 2 0 " l l l rui1.n
ANGLES
CHANNELS
W SHAPES
Table 3. Permissible variations in camber and
Sizes
Permissible variation, in
Length
I
Camber
Sizes with flange width quai to
or greater than 6 in
All
Sizes with flange width less
than 6 in
All
sweep
1/8 in x (total length, ft)
1/8 in x (total length, ft)
10
1
10
1/8 in x (total length, ft)
5
~
Certain sections with a flange
width approx. equal to depth
and specified on order as
COlUmIIs'
1/8 in x (total length' ft) wiîb 3/8 in max.
10
4
-
3/8 in + 1/8 in x (total length, ft
10
over 45 ft
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- 45)
-
Ob39804 0 0 3 3 5 4 8 4 8 5
AASHTO T I T L E CHBTW 95
Imperfections on the top and bottom surfaces of plates may be removed by chipping, grinding, or arc&
gouging and then by depositing weld metal subject to the following limiting conditions:
The chipped, ground, or gouged area shall not exceed 2 percent of the area of the surface being
conditioned.
After removal of any imperfections in preparation for welding, the thickness of the plate at any
location must not be reduced by more than 30 percent of the nominal thickness of the plate.
(ASTM A131/A131M restricts the reduction in thickness to a 20-percent maximum.)
The edges of plates may be conditioned by the manufacturer or processor to remove injurious
imperfections by grinding, chipping, or arc-air gouging and welding. Prior to welding, the
depth of depression, measured from the plate edge inward, shall be limited to the thickness of
the plate, with a maximum depth of 1 in (25.4 mm).
Structural Shapes and Steel Sheet Piling Conditioning - These products may be conditioned by the
manufacturer for the removal of injurious imperfections or surface depressions by grinding, or chipping
and grinding, provided the area ground is well faired without abrupt changes in contour and the
depression does not extend below the rolled surface by more than: (1) 1/32 in (0.8 mm), for material
less than 3í8 in (9.5 mm) in thickness; (2) 1/16 in (1.6 mm), for material 318 to 2 in (9.5 to 50.8 mm)
inclusive in thickness; or (3)
118
in (3.2 mm) for material over 2 in (50.8 mm) in thickness.
Imperfections that are greater in depth than the limits previously listed may be removed and then weld
metal deposited subject to the foliowing limiting conditions:
The total area of the chipped or ground surface of any piece prior to welding shall not exceed
2 percent of the total surface area of that piece.
O
The reduction in thickness of the material resulting from removal of imperfections prior to
welding shall not exceed 30 percent of the nominal thickness at the location of the
imperfection, nor shall the depth of depression prior to welding exceed 1%in (32 mm) in any
case except as follows:
The toes of angles, beams, channels, and zees and the stems and toes of tees may be
conditioned by grinding, chipping, or arc-air gouging and welding. Pnor to welding, the depth
of depression, measured from the toe inward, shall be limited to the thickness of the material at
the base of the depression, with a maximum depth limit of 2 percent of the total surfaœ area.
Welding - Most of the ASTM-specificationconstruction steels can be welded without special
precautions or procedures. The weld electrode should have properties matching those of the base metal. When
properties are comparable, the deposited weld metal is referred to as “matching” weld metal. Table 4 provides
matching weld metal for many of the common ASTM-designated structural steels. In general, welding of
unidentified structural steel is not recommended unless weldability is determined.
Most of the readily available structurai steels are suitable for welding. Welding procedures can be
bas& on specified steel chemistry because most mili lots are usually below the maximum specified limits.
Table 5 shows the ideal chemistry for carbon steels.
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A A S H T Q T I T L E CHBTW 95
0639804 0033549 33l
Group
Base metal steel specification'
I
ASTM A36, A53 Grade B,
M
O
O
, Mol, A529,
A570 Grades 4û,45, and 50
A709 M e 36
II
III
IV
V
Welding fimess*
Shielded metal arc
Submerged arc
Gas metal arc
Flux cored arc
welding (SMAW)
welding (SAW)
welding (GMAW) welding WAW)
AWS A5.1 or A5.5 AWS A5.17 w A5.23 AWS A5.18
AWS A5.20
EóOXX or E7OXX F6X or Fix-EMLX ER70S-X
ESXT-X and
E7XT-X (except
-2, -3, -10, -GS)
AWS A5.1 or A5.5 AWS A5.17 or A5.23 AWS A5.18
E70XX'
Fix-Exxx
ER70S-X
ASTMA242P
A572 Grades 42 and 50
A588
A709 Grades 50 and SOW
ASTM A572,
Grades 60 and 65
ASTM A514 (over 2% in thick),
A709 Grades 100 and 1OOW
(2% in and under)
AWS A5.20
E7XT-X
(except -2, -3, -10,
-GS)
ASTM AS14 (2% in and under),
A709 Grades I00 and 1oOW (2% ia and
under)
AWS A5.5
AWS A523
AWS A5.28
EOXX'
AWS A55
FIX-EXXX'
ERBOS'
ElOOXX'
AWS A5.23
F I O X - E ~
AWS A528
ER~OOS~
AWS A5.5
EllOMC
FIIX-~m
ER110s'
AWS A5.23
AWS ~ 5 . 2 8
AWS A5.29
E8XT'
AWS A5.29
EIOXT'
AWS A5.29
E l 1Xl'
~
Notes:
(a) When welds are to be stress relieved, the deposited weld metal shall not exceed 0.05 percent vanadium.
(b) See AWS Dl.1-92. Sec. 4.20 for electrarlag and electrogas weld metal requirements.
(c) In joints involving base metals of two different groups,low-hydrogen filler metal electrodes applicable to the lower strength group
metal may be used. "he low-hydrogen processes shall be subject to the technique requirements applicable to the higher strength group.
(d) Special welding materials and procedures may be required to match the notch toughness of base metal or for atmospheric corrosion and
weathering charactexistics.
(e) Low hydrogen classifrcations only.
(0 Deposited weld metal shall have a minimum impact strength of 20 ft-lbf (27 J) at O O F (-18 "C)when C h q y V-notch specimens are
used. ?his requirement is applicabte only ta bridges.
(s) Conversion: 1 in = 25.4 mm
Table 5. Preferred analysis of carbon steel
for good weldability.'5)
Element
Normal Range (96)
Carbon
0.06 - 0.25
Manganese
0.35 - 0.80
Silicon
0.10 max
Sulfur
0.035 max
Phosphorus
0.030 max
Guidance with respect to worlananship, qualification, i d inspection of weldable steel can l
x obtained
from Structural Welding Code, AWS D1.1-92,'4' Acceptable and unacceptable weld profiles prescribed by AWS
are illustrated in figure 1.
6
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Table 4. Matching fiiier metal req~irernents.'~)
AASHTO T I T L E CHBTW 95
0639804 0033550 033
S
(B) ACCEPTABLE FILLET WELD PROFILES
(A) DESIRABLE FILLET WELD PROFILES
Note:Convexity. C. of a weid o( individurl suñaca bead SMnot exceed the value of the following table:
~ n d L l g ~ a
wmh of Individual sumcr &ad.
Max. convexity
L
1/16 in (1.6
L<!316in (6mm)
Y16in < L ' l i n
mm)
l a i n (3mm)
(25mm)
3/16 in (5 mm)
L > 1 in
INSUFFICIENT EXCESSIVE
CONVEXITY
THROAT
OVERUP
EXCESSWE
UNDERCUT
INSUFFICIENT INCOMPLETE
LEO
FUSION
(C) UNACCEPTABLE FILLET WELD PROFILES
Figure 1. Acceptable and unacceptable weld profiles.")
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Timber
Timber Quality - The design values for new lumber are obtained from grading rules published by
several agencies, including: National Lumber Grades Authority (a Canadian agency), Northeastem Lumber
Manufacturers Association, Northern Sofnivood Lumber Bureau, Southern Pine Inspection Bureau,West Coast
Lumber Inspection Bmau, and Western Wood Products Association. Design values for most species and gmdes
of visually graded dimension lumber are based on the provisions of ASTM D1990-91, Establishing Allowable
Properties for Visually Graded Dimension Lumber From In-Grade Tests of Full-Size Specimens. Design values
for visually graded timbers, decking, and some species and grades of dimension lumber are based on the
provisions of ASTM Dî45-88,
Establishing Structural Grades and Related Allowabie Properties for Visually
Graded Lumber.
The methods in ASTM Dî45-88involve adjusting the strength properties of small clear specimens of
wood, as given in ASTM D2555-88,Establishing Clear Wood Strength Values, for the effects of knots, slope of
grain, splits, checks, size, duration of load, moisture content, and other influencing factors, to obtain design
values applicable to n o d conditions of service. ASTM DU5 describes the procedures for rating lumber on
the basis of strength ratio. Strengtb ratio of a stnichital timber is the ratio of its strength to that which it would
have if no weakening characteristics were present.
Used Lumber - Where the origin and grading of the matenal is no longer known, it should be regraded
by a qualified agency or individual. Timber should be discarded if it has been painted such that it prevents
assessment, if there is any sign of rot (fungal or chemical), if there is mechanid damage, or if there is any
undue distortion of shape. Timber should never be reused without careful inspection.
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AASHTO T I T L E CHBTW 95
Timber Characteristics - Because wood is an organic material, it is subject to variations in structure or
properties or both. Some important anatomical characteristics of wood and their effects on the strength of wood
members are as follows:
Knots - A knot is a portion of a branch or limb. which has been surrounded by subsequent
growth of the wood of the bunk. Knots reduce the strength of wood because they intempt the
continuity and direction of wood fibers. They also cause local stress concentrations where
grain patterns are abruptly altered. The influence of a knot depends on its size, location, shape,
soundness, and the type of stress considered. In general, knots have a greater effect in tension
than in compression, whether srresses are applied axially or as a result of bending. Shapes of
knots in various structural members and methods of measurement are illustrated in figure 2.
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
Slope of Grain - Slope of grain or cross grain are terms used to describe the deviation in wood
fiber orientation from a line parallel to the edge of the specimen. It is expressed as a ratio such
us 1 in 6 or 1 in 14, and is measured over sufficient distance along the piece to be
representative of the generd slope of the wood fibers. Slope of grain has a significant effect
on wood mechanical properties. Strength, for example, decreases as the grain deviation
increases. Specimens with severe cross grain are also more susceptible to warp and other
dimensional deformations due to changes in moisture content. The technique to measure slope
of grain is illustrated in figure 3.
Checks and Splits - Checks and splits are separations of the wood across or through the rings
of annual growth, usually as a result of drying shrinkage during seasoning. Checks are partial
depth fractures, while splits extend through the full cross section. If members are subject only
to tension or compression, checks and splits do not greatly affect strength, unless they occur in
zones of severe grain slope.
Moisture Content - Design values prescribed by the National Design Specification for Wood
Construction (NDS) are for normal load duration under dry conditions of service.'*' Dry lumber is defined as
lumber that has been seasoned to a moisture content of 19 percent or less by weight. Green lumber is defined as
lumber having a moisture content in excess of 19 percent. Because the strength of wood varies with the
conditions under which it is used, these design values should only be applied in conjunction with appropriate
design and service recommendations from the National Design Specification.
Member Size - Timber members should be generally assumed to be standard dressed (S4S)sawn
lumber unless otherwise shown on the falsework drawings. Section properties of S4S lumber are furnished in
appendix A. While these sizes are generally available on a commercial basis, it is good practice to consult the
local lumber dealer(s) to determine local availability.
Typically, the dimensions of rough-cut lumber will vary appreciably from nominal, particularly in the
larger sizes commonly used in falsework construction. If the use of rough-cut material is required by the
falsework design, the actual member size should be verified prior to use.
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AASHTO T I T L E CHBTW 95
Ob398OY 0031552 9 0 6
4
BOXED
HEART
'MEASURE
LiF
SIDE
cur
IVERAGE
o, ANO oz
.. .
Measuremeat of' Knots in doists and Planks.
<
MEASURE D, OR 0 2
wH,CHEYER IS GREATEST
Measurement of' Knots in Posts and ï i m ò e r s
or Other Compression Members
Measurement of Knots in Beams and Stringers.
Figure 2. Shapes in which knots appear in various structural
members and methods of measurement.(6)
w
That
Figure 3. Determination of combined slope of grain.")
9
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AASHTO T I T L E CHBTW 75
m
Ob39804 0031553 842
m
Manufactured Components
As defined in the Guide Design Specifictltionfor Bridge Temporary Works,manufactured components
include proprietary shoring systems and manufactured assemblies. The proprietary shoring systems consist of
verticai and horizontal shoring. Cantilever overhang brackets, hangers, and circular friction collars, are examples
of manufactured assemblies commonly used in bridge construction.
Vertical Shoring - The vertical shoring systems can be divided into three general categories. The socalled "pipe-frame" systems consist of ladder or cross-braced welded frames, with maximum allowable leg loads
from 10,OOO to 15,000 Ibf (44,OOO to 66,700 N). When externally unbraced, pipframe assemblies are generally
limited to 30-ft (9-m) heights. Intermediate frames are described as cross-braced welded frames with an
allowable load-carrying capacity of up to 30,000 Ibf (133,000 N) per leg. Heavy-duty shoring systems are
defined as having dowable leg loads of greater than 30,000 lbf (133,000 N). This terminology and the
corresponding allowable leg loading can vary from one manufacturer to the next.
In the United States, there are several manufacturers of proprietary shoring systems. However, no
industry standards exist for the various components of these systems and, as a general rule, towers or
components produced by different manufacturers should not be intermixed. Some other limitations or general
characteristics of modular systems are as follows:
o
Allowable leg capacities are generally reduced when the screw jacks, or extension legs, are
fully extended.
0
Multi-tiered towers stacked in excess of two frames high have lower allowable leg capacities
than single- or double-tier towers.
o
External bracing is generally recommended when the height to width ratio exceeds 4 to 1.
o
The drift characteristics of proprietary systems can vary considerably, depending upon their
bracing configurations. Ladder frames exhibit the least lateral stiffness, and very little benefit
is derived from the horizontal braces. The buckling mode of ladder-type and cross-braced
frames are illustrated in figure 4.
Horizontal Shoring - The term "horizontal shoring" can describe a variety of adjustable or fxed length
beams or trusses used as load-carrying members in falsework systems. Unlike vertical shoring systems where
the prescribed factor of safety is 2.5, horizontal shoring beams are specified to have a safety factor of 2.0j9'
Institute and are commonly found in manufacturer's literature!")
An application of horizontal shoring is shown in figure 5. For this particular bridge structure, temporary
timber pile bents were constructed between the permanent bridge piers and abutments. Horizontal shoring beams
spanned half the normal bridge spans and supported the formwork.
Cantilever Overhang Brackets - Adjustable, proprietary overhang brackets are common in bridge
construction due to their adaptability to various bridge dimensions. In general, the longer the vertical leg, the
farther apart the brackets can be spaced. In cases where a bridge deck finishing machine is supported at the
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General safety rules for the use of horizontal shoring are prescribed by the Scaffolding, Shoring, and Forming
A A S H T O T I T L E CHBTW 95
Ob39804 0033554 7 8 9
outer edge of a cantilevered deck overhang, however, particular care must be taken to prevent excessive
deflection of the overhang bracket and excessive lateral load on the bridge stringer web. For design purposes,
load-deflection curves or tables are generally produced by the manufacturer. An example of this data is shown
in figure 6.
Load
Load
1
1
/i
/
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1
\
l
\
\ \
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A \\
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Reactions
Reactions
Figure 4. Frame and braced tower buckling modes.
9 3/8"
1.
rI
100'-4 1/2"
F
TO Q ABOUT. BENTS
r
27'-10"+(L1)
., . - . .-
,
3/81'
J
r
I
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I
$ BENT 2
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.
11 9
n.
!
SHUKI
I
,
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1%
IT
Conversion: 1 in = 25.4 mm; 1 ft = 0.305 m.
Ergure 5. Adjustable horizontai shoring beams spanning between
bridge piers and temporary timber bents.
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k-I
BENT
ABUT
A A S H T O T I T L E CHBTW 95
Slotled 2x6 To
Clear Cad Rod
Ob39804 003355.5 bL5
--
Based on “A” = 9w”
O
25
50
75
1 ûû
125
Deflection (in)at outboard End of Bracket
Conversion: 1 in = 25.4 mm; 1 lbf = 4.45 N
Figure 6. Adjustable overhang bracket for precast concrete stringer.‘”’
FOUNDATIONS
Foundations for falsework are generally temporary in nature. Depending upon the site conditions,
foundation support is most easily provided by simple spread footings unless pile foundations are required. The
objective of this section is to familiarize the field engineer with available methods of onsite testing. Field
monitoring should also include all site features likely to influence foundation behavior.
Shallow Foundations
Falsework foundations are designed to limit the stress levels in the soil to provide an adequate factor of
safety against bearing capacity failure and to limit settlement. The design soil pressures may be selected on the
basis of measured or assumed properties of the foundation soils that were determined from a site investigation.
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
The construction engineer should familiarize himself with the site investigation data and assumptions used in the
design of the foundations to confm that the site soil and design assumptions are consistent with the conditions
in the field.
In many instances, there wili be no doubt as to the adequacy and uniformity of the soil or rock
supporting the falsework to receive the applied loads safely. In cases of variable strata, or where any doubt
exists concerning the adequacy of the soil or rock, or the general stability of the site, additional investigation
involving in situ tests, test pits, proof-rolling, and/or load tests should be undertaken to obtain sufficient
information to ensure the safety of the foundations of the falsework.
In Situ Testing - In situ tests can provide sufficient information for confirmation of foundation design.
In cohesive soils, a hand penetrometer can be used to estimate the unconfined compressive strength of the
deposit. Tests should be made at various locations within the bearing area and compared to the results of the
previous site investigation and/or the designer’s assumed value of unconfined compressive strength. In granular
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0639804 0 0 3 3 5 5 6 551
AASHTO T I T L E CHBTW 95
soils, field verification of soil strength and compressibility is more difficult. Simple tests include a dynamic
cone penetration test where the number of blows to advance a rod with a cone at the tip are recorded. The
number of blows are a rough indicator of relative density. Field density tests (AASHTO T191-86) can also be
performed to detemine the unit weight of the soil from which an estimate of relative density can also be
obtained. The measured unit weight can be compared with published information on maximum or minimum unit
weights for various soil types. Alternatively, the maximum and minimum unit weights can be determined by
performing laboratory tests AASHTO T180-86 for maximum unit weight and ASTM D-4254 for minimum unit
A better and more sophisticated procedure for determining the suitability of granular and mixed soil
deposits to support the footings is to perform pressuremeter testing (ASTM D4719-87) or dilatometer testing in
shaliow hand auger holes extended below bearing level.
Test Pits - Test pits can be dug throughout the area to investigate the various soil or rock formations.
Test pits should be used to supplement other field monitoring wherever erratic or discontinuous subsurface
conditions are present. Determining the thickness and character of these deposits from a large excavation is
more accurate than from examination of small diameter samples from borings. Block samples can also be
obtained for laboratory testing.
Proof-Rolling - Proof-rolling is a field observation test that can be used to indicate if and where
problem soils are located at shallow distances below grade. The procedure consists of making multiple passes
over the area with a fully loaded dump truck having a minimum weight of 20 tons (18,000 kg). As the dump
truck traverses the area, the amount of ground deflection under loading shall be observed Deflections of 2 in
(50.8 mm) or less are indications of reasonably good support conditions. Large deflections and severe rutting are
indicative of very poor support conditions. The depth of influence of proof-rolling is likely to be on the order of
2 to 5 ft (0.6 to 1.5 m). Any weak soil below this depth will remain undetected.
Load Testing - The procedures for performing plate bearing tests are described in ASTM D1194. The
plate load test consists of a loading plate with a minimum 12411 (305-mm) diameter with a jack to provide a
force, and with a mck or other heavy object used as a reaction. Deflections are measured with either survey
instruments or dial gauges. As the jack loads are applied, deflection readings should be taken at the design load
and at twice the design load. The test results are analyzed in accordance with figure 7. The depth of influence
of a plate load test is only about 1.5 times the diameter of the plate. Thus, larger foundations that smss the soil
to greater depths may perform differently than the plate load test would indicate.
Deep Foundations
If piles are driven to support the falsework, the driving resistance of each pile should be recorded and
compared to the required driving resistance that has been developed for the project using either a wave equation
analysis or acceptable driving formula. Plumbness, length of pile installed, type of hammer and cushion, surface
alignment of the driven pile, and any other observations that could affect pile performance should also be
recorded. This data should be given to the designer for review. If a load test is required. it should be performed
13
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weight and calculation of relative density.
0639804 0033557 498
AASHTO T I T L E CHBTW 95
in accordance with ASTM D1143-87. The procedure to calculate the failure load from a load test is identified in
figure 8.
If drilled piers are used to support the falsework, the strength of the soil at bearing level should be
determined in a manner discussed in the previous section for shallow foundations. The length and diameter of
the drilled shaft and bell (if used) should be recorded along with the plumbness and surface alignment Any
other pertinent field observations tbat could affect performance, such as the presence of squeezing or caving
soils, water inflow, or accumulation of debris at the base of the drilled pier should be brought to the attention of
the drilled pier contractor and reported to the design engineer.
Protection of the Foundation Area
Falsework foundations, in general, are set at a very shallow depth compmd with those of permanent
structures. This places them within the zone affected by seasonal moisture content changes, frost action, scour,
and so forth. The area covered by the foundations under the falsework should be considered in relation to the
general topography of the surrounding ground and the likelihood of outside influences affecting it. Steps should
be taken to safeguard it, and avoid undermining conditions such as shown in figure 9. The stability of the
ground under and around the falsework foundations will depend on the ground remaining unaffected by the
following: local influences of water from water courses, extreme rainfall, melting snow, or burst water mains;
severe frosts or excessively dry and hot weather; movements of surrounding ground subjected to excavation,
filling, or other changes; and all pressures applied by adjacent construction operations.
Falsework in Streams - Where supports (usually consisting of piles or piers) are installed in rivers or
streams, they should be designed to withstand the horizontal loads arising from flood conditions, applied to an
area of resistance substantially greater than that offered by the supports alone. This increase should account for
the accumulation of river debris. To minimize this accumulation and avoid the impact of larger pieces, measures
should be specified and installed upstream to divert such dews from the supports or to retain it independently.
The measures adopted will depend on the circumstances. The use of fenders, floating booms, and cutwaters
should be considered for this purpose.
Scour is likely to occur in areas of increased stream velocity. It is likely to affect the bed of the
waterway around and under the falsework and any banks, channels, or other existing features of the waterway.
Protection should be provided where such scouring forces are likely to occur.
Foundations on Sloping Ground - The stability of foundations on sloping ground should be examined
by a qualified engineer specializing in soil mechanics. For rock slopes, special attention should be given to the
geometry of bedding, cleavage planes, or joint planes that might provide a sliding surface for block failure. In
many sandstone, siltstone, and mudstone formations. it is not possible to predict the shear strength at bedding
planes. Here, it is necessary to ensure that the bedding does not intersect the slope in a manner that would
permit blocks to move out of the face.
Where the requirements are such that foundation members need to be set other than level, appropriately
shaped packs should be used at the base of the vertical member. The foundation member should be effectively
prevented from moving down the slope as shown in figure 10.
14
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AASHTO T I T L E CHBTW 95
0639804 0031558 324
*
---zl
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AR I T M M é r I C
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LOAD
DEF IN I T IONS
U V , I MODULUS O f SUB6RADE REACTION FOR
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MODULUS O f SUIGRADE RLAC110N FOR
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8 CORRECIED Sf I T L E U E N T = MEAfURhD
SCfTLEMENT -ac
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R 9 RADIUS O 1 C I R C U I A R ô E A R t N 6 ? l A r f d
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-
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2 . ?LOT I O G A R I T N U G
LOAD-SETTLEMENT
CURYE A N D DE TERYINE I l E l 0 ? O l M T LOAD.
3. O E T E R M I N E & A N D ?
A T 1/2 YIELD POlNT
LOAD.
1. Kv'
TO O E f t R M l N C u,,, :
FOR FIRM COHESIVE SOILS:
FOR COMESIONLESS COARSE
6 R A l N C O SOILS:
Q/S
-
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U,,,-PM,
E.
ISWAKmrti
(ClRCUtAR WTti
a9sHvl(i-Cca)
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FOR S O I L S COMBININ6
IRICTlON ANO COWCSlOü:
K y - 2.
d * C , + Cx.
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N0TE:ABOVE RELATIONSHIPS APPLY AT
SAME CONTACT PRESSURE.
Kv,
~~
Conversion: 1ft = 0.305 m
Figure 7. Analysis of piate bearing tests.(")
15
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FERFORI 2 fists ON
?LATES OF O I F f E R E W r
wiorns. SOLVE roa
C A R A # € f E R f C, ANO ..C
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-
c, + c,
~
~.
AASHTO T I T L E CHBTW 95
m Ob39804 0033559 260 m
C
t . I f S K I N F R I C T I O N A C T I N G ON TCST ? I l € M A T &E REVERSEO I I TUE ?ROfOTr?E 8Y
C O N S O l I D A T l O N OF M A l E R I A L S AdOVE TU€ I E A I l W 6 S f R A t U Y , A N A L I Z E LOAD TEST i0
oErEnmInE R E L A T I O N OF LOAD YS S E ~ T L E M E N TFOR P I L E T I P ALONE.
2 . C O M I U f E T U E D R E T I C A I E L I S T I C SMORlENIWG A S S U M I U 6 SEVERAL ? O S S l l ) l E V A R I A r l O W f
Of S K I N F R I C T I O N OW ? / L E AS S U O I N dELO1 FOR A C I I I W D R I C A L ? I L € .
3 . COMPARE f U E O R E f I C A L rllM O d f E R V E D E L A S T I C SUORIEWIWG A N D D E f E R M l N E CROûAôLC
V A R I A ~ I O äOF S K I N f R I C T l O N ON ? / L E . U S I N 6 t U I S V A R I A T I O N OF S K I N F R I C T I O N ,
COMPUlL 1010 A T T I ? .
IO4
d;
CTUIIDR / C A L ? I LE: MODULUS
(OF E L A S T I C I I Y æ E
RADIUS * R AREI
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-
-
ELASTIC SUORIEülN6
I I T U LOAD 04 A T
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0.
S K I N FRICTION
DECREASING TO #C. A T T I ? :
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SKIN FRICfIOü
(3) W E 35 TONS W
ST#CTURf
A LfSS4-R LOA0 If THf Y T T L f M f N T ioTourRfMCMS Oc TME
50 LnCIAlf.
Conversion: 1 in = 25.4 mm; 1 ton = 907 kg
Figure 8. Analysis of pile loading tests.(")
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
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A A S M T O T I T L E CHBTW 95
m
Ob39804 003L5b0 T82
Figure 9. Washout under sill support.
(Courtesy of Scaffolding, Shoring, and Forming Institute)
Figure 10. Sole plate and bracing details for
falsework supported on a sloped surface.
17
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m
AASHTO T I T L E C M B T U 95
Ob3980ir 003L5bL 919
FiIl Material - Where falsework is to be carried on fill of unlaiown origin or quality, the fill should be
investigated. Fill may have abrupt variations in composition, compaction, and strength. Where falsework is
supported on a compacted fill whose properties bave been determined, it is important to ensure that both the fill
water or environmental changes. In cases where the fill material is variable in consistency, and unable to receive
and transmit loads uniformly, a minimum depth of 18 in (457 mm) of the fill should be removed and replaced by
well-compacted and stabilized granular matenal of known M n g capacity.
Heavy Vibrations - Deposits or layers of granular
materials, if not fully compacted, are susceptible to
consolidation and settlement if subjected to vibrations either from íhe falsework above, from adjacent operations
(for example, piling), or the passage of heavy traffic. This condition is not accounted for by modification factors
applied to the presumed bearing pressures. Either the granular materials should be compacted, or the sources of
vibration stopped during critical stages of construction. Some uniformly graded sands and silts may also be
adversely affected by vibration from the compaction of concrete above the falsework.
18
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and the underlying ground are protected, so that no disturbance or loss of material resuits from îhe movement of
AASHTO T I T L E CHBTW 95
0639804 0033562 8 5 5
=
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
CONSTRUCTION
General
In falsework construction, overall stability is a function of botb internal (local) and external (global)
conditions. Internally, falsework can be subject to a wide variety of local horizontal forces produced by out-ofplumb members, superelevation, differential settlement, and so forth. TheEfote, it is necessary for the falsework
assembly to be adequately connected to resist these forces. Although friction often provides means of load
transfer, so-called "positive connections" eliminate, or at least reduce, the probability of underestimating the
necessary restraint. The need for positive load transfer is particularly apparent when superelevation exists or the
soffit is inclined.
Timber cross-bracing between adjacent steel beams, shown in figure 11, is commonly used for flange
support in falsework construction. In this method, timber struts are set diagonally in pairs between the top
flange of one beam and the bottom fiange of the adjacent beam, and securely wedged into place. However,
timber cross-bracing alone will not prevent flange buckling because the timber struts resist only compression
forces. A more effective flange support method uses steel tension ties welded, ciampeù, or otherwise secured
across the top and bottom of adjacent beams in combination with timber cross-bracing between the beams.
Uplift can occur when falsework beams are continuous over a long span, coupled with a relatively short
adjacent span. Two common examples of this condition are longitudinal beams with short end spans and a
transverse beam with a relatively long overhang. In the longitudinal example, uplift can occur at the end
support. For the latter case, shown in figure 12, uplift can occur at the fwst interior post (support). Both of
these conditions can contribute to instability and, therefore, should be avoided. If uplift cannot be prevented by
loading the short span first, the end of the beam must be tied down or the span lengths changed.
In order to ensure longitudinal stability, it is necessary to provide a system of restraint to prevent the
falsework bents from overturning when ;he horizontal design load is applied in the longitudinal direction. This
type of restraint can be furnished by diagonal bracing between pairs of adjacent bents, or by direct transfer of
horizontal load into the permanent piers.
,
c
Timber Construction
Lateral Support of Wood Beams - Deep, narrow beams may fail by buckling before the allowable
bending stress is reached if they are not laterally restrained. The mount of restraint needed to ensure beam
stability is a function of the depth-to-width ratio.
Section 4.4.1 of the National Design Specificariian fur Wood Construction provides approximate
guidelines regarding the laterai restraint of rectangular a w n lumber beams.") These guidelines, modified to
reflect the temporary nature of falsework constniction, are as follows:
If the nominal depth-to-width ratio of a timber beam is 3:l or less, no laterai support is needed.
If the nominal depth-to-width ratio exceeds 3:1, but is not more than 4:1, the ends of the beam
should be braced at the top and bottom.
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M Ob39804 003L5b3 791
Figure 11. Timber cross-bracing between longitudinal stringers.
=
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
AASHTO T I T L E CHBTW 75
.
Figure 12. Cantilevered ledger beam at temporary pile bent.
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063980i.i 0 0 3 3 5 6 4 628
AASHTO T I T L E CHBTW 75
O
If the ratio exceeds 41, but is not more than 6:1, the ends of the beam should be fully
supported by blocking between beams.
e
If the ratio exceeds 6:1, in addition to blocking at the beam ends, diagonal bridging should be
used at midspan for spans up to 16 ft (4.9 m), and at midspan and quarter points for spans
greater than 16 ft (4.9 m).
When reviewing falsework drawings, these guidelines should be applied to timber beams and stringers,
but not to joists or other light members having a depth of 8 in (203 mm) or less.
Nail Spacing - The NDS guidelines for spacing between nails or spikes in a timber connection are very
subjective. Tbe requirement is that the spacing be sufficient "...to avoid unusual splitting of the wood."
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
In recent years, there has been a trend toward the use of a greater number of nails in timber connections
than would appear warranted by prudent design considerations. In view of this, Caltrans has established the
following guidelines to govern the spacing of nails and spikes in timber connection^:('^)
e
The average center-to-center distance between adjacent nails, measured in any direction, shall
not be less than the required penetration into the main member for the size of nail being used.
e
The minimum end distance in the side member, and the minimum edge distance in both the
side member and the main member, shall not be less than one-half of the required penetration.
Toe-nailing has little or no reliable structural connection value and should not be relied on for any
construction load capacity. By splintering areas needed for bearing, toe-nailing often does more harm than good.
Vertical Shoring Systems
The following is a general discussion of vertical shoring systems that addresses their general
characteristics and utilization. Discussion of specific products has been avoided. It is recommended that the
reader refer to the manufacturer's literature as it relates to specific applications. As noted earlier, there are
several manufacturers of proprietary shoring systems in the United States, but no uniform industry standards exist
for these products.
Erection Tolerances - Most available guidelines, such as those produced by the Scaffolding, Shoring,
and Forming Institute (SSFI), simply recommend that vertical shoring be erected "plumb." A maximum
allowable deviation from vertical equal to
i18
in (3.2 mm) in 3 ft (0.9 m), but not greater than the radius or least
dimension of the vertical member, is a recurring tolerance found in some literature. However, the source of this
quantity is not welldocumented. For reference, the AISC Code of Standard Practice prescribes that the
deviation of the working line from a plumb line does not exceed 1500 for individual columdi4) Similarly, the
Precast/Prestressed Concrete Institute recommends an out-of-plumb tolerance of 1:480."')
Bracing - As a general rule, manufacturers of modular frames recommend external bracing when the
height exceeds four times the least base dimension. Therefore, it is common practice to connect rows of towers
to each other with tube and coupler horizontai lacing members or, where practical, with additional cross-bmmg,
so that rows of frames are continuously cross-braced in one plane. Braces should be connected as near as
possible to nodes and, where continuous diagonal bracing tubes are used, connections at the node points or
intermittent node points are recommended. Examples of plan bracing are shown in figure 13.
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AASHTO T I T L E CHBTW 75
Ob39õ04 0033565 564 W
OW OF SHORING
I
UBE COUPLERS
BULAR TOWER TIES
SPACING
ROW OF SHORING
L CONTINUOUS CROSS-BRACING
IN EACH ROW
(a) Lacing of shoring tower rows in one direction,
continuous cross bracing in other direction.
ROWS OF INDIVIDUAL
SHORING TOWERS
TUBULAR LACING
TUBE AND
COUPLERS
ROW SPACIN
I
I
I
(b) Shoring tower laced in both directions.
Figure 13. Examples of plan bracing of modular frames.(I6)
22
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AASHTO
TITLE CHBTW 75 m Ob37804 0031566 4 T O m
In general, plan bracing should be provided at the top lifts (tiers) and at least every third intermediate
lift. The plan bracing at the top tier may be omitted when the grillage used to support the permanent structure is
capable of acting as a diaphragm. When shoring a sloped surface, the tube bracing illustrated in figure 14 is
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
recommended.
Figure 14. Bracing detail for screw leg supporting a sloped soffit.
Screw-leg Extensions - Leg load capacity for modular frames generally decreases as the screw-leg
extensions increase. Eccentric loads on screw (extension) heads should also be avoided. Variations between
various proprietary systems preclude generalizations regarding the extent of load reduction for screw-leg
extension. However, extensions at the top and bottom of a frame totaling 12 in (305 mm) generally do not
significantly affect the allowable leg capacity.
23
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AASHTO T I T L E CHBTW 75
Ob37804 0033567 337
Cable Bracing
Bracing systems consisting of securely anchored cable guys are widely used to resist overturning of
falsework. In particular, cable systems provide an effective means of ensuring the stability of heavy-duty
shoring and are relatively inexpensive when compared to other bracing methods. Cable is also used extensively
as temporary bracing to stabilize falsework bents being erected or removed adjacent to Wit. However, the
effect of preloadmg the tower legs should be carefully analyzed before implementing this bracing technique. The
cable bracing should also be applied symmetrically to a shoring assembly to avoid unbalanced loading or
overturning.
Cables, with their fastening devices and anchorages, are "manufactured assemblies" as defined in the
Guide Design Specijication for Bridge Temporary Works. Accordingly, and in addition to information that may
be shown on the falsework drawings, the contractor should be requested to furnish a manufacturer's catalog or
brochure showing technical data pertaining to the type of cable to be used. Technical data should include the
cable diameter, the number of strands and the number of wires per strand, the ultimate breaking strength or
recommended safe working strength, and such other information as may be needed to identify the cable in the
field.
Prior to instaliation, cable should be inspected to verify that the type and size of the cable and its
condition (new or used) is consistent with design assumptions. Used cable should be inspected for strengthreducing flaws, such as obviously worn, frayed, kinked, or corroded cable, which should not be permitted in
construction.
U-bolt clips must be placed on the rope with the u-bolts bearing on the short or dead end of the rope,
and the saddle bearing on the long or live end of the rope. Improperly installed clips will reduce the safe
working load by as much as 90 percent. Also, the omission of the thimble in a loop connection will reduce the
safe working load by approximately 50 percent. After installation, clips should be inspected periodically and
tightened as necessary to ensure their effectiveness. General guidelines regarding the number of wire rope clips
and their spacing are shown in figure 15. However, efficiency factors and prescribed clip spacings can vary, and
the manufacturers literature should be consulted for a given application.
24
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
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0637804 0033568 273 W
AASHTO T I T L E CHBTW 95
Thimble
7
Dead end of wire rope
Recommended number of clips varies:
Live end of
wire rope
Three clips for 1/2 to 518 in
(12.7 to 15.8 mm)
For clips for 3/4 and 718 in
(19 and 22.2 mm)
Five clips for 1 in (25.4 mm)
L Recommended spacing varies :
3-314 in (95 mm) for 5/8 in (15.8 mm)
4-1/2 in (1 19 mm) for 314 in (19 mm)
5-1/4 in (133 mm) for 718 in (22.2 mm)
6 in (152 mm) for 1 in (25.4 mm)
Figure 15. Typical installation of wire rope clip."6)
Bridge Deck Falsework
Multiple girder bridges rarely have ground-supported deck formwork. Deck casting is usually
performed using hanger beams atîached to the interior girders, and cantilever brackets affixed to the exterior or
fascia girders. Figure 16 iliusîrates this forming method, with examples for both steel and concrete girders.
Design examples of bridge deck falsework are provided in appendix B.
The deck forms between interior stringers are generaliy set on joists hung from the top flange or
supported from the bottom flange. Proprietary hangers include removable brackets or coil-bolt assemblies that
stringer, or welded to stirnips or shear studs projecting from the top surface. Welding the hangers creates a
positive connection that will prevent movement during casting. However, several States prohibit welding these
devices to the permanent structure.
In order to form the cantilevered portion of the deck slab, a needle beam arrangement or overhang
bracket can be used. As shown in figure 16, a needle beam works weli for shallow steel girders where bottom
flange tension hangers can be easily attached. This support arrangement is temporarily attached to the steel
members, with no embedment anchors required in the slab.
A more common method of forming the overhang consists of an overhang bracket tied to the fascia
girder with a hanger support. Gravity loads from the formwork, concrete deck, and screed machine act
downward on the bracket. These loads create a force couple on the bracket, where tension is resisted by a
hanger support rod and compression is applied horizontally to the girder web. This compressive force is resisted
by bending in the beam web. For steel stringers, the web could buckle inward due to this out-of-plane force if
25
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--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
remain permanently embedded in the deck slab. The embedded hangers are generally hung over the top of the
AASHTO T I T L E CHBTW 95
= Ob39804
SCREED
SUPPORT
0033569 L O T
PLYWOOD
r SHEATHING
PROPRIETARY
ADJUSTABLE
STEEL OVERHANG
BRACKET
r STRwRS
I
ADDITIONAL STRUTS AS
REQUIRED TO PREVENT
GIRDER ROTATION
FASCIA
GIRDER
FIRST
INTEROR
GIRDER
(a) Bridge deck forming methods with steel stringers.
SCREED
~
~
~
~
O
S
POUR
S 2nd
I B
L E
RIETARY
szm
7
PLY w
re-.,
STRINGERS
PROPRIETARY STEEL
OVERHANG BRACKET
WITH HANGER SUPPORT
COD SPACER AS REQUIRED
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
PLYWOOD SHEATHMG
SUPPORT FOR POST
FASCIA
GIRDER
FIRST
MERIOR
GIRDER
(b) Bridge deck forming methods with precast AASHTO girders.
Figure 16. Bridge deck faisework.
26
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AASHTO T I T L E CHBTW 75
= Ob39804 0033570 921
the magnitude of compressive force is large enough. Therefore, bracket reactions should be investigated with
respect to compressive force resistance, and interior diagonal struts may be required to prevent bottom flange
rotation.
Some general guidelines regarding the use of overhang brackets are as follows:
o
The diagonal leg brace should bear on the web within 6 in (152 mm) of the bottom flange.
o
The exterior stringer should have its top flange tied at regular intervals to prevent outward
rotation. Recommended maximum spacing intervals are 2 ft (0.6 m), when finishing machine
rails are located on the bracket-supported formwork, and 4 ft (1.2 m) when finishing machine
rails are on the top flange of the stringer.
Precast, prestressed concrete I-girders should have ties at 8-ft (2.4-m) maximum spacing.
Steel girder diaphragm cross frames are not to be considered as ties if they do not have a top
horizontal strut.
Hardwood blocking [4 in by 4 in (102 mm by 102 mm) minimum] or the equivalent should be
wedged between webs of the exterior and interior stringer within 6 in (152 mm) of the bottom
flange, located below the top ties.
Traffic Openings
The width of a uaffic opening is generally defined as the distance between the temporary railings and,
as illustrated in figure 17, the clear distance between falsework posts will be considerably greater than the
prescribed width. For a vehicular opening, no portion of the falsework should encroach into the clearance zone
established by: a vertical plane located 3 in (76 mm) behind the back edge of the temporary barrier at its base
and extending upward to a horizontal plane at the top of the rail; and a second vertical plane located 9 in
(230 mm) behind the frst plane and extending from the horizontal plane, at the top of the rail upward to the
falsework stringer.
Temporary construction clearances often govern layout of spans. A typical example is the required
.
vertical clearance over freeways in California, shown in table 6. The usual requirement is a clearance of 16 ft-
6 in (5.0 m) over the traveled way, but the temporary construction clearance may be as low as 14 ft-6 in (4.4 m).
However, for a structure constructed on ground-supported falsework where a 40-ft (12-m) wide opening for
traffic is needed, an adequate depth of falsework may be 2 ft-6 in to 3 ft (0.8 m to 0.9 m). This results in a
finai clearance of 17 ft-O in to 17 ft-6 in (5.2 m to 5.3 m).
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
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AASHTO T I T L E CHBTW 95
Ob39804 0031.571 8b8
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
BOTTOM
OF STRINGER
1
CLEARANCE ZONE
>
t
I3
4
I-1
-
I
4c
WIDTH OF TRAFFIC
OPENING
Conversion: 1 in = 25.4 mm; 1 ft = 0.305 m
(a) Minimum clearance diagram.
(b) "Set-back distance between traffic barrier and vertical shoring.
Figure 17. Traffic openings.
28
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O
U
o
O b 3 7 B 0 4 00311572 7 T 4
AASHTO T I T L E CHBTW 95
Min. width of
traffic opening
Opening width
provides for
Required falsework
span 0)
Min. depth required
for falsework
Freeway
25 ft (7.6 m)
37 ft (11.3 m)
49 ft (14.9 m)
61 ft (18.6 m)
1 lane + (d)
2 lanes + (d)
3 lanes + (d)
4 lanes + (d)
32 ft (9.8 m)
44 ft (13.4 m)
56 ft (17.1 m)
68 ft (20.7 m)
1 ft-9 in
2 ft-2 in
2 ft-8 in
3 ft-3 in
Non-freeway
20 ft (6.1 m)
32 ft (9.8 m)
40 ft (12.2 m)
53 ft (15.8 m)
64 ft (19.5 m)
1 lane + (e)
2 lanes + (e)
2 lanes + (f)
3 lanes + ( f )
4 lanes + (f)
27 ft (8.2 m)
39 ft (11.9 m)
47 ft (14.3 m)
59 ft (17.9 m)
71 ft (21.6 m)
1 ft-9 in (.53 m)
1 ft-11 in (S8 m)
2 ft-4 in (.71 m)
2 ft-9 in (.84 m)
3 ft-5 in (1.04 m)
Special (a)
roadways
20 ft (6.1 m)
32 ft (9.8 m)
1 lane + (e)
2 lanes + (e)
20 ft (6.1 m) (c)
32 ft (9.8 m) (c)
1 ft-7 in (.48 m)
1 ft-9 in (.53 m)
Facility to be
spanned
(.53 m)
(.66 m)
(31 m)
(1.0 m)
When checking vertical construction clearances, remember that deflection of the falsework stringers
under the dead load of the concrete will reduce the theoretical clearance.
The Guide Design Specijication for Bridge Temporary Works requires the use of temporary bracing
while falsework is being erected or removed, to prevent any falsework member from falling onto an adjacent
any sidewalk or shoulder of a roadway that is open to the public, or to a point 10 ft (3.0 m) from the centerline
of any railroad track. Temporary bracing should be installed at the same time the member being restrained is
erected or, if traffic is being detoured during falsework erection, before any traffic is permitted to pass through
the opening.
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roadway. Such temporary bracing is required for falsework whose height exceeds its clear distance to the edge of
AASHTO T I T L E CHBTW 95 M Ob39804 00311573 630 M
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
LOADING
Loads During Falsework Erection
During erection of the falsework, the lower tiers or framing are subject to a steady increase of dead load
plus live load, as weil as wind and impact forces. It is when the structure or component becomes of appreciable
height in comparison with its plan dimensions that wind loading and other lateral loads are of consequence. At
this point, the potential for overturning is perhaps greater than at any other time during construction and,
therefore, adequate bracing is required to ensure stability.
The next criticai stage corresponds to construction of the formwork. This will usually entail the loading
of formwork, closely followed by reinforcement onto the falsework. The falsework will also be subjected to the
additional loads of the labor force and stacked materials. The contractor should be cognizant of the potential for
locally overloading the falsework or components, and should take adequate precautions to avoid unstable
conditions due to unbalanced loading(s).
Concrete Placement
Control of the sequence and rate of placing of the concrete is necessary so that adverse pressures are not
allowed to develop. While it is desirable to load the falsework system as uniformly as possible, the rate of
placement and location of construction joints is generaily dictated by the area of the pour. Hence, the likelihood
of some non-uniform loading is inherent in almost any cast-in-place concrete construction project. The effect of
any proposed changes in the method or sequence of concrete placement requires careful consideration.
The method of placing the concrete on the formwork, and its distribution, can impose impact or surge
effects on formwork and falsework that should be avoided or minimized. It is important that wedges and props
are properly nailed or otherwise restrained so that they do not work loose due to impact or vibration. Any
potential uplift forces should also be adequately considered. The concrete discharged onto the formwork should
not be allowed to accumulate and cause local overloading.
Load Redistribution
For post-tensioned consmction, it is generally recognized that redistribution of gravity load occurs after
the superstructure is stressed. The distribution of load in the falsework after post-tensioning is dependent on
factors such as spacing and stiffness of falsework supports, foundation stiffness, superstnicture stiffness, and
tendon profile and loads. In practice, the loads superimposed on the falsework from post-tensioning operations
will only occur at locations where the stressing tends to sag the span between bearings, as illustrated in
figure 18(a). This type of redistribution is not generally accounted for by the equipment supplier and, therefore,
the magnitude of the forces should be clearly identified on the falsework plans. In the simple span structure
illustrated in figure 18(b), the load is transferred to the point of bearing after the bridge is post-tensioned.
Similar conditions can develop where there is a redistribution of vertical load due to deck shrinkage.
Caltrans has found that depending on the falsework configuration, type of consmction, and construction
sequence. the maximum load imposed on the falsework developed within 4 to 7 days after the concrete was
placed, and varied between 110 to 200 percent of the measured load at 24 hours.('8)
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Ob39804 0031i574 5 7 7
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
AASHTO T I T L E CHBTW 95
(a) Deformation of post-tensioned cantilever.
(b) Deformation of two-span post-tensioned structure.
Figure 18. Deformation of spans subject to post-tensioning.
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A A S H T O T I T L E CHBTW 95 W 0639804 0033575 403
Other Conditions
Where precast concrete, structural steel, or other such components are applied as load to the falsework,
care should be taken to minimize impact forces in both the vertical and horizontal directions. Dragging of
sections into final position should be avoided, unless specific provisions have been incorporated into the design.
Any restrictions on the loading of the falsework so as to avoid placement of concrete or other loads when high
winds, heavy rains, snow, or flooding occur should also be defuied.
INSPECTION
General
As a minimum, inspection of the falsework is recommended during erection, prior to loading, and before
dismantling. A punchlist of items to be checked are as follows:
a
Ail the drawings and written instructions have been strictly complied with.
o
Only the correct materials in serviceable condition have been employed, especially if specific
types or qualities are required.
o
The ground has been adequately prepared and steps taken to prevent erosion.
o
Suitable foundation pads or other bases have been provided and have been properly leveled.
Foundation pads, "sleepers," and other load-distributing members laid on a slope are adequately
prevented from movement down the slope.
o
Any chocks or other supports are the correct shape and are adequately secured.
o
Baseplates have been used and are properly spaced and centered on the supporting foundation
pad.
o
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
o
The extension of each screw or adjustable base is within the permitted limits and b r a d if
necessary.
Vertical members are plumb.
o
Joints in vertical members are properly butted and aligned, and reinforced if required.
o
The spacing and level of each lift of bracing members are correct.
o
The number and position of all bracing members (longitudinal, lateral, and plan) are correct
with connections close to node points.
Vertical Take-up
Readiiy identifiable components of the deflection arise from elastic shortening of support members and
foundation settlement, but additional and often more significant deflections may occur due to take-up arising
from the straightening of bent sole plates, crushing of timber packers, and other causes. The magnitude of
deflections arising from take-up is largely dependent on the properties of packing materials and joint details. As
a general rule, the vertical take-up may be on the order of
1/16
in (1.6 mm) for every lumber surface in contact
with another wood member or steel component.
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AASHTO T I T L E CHBTW 95 D Ob39804 0033576 3 4 T D
Inspection During Concrete Placement
As concrete is being placed, the falsework should be inspected at frequent intervals. Settlement of
falsework should be monitored by telltales and referenced to stationary points such as the permanent bridge piers.
Inspection should not be conducted beneath the area of a pour as connete is being placed.
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
If it appears that a serious problem is developing, concrete placement should be temporarily
discontinued in the affected area until satisfactory corrective measures have been provided by the contractor.
Concrete placement should not be resumed until the engineer is satisfied that further settlement will not occur.
Reference is made to the Guide Design Specification for Bridge Temporary Works regarding the placement of
construction joints, but these policies tend to vary from State to State.
Indications of distress or potential problems are as follows: excessive compression at the tops and
bottoms of wood posts and wood blocking; pulling of nails in lateral bracing; movement or deflection of braces;
excessive deflection of stringers; tilting or rotation of joists or stringers; excessive movement of telItales; posts or
towers that are moving out of plumb; and the sound of cracking timbers.
Inspection After Concrete Placement
Falsework inspection should not stop with completion of the concrete pour, but should be continued
periodically until the falsework is removed. Continuing inspection is particularly important in the case of castin-place continuous structures and post-tensioned structures because of the load redistribution that occurs as the
deck concrete shrinks or when the post-tensioning forces are. applied.
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~
AASHTO T I T L E CHBTW 95
0 6 3 9 8 0 4 0033577 28b
CHAPTER 3. FORMWORK
INTRODUCTION
Formwork is a temporary srrucm that retains plastic or fluid concrete until it gains sufficient strength
to support itself. The formwork system includes the sheathing that is in direct contact wirb the concrete, the
supporting members, hardware, and bracing.
The cost of formwork is significant relative to the cost of the in-place concrete. Therefore, the selection
and design of formwork can significantly affect the overall cost of the structure. Formwork selection is
influenced by many factors, including concrete pressures, uniformity of the structure shape, accessibility to the
structure, crane capacity, material availability and cost, anticipated number of reuses, and crew experience.
This chapter presents an overview of formwork components and corresponding information for design.
Formwork for Concrete, published by the American Concrete Institute. provides extensive data for design.(")
Allowable stresses for formwork materials are those used in standard structural design, except when test data
give different values for proprietary products. Precautions to be taken in the erection. maintenance, and removal
of forms are also discussed in this chapter.
FORM COMPONENTS
Vertical forms are constnicted from five basic components: (1) sheathing, (2) studs to support the
sheathing, (3) walers to support the studs and align the forms, (4) braces to prevent shifting of the forms under
construction and wind loading, and ( 5 ) form ties and spreaders to hold the forms at the correct spacing under the
pressure exerted by the fresh concrete. The formwork srructural components and accessories should be
integrated to provide sufficient capacity in addition to easy assembly and disassembly. Typical vertical form
components are illustrated in figure 19.
Boord Sheothing
Plywood Sheothing
Double W o l e r s
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Figure 19. Formwork c~mponents.('~)
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--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
Wood Spreoder
A A S H T O TITLE
C H B T W 95
m
O L Y J A O ~ 0031,578 112
m
Sheathing
Sheathing is the supporting component of the formwork closest to the concrete. Sheathing materials
consist of wood, plywood, metal, or other products capable of transferring the load of the concrete to supporting
members such as joists or studs. The following factors should be considered when selecting a type of sheathing:
strength; stiffness; ease of removal; initial cost; reuse potential; surface characteristics; resistance to damage
during concrete placement; workability in cutting, drilling, and attaching fasteners; weight; and ease of handling.
The design information provided here relates to plywood sheathing because it is the most common sheathing
material. Figure 20 shows horizontal plywood sheathing for a concrete bridge deck to be supported on steel
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
girders.
Figure 20. Plywood sheathing for horizontal formwork.
Plywood is widely used for both job-built forms and prefabricated form modules. Virtually any exterior
type of American Plywood Association (APA) plywood is appropriate for forming concrete since this plywood is
manufactured with waterproof glue. However, the plywood industry produces a product called Plyform that is
intended specifically for concrete forming. Plyform differs from conventional exterior plywood grades in that
Plyform is constructed only from certain wood species and veneers, and its exterior face panels have thicker face
plies for greater stiffness and are sanded smooth. Typical Plyform trademarks, which indicate class, veneer
grade, and conformance with applicable standards,are given in table 7.
Plyform is available in Class I and Class II, with Class I being the stronger and stiffer panel.
Structurai I Plyform is stronger and stiffer than either Class I or II, and is often recommended for higher
concrete pressures. High Density Overlaid (HDO) Plyform is available in any of the three classes. The face
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A A S H T O T I T L E CHBTW 9 5
Ob37804 O033579 059 M
plies of HDO Plyform are bonded with a resin-impregnated fiber overlay, forming a hard, smooth surface that
eases removal and improves moisture resistance.
Table 7. Grade-use guide for Plyform
Veneer grade
Use these terms when
specifying plywoob
APA
B-B PLYFORM
Class I & II'
Typical trademarks
Description
Specifically manufactured for
concrete forms. Many reuses.
Smooth, solid surfaces. Müi-oiled
unless otherwise specified.
Inner piics
-APAPLVFORM
B-B
CLASSI
EXlERIOR
C
B
15 l d l
APA
High Density Overlaid
PLYFORM
Class I & u b
Hard, semi-opaque resin-fiber
overlay, heat-fused to panel faces.
Smooth surface resists abrasion. Up
to 200 reuses. Light oiling
recommended between pours.
c-Plugged
B
APA
STRUCTURAL I
PLYFORM
Especially designed for engineered
applications. All Group 1 species.
Stronger and stiffer than PLYFORM
Class I and II. Recommended for
high pressures where face grain is
p a d e l to supports. Also available
with High Density Overlay faces.
dPASTRUCTURAL I
PLVFORM
CLASSI
EXTERIOR
E-E
-000-
C or
c-Plugged
B
C
C
SSlU
~
Special Overlays,
proprietary panels, and
Medium Density
Overlaid plywood
specifically designed for
concrete forming!
Produces a smooth uniform concrete
surface. Generally mill-treated with
form release agent.
APA
B-C EXT
Sanded panel often used for concrete
forming where only one smooth,
solid side is required.
No standard grading; for
details of proprietary
versions, see manufacturers'
specifications.
-AMB-c
GRWPl
EXTERIOR
-0004 id3
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
Notes:
(a)
It>)
Commonly available in 19BZ-in (15. I-mm), 5/8-in (15.9-mm), 23nZ-in (18.3-mm), and %-in (19.1-mm) panel thicknesses
[4-ft by 8-ft (1.2-m by 2.4-m) size].
Check dealer for availability in your area.
Plywood manufactured in the United States is built up of an odd number of layers, with the grain of
adjacent layers perpendicular. Alternating the grain direction of adjoining layers minimizes shrinkage and
warping. In determining the flexural strength, shear strength, and stiffness of a panel, only those layers having
their grain perpendicular to the supporting stud are assumed to be stressed. The safe span of plywood is
therefore dependent not only on the type of the plywood, but also on whether it is useü in the weak direction
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AASHTO T I T L E CHBTW 75
Ob37804 003l1580 870
(the face grain runs parallel to the supports) or in the strong direction (the face grain runs perpendicular to the
supports).
Formulas for calculating the maximum allowable pressures for plywood members based on stress and
deflection are given in table 8. Table 9 summarizes section properties for Plyform Class I and Class II, and
Structural I Plyform. Design stresses and moduli of elasticity for these plywood classes are given in table 10.
Due to the nature of plywood, the moment of inertia cannot simply be divided by half of the plywood thickness
to get the section modulus. Therefore. the moment of inertia, I, is to be used to calculate deflection and the
section modulus, KS, to calculate bending stress.
Bending stress and rolling stress may each be increased by 25 percent under loads of short duration, though this
applies only when the number of reuses is limited. Since the limit on the number of reuses is not well defmed,
the designer must decide whether to use this factor. Also, the design stresses may be higher if special conditions
exist, such as if the Plyform is weil sed& against moisture so that the moisture content always remains below
16 percent. In addition to plywood strength, the designer must consider the effect of reuse on the permanent set
or deflection of the plywood.
3 spans
2 spans
Maximum allowable pressure, w, (lbf/fS)
based on bending stress
w, =
Maximum allowable pressun, w, (lbf/ftz)
based on shear stress
Bending deflection,
96F,KS
-
120F,,KS
11"
11"
19.2F,( Ib/Q)
w, =
I,
12
wi:
4 (in)
Shear deflection, 4 (in)
w1:
A p - 1743EI
A, =
~~
~~
20Fs(lb/Q)
w, =
Cwt 21;
1270EeI
~
To calculate the maximum allowable pressure based on maximum allowable deflection, Ad, calculate 4 and A,
with w = 1.0 lbffft2. Then the maximum allowable pressure based on deflection, w, (in ibffft*) is calculated as
follows:
w, =
A*.
4
+Ab
~
~
~
~~~
Convasion: 1 ibffff = 47.9 N/m*; Loo0 lbUm2 = 6.89 Nlmn?; 1 in = 25.4 mm; 1 ít = 0.305 m.
I, = span. c a t a - t w e n t a of supports, in
1, = clear span, (in)
I, = clear span + Y in for 2-in framing, or clear span
+ S/S in for 4-m framing, in
A = ddection, in
C = CoI1-t
= 120 for face grain perpeodicular to the
supports. or 60 for face grain parallel to supports
t = plywood thichicJs. in
w = uniform load, IbVff
FI = bending sircas. ibf/in*
F. = roiling shcar stress. Ibf/ïm2
Ib/Q = rolling shear constant, in*/ít
KS = effective section modulus. in3/ít
I = momait of i n d a , ¡n'/fi
E = modulus of elasticity. adjusted, IbVfr'
E. = modulus of elasticity. unadjusted, IbUR?
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--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
The design smsses in table 10 are given for Plyform used in wet conditions such as concrete forming.
A A S H T O T I T L E CHBTW 95
= Ob39804
0031581 707
=
Table 9. Section properties for Plyform Class I and Class IT, and Structural I Plyform.@)
I Properties for
I
stress
I
applied parallel with face grain Properties for stress applied perpendicular to face grain
Effective section
modulus KS
(in’lft)
Rolling shear
4.743
5.153
5.438
5.717
7.009
7.187
1.4
1.5
1.7
1.8
0.066
0.077
0.115
O.130
0.244
0.268
0.335
0.358
2.1
2.2
2.6
3.0
3.3
O.180
O.199
0.296
0.427
0.554
0.430
0.455
0.584
0.737
0.849
1.4
1.5
1.7
1.8
2.1
2.2
2.6
3.0
3.3
0.063
0.075
0.115
0.130
O.180
0.198
0.300
0.421
0.566
0.243
0.267
0.334
0.357
0.430
0.454
0.591
0.754
0.869
1.4
1.5
0.067
0.078
0.246
0.271
1.7
1.8
0.116
0.131
0.338
0.361
1
2.1
2.2
2.6
3.0
1-1B
3.3
O.183
0.202
0.317
0.479
0.623
0.439
0.464
0.626
0.827
0.955
15/32
1R
19ß2
518
2302
314
718
1
1-118
0.018
0.024
0.029
0.038
0.072
0.092
Effective section
Rolling shear
constant 1WQ
Wft)
0.107
O.130
0.146
O. 175
0.247
0.306
0.422
0.634
0.799
2.419
2.739
2.834
3.094
8.555
9.374
10.430
0.151
0.270
4.499
4.891
5.326
5.593
6.504
6.631
0.015
0.020
0.025
0.032
0.060
0.075
7.990
8.614
9.571
0.123
0.220
0.323
4.503
4.908
5.018
5.258
6.109
6.189
7.539
7.978
0.021
0.029
0.147
0.178
0.034
0.045
0.085
0.108
0.179
0.321
0.199
0.238
0.338
0.418
0.579
0.870
8.841
0.474
1 .O98
0.398
3.798
4.063
6.028
7.014
8.419
class 11
15132
IR
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
19/32
518
23/32
314
718
1
1-118
0.138
0.167
0.188
0.225
0.317
0.392
0.542
0.812
1.O23
2.434
2.727
2.812
3.074
3.781
4.049
5.997
6.987
8.388
Structural i
15/32
IR
19/32
518
23/32
314
718
2.405
2.725
2.811
3.073
3.780
4.047
5.991
6.981
8.377
Notes:
(a) All properties adjusted to account for reduced effectiveness of plies with grain perpendicular to applied stress.
(
I
Conversion:
I
)
1 in = 25.4 mm; 1 ft = 0.305 ft; 1 Ibfíft’ = 47.9 Nim’.
Table 10. Design stresses for Plyform.(”’
Plyform Class I
Plyform Class II
Modulus of elasticity - E (lbflin’, adjusted, use for bending
deflection calculation)
Modulus of elasticity - E. (Ibflin’, unadjusted, use for shear
deflection calculation)
Bending stress - Fb (lbfIin*)
Rolling shear stress - Fs (Ibfíin’)
1,430,000
1,650,000
1,500,000
1,300,000
1,500,000
1,930
72
Conversion: 1,000lbf/in2 = 6.89N/mmz
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Stnictural I Plyform
1,650,000
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1,330
72
1,930
102
AASHTO T I T L E CHBTW 75 W 063780q 0033582 b y 3
In addition to plywood, reconstituted wood materials are available for use as sheathing or as form liners.
Only those materials manufactured for forming applications, with edge sealing and surface treatment, can be
expected to endure as well as treated plywoods. Forms that are built similarly to steel plate girders, described
later in this chapter, are composed of webs, flanges, and stiffeners, with the webs in direct contact with the
concrete. Steel has high suength, stiffness, and durability, but is heavier and therefore more cumbersome to
work with. For pier caps and other applications where conduit and plumbing penetrations are limited, however,
steel formwork is often utilized if enough reuses to justify the cost of steel forms are anticipated. Fiberglasreinforced plastic forms are strong, lightweight, can be readily fabricated to nonstandard shapes, and can be
extensively reused. These forms are common in the construction of round columns, as are spiral wound waxed
paper tubes and ail-steel, two-piece column forms.
Structural Supports
For vertical wail forms, the form ties and sheathing transfer the lateral loads from fluid concrete to studs
and walers. As with sheathing, important considerations in the selection of structurai support members include
strength, stiffness, dimensional accuracy and resistance to permanent deflection, workability, weight, cost, and
durability. In proprietary modular forms, these structural supports and aligners may be made of steel, aluminum,
magnesium, or lumber. Design information for proprietary systems are available from the manufacturer.
Almost ail formwork jobs, regardless of the types of primary materials selected, usually require some
economical for ail types of formwork. Partially seasoned stock is usually preferred for concrete forming,
because dried lumber can swell excessively when wet and green lumber tends to dry out and warp during hot
weather, thus causing problems in form alignment. Information on the design of structurai lumber is presented
in this chapter. Since lumber species, grades, sizes, and lengths vary geographically, local suppliers wiil be the
primary source of advice for the specific materials and sizes that are available.
Lumber may be finished on all four sides and is then referred to as "standard dressed or S4S lumber.
When it is used directly as it comes from the sawmill, the lumber is designated as rough. Properties of standard
lumber sizes common in formwork construction are identified in appendix B.
Guidelines discussed in Chapter 2. Falsework to ensure correct timber quality and size of material are
also applicable to formwork. Expressions commonly used to determine support spacing are provided in table 11
and general beam formulas are provided in table 12. Allowable stresses and strength factors are specified in the
NDS Supplement - Design Valuesfor Wood Construction.(2')
In addition to designing structural lumber to withstand bending and shear stresses, consideration must
also be given to bearing stresses. Ailowable bearing stresses for loads applied parallel to the grain and loads
perpendicular to the grain are also given in the NDS Supplement.
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--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
lumber. Lumber that is straight and free from defects may be used for formwork. Softwoods are generally most
AASHTO T I T L E CHBTW 95
0639804 0033583 5 8 T
Table 11. Formulas for safe support spacings of joists and ledgers.'Ig)
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
To check
for single span beam
La, = Y360
1 = 1.37
Lm
= y270
1 = 1.51
(Gr
(GI
1 = 1.83
n
1 = 2.02
r!(
4,
=
1/16 in
1 = 2.75
A-
= 1/8 in
1 = 3.27
A-
= 1/4 in
1 = 3.90
1 = 3.43
(Gr
1 = 4.08
-7"
1 = 4.85
(EI
W
HORIZONTAL SHEAR
1 = 9.80
1 ?!
I = - 16Hbh + 2 h
(+r
for three or more span beam
1 = 1.69
(q
(q"
1 = 1.86
1 = 3.23
1 = 3.84
r[:
1 = 4.57
(+IB
(gr
(.r
[
!
J
4
I
I
I
BENDING
(r:
for two-span beam
i = 9.80
1 =-
W
JE
W
192Hbh
+2h
15w
1
=
10.95
40Hbh
1=-+2h
3w
Conversion: 1 in = 25.4 nun; 1 ft = 0.305 m; 1,OOO Ibf/in2= 6.89 N/mm2
1 = safe spacing of supports, in w = load, Ib per lineal fi
E = modulus of elasticity, lbfhn'
h = depth of section, in
I = moment of inertia, in'
b = width of section, in
A = deflection, in
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S = section modulus, in3
f = design value for extreme fiber stress in bending, lbf/inz
H = design value for horizontal shear stress, lbfhn*
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AASHTO T I T L E CHBTW 95
Ob39804 0 0 3 1 5 8 4 4Lb
Table 12. Beam formulas.
Simply supported beam, uniform load
ha,
A,
v,
2.
=
w1 at support
2'
=, :
at center
A,
=
-,pi 3
,V
=
i,
at support
Cantilever beam, uniform load
2
A-
,V
at support
WI4
at free end
8EI
= wl, at support
= -,
Beam continuous over two equal spans, uniform load
ha, = -w,l
8
5.
at center
48EI
ha, = -w12
,
4.
W
Simply supported beam, concentrated load at center
M-
3.
-w,l
at center
8
5wl
= -,
at center
384EI
=
W
at center support
-,WI4
185EI
l
I
I
I
I
I
I
I
I
I
I
0.229(1) from exterior support
A-
=
,V
5
= -wl,
at center support
at
I
8
Beam continuous over three equal spans, uniform load
ha, = -wl
,
10
A,
V,
at
W
an interior support
wi 4
= -,
at 0.446(1) from an exterior support
145EI
= ;wi, at an interior support
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--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
1.
A A S H T O T I T L E C H B T W 95
m
Ob39804 0033585 352
m
Table 12. Beam formulas (Continued).
6.
Beam overhanging one support, uniform load between supports
wl'at center between supports
M- = 8
A-
=
W
-,5wl
384EI
wi 3a
A, =
24EI
v- = wl
at center between supports
-
A
L
I
I
+
+-l.,II
I
T
7.
Beam overhanging one support, uniform load
M,
w2
wa
M, = -,
2
=
-(i4
2
at support B
wx
24EI(1)
wa
Ac = -(4a21
%EI
A,
\
W
1
-(i + a)*(l - a)2, at x = -
A-B
- 212x2 + 1x3 -
~
i
+
2
W
- l 3 + 3a3)
V, = wa
Conversion: 1 in = 25.4 mm; 1 kip = 4.45 kN; 1 kipin = 113 kN-mm
M = bending moment, kip-in
A = bending deflection, in
V = shear, kip
w = uniform load per unit of length, kip/in
P = concentrated load, kip
E = modulus of elasticity, kip/in2
I = moment of inertia, in'
1 = length of beam between reaction points, in
a = beam dimension shown in figures,in
x = variable distance dong beam, in
Form Accessories
Forming hardware associated with bridge construction is generally proprietary. Formwork accessories
include form ties, form hangers, and cantilever overhang brackets. Manufacturers publish safe working loads for
their proprietary form accessories. The formwork designer should beaware of the safety factors used to
determine the allowable loads. Since most formwork accessories are designed and tested as a system,
components from different manufacturers should not be interchanged.
Form Ties - A form tie is a tensile unit used to maintain form alignment against the lateral pressure of
unhardened concrete. A form tie generally consists of an inside tensile member and an external holding device.
Two basic types of form ties include continuous single member ties and threaded internal disconnecting ties.
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
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AASHTO T I T L E CHBTW 95
m
Ob39804 003158b 299
Examples of various form ties in these two categories are shown in figure 21. The safe tension loads for these
ties range from 1,000 Ibf (4.400N) to over 50,000 lbf (220,000 N). Selection of form ties for a given
application is mainly dependent on the required capacity. The advantages and limitations of the various form
ties may also affect form tie selection.
Continuous single member form ties consist of a single piece tensile unit and a device for holding the
tie against the exterior face of the form. These ties are typically made from rods or bands. They may be cut to
length on the job or completely prefabricated. Many ties are equipped with spacers, to keep forms apart a
specified distance, and breakbacks, to facilitate partial removal of the ties so that no metal is near the concrete
surface. A portion of a tie equipped with a breakback remains in the concrete. Some single member ties, such
as taper ties and threaded bar ties, can be completely removed from the concrete and reused. Taper ties and
threaded bar ties (figures 21(e) and 21(f)) may be manufactured for light to heavy construction, whereas rod- or
band-type ties (figures 21(a), 21(b), 21(c), and 21(d)) are generally used only for light construction.
The advantages of continuous single member form ties include ease of installation, low cost, and ready
availability through the United States. Also, carpenters and laborers are generally familiar with the installation
procedures for these ties. Taper ties and threaded bar ties are entirely removable and reusable. Many of these
types of ties, however, are not equipped with form spacers. One limitation of a taper tie is that after removal, a
hole is left through the concrete, requiring patching, the cost of which may be relatively high. The
watertightness of a filled hole depends on the filler and on membrane materials. While ties may be equipped
with breakbacks, rust stains from the metal left in the concrete m a y eventually appear on walls exposed to
weather, an important issue if there are architecturai concerns associated with the construction of the bridge.
She-bolts (figure 21(g)) bave external threads on the components that remain outside of the form. This
outside component of the she-bolt allows adjustment for variable form thicknesses. The various components of
the she-bolt can be pre-assembled and then fed through the forms. Stripping of forms is relatively easy with
these form ties. The internal tensile unit is left in the concrete and can be used as an anchor for subsequent
concrete-placing operations. The internal component can be set back further from the concrete wall surface than
other types of ties, thus providing the greatest corrosion resistance with the least surface finishing problems.
Also, the inner rods can be made of stainless steel if more protection is required. She-bolts are also convenient
to use in the construction of tapered walls since the inner rod can be cut in the field to any length.
The limitations of she-bolts include the high initial cost of the hardware, though some components of
the she-bolt may be reused hundreds of times if properly maintained. This tie is not equipped with internal form
spacers, although spacer cones are available. With spacer cones, however, a she-bolt tie requires the assembly of
seven individual pieces. Also, a she-bolt assembly equipped with spacer cones cannot be fed through the foxms.
If no cones are used and the she-bolt assembly is fed through the forms, reinforcing steel may interfere with the
installation of the ties. It is also difficult to inspect the internal threads of the she-bolt for wear.
Coil tie systems (figure 21(h)) may include plastic cones that act as form spacers and that set the coil tie
a specified distance from the concrete surface. Coil threads are self-cleaning, are not prone to cross threading,
and are easy to examine. The coil tie remains in the Concrete and provides an anchor for subsequent pours.
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
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AASHTO T I T L E CHBTW 95
Ob39804 0031587 125
1-
O
=.
/
4-
Woll thickness -Form
4
+
I
-
.-.+[
Form
--Notched foi
tic bimLbOCk
o : o
AO
)
mnci connecting hotdwrc
secure5
toc thrwqh 5lOtS
4
-
(a) Fiat tie.
(e) Taper tie,
(b) Snap tie.
(0 Threadedbartie.
r
Breokbock dmensim
tic t w i s t off otthc
\c,,t;;nq
er rod flots pfevcnt
dutinq stripping
(c) Wire panel tie.
(g) She-bolt.
(d) Puli-out tie.
(h) Coil tie.
Figure 21. Form ties.
45
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--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
Concrete corei or
A A S H T O T I T L E CHBTW 95
Ob39804 003L588 Ob1
Construction of forms with coil tie systems begins with the erection of one side of the form and
installation of the coil tie system as shown in figure 22. The reinforcing see1 is then positioned, the closure
forms erected, and the remaining tie hardware installed. With this installation technique, the reinforcing steel is
not positioned in ftont of tie holes and therefore does not interfere with the tie installation. However, the coil tie
system does not provide the option of being fed through the forms. The external hardware has a high initial
cost. but can be reused.
Figure 22. Coil tie system.
Form Hangers - The proprietary form hangers used with bridge deck formwork are generally the same
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
for cast-in-place decks supported on steel girders and on precast girders. A variety of formwork hangers are
available for the construction of bridge decks. Examples of an exterior hanger and of an interior hanger are
illustrated in figure 23.
Exterior hangers are designed to support the overhanging portion of a bridge deck on the fascia beam of
the bridge. Exterior hangers generally consist of a vertical support on the interior side of the fascia beam and an
exterior angled support typically used to support an overhang bracket on the exterior face of the beam. An
interior hanger, as shown in figure 23, may be equipped with a fixed length or adjustable coil bolt assembly.
Form hanger capacities generally range from 2,000 Ibf (8,800 N) to 6,000lbf (26,400 N).
46
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B A S H T O T I T L E CHBTW 95
L LUC
FLANGE WIDTH
3w-I
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
114=
3' MIN.'
Ob39804 0031589 T T B
,318"
BRACKET
Conversion: 1 in = 25.4 mm
(a) Exterior hanger.
(b) Interior hanger.
Figure 23. Exterior and interior formwork hangers.'=)
LOADS
Loads acting on formwork include the weight of fomwork, reinforcing steel and concrete, the horizontal
pressures exerted by plastic concrete, and various construction live loads, impact, and environmental loads.
During construction and use of formwork, it is necessary to know and understand the assumptions made in the
design of the formwork. Violation of these assumptions could lead to overloads and subsequent failure of the
formwork.
Construction activities that must be controlled to avoid overloading the f m w o r k include concrete
dumping onto the forms, movement of wohnen and equipment, temporary materiai storage, concrete pumping,
internai or externai vibration of concrete, and concrete placement sequence.
The laterai load imposed by fresh concrete against wall or column f m s is a function of concrete unit
weight, vibration and revibration of concrete, initial concrete temperatures, rate of concrete placement, and use of
retarding admixtures and plasticizers.
The Guide Design Specflcaion for Bridge Temporary Works provides several equations for calculating
lateral pressure against forms due to newly placed concrete.") The design pressure envelope in figure 24 and the
47
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AASHTO T I T L E CHBTW 95 W Ob39804 O033590 7LT W
design lateral pressures in figure 25 are based on these equations and are applicable only for the following
conditions:
The concrete weighs 150 Ibf/e (23.6 kN/m3). is made with Type I cement, and has a slump of
not more than 4 in (100mm). For concrete weighing other than 150 lbf/fs (23.6 W/m3), the
resulting pressure from the equations is multiplied by the ratio of actuai unit weight to 150
Ibflfe (23.6 lcN/m3).
The concrete contains no admixtures or pozzolans. When a retarding admixture, or fly ash or
other pozzolan replacement of cement is used in hot weather. an effective temperature less than
that of the concrete in the forms should be used.
The temperature of the concrete is between 40 OF &d 80 O F (4.4
"Cand 26.7 OC).
The concrete is consolidated by internal vibration to a depth of 4 ft (1.2 m)or less.
Column forms are assumed to have a maximum plan dimension of 6 ft (1.8 m), othemise they
are classified as waii foms.
If these conditions do not apply, the foms must be &signed for the full hydrostatic pressure (unit
weight x height) of the newly poured concrete layer.
DESIGN PRESSURE ENVELOPE
HEIGHT OF
CONCRETE
ENVELOPE OF PRESSURE IF
CONCRETE ACTED AS FLUID
FORMWORK
/
-.t
'\.
/
pmex+
\.
LATERAL PRESSURE
Figure 24. Distribution of concrete pressure with form height.
48
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--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
TYPICAL ENVELOPE OF
PRESSURE ON FORMWOAK
m
AASHTO T I T L E CHBTW 75
Ob39804 003159L 656
m
3200
2800
2400
-3
IL
w
2000
U
3
8
1600
w
U
a 1200
r3
o m
U
800
400
O
2
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
O
4
6
10
8
12
14
16
18
20
POUR RATE (FTM)
Conversion: 1 ibf/ft! = 47.9 Nh2; 1 ft = 0.305 m; (OF- 32)/1.8 = “C
Figure 25. Lateral pressure of concrete on formwork.
FORMWORK TYPES
Bridge formwork can be divided into two categories: vertical and horizontal formwork. Vertical
formwork can be constructed using job-built systems or prefabricated systems. Horizontal formwork can be
constructed utilizing job-built, prefabricated, or permanent stay-in-place systems. These systems are defined as:
o
Job-Built Formwork - a formwork system designed and built for a specific application, most
commonly using plywood and lumber.
o
Prefabricated or Modular Formwork - a modular system that has the durabiiity for multiple
reuses and normally is built with plywood with a metai framing. Prefabricated formwork can
be built for custom uses on special projects.
49
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0 6 3 9 8 0 4 0033592 592
AASHTO T I T L E CHBTW 95
0
Stay-in-Place Formwork - a formwork system designed such that the formwork is not removed
after construction. This system most commonly consists of stay-in-place metal decks or precast
concrete planks for forming concrete deck systems.
Job-Built Formwork
Job-built wood forms have a low initial material cost, but generally require much labor and can only be
reused 10 to 15 times. The labor cost to repair and erect job-built wood forms is high compared to that for
prefabricated modular forms that have much greater reuse potential. An example of a job-built form in bridge
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
consmction is given in figure 26.
Figure 26. Job-built formwork.
Modular Formwork
The term "modular form" refers to all-metal forms or metal-supported-plywood systems whose
integrated design of tie and connecting hardware is engineered to assure dimensional control, speed of erection,
and ease of stnpping as well as stnictural integrity. Care must be taken when assembling modular forms to
ensure tight and well-aligned joints with no offsets. Also, these forms must be inspected for permanent set or
deflection that may occur after many reuses.
The most common modular forms consist of steel frames with replaceable plywood faces. This
combination provides the job-site workability of plywood and the iarge tie spacing and form durability of steel.
Overlaid plywood furíher extends the form-face wear, and yet can be nailed or cut. The most successful of &se
systems utilize high-carbon steels to minimize weight. The steel portion of the form is generally designed to
protect the edges of the plywood and absorb tie loads and stripping, wracking, and lifting stresses. Since ties fit
between panel joints (instead of through the plywood), the steel firame absorbs the tie loads and the wear. AU-
steel forms are practical for piers and columns since they provide great rigidity and strength and can be rapidly
50
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AASHTO T I T L E CHBTW 95
0639804 0033593 4 2 9
m
erected, disassembled, moved, and re-erected. A sufficient number of reuses must be expected to justify tbe high
initiai cost. Also, special precautions must be taken when placing concrete in cold weather since the ali-steel
forms provide littìe or no insulation protection to the concrete.
Lightweight modular forms are also made of aluminum and magnesium, but are susceptible to
deterioration from contact with fresh concrete. They should, therefore, only be used if suitably coated or as a
stnictural support with a separate sheathing material. Aluminum exwsions can provide bolt slots, nailer pockets,
and other special features. Aluminum beams and double-channel walers provide large gang-wail forms that are
exceptionaliy lightweight and suaight due to the nature of the extrusions.
Stay-in-Place Formwork
In areas where form removal is expensive or hazardous, the use of stay-in-place (SIP) forms may be
desirable. SIP forms help facilitate the construction of bridge decks over high-traffic areas. The additional dead
weight of the deck slab, appearance, and corrosiveness of the environment are some of the factors that should be
considered when deciding if metal or precast concrete SIP forms should be used. Ribbed metal deck and precast
concrete elements may act solely as formwork for cast-in-place concrete, or may act compositely with the
elements fabricated from nonweldable grades of steel is generally prohibited.
Gang Forms
Gang forms consist of prefabricated formwork panels bat include sheathing, studs, and walers, joined
into larger units for ease in erecting, removal, and reuse. These systems are quickly assembled and permit
repetitive uses without rebuilding for efficient wall constniction. Modular units are fastened to each other and to
lift brackets, lift beams, tag lines, and possibly a work platform while still on the ground. Vertical angles may
also be provided along the edges in order to attach individual gang forms with bolts or special steel clamps.
Although gang forms may be used as hand-set units, they are more commonly lifted into place by
cranes and are therefore limited in size only by the crane capacity. The use of iarge gang forms helps to offset
the high cost of labor, though large forms do not easily accommodate odd shapes or field adjustments.
Integration of relatively small modular panels with large gang forms maximizes the benefits of both systems.
After the concrete becomes self-supporting, the forms can be removed as large units and efficiently reused.
Lift brackets are attached to a lift beam or directly to gang form structural elements that must have
sufficient strength ta withstand the inclined loads from the slings during lifting. Gang forms used in multi-lift
applications must be supported by specially designed inserts, anchors, and brackets because these are in t u n
supported by freshly cured concrete.
A gang form, equipped with a working platform, is shown in figure 27. The entire unit is lifted into
place and then removed as a unit when the concrete has gained sufficient strength. Gang forms are well suited
to the construction of walls as shown in figure 28.
51
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--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
concrete and become part of the load-bearing structure. Welding to flanges in tension zones or to structurai
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
Figure 27. Assembled gang form.
Figure 28. Gang form for wall construction.
52
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A A S H T Q T I T L E CHBTW 95
= 0639804 0033595 2TL M
Plate Girder Forms
Plate girder forms, such as the one shown in figure 29, are well suited for the construction of bridge
pier caps. These systems are capable of forming concrete while srnicturaîiy spanning between supports witb no
intermediate shoring. In many applications, these panels also do not require external waiers. The large tie
spacing and high pressure capacity provide form tie cost advantages in spite of the high form cost and weight.
Large plate girder modules create fewer joints to seal, align, and finish. The most significant cost-savings result
from the self-spanning capabilities of this system, which makes bridge pier construction possible while
minimizing the amount of falsework.
In plate girder form systems, the web of the steel girder doubles as a form face. The steel ribs of the
girder serve as web stiffeners to support the weight of the form and concrete. They also act as beams to transfer
the horizontal pressures of the liquid concrete from the form web to the form top and bottom flanges. The plate
girder forms come in modules that are bolted together, as needed, for the specific project. Proprietary bolting
hardware ailows the uansfer of fiange forces between individual modules, thereby allowing the formwork system
to span between supports without intermediate shoring. Examples of plate girder forms are given in figures 29
and 30.
/
TOP YOKE TY-4 @
I4 '-0 " CTRS MAX
'A "O x 2 " D BOLT
@ EACH PLATE JOINT
1 '-0" CTRS MAX
/
( 2 t l "O x 4 " A325
BOLTS @ EACH P G
CORNER BOLTING
BLOCK TOROUE TO
700 FT LBS
4
T
i.
L
B C 3 6 " R x 10'-O"
BC 36"R x 10'0"
36"R x lO'-O''
BRACKET
X E W JACK
SJ 10 (TYP)
WINDBEAM WE1
WITH PIPE BRACE
PB 10-15 (TYP)
Conversion: 1 ft = 0.305 m; 1 in = 25.4 mm
(Courtes, of Economy Forms Corporation)
Figure 29. Plate girder form spanning between two supports.
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
53
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AASHTO T I T L E CHBTW 95 H Ob39804 0033596 138
(Courtesy of Economy Forms Corporation)
Figure 30. Plate girder forms used to form a bridge pier.
CONSTRUCTION
It is essential that fonnwork is erected as designed. The assumptions made in the design of the
formwork, such as rate of concrete placement, should be designated on the shop drawings and confumed during
construction. Guidelines that apply to the safe construction of formwork are as follows:
In addition to inspection prior to concrete placement, inspection should continue during the
pour to ensure early recognition of possible form displacement or failure. A supply of extra
bracing materiais necessary in an emergency should be readily available.
8
Construction materiais, including concrete, must not be dropped or piled on tbe formwork in
such a way as to damage or overload it.
0
Safe working loads as provided by the manufacturer should never be exceeded. These
dowable loads are based on the assumption that the component is in good condition. Products
that have excessive thread wear or have been bent, overloaded, or damaged in any way should
be discarded or, if possible, reconditioned by the manufacturer. Products from different
manufacturers should not be interchanged.
Lift height, rate of concrete pouring, and use of admixtures must not differ from the
assumptions used in the design of the fonnwork.
54
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
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Ob39804 0033597 074 H
A A S H T O T I T L E CHBTW 95
e
Any welding of formwork should be performed only by a certified welder. Extreme caution
must be used when field welding any item. This produces residual stresses and embrittiement
that may lead to sudden failure. in general, manufacturers do not guarantee products that have
been altered in any way after leaving the factory.
0
Improper instaliation of form ties, including failure to install the required type or number of
ties, misalignment of form ties, and incorrect form tie lengths should be avoided.
O
External vibration should not be used if the forms were not designed for this method of
concrete consolidation. Excessive vibration of new concrete or deep vibration into semihardened lifts mz7y place higher than expected loads on formwork. Depth of vibration should
be limited to the top 4 ft (1.2 m) of new concrete. Also, new concrete should not be placed
while lower lifts are still in a plastic state (see figure 31).
Liquid Concrete
Wrong
Right
Figure 31. Vibration of concrete.'")
b
Steel wedges for securing form ties should be nailed into position to avoid movement of the
wedges during vibration of the concrete. The snap tie head must be positioned at the midpoint
of the wedge or higher; that is, on the thicker portion of the wedge (see figure 32).
Double
Head
Wedge Loosens and Bounces of1
pj
o
Wrong
Right
Figure 32. Installation of wedges.'")
b
To avoid crushing of walers or bending of wedges or washers when using a double waler
system, the manufacturer's recommended spacing between a pair of waiers should be
maintained. In general, this spacing should equal 518 to % in (16 to 19 mm) when snap ties are
55
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
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AASHTO T I T L E CHBTW 95
Ob39804 0033598 T O O
=
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
used. For use of double walers with coil bolts, coil ties, coil hanger saddles, hebolts, taper
ties, and other coil threaded items, the spacing between double walers should be equal to the
nominal diameter of the bolt plus i/2 in (13 mm).(=)
Plastic cones and metal washers are designed to act only as form spreaders. Attempts to
straighten warped walers using the form tie wedge or to over-tighten the wedge may shatter the
plastic cones, bend the metal washers, or cause premature failure of the tie at the breakback.
a
Joints or splices in sheathing, plywood panels, and bracing should be staggered. Forms should
be sufficiently tight to prevent the loss of mortar through the joints.
a
No attempt should be made to plumb forms after concrete has been placed.
The full capacity of coil tie assemblies can be obtained only when the required bolt penetration
is achieved. Installing coil bolts with less than the required minimum coil penetration causes
excessive wear on the first few threads of the bolt and places the entire load on a smaller
portion of the coil. This may cause the coil welds to break and the coil itself to unwind (see
figure 33).
I
Right
Insufficient
Form Failure
Wrong
Figure 33. Coil bolt assembly.'")
FORM MAINTENANCE
General guidelines for form maintenance developed by the American Plywood Association are as
follows:'2o'
Cleaning and Oiling - Soon after removal, plywood forms should be inspected for wear, cleaned,
repaired, spot-primed, refinished, and lightly oiled before reusing. Use a hardwood wedge and a stiff fiber brush
for cleaning (a metal brush may cause wood fibers to "wool"). Light tapping with a hammer will generally
remove a hard scale of concrete. On prefabricated forms, plywood panel faces (when the grade is suitable) may
be reversed if damaged, and tie holes may be patched with metal plates, plugs, or plastic materials. Nails should
be removed and holes filled with patching plaster, plastic wood, or other suitable water-resistant materials.
Handling and Storage - Care should be exercised to prevent form panel chipping, denting, and comer
damage during handling. Panels should never be dropped. The forms should be carefully piled flat, face to face
and back to back, for hauling. Foms should be cleaned immediately after stripping and can be solid-stacked or
56
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AASHTO T I T L E CHBTW 95
Ob39804 0031599 947 W
stacked in small packages, with faces together. This slows the drying rate and minimizes face checking of the
sheathing.
Plywood stack handling equipment and small trailers for hauling and storing panels between jobs will
minimize handling time and damage possibilities. During storage, the plywood panels should be kept out of the
sun and rain,or covered loosely to allow air circulation without heat buildup. Panels no longer suited for
Specially coated panels with long-lasting finishes are available that make stripping easier and reduce
maintenance costs. They should be handled carefully to ensure the maximum number of reuses.
Hairline cracks or splits may occur in the face ply. These "checks" may be more pronounced after
repeated use of the form. Checks do not mean the plywood is delaminating. A thorough program of form
maintenance including careful storage to assure slow drying will minimize face checking.
Oils and Compounds - Protective sealant coatings and parting agents for plywood increase form life
and aid in stripping. Mill-oiled Plyform panels may only require a light coating of oil or parting agent between
uses. Specifications should be checked before using any oil or compound on the forms. A frequently used mili
oil is 100 or higher viscosity pale (color) oil.
A liberai amount of oil or parting agent, applied a few days before the plywood is used, then wiped so
only a thin film remains, will prolong the life of the plywood form, increase its release characteristics, and
minimize staining.
When selecting and using a parting agent, one must be aware of requirements relating to fire safety,
personnel safety, environmental concern, and the effect of the parting agent on concrete finishing or painting.
Plywood Form Coatings - Lacquers, resin, or plastic base compounds and similar field coatings
sometimes are used to form a hard, dry, waterproof film on plywood forms. The performance level of the
coating is generally rated somewhere between B-B Plyform and High Density Overlaid (HDO) plywood. In
most cases, the need for oiling between pours is reduced by the field-applied coatings, and many contractors
report obtaining significantly greater reuse than the B-B Plyform, but generally fewer than with HDO plywood.
Mill-coated products of various kinds are available in addition to mill-oiled Plyform. Some plywood
manufacturers suggest no oiling be used with their proprietary concrete forming products, and ciaim exceptional
concrete finishes and a large number of reuses. In any event, the selection of a release agent should be made
with an awareness of the product's influence on the finished surface of the concrete. For example, some release
agents, including waxes or silicones, would not be used where the concrete is to be painted. The fmished
architectural appearance should be considered when selecting the form surface treatment.
57
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--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
formwork may be saved for use in subflooring or wall and roof sheathing if their condition permits.
AASHTO T I T L E CHBTW 95
Ob39804 0031b00 499
CHAPTER 4. TEMPORARY RETAINING STRUCTURES
INTRODUCTION
This chapter addresses a wide variety of temporary retaining structures common to bridge construction,
with specific emphasis on cofferdam construction. A cofferdam is a watertight structure that allows foundations
to be constructed under dry conditions. When built in water, cofferdams are usually constructed from cells that
are filled with soil to form a free-standing gravity wall. The term cofferdam also covers land-based operations
and systems located partially in water, where a temporary earth fill is placed to create a dry working platform
adjacent to another cofferdam. Steel sheet piling is most commonly used in the construction of medium to large
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
cofferdams. For land-based operations, steel soldier piles, driven or predrilled, combined with wood lagging is
also a popular system. For shallow excavations and smaller operations, wood sheeting is a viable aitemative.
Many other systems that combine steel, concrete, wood, and soil stabilization schemes are available for the
construction of cofferdams. Many new innovative patented systems, such as the prefabricated "Portadam," are
also being utilized.
CLASSIFICATIONS
Cofferdams can be classified in various ways. Several classification methods are outlined below:
By type of environment
o
o
o
in water
in soil
partially in water and partially in soil
By type of construction
o
o
o
o
o
wood sheeting
soldier piles and wood lagging
soldier piles with concrete in411
soldier pile tremie concrete
steel sheet piles
e
e
o
tangent piles or contiguous piles of concrete
precast concrete elements
cast-in-place concrete diaphragm walls
cast-in-place shotcrete
By method of support
self-supporting
o
cantilever system
o
double-wail sheet-piled dam
o
cellular cofferdam
externaliy supported
o
deadman system
o
soil and/or rock anchors
o
batter pile bents
internally braced
o
strut waier systems
o
compression rings
stabilized soil systems
o
jet-grouted
o
chemically stabilized
o
frozen ground
boxed caissons
Not ail of these systems are applicable to bridge construction. Sketches of several systems are shown in
figures 34 through 36.
59
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A A S H T O T I T L E CHBTW 9 5
Ob39804 0 0 3 L b 0 1 325
Structural bracing frame
Structurai bracinq
Sheet pile
Concrete seai caurse
ûearing oiles
(a) Without seal.
(b) With seai.
Figure 34. Typical cofferdams.
X= 1.5 in hard clays. 2 IO 2.5
in sliff to medium clays
/--
Ground surface 7
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
ûracinq ta be installed
Ercavation subqradt
inslrlled or grade beam lo extend
r on opposne side of excavation
U
LEorth berm t o remain in place
until replaced by temporary
bracing system
Raker-supported internai bracing.
Sheeting
Compression Ring
u
(b) Internai bracing with compression rings.
Figure 35. Internally braced cofferdam systems.
60
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AASHTO T I T L E
CHBTW 95 H Ob39804 0031b02 261 H
Construction Area
Possible Earth Fill
(b) Double-walled sheet pile dam.
(a) Cantilever system.
EOUIVALENT RECTANGULAR SECTION
EOUIVALENT RECTANGULAR SECTION
,
EOUIVALENT RECTANGULAR SE,CTION
- Id CIRCULAR CELLS
(cl CLOVERLEAF TYPE CELL
Ib) DIAPHRAM CELLS
(c) Cellular cofferdams
ORIGINAL CIOUNO
OWDGE
STEEL
PILING
S"EET
U
(e) With deadman anchor.
(d) With grouted anchor.
Figure 36. Self-supporting and externally anchored cofferdam systems.
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--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
Construction Area
Ob39804 0033603
A A S M T O T I T L E CHBTW 95
LTB m
Wood Sheeting
Wood sheet piles are constructed from wood planks 2 to 4 in (51 to 102 mm) thick, 8 to 12 in (203 to
305 mm) wide, with lengths varying up to 24 ft (7.3 m). In their simplest form, the planks are driven with the
nmow edges abutting. The connections may consist of mill-cut tongue and grooves or the planks may be
staggered and nailed together to fom lapped joints. Wakefield type sheeting is constructed by nailing together
three rows of planks, with the center row offset to obtain lapped joints. These various schemes for constructing
wood sheeting are illustrated in figure 37.
In order to drive wood piles into soil, the lower end of the piling is beveled and provided with a dnving
shoe ma& of
1116-
to
i/rc-in
(1.6- to 3.2-mm) thick metal. Even so, this type of sheeting is hard to drive into
very stiff or dense formations. Also, wood sheeting can span only limited lengths and therefore requires fairly
cumbersome bracing. When a single plank 3 to 4 in (76 to 102 mm) thick is used, bracing is required at a 5- to
7-ft (1.5- to 2.1-m) spacing. Bracing may be spaced at larger intervals if heavy or builtup members are used.
Soldier Piles
Soldier piles are isolated vertical elements, usually spaced at 5 to 10 ft (1.5 to 3.0 m), and driven or set
in predrilled holes and bacHilled with lean grout or concrete. The soil between the piles is supported by
lagging, shotmete, or cast-in-place reinforced concrete, The soldier piles must carry the full earth pressure, while
the lagging must resist earth loads that are relatively minor due to the soil arching effects. Because of this soil
arching phenomenon, lagging is designed empirically for a soil pressure reduced by 50 percent or more. The
design of the lagging may also be based on experience for the type of soil and span. A table giving
recommended lagging thicknesses is included in appendix C.
The most common soldier piles are rolled steel shapes, bearing piles, or H-sections. However, soldier
piles can be formed from precast sections, steel pipes, rails, double channels, or even sheet piles. Wood lagging;
usually 2 to 4 in (51 to 102 mm) thick, is the most common element used to span between the soldiers. Lagging
can also consist of light steel sheeting, corrugated metal, or precast concrete planks. Lagging can be placed
behind or in front of the front flange by using welded studs or bolts, or a J-type or C-type bolt hooked to the
front flange. Each bolt will engage two planks with a washer plate. Lagging can also be placed behind the back
flange. However, this reduces the soil arching effects and is therefore not a desirable method. Some schemes
for attaching the lagging to the piles, such as Contact Sheeting, are patented. Lagging placed behind the front or
back flange stays in position by soil pressure. Various soldier pile shapes and methods for attaching lagging are
shown in figures 38 through 41. Other schemes can be devised to suit a particular field situation. Spacers are
often placed between the lagging boards to allow drainage of seepage and backpacking of overcut zones. The
space is sometimes filled with excelsior, hay, or a geotextile to prevent soil washout.
In hard clays, shales, or cemented materials, lagging can be omitted or only a skeleton system provided
(widely spaced lagging), if the soldier piles are spaced sufficiently close. Spalling of the soil can be prevented
by attaching wiremesh to the soldiers. Soil raveling can also be controlled by spraying a bituminous compound
or shotcrete.
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
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62
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~
~~
AASHTO T I T L E
C H B TW 9.5
Ob39804 003Lb04 034
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(91
OF
TIMBER
Swær Rmat
(4flan butt-jointed sheeting.
(b) Lapped butt-joint.
(e) Tongue and groove joint.
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
( d ) Joint formed by nailing or spiking strips to sheet pila.
(e) K y e d joint with keyr inserted znd driven a t l a driving p i l a
(f)Birdsmouthjoint formed by bolting together doublabeveilsd p L . d u
( E ) Wake6cld shoot piling.
W-IELO
SHEETING
Soikad ar bQlIed
pionninq
PLAN TBG SHEETING
HEAVY TIMBER SECTIONS
with added TandG ar
dovetail joints
Figure 37. Types of timber sheet piling,'")
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A A S H T O T I T L E CHBTW 95
m
Ob39804 O031605 T 7 0
m
PLAN
I-
WOOD CLEAT
t
"LOUVER" OR
SrnCI
FRONT ELEVATION
SOLDIER PILE
Conversion: 1 in = 25.4 mm
Figure 38. Louver effect for wood lagging.")
.
.
LAGGING CAN ALSO BE
ATTACHED TO FRONT
F iANGE.
.
LAGGING CAN ALSO
BE ATTACHED TO FRONT
FLANGE ADAPTABLE
OR TIEBACKS
BEHIND FRONT FLANGE
(a) WF section of H-pile section.
01)Channel section.
LAGGING
TO FRONT
WELDED BOLT, OR
S T SECTION
t TIEBACK+
(c) Pipe section.
Figure 39. Steel soldier piles.(24)
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A A S Y T O T I T L E CHBTW 95
= 0639BOY
003LbOb 907 W
CONCRETE WALL
STEEL W SECTION
(a) Cast-in-place.
(b) Shotcrete.
Figure 40. Concrete in-fill between soldier piles.(")
T
I
II
&\\\\\\Y\\\\\'
THREADED BOLT ATTACHED BY
NELSON STUD OR RAM SET.
CONTACT SHEETING
INCORPORATED
(NYACK. N.Y.)
'\.-i,
%BOLT
PASSES BETWEEN AND PLATE
HOLDS THE TWO LEVELS OF
LAGGING BOARDS
'-PLATE
OR CHANNEL SECTION HOLDS
TOP AND BOTTOM LAGGING
(b) Bolt.
(a) Contact sheeting.
SPLIT "TO' WELDED TO FACE
(c) Split T-section.
Figure 41. Wood lagging to front flange.'")
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
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AASHTO T I T L E CHBTW 95
Ob39804 0033607 843
Steel Sheet Piles
Steel sheet piles are rolled 2-shaped or arch-shaped members, with interlockings to engage each other.
A variety of steel sheet piles are available in different forms from various manufacturers. Hot-rolled and cold-
rolled sections are manufactured with various types of interlocks. Generally, hot-rolled sections have stronger
interlocks and tighter joints as compared with cold-rolled shapes. Pieces at corners and joints are fabricated
either by riveting, bolting, or welding. Common sheet pile shapes are shown in figure 42, and section properîies
of some common sheet piles are included in appendix D. Further information on specific sheet piles can be
obtained from the nmufacturers' catalogs. If sheet piles from various manufacturers or different shapes are
mixed, their interlocking capability should be verified. If adjacent sheet piles are to be installed at an angle to
one another, the maximum angular change that can be accommodated by the interlocks should be verified by the
manufacturer. Straight sheet piling permits about a 10" angular change. For larger changes, bent sheet piling
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can be utilized. The arch-shaped sheet pile sections (PDA and PMA) interlock at the mid-line of the wall,
whereas the 2-sections or the straight web sections interlock on the inside and outside line of the wall.
!i
I-
/
14.9'7
\
0.60"
/
i
t
I
19.69"
1i
-
16.1"'
/K0.50"
-16.1"
+/i-
_____--
-4
-
0.50"
0.60"
-
i
t
7
19.69"
PS27 5'
0.40
t
Conversion: 1 in = 25.4 mm
Figure 42. Typical steel sheet-piling sections.'z)
66
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A A S H T Q TITLE
C H B T W 95
m
OO~IAOB ~ B w
T
OLMBOY
Tangent Piles
Tangent or contiguous piles consist of a single TOW of tangentially touching piles. The piles are
constructed by drilling and backfîlling with concrete. In water-bearing soils, the hole may be drilled using a
bentonite slurry and the concrete placed by the tremie method. Reinforcement may consist of a cage of several
rebars, a single bar, or a wide flange or I-beam section, placed in the hole before concreting, or mucked in the
wet concrete if no rebar cage is used. A watertight connection is not usually obtained because small gaps of up
to a couple of inches (approx. 50 mm) could remain between adjacent piles. A closed joint can be achieved by
constructing alternate piles, followed by intermediate ones that cut away a part of the first piles. This system is
usually referred to as a "secant-pile" wall. Another method of achieving a tight system is to instail a second row
of smaller piles behind the fmt row (see figure 43 for layout of piles).
TANGENT PILES
SECANT PILES
DOUBLE LINE OF FILES
Figure 43. Typical pile arrangements.
A concrete diaphragm wail is a continuous concrete wall built downward from the ground surface. The
wall m a y consist of precast or cast-in-place concrete panels cast within a trench that is stabilized with bentonite
slurry as tbe excavation proceeds. The trench is usually 24 to 36 in (610to 914 m)wide and is excavated
using a clamshell bucket or by a rotary cutting system within guide wails that range in lengths from 10 to 20 ft
(3.0 to 6.1 m). After an individual panel is excavated, the trench is cleared of sediments at the base and those
suspended in the slurry by a desanding process. A reinforcing cage is then inserted in the trench. End stops are
installed at the ends and concrete is placed by the tremie method. Soon after the initial set of the concrete, the
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AASHTO T I T L E CHBTW 95
Ob39804 0033bOq bLb
end stops are removed. Once the concrete is set and attains a specified strength, neighboring panels can be
excavated. In one system, precast concrete panels are inserted in the excavated trench and the remaining
bentonite slurry is replaced by a bentonite-cement slurry. The slurry attains a strength of 200 to 600 lbf7in2
(1,380 to 4,140 kN/m2). which is better than the adjacent soil.
SELECTION OF COFFERDAM SCHEME
Size and layout of the cofferdam will depend on the shape and size of the foundation and the layout of
the supporting piles and foundation seal, if any. The entire construction must be accommodated within the
cofferdam, including any batter piles. Where space is available and soil conditions are suitable, a sloping cut can
be made to the foundation level or to a higher level to reduce the depth of a cofferdam.
Factors that should be considered in the selection of a cofferdam scheme are listed below:
Soil type and strength.
o
Ground water.
Available installation equipment.
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
e
Static or flowing water.
o
o
Tolerable movement.
o
o
Environment and neighboring conditions.
e
Duration of the work.
e
o
o
Relative cost of the system.
External loading (e.g., from railroad
surcharge, barge impact, currents, etc.).
o
Local experience.
Space available for benching and sloping
cuts and external anchorage.
Construction staging.
Interference with existing facilities and
obstructions.
Availability of materials.
Size of internal structure (foundation, batter
piles, etc.).
Wood sheeting is suitable only for relatively shallow depths. It cannot be driven very hard and so it is
often driven as the bottom is excavated, especially in stiff or dense soils. Some overdigging may occur behind
the sheeting, and so wood sheeting is appropriate only in cohesive soils that can stand temporarily unsupported.
As the sheeting is pushed down, it slides against the walers and the face of the excavation. It is necessary to
backfill all voids soon after the sheeting is installed.
Soldier piles and wood lagging are suitable for use in cohesive soils, except when the soils are soft or
loose and have a tendency to flow after exposure and before the lagging boards can be installed. Usually this
system is not suitable for wet granular soils unless they are predrained. However, predraining may be difficult
when the wet soils (silts, sands, gravel, etc.) overlay an impervious stratum or rock. Installation of wood lagging
requires some overcut behind the lagging, which causes additional ground movements in the retained soil.
Interlocking steel sheet piles are most commonly used when a water cutoff is required. Although
seepage through the interlocks will occur, the amount of ground water flow will be reduced. Sheeting is also
used to provide a cutoff below the excavation level or to reduce seepage gradients below the bottom of the
68
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AASHTO T I T L E CHBTW 95 W Ob39804 0031610 3 3 8 W
excavation. Where rock exists within the excavation, a tight seal may be difficult to attain between the toe of
sheeting and the top of the rock. The presence of boulders or obstructions can lead to ripping of sheet piles or
jumping of interlocks, which will seriously impair the effectiveness of the sheet pile wall. Steel sheeting walls
are most often used for water-retaining cofferdams built in soft clays, organic soils, silts, wet granular soils, and
dilatant soils of low plasticity for which a soldier pile scheme is inappropriate. Hard driving conditions may
preclude the use of sheet piles and suggest predrilled soldier piles, slurry construction, or tangent piles.
Cast-in-place diaphragm walls are suitable for virtually any type of soil. However, their use for
temporary bridge construction is rare due to high costs and the permanent nature of this type of cofferdam.
Their use would be more economically feasible if they could be combined with the permanent structure elements,
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
such as abutments and wing walls, or counterweight pits or anchor pits for cable suspension bridges. Diaphragm
walls with precast concrete elements are rarely used for bridge temporary works.
Contiguous or tangent piles are constructed by boring or drilling a circular hole, generally 12 to 36 in
(305 to 914 mm) in diameter, placing a reinforcing cage or a steel beam, and backfilling with concrete.
Reinforcing elements may be placed in alternate piles or every third pile, depending on design requirements.
This system is useful in dense or extremely dense wet granular soils in which it would be difficult to drive sheet
piles. The system is, of course, more attractive if it is incorporated into the permanent structure for a loadbearing or retaining wall.
A me caisson is typically a prefabricated boxlike structure, that is sunk from the ground or water
surface to the desired depth. Its use is more common in marine construction where it can be installed in a
predredged location and then sunk by removing soil from inside without dewatering. The most common shapes
are circular and rectangular, with compamnents for bridge piers of all sizes. For smaller sizes, the shell may be
of steel, reinforced to prevent buckling. Larger sizes are made of reinforced concrete with a steel cutting edge
consisting of angles and plates. After installation to the required stratum, it will typically form a permanent deep
foundation or bridge pier. These are useful in relatively shallow [15 to 30 ft (4.6 to 9.1 m) deep] foundations
where the soil or rock is too hard for sheet pile driving, or in highly porous soil that cannot be dewatered by
conventional methods. They are not suitable for very deep excavations, or where the skin friction on the walls
of the caisson is excessive for sinking, or the foundation structure is a sloping rock surface. Great care stili is
required to ensure even sinking to the required depth, to overcome friction, and to prevent tipping that may be
hard to correct. The caisson bottom will need to be properly sealed prior to dewatering by placing tremie
concrete of sufficient thickness to withstand hydrostatic pressures. A complicated marine installation of a
caisson can sometimes be converted to a land job by creating a sand island at the pier site. The island is made
by earth filling to a level above the high-water level. Thereafter, a more conventional cofferdam of driven steel
sheet piles can be installed.
69
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AASHTO T I T L E CHBTW 95
Ob39804 003LbLl 274
RELATIVE COSTS
Relative costs of the various cofferdam systems are typically in the following ascending order:
Wood sheeting
Soldier piles and lagging
Steel sheet piles
Drilled in soldier piles and cast-in-place concrete or shotcrete
Contiguous or tangent piles
Cast-in-place diaphragm walls
Caissons
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
SELECTION OF SUPPORT METHOD
The simplest support system is a cantilever wall, where the wall element (sheeting, soldier pile, pile, or
diaphragm wall) is installed to a sufficient depth in the ground to become fixed as a vertical cantilever. ?bis
type is suitable for a moderate height, generally less than 15 ft (4.6 m), where the embedding medium is
sufficiently strong to restrain the wall. The embedment length should be designed to accommodate any scour or
erosion in front of the wall.
For deeper cofferdams, or those with insufficient or inadequate subgrade soils, a bracing system is
required. A conventional internal bracing consists of walers and struts. Various layouts of struts are possible to
suit the shape of the cofferdam and desired open space. For a relatively wide excavation, the wall can be braced
with inclined rakers reacting on a deadman or on one or more foundation units connected by grade beams. A
circular cofferdam can be braced with compression rings of rolled-steel W-shapes or by cast-in-place reinforced
concrete beams. These beams will require laterai support.
A self-supported system or an externally braced system provides an unobstructed working area For
narrow cofferdams, internal bracing is usually more economical although it may restrict working space. Grouted
soil anchors are feasible in granular soils that are at least medium dense and in cohesive soils with unconfined
strengths of over 1.5 ton-forcdf? (144 kN/m2) and where sufficient space is available for anchorage beyond a
45" slope Bom the bottom of the wall. For installation of grouted anchors, a bench about 50 ft (15 m) wide is
required for the anchor installation equipment.
Ground freezing can be used as a means of ground support, water cutoff, or a combination of both.
However, the process of ground freezing is expensive and takes a fairly long time. Unless other systems cannot
be used, this metbod may not be a viable scheme for bridge temporary works.
Ground stabilization by injection is also not a common method for cofferdams in bridge temporary
works. However, grouting is utilized for minimizing seepage through sheet pile interlocks and where the sheet
pile or other retention system has been damaged by obstructions or hard-driving resistance.
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Ob39804 003LbL2 L O O
A A S H T O T I T L E CHBTW 95
=
SEALINGANDBUOYANCYCQNTROL
When conditions are encountered that render it impractical to dewater a cofferdam for the construction
of the structural unit, a foundation seal is usually placed under water to resist buoyancy, to act as a lower
support for the sheet piles, to tie the driven piles together to resist uplift, and to provide subsequent support for
the construction of the pier footing. Foundation seal concrete is placed underwater by the tremie method. The
size and thickness of the seal concrete should be sufficient to permit subsequent dewatering without the risk of
buoyancy.
For tremie concrete, the mix selected should be highly flowable. A slump of 6 to 8 in (152 to 203 mm)
is desirable. Coarse aggregates should be under W in (19 mm) and preferably rounded for better workability.
An appropriate mix design and trial batches should be prepared. Air entrainment is not necessary. Use of
pozzolan as a partial replacement for cement improves flow and reduces heat of hydration.
Placement of tremie concrete is best initiated in a sealed tube or tremie pipe 10 to 12 in (254 to
305 mm) in diameter. The bottom is closed by a plate with a gasket, tied to the pipe with twine. The plate is
held in place tightly by the static pressure of water as the pipe is lowered. Concrete is placed into the pipe, just
sufficient to balance buoyancy. The pipe is then raised about 6 in (150 mm) off the bottom. This is usually
sufficient to break the seal and the concrete flows out. Additional concrete is constantly kept flowing into the
uemie pipe. The concrete continues to flow out and fill the cofferdam, maintaining a surface slope of about
6 (horizontal):l (vertical) to 10 (horizontal):l (vertical). The tip of the pipe must be kept immersed a depth of
3 to 5 ft (0.9 to 1.5 m) in the concrete. If the tip is raised out of the concrete, the seal will be lost, the flow rate
will increase, and water will be mixed with the concrete, causing segregation and loss of strength.
The tremie pipe layout and sequence should be such as to maintain acceptable flow distances of about
25 to 35 ft (7.6 to 10.7 m) and to prevent cold joints. The latter requires relatively high pour rates of about 50
to 100 yd3/h (38 to 76 m3/h). Retarding admixtures have been found to be helpful in preventing cold joints. The
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
tremie concrete surface will be somewhat irregular, with a mound at the location of the urnie pipe. The valleys
can be filled after dewatering, when any laitance is also removed. Horizontal lifts are not desirable in a tremie
concrete placement as the surfaces will have laitance. If a large cofferdam is to be subdivided, it should be done
with a vertical bulkhead.
Buoyancy is resisted by the weight of the seal concrete, the cofferdam elements (if they are anchored
with the seal), and from uplift resistance of the foundation piles embedded in the seal. However, the weight of
the cofferdam elements should not be included in the resistance of hydrostatic uplift pressures. Also, since it is
difficult to estimate the frictional resistance of sheet piles and to engage ail of the sheet piles in the seal coat, the
resistance of the sheet piles to buoyant forces should not be included. When conditions are encountered that
render it impractical to place seai coat concrete of sufficient thickness to resist buoyancy, additional resistance
can be provided by rock or soil anchors drilled to sufficient depth below the seal coat and embedded in the seal
coat or anchored to it. Anchors can be installed through sleeves cast in the seal coat and grouted after the
anchors have been preloaded.
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AASHTO T I T L E CHBTW 95
Ob39804 0033633 047
Where the foundation is supported on the subgrade soils, it will be necessary to remove al disturbed
soil from the excavated subgrade. In underwater excavation, this is difficult to verify. Dry excavation is
possit4e only in cohesive soils or in cohesionless soils if the water table is sufficiently depressed. Even i€ the
disturbed soil is removed while it is dry, piping and sloughing can occur under any seepage head. Hence, the
subgrade should be protected by placing a layer of clean gravel or crushed rock, Flooding of the cofferdam to a
level equal to the outside water level will also ensure subgrade integrity.
SEEPAGE CONTROL
When the water level outside the sheeting is higher than the excavation level within the cofferdam, the
water percolates through the soil behind the sheeting and then upwards in front of the sheeting. The upward
seepage reduces the effective weight of the soil and consequently the passive resistance. Seepage forces per unit
volume equal the unit weight times the seepage gradient. When the gradient is high, seepage forces can equal
the buoyant weight, and sand boiling or piping can occur. Piping is controlled by dewatering outside the
cofferdam (lowering the water table), by pressure relief using dewatering wells within the cofferdam, and by use
of a cutoff (extending sheeting deeper to reduce gradients). Deepening of the cutoff is particularly effective if
the toe is embedded in an impervious layer that will stop or reduce flow around the bottom of the cutoff. The
design of sheeting penetration to control piping for various subsurface conditions is presented in figures 44
and 45.
In order to prevent blow-up of a relatively thin impervious layer penetrated by the sheeting, pressure
relief of underlying pervious soils may be necessary using deep wells. If interlocks between sheet piles are poor,
high water pressures may be created in the soil sandwiched between two impervious layers by the head of water
outside the sheeting.
Minor seepage within the cofferdam can be removed by sump pumps or wellpoints. The latter is
preferred where excessive seepage through the interlocks creates piping or boiling. In certain situations, grouting
of the interlocks might be a better alternative to control horizontal seepage. Cement-bentonite grout has been
found to be very effective in sealing the interlocks within the soil, For the section within the water, cinders
sprayed on the surface flow into the interlocks and help to seal them. Alternate measures include caulking and
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
welding.
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ü b 3 9 8 0 4 O033634 T 8 3 W
AASHTO T I T L E CHBTW 95
P E N E T R A T I O N R E Q U I R E D FOR S H E E T I N G
I N DENSE S A N D OF l l M / T E û DEPTH
$
$
\
\
f
\
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\
\
\
?/H#./
A FACTOR
OF
AGA I NS T P I P I N 6
1
O. 5
RATIO
w/n,
9
1.0
R A T I O OF
1,s
r i o r n OF E X C A V A T I O N T O N E T HEAD
Figure 44. Penetration of sheeting required to prevent piping in isotropic sand.(12)
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2.
m
A A S H T O T I T L E CHBTW 95
Ob39ô04 003Lb15 91T
m
COARSE SAND UNDERLYING f I N € SAND
. a r
. . I
I f TOP O f COARSE LAYER I S A T A DEPTH BELOW S U E E T I N G
T I P S GREATER TMAN W I D T H OF EXCAVATlON,SAFETY
FACTORS O f F I G . 8 - 2 f o b I Y f I N l T E DEPTH A P P L Y .
I f TOP O f COARSE LAYER IS A T A DEPTH BELOW S H h E T I U G
T I P S LESS T H A N W I D T U O f EXCAVATION, TUE U P L I F T
PRESSURES ARE 6ûEATER T U A N FOR TME UOYOGENEOUS
CROSS-SECTION. I f PERMEABIL I TY O f COARSE LAYER I S
YORE TUAN T E N T I M E S T H A T O f F I N E L A Y E R , f A I L U R E
HEAD In,)
TUICKNESS OF F I N E LAYER I U z ) .
I MPER v I OUS
f
f l N E SAND UNDERLYING COARSE SAND
o
0.0
PRESENCE O f F I N E LAYER CONSTRICTS f L O 8 BENEATH
.* o
COARSE
S H E E T I N 6 AND SENERALLY DECREASES SEEPAGE GRADIENTS
l n THE COARSE u m .
I F TOP OF F I N E LAYER L I E S BELO8 S H E E T I N 6 T I P S ,
SAFETY f A C T O R S ARE IWTERMEOIATE BETWEEN TUOSE FOR
AN IM?ER#EABLE BOUNDARY A T TOP OR BOTTOM O f TUE
F I N E LAYER I N f l G . 8 - 2 .
I f TOP O f THE F I N E LAYER L I E S A I O V E S U E E T I N G T I P S
THE SAFETY FACTORS O f F I G . 8 - 2 ARE SOMEWUAT
CONS E UVA T I VE FOR PENETRA T I ON REQUIRED.
I MPERV IOUS
F I N E LAYER I N HOMOGENEOUS SAND STRATUM
I f THE TOP OF F I N E LAYER I S A T A DEPTH GREATER T H A N
WIDTH OF E X C A V A T I O N BELOW 5 H E E T f N 6 T I P S , SAFETY
FACTORS O f F I G . 8 - 2 APPLY, ASSUMING IMPERVIOUS
BASE A T TOP OF F I N E L A Y E R .
I F TOP O f F I N E LAYER I5 A T A OEPTU L E S 5 T H A N W l D r U
O f E X C A V A T l O ü BELOW S H E E T I N 6 T l ? S , PRESSURE R E L I E F
I S REQUIRED SO T M A T U N B A L A I C E D MEAD BELO8 F I N E
LAYER DOES NOT EXCEED H E I G H T OF S O I L ABOVE BASE Of
LAYER.
I f F I N E L A Y E R L I E S ABOVE SUBGRADE O f EXCAVATION,
F I N A L C O N D I T I O N I S SAFER THAN UOMOGENEOUS CASE,
BUT DAN6EROUS C O N D l T l O N M A ï A R I S E D U R I N 6
EXCAVATION ABOVE TUE f I N € LAYER AND PRESSURE
R E L I E F I S REQUIRED AS I N TUE PRECEDlN6 CASE.
Figure 45. Penetration of sheeting required to prevent piping in stratified sand.('')
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--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
PRESENCE O f COARSE LAYER MAKES f L O 8 II F I N E M A T E R I A L
MORE NEARLY U E R T I C A L A Y O G ~ N E R A L L Y INCREASES
SEEPAGE GRADIENTS I N TUE F I N E LAYER C O M P A R E D TO
TME HOMOGENEOUS CROSS-SECTION O f f I t . 8 - 2 .
AASHTO T I T L E CHBTW 95
O639804 0033636 856
PROTECTION
A cofferdam constructed in a channel creates an obstruction to flow and therefore creates a higher
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
stream velocity. Erosion along the cofferdam should be prevented by placing riprap on the bed unless the
anticipated scour is provided for in the design. Scour depths and required riprap for protection can be estimated
from formulae given in standard textbooks. Additional scour can occur from obstructions created by floating
debris, bushes, and logs during floods. Scour is most severe in fine sand or silt, and at comers of the cofferdam
due to eddy currents.
Stability of the cofferdam must be investigated for ail conditions of loading. Differential pressures on
the sides of the cofferdam can occur when variations exist in the height of soil or in surcharge loadings. In the
cofferdams for abutments, there is often a higher ground level on the land side and a lower ground level on the
channel side. The cofferdam bracing must be designed for this unequal loading. The higher pressures can be
balanced by use of external deadman anchors, grouted tiebacks, tension piles, or a batter pile frame system.
Pressures can also be equalized by excavating from the high side and filling on the low side. It is sometimes
feasible to create a rigid truss by welding the waiers to the sheet piles and providing diagonal bracing in a
vertical plane. A deep truss can also be created by welding the interlocks of the sheet piles.
In marine construction, the cofferdam will need protection from impact by barges and waterway M i c .
Lateral loads from collision of work barges and loading from construction equipment should be accommodated in
the protection scheme. During winter construction, loading from ice forces, if applicable, needs to be considered.
When water levels exceed the design level, cofferdam stability will be affected not only by increased lateral
pressures, but also by uplift pressures on the seal coat, if any, and by reduced passive pressure on the embedded
sheet pile section due to increased seepage gradients. These situations can be negated by flooding the cofferdam
until the water level drops below the design level. The correct level of flooding can be obtained by cutting a
slot in the sheet pile at the design water level.
After completion of the footings and the pier or abutment within the cofferdam, intemai waiers and
braces can be removed in stages as the backfilling progresses. The cofferdam must remain stable during these
stages with the reduced number of supporting members.
Conditions can exist where the sheet pile toe remains higher than the excavation level, such as from the
presence of obstructions, boulders, and cobbles. in such cases, the sheet piles require protection from kickout of
their toe from lateral pressures. Lateral support can be provided by internal braces or external anchors,
depending on the site and ground conditions. For vertical support, the cofferdam can be supported by grouting
and stabilizing the lower strata or by leaving an adequate bench near @e toe of sheet piles and stepping in for
the remaining excavation. These situations frequently occur where zones of gravel, cobbles, boulders, or
extremely dense tills overlay weathered rock and the foundations are designed to bear on sound rock.
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A A S H T O T I T L E CHBTW 95
=
Ob39804 003LbL7
CONSTRUCTION
Timber Sheet Pile Cofferdam
Timber sheet piles are used only for small and relatively shallow excavations. Their use is more
common in utility trenches. Various types of wood sheet piling are shown in figure 46. The difficulty in the
use of wood sheeting is the need for bracing at spacings of 5 to 7 ft (1.5 to 2.1 m), therefore a cluttered
cofferdam results from the obstructions of walers and struts. Furthermore, the wood sheeting cannot be driven
through hard or dense soils, requires progressive excavation, and can sustain only light driving. Hard driving
results in splitting or brooming and damage to the sheeting. They are most useful in low headroom situations,
for low heads of water and for cofferdams founded on an irregular bedrock surface, and where hand excavation
is necessary due to obstructions, such as utilities. Wood sheeting can also be advantageous in circular
cofferdams in cases where they can be braced with rolled-steel compression rings. The maximum length of
available wood sheeting is approximately 24 ft (7.3 m). For deeper excavations, a telescopic arrangement with
lower shafts progressively reduced in size, can be utilized. The step-in at each level will depend on the size of
the bracing waiers or rings at the upper shaft.
TIMBER
Ca
STEEL
R A N K S . SHEET
PILES. TRENCH
SHfETS ETC
cha
eLoci<s
Timbered cofferdam
xcowotion subgrade
Figure 46. Wood sheeting systems.'w
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
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0 6 3 9 8 0 4 0 0 3 L b L B 629
AASHTO T I T L E CHBTW 95
Wood sheeting is usually driven with light, air-driven hammers. An excavation is usually carried out to
the bottom of the sheets which are then driven 2 to 3 ft (0.6 to 0.9 m) below the cut level. After driving to
proper depth, bracing consisting of heavy timbers, is installed. Often, steel beam walers are used to aiiow wider
spacing of struts. Typical wood sheeting installation is illustrated in figure 48.
Loations of soldier piles are marked and sometimes set with a template. Piles are installed by
conventional pile-driving equipment or installed in predrilled holes. Both impact-type and vibratory hammers are
used. Vibratory hammers are more appropriate in granular soils, and they cause less noise than an impact
hammer. Hammer energy is selected based on experience considering the type of soil and length of the pile.
Maximum hammer energy for steel piles is on the order of 1,500 to 21000 ft-lb/in2 (3.2 to 4.2 N-m/mm2) of
cross-sectional area of the pile. Variation in vertical alignment of about 1 percent of pile length should be
expected. Underground obstructions and very hard driving resistance could cause greater misalignment or
twisting. Where hard driving or cobbles/gravel is expected, it may be preferable to provide a driving shoe at the
pile tip.
Where vibrations and noise must be limited or hard ground conditions are anticipated, piles can be
installed by predrilling. This method also allows use of less compact pile sections or fabricated double-channel
sections that are too flexible to be driven. The hole size must be a few inches larger than the pile size. Drilling
may be performed by augers or by a rotary method using drilling fluid or a temporary casing, depending on the
soil and ground water conditions. In rock strata, percussion drilling may be more appropriate. After drilling, the
structural steel section is inserted and the hole backfilled with lean concrete or cement-sand grout of a 1- to 2bag cement mix. A higher strength backfill is also used in the section below the excavation levei. For dry
holes, concrete is placed by the free-fall method using a funnel or an elephant trunk. In wet holes, it is placed
by the trernie method. The tremie pipe must be kept immersed at least 5 ft (1.5 m) into the grout to prevent
entrapment of slurry in the grout. Usual slump for the free-fall concrete is 5 to 6 in (127 to 152 mm) and for
the tremie grout, 8 to 10 in (203 to 254 mm). A temporary casing, if used, can be withdrawn as the hole is
filled with grout.
Lagging is installed in the space between the soldier piles. Wood lagging boards may be placed behind
the front flange or attached to the front flange with welded studs or a J-type bolt engaging the front flange. Soil
needs to be trimmed carefully, usually by handtools, to fit the lagging boards. The typical procedure is to dig
below the last section of installed lagging and to rotate and slide the next lagging board in place. The boards
require a bearing of about 2 in (51 mm) on the flanges of the pile. Digging below the installed boards will
depend on the type of soil. It may be about 1 ft (0.3 m) in silts or sands and as much as 4 to 5 ft (1.2 to 1.5 m)
in stiff cohesive soils.
Wood boards may be installed tightly or with a small space of 1 to 1% in (25 to 38 mm) left between
them using a cleat, called a louver. This space allows drainage of any seepage and also allows backpacking of
overcut voids behind the iagging (see figure 38). In wet soils, a geotextile or excelsior is placed at the openings
to prevent
soil washout with the seepage.
77
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--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
Soldier PileiWood Lagging Cofferdam
AASHTO T I T L E CHBTW 95
m
Ob39804 003LbL9 565
m
Lagging boards are usually 8 to 12 in (203 to 305 mm) wide. They are made from untreated and
usually undressed lumber. For temporary works, they may be left in place or removed at the contractor’s option
at the time of backfilling upon completion of the structure. It is difficult to remove the boards placed behind the
flanges. However, those attached to the front flange can be easily removed by loosening the bolt nuts.
In dry sandy soils, the soils tend to flow and cave before the lagging boards can be installed. This can
be controlled by moistening the soils to impart some apparent cohesion. Flowing soils are sometimes controlled
by driving horizontal boards (spiling) into the soil below the previously installed board to support the soils and to
prevent raveling. Figure 47 shows a photograph of a soldier pile retained with soil anchors.
Figure 47. Soldier pile retained with soil anchors.
Steel Sheet Pile Cofferdam
In order to maintain alignment in plan and vertically in pitch and also to prevent the sheet piles from
misalignment when driving past obsüuctions, it is important ilnt the sheet piles be driven through a template or
1
guides (see figure 48). In land-based operations, this can be achieved by a waler at ground level and another set
up at a higher level secured to piles already driven or by a fabricated trestle. For marine operations, the template
must be well secured by spud piles.
Sheet piles are driven in panels of 6 to 10 pairs in order to maintain accuracy both vertically and
be attached. A crane is then used to lift and pitch the pile. Forming the initial interlock can be troublesome,
especially in windy conditions.
The sheet piles are pitched, usually in pairs, to form a panel and the first and last pairs are partially
driven first, helping to prevent creep due to play in the interlocks. The ball end should always lead to prevent
plugging of the socket. This helps to protect the interlocks from tearing or dragging downward previously driven
78
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--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
horizontally. Each pile is usually supplied with a hole drilled near the top to which a quick release shackle can
AASHTO T I T L E CHBTW 95
=
Ob39804 0031b20 287
sheets. The remaining piles of a panel are then driven. The general installation technique is to drive the
sheeting in waves, always maintaining the tips of adjoining piles no more than 5 to 6 ft (1.5 to 1.8 m) apart.
The last and partly driven pile serves as the guide pile for the next panel. Figure 48 illustrates staged driving of
each panel of sheet piles.
Sheet piles may be driven by a drop hammer, a vibratory hammer, or a single-acting or doubleacting
hammer. The double-acting hammers are preferred for speed and efficiency. This type functions well in most
soils, especially in sandy and gravelly soils. Vibratory hammers also work well in granular soils. Single-acting
hammers are preferred in heavy clays. Diesel-operated single-acting hammers are effective for hard driving
conditions. Some patented "silent" hammers are also available, such as one type that employs a drop hammer
within a sound-absorbent box. Another type operates on the principle of pushing sheet piles by hydraulic rams
against a reaction provided by the skin friction developed on piles already in place. Initially, the reaction is
obtained from the weight of the equipment.
GUIDE WALERS FOR DRIVING SHEET PILES
STAGE 3
PANEL RTCHEO
GE PILES
TO GQW
NOTE F R H S NOT SMWN
FOR SlMWCITY
Figure 48. Sheet pile driving procedure.'")
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
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AASHTO T I T L E CHBTW 95
Ob39804 003Lb2L LI3
The required hammer energy depends on the type and density of the subsoils and type and length of
sheet piles. In denser soils, a sheet pile with a larger cross-sectional area would be easier to drive. Lighter sheet
piles could buckle or tear at the interlocks. Maximum hammer energy is on the order of ZOO0 ft-lbf for each
square in (4.2 N-mhnm2) of cross-sectional area of the sheet pile.
The heads of sheet piles are protected during driving by an anvil block or a cap of cast steel. If
obstructions are encountered near the ground surface, they are dug out and backfilled with soil. If the
obstructions are too deep or cannot be removed, then tbe procedure is to drive flanking piles to their full depth.
The gap is laîer filled with lagging as the excavation progresses. Hard driving in soil can sometimes be
overcome by jetting with water under high pressure. Jetting pipes can be prewelded to the sheet piles and jetting
applied when resistance builds up.
,
In wet, coarse-grained soils, potential ground water seepage and soil wash-inthrough the interlocks can
be minimized by grouting with cement-bentonite. Grout pipes can be prewelded to the sheet piles near the
socket and grout injected after their being driven to full depth. Hot-rolled sheet piles provide a tighter interlock
as compared with cold-rolled sheet piles and so the former experiences less seepage problems. Patented joint
systems with good leak resistance are also being advertized, but their usage is minor.
When driving piles in soft clay, previously driven adjacent piles might be dragged down due to
adhesion. This can be prevented by cross-bolting the piles or bolting them to walers. Limited headroom may be
overcome by driving short lengths and welding on extension pieces. Due to the pitching of piles through the
interlocks, each pile length can equal only half the available headroom.
For removal of sheet piles, conventional extractors can be used. Both impact type and vibratory
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
extractors are utilized.
Vibrations from sheet pile driving or exíraction will result in consolidation and settlement in adjoining
ground, especially in loose granular soils. This influence could be significant to about twice the sheet pile
length. In cohesive soils, clay may adhere to the sheets and this could contribute to future settlement. Adhesion
can be reduced by applying bituminous coating to the sheet piles.
Soil and Rock Anchors
There are various types of anchors, and their installation methods vary. Table 13 gives a summary of
main features of consîruction for several types of anchors. Various specialty contractors have developed their
own equipment and methods for construction.
Augered-type anchors are installed by the use of continuous flight solid stem or hollow stem auges.
The augers are guided by a kelly bar arrangement. If a solid stem auger is used, the augers are withdrawn after
drilling to the required depth, the tendon is inserted in the hole using suitably positioned spacers, and the hole is
filled with concrete under gravity. In hollow stem auger-drilling, a detachable point is often placed at the auger
tip to which the tendon bar is attached, which then extends the entire drilled length. The grout is pumped
through the hollow stem while the augers are withdrawn. The point remains in the ground. Grouting is
performed under pressures up to 150 Ibf/in2(1,030 kN/m2) through the hollow stem.
80
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AASHTO T I T L E C H B T W 75
64
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s
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-
AASHTO T I T L E CHBTW 75
Ob37804 0 0 3 3 6 2 3 T ï b
8
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82
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Single-belled anchors are constructed similar to a solid stem drilled anchor. After drilling to the design
depth, the augers are withdrawn and belling equipment is inserted to form the bell. Thereafter, the tendon is
placed and the hole filled with concrete under gravity. Multi-belled anchors are installed by patented equipment
designed to cut several bells simultaneously. Anchors with gravity or low pressure grouting should have a
downward inclination of about 15" to prevent entrainment of air.
Small diameter anchors in sand are installed by driving a heavy-wall casing pipe with a sacrificial
detachable point, using a percussion hammer. After driving to the required depth, an anchor rod is inserted in
the pipe and attached to the point and the point is separated from the casing. Grout is then injected through the
casing that is withdrawn slowly as the grouting progresses. Grout pressures are on the order of 150 to
300 lbf/inz (1,030 to 2,070 kN/m2). The anchors may also be installed by drilling instead of by driving the
casing. The soil cuttings are removed by air or by water.
In the case of regroutable anchors, drilling methods are the same as mentioned above. After drilling the
hole, the anchor rod or strand is inserted with an attached grout pipe. Grout is pumped at low pressure as the
casing is withdrawn to fill the void outside the grout pipe. Second stage grouting is conducted through the grout
pipe that has rubòer sleeve-covered ports spaced at about 3 ft (0.9 m). Grouting can be done over the entire pipe
fracture of the initial grout and grout penetration into the surrounding medium through the ports. If the grout
pipe is cleaned after each stage of grouting, additional grouting can be performed in subsequent stages over the
entire anchor or in certain isolated sections to improve the anchor capacity.
Anchors in rock are installed by drilling with an air percussion-type bit, a small diameter hole 3 to 8 in
(76 to 203 mm) in diameter. Grouting of the tendon is done by gravity or at low pressure. Grouting mixtures
generally combine regular cement with common admixtures for flowability and quick set. Nonshrink or
expansive admixtures are rarely used.
All anchors are proof-tested to at least 120 percent of the design load. Some specificationsrequire
proof-testing to 140 percent of the design load. Figure 49 shows a typical sheet pile installation with tiebacks
and tieback details.
Internal Bracing
Rectangular cofferdams are the most common type for bridge piers. Internal bracing consists of waiers
and struts. The bracing system must meet many other requirements besides initial economy and strength design.
The following are some of the factors in the selection of a bracing system:
e
e
Depth of the footing foundation and pile cut-off level.
Outside ground elevation.
e
Highest water level, tide levels, and normal levels.
e
e
Construction joints in the pier.
Tremie seal level.
e
Dimensions of the cofferdam.
e
Outline of the footing and pier.
83
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--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
or in sections isolated by packers. Grout pressures are high [300 to 600 lbf/in2(2,070 to 4,410 kN/m2)], causing
AASHTO T I T L E CHBTW 95
Ob39804 003Lb25 869
Soldier piles------
Tieback
tendon7
/
Weld as required
for verticol comoonent
o f tieback load
(b) Tieback details.
(a) Sheet piles with tiebacks.
The number of bracing levels are selected by Uial to suit the suength of the sheet pile section selected
and the position of the construction joints. One set of bracing is desired near the top because it is useful in
aligning the cofferdam sheet piles, provides a skeleton for support of minor consüuction equipment, and also
resists lateral forces applied near the surface such as impact from floating logs, ice, and floating equipment.
Other bracing levels will be based on consideration of design loads during the various stages of construction and
on other factors listed above. A critical stage may occur when all excavation is completed inside and before the
seal coat is placed. This condition usually requires that a lower set of bracing be installed under water when the
inside dredging is at a certain level.
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--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
Figure 49. Sheet pile installation.
AASHTO T I T L E CHBTW 95
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Ob39804 0033626 7T5
m
The bracing frames should be prefabricated and tied together as a unit before the sheet piling is set or
the lower set can be hung from the upper set. Once dredging is done to the next bracing level, the fabricated set
is lowered and wedged to the sheet piles. Full connections with brackets, etc., can be made when dewatering is
done. The second bracing set, wedged tightly to the sheeting, provides sufficient temporary support of the sheet
piles.
In order to avoid shifting of the bracing during construction of the pier, bracing levels should be located
above the construction joints. Waler sections should be approximately square with wide flanges to prevent their
rolling or twisting from strut loads. Rugged brackets provided to support the walers at a frequent spacing of 10
to 15 ft (3.0 to 4.6 m) are most beneficial in preventing rolling of the walers. Adequate stiffeners must be
provided at strut connections to prevent web crippling. Connections Óf comer diagonals to the walers must be
designed to transfer axial thrust and shear. To make a rigid comer with a full moment connection, a short Hbeam with stiffeners between the flanges of the wales and the flanges of the diagonals may be used,
supplemented with cover plates if necessary. This type of comer helps to reduce the bending stress in the wale
at the location of greatest axial stress.
Struts act essentially as columns and require adequate lateral stiffness both vertically and horizontally.
Struts
made from pipes or square tubings have the same stiffness in both planes. However, wide flange beams
require bracing in their weak plane. For large cofferdams, the lateral bracing may be in the form of a truss.
Pipe struts need little lateral support, but intersection details and connections to walers are difficult. An end
plate with stiffeners is often used for connecting the strut to the waler. Some details used for connections of
walers and struts are shown in figure 50. Also included is a detail for preloading of the strut.
85
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AASHTO T I T L E CHBTW 95
ILAM
al WEDGING
=
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I
0639804 0031627 631
PIPE 0 . D - A P ~ I M A ~ L Y
EOUAL TO 1.0. OF ORICE.
BLARIIIC AûAINST ORACE
PUT SECTION OF PIPE WELDED IN PLACE
AFTER PRESTRESSING. LOA0 STILL I N U K .
ESCOPING PIPE
(b) Prestressing of pipe brace at comers usmg
(a) Prestressing details for braces.
brackets as reaction.
,-StNt
0)
BRIICE DEPTH SMALLER THAN
WâLE R A N G E WIDTH.
-
(d) Typicai detail for horizontal brace with brace
web vertical.
(c) Typicai detail for horizontal brace with brace
web horizontal.
Figure 50. Typical framing
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(TICI(STICMNCRS
iNPucsr
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I
= Ob37804 0033628 5 7 8 W
AASHTO T I T L E CHBTW 75
Bracing in the cofferdam should be removed after completion of the structure within it. Sometimes
bracing below the mud line is left in place. When it becomes necessary to have struts pass through the structure,
the most common method of removal is to flood the cofferdam and bum off the struts. The struts may
sometimes pass through a box-out in the structure. Then, the struts could be bumt off and pulled out sideways.
A more common method is to remove the braces piece by piece before flooding the cofferdam. In this case, the
sheeting is blocked to the structure just below the braces to be removed. It is necessary to evaluate if the
structure can resist additional loads from the sheeting.
For land-based excavations, the lowest bracing waiers may be supported by inclined rakers reacting on a
heel block or on a partiaily completed footing. A typical detail for connection of an inclined brace with a
horizontal wale is shown in figure 5 1.
When a cofferdam wail is supported by an extemai dead man, the location of the dead man should be
beyond the "active" wedge of the cofferdam sheeting plus the "passive" wedge of the dead man. A typicai
connection of waiers to an extemai deadman sheeting is shown in figure 52.
INCLINED 'KICKER' OR
>SPUR ORACE.
-
WELD A = WELD B I 3 C
VERTICAL COMPONENT
OC BRIICELOAO
SHEETING
(3"THICK1
v
Conversion: 1 in = 25.4 mm
Figure 51. Typical connection for inclined brace and horizontal wale.'")
87
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A A S H T O T I T L E CHBTW 95
m
Ob39804 0033b29 404
m
INSIDE WALE
TE WASHER)
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STEELSHEET
PILE ANCHOR
FIXING BOLTS
LATE WASHER)
SECTION A.A
Figure 52. Typical wale and anchor rod de@
ats.li )'
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OUTSIDE
WALE
AASHTO T I T L E CHBTW 95
0639BOY 0033630 3 2 6
APPENDIX A
SECTION PROPERTIES OF STANDARD DRESSED (S4S) AND ROUGH SAWN LUMBER
Nominal
size
bxh
in x in
1 x4
1 x6
1x8
1 x 10
1 x 12
Table
14. Section properties of standard dressed (S4S) lumber
Actual
b
in
Actual
h
in
34
34
3%
%
Area
A
in2
-
Axis
Moment of
inertia, in'
I
x-x
Axis Y-Y
Approximate
Section modulus,
S , in'
Moment of
inertia, F, in4
Section modulus,
S , in3
weightb
Ib/ft
%
7%
9%
11w
2.63
4.13
5.44
6.94
8.44
2.68
10.40
23.82
49.47
88.99
1.53
3.78
6.57
10.70
15.82
0.12
0.19
0.25
0.33
0.40
0.33
0.52
0.68
0.87
1.05
0.7
1.1
1.5
1.9
2.3
2x4
2x6
2x8
2 x 10
2 x 12
1%
1142
11%
1%
1%
3%
5%
7%
9%
11%
5.25
8.25
10.88
13.88
16.88
5.36
20.80
47.63
98.93
177.98
3.06
7.56
13.14
21.39
31.64
0.98
2.04
2.60
3.16
1.31
2.06
2.72
3.47
4.22
1.5
2.3
3.0
3.9
4.7
3x4
3x6
3x8
3 x 10
3 x 12
2'42
2%
2%
2%
2%
3%
7%
9%
11%
8.75
13.75
18.13
23.13
28.13
8.93
34.66
79.39
164.89
296.63
5.10
12.60
21.90
35.65
52.73
4.56
7.16
9.44
12.04
14.65
3.65
5.73
7.55
9.64
11.72
2.4
3.8
5.0
6.4
7.8
4x4
4x6
4x8
4 x 10
4 x 12
3%
3'42
3%
3142
3%
3%
5%
7%
9%
11%
12.25
19.25
25.38
32.38
39.38
12.51
48.53
7.15
17.65
111.15
30.66
230.84
4 15.28
49.91
73.83
12.51
19.65
25.90
33.05
40.20
7.15
11.23
14.80
18.89
22.97
3.4
5.3
7.0
9.0
10.9
6x6
6x8
6 x 10
6 x 12
5%
5%
5%
5%
5%
7%
9%
11%
30.25
41.25
52.25
63.25
76.26
193.36
392.96
697.07
27.73
51.56
82.73
121.23
76.26
103.98
131.71
159.44
27.73
37.81
47.90
57.98
8.4
11.5
14.5
17.6
8x8
8 x 10
8 x 12
7%
7%
7%
7%
9%
11%
56.25
71.25
86.25
263.67
535.86
950.55
70.3 1
112.81
165.3 1
263.67
333.98
404.30
70.31
89.06
107.81
15.6
19.8
24.0
lox 10
l o x 12
9%
9%
9%
11%
90.25
09.25
678.76
1204.03
142.90
209.40
678.76
821.65
142.90
172.98
25.1
30.3
12x 12
11%
11lh
32.25
1457.51
253.48
1457.51
253.48
36.7
%
5%
5%
1.55
Notes:
(a)
fi)
(c)
This table is based on information from references 19 and 21. The section properties are given for dry lumber,which is
defined as lumber îhaî has been seasoned to a moisture content of 19 percent or less.
Based on a unit weight value of 40 lb/f?. Actual weights vary depending on species and moisture content. At 15-percent
moisture content, the unit weight of coastal region Douglas Fir is 34 lWfI3 and that of Southern pine ranges between 36
and 44 IWft?. The other species commonly used for formwork in North America weigh less.
Conversion: 1 in = 25.4 mm; 1 Ibf/fP = 157 N/m3: 1 Ibfm = 1.49 kglm.
89
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Table 15. Section properties of rough sawn lumber
Nominal
size
Actual
b
in
Actual
h
in
Area
A
in2
718
718
3 518
5 SI8
7 318
9 3/8
11 3/8
2x4
2x6
2x8
2 x 10
2 x 12
1 518
1 518
1 518
1 518
1 SI8
3x4
3x6
3x8
3 x 10
3 x 12
bxh
Axis Y-Y
Axis X-X
Approximate
weight'
1wt
Section modulus,
S, in'
1.92
4.61
7.93
12.82
18.87
Moment of
inertia, F, in'
0.20
0.3 1
0.4 1
0.52
0.64
Section modulus,
3.17
4.92
6.45
8.20
9.95
Moment of
inertia, I in'
3.47
12.98
29.25
60.08
107.32
0.46
0.72
0.94
1.20
1.45
0.9
1.4
1.8
2.3
2.8
3 518
5 518
7 3/8
9 318
11 3/8
5.89
9.14
11.98
15.23
18.48
6.45
24.10
54.32
111.58
199.31
3.56
8.57
14.73
23.80
35.04
1.30
2.01
2.64
3.35
4.07
1.60
2.48
3.25
4.13
5.01
1.6
2.5
3.3
4.2
2 518
2 SI8
2 518
2 SI8
2 518
3 SI8
5 SI8
7 318
9 318
11 3/8
9.52
14.77
19.36
24.61
29.86
10.42
38.93
87.75
180.24
321.96
5.75
13.84
23.80
38.45
56.61
5.46
8.48
11.12
14.13
17.15
4.16
6.46
8.47
10.77
13.06
2.6
4.1
5.4
6.8
8.3
4x4
4x6
4x8
4 x 10
4 x 12
3 SI8
3 518
3 518
3 518
3 518
3 SI8
5 518
7 318
9 318
11 3/8
13.14
20.39
26.73
33.98
41.23
14.39
53.76
121.17
248.91
444.61
7.94
19.12
32.86
53.10
78.17
14.39
22.33
29.28
37.21
45.15
7.94
12.32
16.15
20.53
24.91
3.7
5.7
7.4
9.4
11.5
6x6
6x8
6 x 10
6 x 12
5 518
5 518
5 518
5 SI8
5 518
7 518
9 SI8
11 SB
31.64
42.89
54.14
65.39
83.43
207.81
417.97
736.41
29.66
54.51
86.85
126.69
83.43
113.09
142.75
172.42
29.66
40.21
50.76
61.30
8.8
11.9
15.0
18.2
8x8
8 x 10
8 x 12
7
7
7
518
7 518
9 SI8
11 5/8
58.14
73.39
88.64
281.69
566.58
998.25
73.89
117.73
171.74
281.69
355.58
429.47
73.89
93.27
112.65
16.2
20.4
24.6
l o x 10
l o x 12
9 SI8
9 SI8
9 518
11 5/8
92.64
111.89
715.19
1260.08
148.61
216.79
715.19
863.80
148.61
179.49
25.7
31.1
12 x 12
115B
11518
135.14
1521.92
261.83
1521.92
261.83
37.5
in x in
1x4
1x6
1x8
1 x 10
1 x 12
718
718
718
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
518
SI8
S, in'
5.1
Notes:
(a)
@)
(c)
This table is based on information from references 19 and 21. The section properties are given for dry lumber, which is
defined as lumber that has been seasoned to a moisture content of 19 percent or less.
Based on a unit weight value of 40 ib/ft3. Actual weights vary depending on species and moisture content. At 15-percent
moisture content, the unit weight of coastal region Douglas Fir is 34 ib/f? and that of Southern pine ranges between 36
and 44 Ib/fl'. The other species commonly used for fomwork in North America weigh less.
Conversion: 1 in = 25.4 mm; 1 Ibf/f? = 157 N/m3; 1 lbflft = 1.49 kg/m
90
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APPENDIX B
FALSEWORK AND FORMWORK DESIGN EXAMPLES
-
EXAMPLE 1 SLAB FALSEWORK WITH OVERHANG BRACKET
Problem Description:
Check the fomwork elements and overhang bracket in figure 53. Specifically, investigate the following
items:
0
e
a
P l y w d sheathing [%-in (19-mm) Plyform Class Il.
maximum allowable pressure
Stringers [2-in by 4-in nominal (50-mm by 100-mm) S4S dimension lumber].
bending stress
horizontal shear stress
bearing stress
deflection
Steel overhang bracket and hangers.
safe load
deflection
e
a
a
a
a
a
The bridge deck will be consîructed from normai weight concrete.
The screed rails will be placed directly over the bridge deck steel girders. Therefore, the
fomwork and falsework will not be affected by the screed loads.
No motorized carts will be driven on the formwork.
The Class I Plyform sheathing will be placed so the stress is applied parallel to the face grain
(that is, the supports will be perpendicular to the face grain). Assume the sheathing will be
placed over two spans.
The stringers span over four supports (three spans). The bearing area of the stringer on each
support is 4.5 in2 (2,900 mm?.
All lumber is to be Douglas Fir - Construction Grade S4S dimension lumber. Assume the
allowable bending stress for the lumber is 1O
, OO 1bWn2(6.9 N/mm2), the allowable horizontal
shear stress equals 95 lbf/in2(0.66 N/mm2), and the allowable bearing stress is 625 lbf/in2
(4.3 N/mm2). The modulus of elasticity (E) equals 1,500,000 lbf/ina (10,300Nhnmz).
References:
Guide Design Specification for Bridge Temporary Works")
National Design Specification for Wood Construction, 1991 Edition")
NDS Supplement - Design Valuesfor Wood Constructionf2')
American Plywood Association Concrete
Dayton-Superior 1985 Bridge Deck Forming Handbook(22)
91
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Design Conditions:
C H B T W 95
A A S H T O TITLE
m
ob39804 o o 3 1 , ~ 3 3935
/-- 2"X4"(@ 12" O.C.
r
m
3 / 4 PLYFORM CLASS I
8 MIN. CONCRETE
1/2" DIA. COIL BOLT (TYP.)
Conversion: 1 in = 25.4 mm; 1 ft = 0.305 m.
Figure 53. Slab falsework with overhang bracket.
Calculation and Discussion:
1.
Calculate the load on the plywood sheathing.
Section 3.2 of the Guide Design Specification for Bridge Temporary Wot,; specifies a minimum live
load of 50 lbf/ft? (2,400 N/m2) be applied to fonnwork for the vertical load of construction traffic. This
load applies to the fonnwork sheathing only, and not to the underlying falsework members. Also,
according to the specification, the combined dead and live loads shall equal at least 100 lbf/ft2
(4,800 N/m2) when no motorized carts are used.
Dead load (calculated where the concrete depth is greatest)
The concrete depth to the left of the exterior girder is approximately 10 in (250 mm).
concrete:
(10 in)(l fU12 in)(150 lbffft?) = 125 lbf/ft? (5,990 N/m2)
plywood:
2.2 lbf/ft? (105 N/m2) from table 9 in chapter 3.
Live load
Live load on fonnwork
50 lbf/ft? (2,390 N/m?
Total dead and live load: 177 lbf/ft! (8,430 N/m?
The total dead and live load exceeds the specified minimum of 100 lbf/ft! (4,800 N/m2).
Therefore, the allowable pressure must exceed 177 lbf/ft? (8,430 N/m?.
92
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m
A A S M T O T I T L E CHBTW 95
2.
Ob39804 0031634 871
m
Determine the maximum allowable pressure on the plywood sheathing based on bending stress, shear
stress, and deflection.
Allowable pressure (wb)based on bending stress:
From table 8 in chapter 3,
wb =
96FbKS
, for two spans
1:
where, for this example,
Fb= 1,930 lbflin’ (13.3 N/mm2), bending stress from table 10
KS = 0.455 in3/ft (24,450 mm3/m), effective section modulus from table 9
1, = 12 in (305 mm), span from center-to-center of supports
=
96(1,930)(0.455)
(12)’
ft
Allowable pressure (w,) based on shear stress:
From table 8 in chapter 3,
19.2 F(lb/Q)
w, =
4
, for two spans
where, for this example,
F, = 72 Ibf/in2(0.50 N/mm2), rolling shear stress from table 10
lb/Q = 7.187 in2/ft (15,200 mm2/m),rolling shear constant from table 9
1, = 12 in - 1.5 in = 10.5 in (267 mm), clear span
ws = 19.2(72)(7.187) = 946 !
!!! [45,300.$]
ft2
10.5
Allowable pressure (w,) based on bending and shear deflection:
According to section 3.3.3 of the Guide Design SpeciJicationfor Bridge Temporary Works,
forms for exposed concrete surfaces should not exceed either 118 in (3.2 mm) or la40 of the
center-to-center distance between joists.
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Wb
AASHTO
TITLE CHBTW 95
Ob39804 0031635 708
Therefore, the maximum allowable deflection ($13 is:
IR
in (3.2 mm)
or (11S40)(12 in) = 0.05 in (1.3 mm)
For this example, & = 0.05 in (1.3 mm).
Determine the bending deflection (A,,) based on a 1.0 lbf/ft2 (47.9N/m? load using the
following equation from table 8 in chapter 3:
wi;
, for two spans
= 2,220 EI
where, for this example,
l3 = 10.5 in + 0.25 in = 10.75 in (273 mm),clear span plus % in (6.4 mm)
E = 1,650.000 lbf/ii2(11,400 Nhnm’), adjusted modulus of elasticity
I = 0.199 in4/ft (272,000 mm4/m), moment of inertia
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
Ab =
(l.0)(10.75)4
= O.oooO183 in (0.000465 mm)
2,220(1,650,000)(0.199)
Determine the shear deflection (AJbased on a 1.0 lbf/fe (47.9 N/m?load using the following
equation from table 8 in chapter 3:
A, =
Cwt 21;
1,270 E,I
where, for this example,
C = 120 for face grain perpendicular U, supports
t = y4 in (19 mm), plywood thickness
i, = 10.5 in (267 mm), clear span
E, = 1,500,000 Ibf/in2 (10,300 N/mm2), unadjusted modulus of elasticity
I = 0.199 in4/ft (272,000 mm4/m), moment of inertia
94
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AASHTO T I T L E CHBTW 75
Ob37804 003Lb3b 644
The allowable pressure (w,) based on deflection is therefore:
Aau. =
0.05
w, = 4 + A, 0.0000183 + 0.0000196
w, = 1,320 ibf/ft2 (63,200 N/mz)
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
The maximum allowable pressure on the plywood sheathing is governed by wb, which is 585 Ibf/f$
(28,000 N/m2). The combined &ad and live load is 177 ibf/f? (8,430 N/m2). Therefore, the capacity of
the plywood sheathing exceeds the total design load.
3.
Calculate the load on the most heavily loaded stringer, which in this example is the fourth stringer from
the left.
The average concrete depth at this stringer is approximately 9 in (230 mm).
Dead load
Section 2.2.2 of the Guide Design Specification for Bridge Temporary Works specifies
that the combined weight of concrete, reinforcing and prestressing steel, and formwork
shall be assumed to be not less than 160 Ibf/fe (25,100 N/m3).
The dead load on the stringer is therefore:
(:,!J
- [y b f ]
/
concrete, formwork: (9 in)(12 in)
\
-(1,750 E)
= l2;?
stringer: 1.5 IbUft (22 N/m) from table 14 in appendix A.
Live load
According to section 3.1.3 of the specification, structurai supports on the soffit of a
bridge deck and slab overhangs are falsework by definition and shall be designed
accordingly.
The construction live load must include the actual weight of any equipment to be
supported on the falsework plus a uniform load of 20 Ibf/f? (960 N h 2 ) over the area
supported and a line load of 75 Ibf/ft (1100 N/m) applied at the outside edge of bridge
overhangs. In this example, the line load is applied to the leftmost stringer.
The stringers in this example must therefore be designed for the following live load:
live load on stringer: -(12
20 Ibf
ft
(-E)
=f
20 t
Ibf 290 N
in)(*
12 in
Total dead and live load: 142 lbf/ft (2,070 N/m)
According to section 2.2.4 of the speciíìcation, the minimum total vertical design load
for any falsework member shall not be less than 100 lbf/ft? (4,790 N/m% which for a
1 2 4 (305-mm) spacing, gives a lW-lbf/ft (1,460-Nhn) load on the sîringer.
Therefore, the design load on the stringer is 142 lbf/ft (2,070 Nh).
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AASHTO T I T L E CHBTW 95 M Ob39804 003Lb37 5 8 0
4.
Determine the maximum moment and maximum shear in the stringer.
The stringers span over four supports (three spans). The center-to-center distance between supports is
4 ft (1.2 m). Beam formulas for calculating maximum moments, shears, and deflections can be found in
table 12 in chapter 3.
The maximum moment &
occm
I) at an interior support for the stringer (assuming four supports):
M = -w12 = (142 Ibffft)(4 ft)2
10
10
M = 2,730 Ibf-in (308 N-m)
The maximum shear (V) also occurs at an interior support:
v = -3wl
5
3(142 lbffft)(4 ft)
5
5
V = 341 lbf (1,520 N)
5.
Calculate the bending and shear stresses in the swinger.
Bending stress:
The section properties for the 2-in by 4-in nominal (50-mm by 100-mm) stringer can be found
in table 14 in appendix A.
The section modulus (S,)
is 3.06 in3 (50,100 nun3).
The bending stress (f,) is:
fb =
M =
-
s,
2,730 lbf-in
3.06 i n 3
fb = 892 lbf/in2 (6.2 N/mm2)
The allowable bending stress is 1,OOO lbf/in2(6.9 N/mm3). The stringer is therefore acceptable
in bending.
Shear stress:
From table 14 in appendix A, the actual dimensions of 2-in by 4-in nominal (50-mmby
100-mm)S4S dimension lumber are:
b = 1.5 in (38 mm)
h = 3.5 in (89 mm)
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Ob39804 0031638 417
AASHTO T I T L E CHBTW 95
The horizontal shear stress (H) is:
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
H = - 3V
2bh
5
3(341 lbf)
2(1.5 in)(3.5 in)
H = 97 lbflin (0.67 N/mm 9
The allowable shear stress is 95 lbf/in2(0.66 N/mm2), which is nearly equal to the calculated
shear stress. The stringer is therefore acceptable in shear.
The allowable spacing of the supports based on h e bending and shear capacity of the stringer may also
be determined using the formulas given in table 11 in chapter 3.
6.
Calculate the bearing stress in the stringer.
The largest reaction (R)occurs at an interior support:
R - , w11
l=
10
1l(142 lbf/ft)(4 ft)
10
R = 625 lbf (2,780 N)
The bearing area (A) is given as 4.5 in2 (2,900 mm2).
The bearing stress (0 is therefore:
R
f=---
-
A
625 lbf
4.5 in2
f = 139 lbtlin' (0.96 Nimm?
The allowable bearing stress is 625 lbf/in2(4.3 N/mm2). The stringer is therefore acceptable in bearing.
7.
Calculate the maximum deflection due to bending of the stringer.
Deflection:
The moment of inertia (h)
is 5.36 in4 (2,230,000 mm4) as given in table 14 of appendix A.
The maximum deflection (A) occurs in an exterior span:
(142 Ibf/ft)(4 ft)4
12 in
A = - w14 145EI
145(1,500,000 lbf/in 2)(5.36 in 4,
(71
A = 0.054 in (1.4 mm)
According to section 2.3.5 of the Guide Design Specification for Bridge Temporary Works,the
calculated vertical deflection of falsework members shall not exceed 1n40of their span under
the dead load of the concrete only. In this example, ia40of the span is 0.20 in (5.1 mm). The
deflection due to the total dead and live load is only 0.054 in (1.4 mm). The deflection of the
stringer is therefore within the limit required by the specification.
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A A S H T O T I T L E CHBTW 95 H Ob39804 0033639 3 5 3
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
8.
Determine the capacity and deflection of the steel overhang bracket,
Safe working loads for steel overhang brackets and hangers can be found in the product catalogs
provided by manufacturers. These allowable loads are dependent on a number of factors including the
bracket dimensions, the deck slab thickness and length of the slab overhang, the design live load, the
screed load (where applicable), and the safety factor used to determine the allowable loads and bracket
spacings. The effects of the brackets on the bridge girder must also be consider4 when using these
brackets to construct deck overhangs. Note rhat increasing the length of the bracket’s vertical leg
generally increases the bracket capacity. More importantly, the load fiom the bracket is transferred to
the bridge girder near its bottom flange, thereby reducing twisting or bending of the girder.
The total vmicai load on the bracket may be estimated as follows. The avenge depth of the concrete
on the overhang is approximately 9 in (230 mm). The total dead and live distributed load is therefore
142 lbf/f? (6800 N/m2) based on the calculations in step 3,
total dead and live load = 142 ib/ft!(3.5 ft)(4 ft) = 1,990 lbf (8,850 N)
line load = 75 lbf/ft x 4 ft = 300 lbf (1,300 N)
Therefore the total load on the bracket is 2,290 Ibf (10,200 N).
The manufacturer’s product technical literature should then be consulted to determine safe
working loads for the given bracket spacings.
To estimate the deflection of the hanger, use the load-deflection curve in figure 54. For the bracket in
this example, the deflection is estimated based on the sum of the vertical loads on the bracket.
I
!
!
!
! !
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
I
-
!
!
!
!
I
6000
4500
3000
1500
O
O
0.25
0.75
0.5
1
1.25
Deflection at outboard end of bracket (in)
Conversion: 1 in = 25.4 mm; 1 lbf = 4.45 N
Figure 54. Load-deflection curve for steel
overhang bracket.
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A A S H T O T I T L E CHBTW 95
Ob39804 0031b40 075
The deflection of the bracket will be estimated only for the concrete load since the deflection due to the
weight of the falsework may be corrected prior to concrete placement.
[-
Total weight of concrete = (8 in i 1 0 in) 3.5 ft)(4 ft) 1 5 t ) bf ~ ~ 2 ~ )
= 1,580 lbf (7,030 N)
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
According to figure 54, the deflection at the outboard end of the bracket based on a total vertical load of
1,580 lbf (7,030 N) is approximately 0.4 in (10 mm).
99
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A A S H T O T I T L E CHBTW 95
Ob39804 003Lb4L T O I W
-
EXAMPLE 2 NEEDLE BEAM
Problem Description:
Check the stresses and defl-ction of the needle beam shown in figure 55. Specifically, investigate the
following items:
e
o
o
o
Bending stress.
Horizontal shear stress.
Bearing stress on piate and washer with contact area A = 15 in2 (9,680 m
').
Maximum deflection.
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
Design Conditions:
o
o
o
o
The 8-in(200-mm) thick bridge deck will be constructed from normal weight concrete.
The screed rails wiil be placed directly over the bridge deck steel girders. Therefore, the
formwork and falsework will not be affected by the screed loads.
No motorized carts will be driven on the formwork.
The needle beam is to be constructed of Douglas Fir - Construction Grade S4S dimension
lumber. Each needle beam consists of two 2-in by 12-in nominai (50-mm by 300-mm)
members spaced at 4 ft (1.2 m) on center. The allowable bending stress for the lumber is
1,OOO lbf/in2 (6.9 N/mm2) and the allowable horizontal shear stress equals 95 lbf/ii2
(0.66 N/mmZ). The allowable bearing stress is 625 1bElin' (4.3 N/mm2). The modulus of
elasticity (E) is 1,500,000 Ibf/in2 (10,300 N/mm2).
Assume the formwork applies a lO-ibf/ft' (480-N/mZ)disuibuted load on the needle beam.
Note that in this example the fascia beams of the bridge are relatively shallow. An overhang bracket
cantilevered from a fascia beam would cause it to rotate significantly. A needle beam is therefore used to
support the overhanging portion of the bridge deck slab. In general, for beams with depths less than 24 in
(610 mm), a needle beam such as the one shown in this example should be considered.
References:
Guide Design Specification for Bridge Temporary Works")
National Design Spec@cation for Wood Construction, I991 Edition("
NDS Supplement - Design Values for Wood Construction""
100
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AASHTO T I T L E CHBTW 95
0639804 0031642 948
SIDE FORM
JOISTS
I
\
c
FASCIA BEAM
SHORE (BRACED
FOR STABILITY)
2
I
NEEDLEBEAM
2-2x12 Q 4 - 0C.C.
DOUGLAS FIR
WEDGES
Conversion: 1 in = 25.4 mm; 1 ft = 0.305 m
Figure 55. Needle beam for slab overhang.
Calculation and Discussion
Calculate the loads on the needle beam.
Dead load
According to section 2.2.2 of the Guide Design Specification for Bridge Temporary Works, the
weight of the concrete, steel reinforcement, and formwork shall be assumed to equal at least a
16O-ibf/ft! (25,100-N/mm3 load due to the falsework members that bear on the needle beam.
The dead load on the needle beam is therefore:
[
concrete, formwork: (8 in) - - 4 ft) - = 640 lbf (2,800 N)
(1~1n13:1
1 6 ~ 9
falsework:
[
m b f 1 3. 2 ; ft + 1 ft
4 ft) = 105 lbf (470 N)
Live load
The construction live loads specified in section 2.2.3.1 of the specification include a 2O-ibfff?
(96O-Nlm3 distributed load and a 75-lbf/ft (l,lOO-N/m) line load applied at the outside edge of
the deck overhang, which in this case is located directly over the shore.
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1.
A A S H T O T I T L E CHBTW 75
m
Ob37804 0031643 884 W
The needle beam must therefore be designed for the following live loads:
distributed load:
line load
(3*2i
+
1
1 ft 4 ft)(20 Ibf/ft? = 210 Ibf (930 N)
(y).
ft) = 300 lbf (1,300 N)
Total dead and live load: 1,260 Ibf (5,600 N)
The load is applied as a concenîrated load on the needle beam at the location of the shore as shown in
figure 55.
2.
Determine the maximum moment and the maximum shear in the needle beam.
Each needle beam is constructed from two 2-in by 1241-1nominal (50-mm by 300-mm). The weight of
one 2 x 12 is 4.7 lbf/ft (70 N/m) from table 14 in appendix A. The concentrated load on each 2 x 12 is
one-half of the load calculated in step 1, that is, 630 lbf (2,800 N).
Maximum moment (M) occurs at the outside coil bolt support:
M = [(630 lbf)(34ft)( +‘ ( 4 7 lbf
ft
ft)’(i/;I- 12 in
ft
M = 23,100 lbf-in (2,610 N-m), per 2 x 12
Maximum shear (V) also occurs at the outside support:
V = 630 lbf
+ (4.7 Ibf/ft)(4 ft)
V = 650 lbf (2,890 N), per 2 x 12
3.
Calculate the bending and shear stresses in each 2 x 12 of the needle beam
Bending stress:
The section properties for the 2 x 12 can be found in table 14 of appendix A.
The section modulus (S,) is 31.64 in3 (518,000 mm’).
The bending stress (f,) is:
M
S,
fb=-=
23,100 lbf-in
31.64 in3
fb = 730 Ibf/in’ (5.0 N/mm2)
The allowable bending stress for each 2 x 12 is 1,000 lbf/in2(6.9 N/mm2). The needle beam is
therefore acceptable in bending.
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AASHTO T I T L E CHBTW 95
m 0637804
0031bY4 710
m
Shear stress:
The actual dimensions of a 2 x 12 S4S member are:
b = 1.5 in (38 mm)
h = 11.25 in (290 mm)
The horizontal shear stress (H) is:
3(650 lbf)
H = - 3v =
= 58 1bWn * (0.4 N/mm 3
2(1.5 in)(11.25 in)
2bh
The allowable shear stress is 95 lbf/in2(0.66 N/mm2). The needle beam is therefore acceptable
in shear.
4.
Calculate the bearing stress on the plate washer.
The bearing reaction at the outside support equals (including both 2 x 12’s):
+ (1,260 lbO(9.25 ft)
R =
= 1,940 lbf (8,600 N)
6.25 ft
The bearing stress (f) is:
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
f=
R
15.0 in’
=
lbf = 130 Ibf/in2(0.9 N / m ?
15.0 in’
The allowable bearing stress is 625 Ibf/in2 (4.3 N/mm?. Therefore, the bearing stress in the needle
beam is acceptable.
5.
Calculate the deflection of the needle beam at the edge of the bridge deck overhang, that is, 1 ft (0.3 m)
from the right end of the needle beam. The needle beam can be set to the correct elevation, after the
deflection due to the weight of the falsework members has occurred. The calculation for deflection,
therefore, includes only the deflection due to the weight of the concrete.
Deflection (calculated for one 2 x 12):
The moment of inertia (I,) for one 2 x 12 is 177.98 in4 (74,000,000 mm4).
(630 lbf)(3 ftY(6.25 ft + 3 ft)
A =
3(1,500,000 Ibffin 2)(178 in4)
= 0.11 in (2.8 mm)
When the deflection is significant, the falsework should be set high at the outer runner to account for
deflection of the needle beam due to the concrete load. In addition, the wood-to-wood surfaœs in the
support falsework tend to seat when the concrete is applied. A commonly used practice is to set the
falsework high by 1/16in (1.6 mm) per wood interface.
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AASHTO T I T L E C H B T U 75
O637804 0031645 657
m
-
EXAMPLE 3 PIER CAP VERTICAL FORMWORK
Problem Description:
Check the pier cap vertical formwork elements shown in figure 56. Specifically, evaluate the following
items:
Sheathing Tys-in (19-mm) Plyform Class I].
bending stress
rolling shear svess
deflection
Studs.
bending stress
horizontal shear stress
bearing stress on walers
deflection
Walen.
bending stress
horizontal shear stress
bearing on tie plates [bearing area (A) is 15 in2 (9,700 mm2)1
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
Design Conditions:
The pier cap will be constructed from normal weight concrete, containing no admixtures or
pozzolans and having a slump less than 4 in (100 mm). These concrete properties have been
chosen in order to demonstrate the pressure equations given in section 3.2.2.2 of the Guide
Design Specijicutionfor Bridge Tempormy Works. When these criteria have not been fulfilled,
the hydrostatic pressure equation of section 3.2.2.1 is used instead.
The sheathing (Plyform Class I) will be placed so that the stress is applied parallel to the face
grain (that is, the face grain will run perpendicular to the supports). The plywood will be
placed across at least four supports (three spans).
All lumber is to be Douglas Fir - Construction Grade S4S dimension lumber. The allowable
bending stress for the lumber is 1,OOO lbflin’ (6.9 N/mm2), the allowable horizontal shear stress
is 95 ibf/in2 (6.6 N/mm2), and the allowable bearing stress is 625 lbf/in’ (4.3 N/mm2).
Note that the conditions assumed for design must be verified in the field.
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~~
~~
AASHTO T I T L E CHBTW 75
Ob39804 0033646 5 9 3
I
I
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
PIER ELEVATION
3/4' K Y F W CLASS I
i
2x4. c w0.c.
+1/2'
DIA. COL BOLT WITH 4x5'
WASHER SPACED @ 4'-O'
TWO 2x6' IWALERSI
4181(3/4' PLYFORM. CLASS 1
2x4' BRACE e
4DC. BOW SDE
xexK)'LONG @ Io-0.c
FRICTION COLLAR
SECTION A
-A
Conversion: 1 in = 25.4 mm; 1 ft = 0.305 m
Figure 56. pier cap on friction collar.
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AASHTO T I T L E CHBTW 95
Ob39804 0031647 42T
Calculate the loads on the pier cap vertical formwork.
1.
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
All the elements of the vertical side forms, including sheathing, studs, and walers, are defined as
formwork and are therefore governed by the provisions of chapter 3 of the Guide Design Specification
for Bridge Temporary Works. The lateral pressure exerted by fluid concrete on the side forms is
determined by the equations in section 3.2.2 of the specification.
Assume the contractor will pour one pier cap in approximately 30 minutes. The rate of placement (R)
is then:
3.67 ft = 7.3 f t h (2.2 mh)
R =30 min
Equation 3-4 from the specificaion applies since the pour rate exceeds 7 f t h (2.1 m/h). Assume the
temperature of the concrete is 70 OF (21.1 OC). The lateral pressure (p) is calculated as follows:
p = 1 5 0 + -43 400
70
+
2,800(7.3)
70
p = 1,060 Ibf/ft (50,800 N/m *)
However, according to section 3.2.2.2 of the specification, this lateral pressure need not exceed
150 times the depth of the fluid concrete:
p = 15q3.67 ft) = 550 lbffft (26.300 N/m
The vertical side forms are therefore to be designed for a lateral pressure that varies linearly from p = O
at the top of the pier cap form to p = 550 lbf/ft2 (26,300 N/m3 at the bottom of the form. This pressure
may also be determined from the graph given in figure 25 in chapter 3 of this handbook.
2.
Determine the maximum allowable pressure on the vertical plywood sheathing based on bending stress,
shear stress, and deflection.
The sheathing capacity should be compared to the maximum pressure, which for this case equals
550 lbf/ft! (26,300 N/m?.
Allowable pressure (w,,) based on bending stress:
From table 8 in chapter 3:
Wb
=
120FbKS
1;
where, for this example
Fb = 1,930 lbf/in2(13.3 N/mmZ), bending stress from table 10
KS = 0.455 in3/ft (24,450 mm3/m),effective section modulus from table 9
I, = 16 in (406 mm), span from center-to-center of supports
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Allowable pressure (w,) based on shear stress:
From table 8 in chapter 3:
w, =
2OF.(lWQ)
, for two spans
'2
where, for this example
F, = 72 lbf/in2(0.50 N/mm?, rolling shear stress from table 10
lb/Q = 7.187 in2/ft (15,200 mm2/m), rolling shear constant from table 9
1, = 16 in
W'
- 1.5 in = 14.5 in (368 mm), clear span
= 2q72)(7'187)
14.5
= 714 lbflft' (34.200 N/m ,)
Allowable pressure (wJ based on bendmg and shear deflection:
According to section 3.3.3 of the Guide Design Spec$cation for Bridge Temporary Works,
forms for exposed concrete surfaces should not exceed either 118 in (3.2 mm) or 1/240 of the
center-to-center distance between joists.
118
in(3.2 mm)
or (in40)(16 in) = 0.07 in (1.8 mm)
For this example, kl,= 0.07 in (1.8 mm)
Determine the bending deflection (AJ based on a i.O-lbtlft? (47.9-N/m2) load using the following
equation from table 8 in chapter 3:
wi:
= 1,743 EI
, for two spans
where, for this example
l3 = 14.5 in
+ 0.25 in = 14.75 in (375 mm), clear span plus '/4 in (6.4 mm)
E = 1,650,000 lbf/in2(11,400 Nhnm'), adjusted modulus of elasticity
I = 0.199 in4/ft(272,000 mm'/m), moment of inertia
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--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
Therefore, the maximum allowable deflection (4,Jis:
AASHTO T I T L E CHBTW 95
= Ob39804
0031649 2 T 2
(i.0)(i4*75)4 = O.ooOo827 in (0.00210 mm)
(1,650,000(0.199)
Determine the bending deflection (AJbased on a l.0-lbf/ft2(47.9-N/m2)load using the following
equation from table 8 in chapter 3:
As =
Cwt '12'
1,270 EeI
where, for this example
C = 120 for face grain perpendicular to supports
t = % in (19 mm), plywood thickness
1, = 14.5 in (368 mm), clear span
E. = 1,500,000 lbf/in2(10,300 N/mm2), unadjusted modulus of elasticity
I = 0.199 in4/ft(272,000 mm4/m), moment of inertia
The allowable pressure (w,) based on deflection is therefore:
WA =
41,.-A,
+
As
0.07
O.oooO827 + 0.0000374
wA= 583 lbf/ft2 (27,900 N/m2)
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
The maximum allowable pressure on the vertical sheathing is acceptable for shear and deflection.
However, the bending capacity [412 ibf/ft2 (19,700 Nhn?)] of the plywood is approximately 33 percent
too low. The required capacity in 530 lbf/fl! (26,300 N/mZ). increasing the plywood thickness from
% in (19 mm) to 1 in (25 mm) increases the bending capacity of the sheathing to:
w,
=
667 IbWft' (31,900 N/m2)
Instead of increasing the plywood thickness, the spacing of the studs may be decreased. The allowable
pressure based on bending of %-in (19-nun) plywood with supports spaced at 12 in (305 mm) is:
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A A S M T O T I T L E C H B T W 75 9 Ob39804 0031b50 TL4 9
Therefore, the plywood thickness must be increased or the stud spacing decreased in order for the
sheathing capacity to exceed the design load. When evaluating the other vertical formwork components,
use a spacing of 16 in (406 mm) for the spacing of the studs, as shown in figure 56.
3.
Check the stresses and deflection in the studs of the vertical formwork.
The studs are loaded by a triangular load distribution that varies linearly from 550 lbf/fi? (26,300 Nhn')
to O. The studs are spaced at 16 in (406 mm) on center. The linear load on each stud is therefore
733 lbf/ft (10,700 N/m) at the bottom of the stud and O at the top. The studs are supported on the
waiers. Using conventional beam theory, calculate the maximum moment, shear, and reaction force in
the stud under these loading conditions. The maximum values are as follows:
M = 155 lbf-ft (210 N-m), maximum moment
V = 556 lbf (2,470 N), maximum shear
R = 1,003 Ibf (4,460 N), maximum reaction
Bending stress:
The section properties for the 2-in by 4-in nominal (50-mm by 100-mm) S4S stud can be found
in appendix A (table 14).
The section modulus (S,) is 3.06 in3 (50,100 nun3).
The bending stress (fb) is:
-
fb = M = (155 Ibf-ft)(l2 idft)
3.06 in3
sxx
fb = 608 1bWin *(4.2 N/mm2)
The allowable bending stress is 1,000 lbf/in2(6.9 N/mm*). The stud is therefore acceptable in
bending.
Shear stress:
From table 14 in appendix A, the actual dimensions of the stud are:
b = 1.5 in (38 mm)
h = 3.5 in (89 mm)
The horizontal shear stress (H)is:
H = -3 v
=
2bh
3(556 lbf)
2(1.5 in)(3.5 in)
H = 159 lbf/in2(1.1 N/mmz)
This stress exceeds the allowable shear stress of 95 lbf/in2 (0.66 N/mm2). The stud spacing
may be decreased or the stud size increased to reduce the maximum shear stress.
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1o9
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A A S H T O TITLE
C H B T W 95
m
O L ~ ~ ~ H 0J L0 , 3 1 , ~
950
~
m
If a 2-in by 6-in nominal (50-mm by 150-mm) S4S stud is used instead, the shear stress
becomes:
3(556 Ibo
= 101 Ibf/in2 (0.70 N/mm2)
2(1.5 in)(5.5 in)
H -
The spacing of the studs would have to be reduced to 9 in (230 mm) in oder to decrease tbe
shear stress to the allowable shear stress. For a 9-in (230-mm) spacing, the shear stress (H)
equals:
H = (9 id16 in) (159 lbfïm2)
H = 89 lbf/in2 (614,000 N/mm*)
Bearing stress:
The bearing area (A) equals:
A = (1.5 in)(2)(1.5 in) = 4.5 in2 (2,900 mm2)
The bearing stress (f) is therefore equal to:
f - , =
R
A
1,003 Ibf
4.5 in2
f = 223 lbf/in2 (1.5 N/mm?
The allowable bending stress is 625 Ibf/in2(4.3 N/mm2). The studs are therefore acceptable in
bearing.
4.
Check the bending deflection of the studs.
The deflection of the studs can be conservatively estimated using the deflection formula for a simply
supported beam. At the midpoint between walers, the linear load equals 383 lbf/ft (5.59 N/m). Use this
average load to estimate the deflection:
5w14
A=-384EI
-
5(383 lbf/ft)(26 in)* (1 fV12 in)
384(1,500,000 ibf/in ‘)(5.36 in4)
A = 0.02 in
The allowable deflection is given in section 3.3.3 of the Guide Design SpeciJicafionfor Temporary
Work. The maximum allowable deflection is ID in (3.2 mm) or 1n40 of 26 in (660 mm), which equals
0.11 in (2.8 mm). The stud deflection is therefore acceptable.
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
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AASHTO T I T L E
5.
063980Y 0033652 897 W
CHBTW 95
Check the bending, shear, and bearing stress in the walers.
Three studs bear on each waler between tie rods. The lower waler carries more load than the higher
waler. The maximum reaction on the stud support (waler) is given in step 3. The load from each stud
is 1,003 Ibf (4,460 N). The maximum values under these loads are as follows:
M = 20,000 lbf-in
V = 1,500 lbf
R = 1,500 lbf (6,670 N)
Bending stress:
The section modulus (SJ of one 2-in by 6-in (50-mm by 150-mm) S4S dimension lumber is
7.56 in3 (124,000 mm3), from table 14 in appendix A. Each waler consists of two 2 x 6‘s.
The bending stress (fb) is:
fb =
M = 20,000 lbf-in
2S,
(2)(7.56 in3)
The walers are therefore not acceptable in bending.
Using a 2-in by 8-in nominal (50-mm by 200-mm) section, with S , equal to 13.14 in3 (215,000 m’),
the bending stress is:
fb =
20.000 lbf-in
(2)(13.14)
fb = 761 Ibf/in2= (5.2 N/mm2)
This calculated bending stress is below the maximum allowable bending stress of 1,OOO ibf/in2
(6.9 N/mm2).
Shear stress:
Check the shear stress using 2 x 8 walers.
The actual dimensions of each 2 x 8 are:
b = 1.5 in (38 mm)
h = 7.25 in (184 mm)
The horizontal shear stress (H) for two 2 x 8’s is:
H=-
3v =
3(1,500 lbf)
(2)2bh
(2)(2)(1.5 in)(7.25 in)
H = 103 Ibf/in2 (0.71 N/mmZ)
111
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--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
f,, = 1,323 Ibf/in2(9.1 N/mm?
A A S H T O TITLE
C H B T W 95
m
o b m o 4 0 0 3 ~ b 5 37 2 3
rn
This exceeds the allowable shear s m s by 8 percent. The designer should determine if an allowable
s m s increase may be taken for shortduration loading.
Bearing stress:
The bearing stress (0 on the tie plaie is:
f = - R=
A
1,500 lbf
15 in2
f = 100 lbffin' (0.69 N / m ?
--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
The allowable bearing stress is 625 lbf/in2(4.3 N/mmZ). The waler is therefore acceptable as
designed. The reaction load (R) must be transferred through the tie rods. Check manufacturer
product data for allowable tensile loads on the tie rods.
112
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A A S H T O T I T L E CHBTW 9 5
Ob39804 0031654 bbT
APPENDIX C
RECOMMENDED THICKNESSES OF WOOD LAGGING'24'
Table 16. Recommended thickness of wood lagging for various soil types.
Unified
classification
Soil descripiion
Silts or fine sand
and silt above
water table
ML
ci
Sands and
gravels (medium
dense to dense)
GW, GP,
GM,GC,
SW, SP,SM
CL, CH
8
clays (stiff to
very stiff); nonfissured
Clays, medium
consistency and
CL,CH
Sands and silty
sands, (ïoose)
SW,SP,SM
Clayey sands
(mediumdense
to dense) below
water table
sc
Clays, heavily
overconsolidated
fissured
CL, CH
Cohesionless silt
or fine sand and
silt below water
table
ML; SM-ML
2
E3
Recommended thickness of lagging
(rough-cut) for clear spans of:
Depth
6R
7A
8ft
9R
loft
2 in 3 in
3 in
3 in
4 in
4
5A
SM-ML
O A to 25 fî
in
3
3
2
i
CI
25 ft to 60 ft
3 in 3 in
3 in
4 in
4 in
5 in
O ft to 25 ft
3 in
3 in
3 in
4 in
4 in
5 in
25 fî to 60 A
3 in
3 in
4 in
4 in
5 in
5 in
O ft to 15 fi
3 in
3 in
4 in
5 in
--
--
15 ft to25 fi
3 in
4 in
5 in
6 in
--
--
25 fi to 35 A
4 in
5 in
6 in
--
--
~
Sofi clays
"
2
CL,CH
Slighîiyplastic
silts &low water
ML
Clayey sands
flocse) below
water table
sc
Notes:
(a) In the category of "potentially dangerous soils," use of lagging is questionable.
(b) Conversion: 1 in = 25.4 mm; 1 A = 0.305 m
113
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A A S M T O T I T L E CHBTW 95
= Oh37804 0031655 5Th m
APPENDIX D
STEEL SHEET PILE DATA
The following information is excerpted from the United States Steel Sheet Piling Handbook.’”’ While
specific sheet pile data provided in this appendix may be dated, information relating to nomenclature, driving
practices, steel grades, and interlock characteristics is still applicable.
Standard Nomenclature System for Sheet Piling
As part of the steel industry’s program for unifying and improving the classification and designation of
structurai steel products, a standardized nomenclature system for steel sheet piling was introduced in 1972. The
following information describes this system.
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Alphabetic and Numerical Designations:
P
Z
= Steel sheet piling
SA
= 2-shaped profile or cross section
= Straight web profile
= Shallow arch profile
MA
DA
X
Number
= Median arch profile
= Deep arch profile
= High-strength interlock
= Weight of sheet piling shape, lbf/ft2of wall
S
For example, the designation PSX32 represents steel sheet piling (P) with a straight web ( S ) and a high-strength
interlock ( X ) and which weighs 32 ibf/ft2(156 kg/m2)of wall.
Driving Practices
The driving dimensions given for the various sheet piling profiles are nominal. Because of normal miii
tolerances and probable variations in onsite conditions, sheet piles may drive either short or long in a waU, even
when they are carefully lined up and driven with a template. This can be anticipated particularly in the case of
2 piles - where a gain or loss of seved inches (per pair of piles) is possible. To a large extent, such
dimensional variations occur as a result of the setting-up position. To compensate for this, standard practice
requires that setting and driving operations be checked frequently. In this way, the position of certain pairs of
piles can be changed whenever it is necessary to compensate.
Steel Grades
The common specification for USS steel sheet piling is ASTM A328. Because this is the most
frequently specified grade, it is the most readily available.
ASTM A328 - This is the basic sheet piling specification and provides for a minimum yield point of
38,500 lbf/in2(265 Nhnmz) and minimum tensile strength of 70,000 Ibf/in2(483 N/mmZ). With this grade, it is
general practice to allow a working stress of at least 25,000 lbf/in2 (172 N/mm2). Because of its applicability to
a majority of piling uses, it is the one grade most readily available either from rollings or from stock.
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TITLE CHBTW 95
ASTM A572 Grade 50 - This grade has a minimum yield point of 50,000 lbf/in2 (345 N/mm2) and an
allowable design stress of 32,000 lbf/in2 (221 N/mmZ). This is almost 30 percent higher than the suggested
allowable design stress for carbon grade (ASTM A328). This grade (ASTM A572 Grade 50) is generally
available on order from planned rollings; it is not a normally stocked grade.
PSX32, the high interlock-strength piling, is available only in 50-kip/in2 ( 3 4 5 - N h 9 minimum yield
point steel. The increased strength offered by this grade increases resistance to bending forces and is used
n o d y for the 2-pile profiles.
Interlock Characteristics
Interlocks of straight web and arch web piling are referred to as the "thumb and finger" type: this design
provides three contact points and helps develop both strength and watertightness characteristics.
adjacent sections for piling lengths up to 50 ft (15 m). PSX32 used in larger structures where swing
requirements are minjmal, has a swing of at least 5". Where lengths are longer than 50 ft (15 m), the swing
requirement should be shown on the order. Where swings in excess of the above must be ensured, it is possible
to use pre-bent pieces. When PSX32 is used in a circular coffer-cell, PS28 or PS32 shapes may be considered in
the arcs that connect the main cells. These latter shapes have the increased swing that may be needed to close
the arcs, if other than T-typeconnectors are used.
Figure 57. Normal interlock swing is at least 10"
on arch web and straight web shapes.
The interlocks of 2 piling is the ball-and-socket type. This interlock has the least driving resistance
(provided that the socket end is driven over the ball end). While no swing is guaranteed in 2-type piling
interlocks, some small yet practical amount may be developed during the actual installation. Again, where swing
must be assured, pre-bent piles can be supplied. It is suggested that if 2 piles are to be used for circular
structures, USS product engineers should be consulted prior to ordering. Interlocks are manufactured so that the
sheet piling will be reasonably free-sliding to grade.
In a given structure where sheet piling from different producers must be mixed, it is suggested that the
number of such connections should either be held to a minimum or that compromise connections be fabricated.
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Arch web and straight web piling interlocks have a swing of at least 10" (figure 57) between two
Ob39804 003Lb57 379
AASHTQ T I T L E CHBTW 95
Nonnal warranties applicable to sheet piie generally do not apply where sheet piling from different
manufacturers are interlocked together.
The interlocks of USS steel sheet piling are designed for the normal joining technique as shown in
figure 58. On occasion, however, a design requirement will call for a reversed position as shown in figure 59.
While the shapes can be joined in such a manner, this reversed position will result in a weaker interlock
connection and will create difficulty in holding the alignment of the sheet piling wail for any extended run.
Performance warranties apply only to the normal method.
Table 17 shows which shapes interlock with one another. Additionally, the ball element of PZ27 will
interlock witb tbe sockets of PZ32 and PZ38.
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Figure 58. Steel sheet piling interlocks in the normal position.
Figure 59. Steel sheet piling interlocks in the reverse position (not recommended).
117
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AASHTO T I T L E CHBTW 95
m
063980Y 003Lb58 205
m
Table 17. Standard sheet piling (circa 1972).
-
Designation
Profile
Weight
per
linear ft,
Ib
Weight
per sq
ft of
wall, lb
57.0
38.0
PZ38
Section modulus
Driving
per ft
Area
A, in2
width,
in
Wall,
in3
per
pile,
in3
16.8
18
46.8
70.2
21
38.3
67.O
-
56.0
pZ32
32.0
16.5
PZ27
P.DA27
~
40.5
27.0
11.9
18
3.O2
45.3
36.0
27.0
10.6
16
10.7
14.3
-
-
Y
36.0
PMA22
22.0
10.6
195/8
5.4
8.8
16
2.5
3.3
16
2.4
3.2
37.3
PSA2a
28.0
11.0
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30.7
PSA23
23.0
9.O
-
~~
PSX32
44.0
32.0
13.0
PS32
PS2a
40.0
32.0
11.8
2.4
3.3
15
1.9
2.4
~
35.0
28.0
10.
Conversion: 1 in = 25.4 mm; 1 Ibf = 4.45 N
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~
15
1.9
2.4
-
AASHTO T I T L E CHBTW 95 W Ob39804 0033657 141 W
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--``,`,`,,``,`,,`,`,,,,```,,``-`-`,,`,,`,`,,`---
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J.F. DUNTEMA", L. EDWIN D U " , S A F D A R GILL, ROBERT G. LUKAS, and MARK K.
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4.
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R.T. RATAY, Ed., Handbook of Temporary Structures in Construction, First Edition, McGraw-Hill
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AASHTO T I T L E CHBTW 95
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ACI-ASCE COMMITEE 343, "Analysis and Design of Reinforced Concrete Bridge Structures (AU
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M.J. TOMLINSON, Foundation Design and Construction, 3rd Ed., John Wiley & Sons, New York NY,
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24.
D.T. GOLDBERG, W.E. JAWORSKI, and M.D. GORDON, Lateral Support System and
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U.S. STEEL CORPORATION, Steel Sheet Piling Design Manual, Pittsburgh, PA, 1972.
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*U.S.
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17.
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