---
•
•
•
PEDE,8TRIAN BRIDGES
© 2009 by the American Association of State Highway and Transportation Officials.
All rights reserved. Duplication is a violation of applicable law.
11
EXECUTIVE COMMITTEE
2008-2009
Voting Members
Officers:
President: Larry L. "Butch" Brown, Sr., Mississippi
Vice President: Susan Martinovich, Nevada
Secretary-Treasurer: Carlos Braceras, Utah
Regional Representatives:
REGION I:
Joseph Marie, Connecticut, One-Year Term
Gabe Klein, District of Columbia, Two-Year Term
REGION II:
Dan Flowers, Arkansas, One-Year Term
Mike Hancock, Kentucky, Two-Year Tenn
REGION III:
Nancy J. Richardson, Iowa, One-Year Term
Thomas K. Sorel, Minnesota, Two-Year Term
REGION IV:
Paula Hammond, Washington, One-Year Term
Amadeo Saenz, Jr., Texas, Two-Year Term
Nonvoting Members
Immediate Past President: Allen Biehler, Pennsylvania
AASHTO Executive Director: John Horsley, Washington, DC
111
HIGHWAYS SUBCOMMITTEE ON BRIDGES AND STRUCTURES,
2009
MALCOLM T. KERLEY, Chair
KEVIN THOMPSON, Vice Chair
M. MYINT LWIN, Federal Highway Administration, Secretary
RAJ AILANEY, Federal Highway Administration, Assistant Secretary
KEN KOBETSKY, AASHTO Liaison
KELLEY REHM, AASHTO Liaison
ALABAMA, John F. "Buddy" Black, William "Tim"
Colquett, George H. Conner
ALASKA, Richard A. Pratt
ARIZONA, Jean A. Nehme
ARKANSAS, Phil Brand
CALIFORNIA, Kevin Thompson, Susan Hida, Barton 1.
Newton
COLORADO, Mark A. Leonard, Michael G. Salamon
CONNECTICUT, Julie F. Georges
DELAWARE, Jiten K. Soneji, Barry A. Benton
DISTRICT OF COLUMBIA, Nicolas Galdos, L. Donald
Cooney, Konjit "Connie" Eskender
FLORIDA, Marcus Ansley, Sam Fallaha, Jeff Pouliotte
GEORGIA, Paul V. Liles, Jr.
HAWAII, Paul T. Santo
IDAHO, Matthew M. Farrar
ILLINOIS, Ralph E. Anderson, Thomas J. Domagalski
INDIANA, Anne M. Rearick
IOWA, Norman L. McDonald
KANSAS, Kenneth F. Hurst, James 1. Brennan, Loren R.
Risch
KENTUCKY, Mark Hite
LOUISIANA, Hossein Ghara, Arthur D'Andrea, Paul
Fossier
MAINE, David B. Sherlock, Jeffrey S. Folsom
MARYLAND, Earle S. Freedman, Robert J. Healy
MASSACHUSETTS, Alexander K. Bardow, Shirley
Eslinger
MICHIGAN, Steven P. Beck, David Juntunen
MINNESOTA, Daniel L. Dorgan, Kevin Western
MISSISSIPPI, Mitchell K. Carr, B. Keith Carr
MISSOURI, Dennis Heckman, Michael Harms
MONT ANA, Kent M. Barnes
NEBRASKA, Mark 1. Traynowicz, Mark Ahlman, Fouad
Jaber
NEVADA, Mark P. Elicegui, Todd Stefonowicz
NEW HAMPSHIRE, Mark W. Richardson, David L. Scott
NEW JERSEY, Richard W. Dunne
NEW MEXICO, Raymond M. Trujillo, Jimmy D. Camp
NEW YORK, George A. Christian, Donald F. Dwyer,
Arthur P. Yannotti
NORTH CAROLINA, Greg R. Perfetti
NORTH DAKOTA, Terrence R. Udland
iv
OHIO, Timothy 1. Keller, Jawdat Siddiqi
OKLAHOMA, Robert 1. Rusch, Gregory D. Allen,
John A. Schmiedel
OREGON, Bruce V. Johnson, Hormoz Seradj
PENNSYLVANIA, Thomas P. Macioce, Harold C. "Hal"
Rogers, Jr., Lou Ruzzi
PUERTO RICO, (Vacant)
RHODE ISLAND, David Fish
SOUTH CAROLINA, Barry W. Bowers, Jeff Sizemore
SOUTH DAKOTA, Kevin Goeden
TENNESSEE, Edward P. Wasserman
TEXAS, David P. Hohmann, Keith L. Ramsey
U.S. DOT, M. Myint Lwin, Firas 1. Sheikh Ibrahim
UTAH, (Vacant)
VERMONT, Wayne B. Symonds
VIRGINIA, Malcolm T. Kerley, Kendal Walus, Prasad L.
Nallapaneni, Julius F. J. Volgyi, Jr.
WASHINGTON, Jugesh Kapur, Tony M. Allen, Bijan
Khaleghi
WEST VIRGINIA, Gregory Bailey, James D. Shook
WISCONSIN, Scot Becker, Beth A. Cannestra, William
Dreher
WYOMING, Gregg C. Fredrick, Keith R. Fulton
GOLDEN GATE BRIDGE, Kary H. Witt
N.J. TURNPIKE AUTHORITY, Richard J. Raczynski
N.Y. STATE BRIDGE AUTHORITY, William J. Moreau
PENN. TURNPIKE COMMISSION, James L. Stump
U.S. ARMY CORPS OF ENGINEERSDEPARTMENT OF THE ARMY, Christopher H.
Westbrook
U.S. COAST GUARD, Hala Elgaaly
U.S. DEPARTMENT OF AGRICULTURE-FOREST
SERVICE, John R. Kattell, Scott F. Mitchell
ALBERTA, Tom Loo
NEW BRUNSWICK, Doug Noble
NOV A SCOTIA, Mark Pertus
ONT ARlO, Bala Tharmabala
SASKA TCHEWAN, Howard Yea
TRANSPORTATION RESEARCH BOARD-Waseem
Dekelbab
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
Table of Contents
I--GENERAL ................................................................................................................................................................. 1
1.1-Scope ................................................................................................................................................................ 1
1.2-Manufacturer-Designed Systems ...................................................................................................................... 1
1.3-Collision Mitigation ......................................................................................................................................... 1
2-PHILOSOPHY ........................................................................................................................................................... 2
3-LOADS ....................................................................................................................................................................... 2
3. I-Pedestrian Loading (PL) ................................................................................................................................... 2
3.2-Vehicle Load (LL) ............................................................................................................................................ 4
3.3-Equestrian Load (LL) ....................................................................................................................................... 5
3.4-Wind Load (WS) .............................................................................................................................................. 5
3.5-Fatigue Load (LL) ............................................................................................................................................ 6
3.6-Application of Loads ........................................................................................................................................ 6
3.7-Combination of Loads ...................................................................................................................................... 6
4--FATIGUE ................................................................................................................................................................... 7
4.1-Resistance ......................................................................................................................................................... 7
4.2-Fracture ............................................................................................................................................................. 7
5-DEFLECTIONS ......................................................................................................................................................... 7
6-VIBRATIONS ............................................................................................................................................................ 7
7---STABILITY ............................................................................................................................................................... 8
7. I-Half-Through Trusses ....................................................................................................................................... 8
7.1. I-Lateral Frame Design Force ................................................................................................................... 8
7.1.2-Top Chord Stability ................................................................................................................................ 9
7.1.3-Altemative Analysis Procedures .......................................................................................................... 11
7.2-Steel Twin I-Girder and Single Tub Girder Systems ...................................................................................... 12
7.2.I-General ................................................................................................................................................. 12
7.2.2-Lateral-Torsional Buckling Resistance-Twin I-Girder ...................................................................... 12
7 .2.3-Lateral-Torsional Buckling Resistance-Singly Symmetric Sections ................................................. 13
8-TYPE SPECIFIC PROVISIONS .............................................................................................................................. 13
8. I-Arches .. ,............................................................................................................... ,......................................... 13
8.2-Steel HSS Members ........................................................................................................................................ 13
8.2.I-General ................................................................................................................................................. 13
8.2.2-Detailing ..................................... ,............................... ,......................................................................... 14
8.2.3-Tubular Fracture Critical Members ...................................................................................................... 14
8.3-Fiber Reinforced Polymer (FRP) Members .................................................................................................... 15
9-DESIGN EXAMPLE ................................................................................................................................................ 16
Illustrative Example of Key Provisions of Guide Specifications ............................................................................ 16
General Information ................................................................................................................................................ 16
v
Truss Members: All Rectangular HSS .................................................................................................................... 17
Floorbeams .............................................................................................................................................................. 17
Dead Load ............................................................................................................................................................... 18
Pedestrian Live Load .............................................................................................................................................. 18
Vehicle Load ........................................................................................................................................................... 18
Wind Load .............................................................................................................................................................. 19
Total Vertical Loads per Truss ................................................................................................................................ 21
Truss Member Design Loads .................................................................................................................................. 21
Truss Top Chord Lateral Support ........................................................................................................................... 21
Top Chord Compressive Resistance ....................................................................................................................... 23
Lateral Force to Be Resisted by Verticals ............................................................................................................... 24
End Posts ................................................................................................................................................................. 25
Deflection ................................................................................................................................................................ 25
Vibrations ................................................................................................................................................................ 25
Vertical Direction ............................................................................................................................................ 25
Lateral Direction .............................................................................................................................................. 26
Fatigue .................................................................................................................................................................... 27
REFERENCES ............................................................................................................................................................... 29
VI
LRFD GUIDE SPECIFICATIONS FOR THE
DESIGN OF PEDESTRIAN BRIDGES
1-GENERAL
l.l-SCOPE
C1.l
These Guide Specifications address the design and
construction of typical pedestrian bridges which are
designed for and intended to carry primarily
pedestrians, bicyclists, equestrian riders, and light
maintenance vehicles, but not designed for and
intended to carry typical highway traffic. Pedestrian
bridges with cable supports or atypical structural
systems are not specifically addressed.
This edition of the Guide Specifications was
developed from the previous Allowable Stress Design
(ASD)- and Load Factor Design (LFD)-based first
edition (AASHTO 1997). An evaluation of available
foreign specifications covering pedestrian bridges, and
failure investigation reports, as well as research results
related to the behavior and performance of pedestrian
bridges was performed during the development of the
LRFD Guide Specifications.
These Guide Specifications provide additional
guidance on the design and construction of pedestrian
bridges in supplement to that available in the AASHTO
LRFD Bridge Design Specifications (AASHTO LRFD).
Only those issues requiring additional or different
treatment due to the nature of pedestrian bridges and
their loadings are addressed. In Article 3 of this
document, the load definitions and abbreviations are
taken from AASHTO LRFD. Aluminum and wood
structures are adequately covered in AASHTO LRFD,
and as such are not specifically addressed herein.
Implementation of the wind loading and fatigue
loading provisions require reference to the AASHTO
Standard Specifications for Structural Supports for
Highway Signs, Luminaires, and Traffic Signals
(AASHTO Signs).
1.2-MANUFACTURER-DESIGNED SYSTEMS
Cl.2
Where manufacturer-designed systems are used for a
pedestrian bridge crossing, the engineer responsible for
the design of the system shall submit sealed
calculations prepared by a licensed Professional
Engineer for that system.
It is
important to clearly delineate the
responsibilities of each party when proprietary bridge
systems are used. All portions of the design must be
supported by sealed calculations, whether from the
bridge manufacturer, or the specifying engineer. The
interface between the manufacturer-designed system
and the project-specific substructures and foundations
needs careful attention.
1.3-COLLISION MITIGATION
Cl.3
AASHTO LRFD Article 2.3.3.2 specifies an
increased vertical clearance for pedestrian bridges
1.0 ft higher than for highway bridges, in order to
In most cases, increasing vertical clearance
most cost-effective method of risk mitigation.
IS
the
LRFD GUIDE SPECIFICATIONS FOR THE DESIGl'I OF PEDESTRIAN BRIDGES
2
mitigate the risk from vehicle collisions with the
superstructure. Should the owner desire additional
mitigation, the following steps may be taken:
•
Increasing vertical clearance in addition to that
contained in AASHTO LRFD
•
Providing structural continuity of the
superstructure, either between spans or with the
substructure
•
Increasing the mass of the superstructure
•
Increasing the lateral resistance of the
superstructure
2-PHILOSOPHY
Pedestrian bridges shall be designed for specified
limit states to achieve the objectives of safety;
serviceability, including comfort of the pedestrian user
(vibration); and constructability with due regard to
issues of inspectability, economy, and aesthetics, as
specified in AASHTO LRFD. These Guide
Specifications are based on the LRFD philosophy.
Mixing provisions from specifications other than those
referenced herein, even if LRFD based, should be
avoided.
3-LoADS
3.l-PEDESTRIAN LOADING (PL)
C3.l
Pedestrian bridges shall be designed for a uniform
pedestrian loading of 90 psf. This loading shall be
patterned to produce the maximum load effects.
Consideration of dynamic load allowance is not
required with this loading.
This article modifies the pedestrian loading
provisions of the Fourth Edition of AASHTO LRFD,
through the 2009 Interim. The previous edition of these
Guide Specifications used a base nominal loading of
85 psf, reducible to 65 psf based on influence area for
the pedestrian load. With the LFD load factors, this
results in factored loads of 2.17(85) = 184 psf and
2.17(65) = 141 psf. The Fourth Edition of AASHTO
LRFD specified a constant 85 psf regardless of
influence area. Multiplying by the load factor, this
results in 1.75(85) = 149 psf. This falls within the
range of the previous factored loading, albeit toward
the lower end.
European codes appear to start with a higher nominal
load (approx 105 psf), but then allow reductions based
on loaded length. Additionally, the load factor applied
is 1.5, resulting in a maximum factored load of
( 1. 5) 105 = 158 psf. For a long loaded length, this load
can be reduced to as low as 50 psf, resulting in a
factored load of (1.5)50 = 75 psf. The effect of
resistance factors has not been accounted for in the
above discussion of the European codes. There are,
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
however, warnings to the designer that a reduction in
the load based on loaded length may not be appropriate
for structures likely to see significant crowd loadings,
such as bridges near stadiums.
Consideration might be given to the maximum credible
pedestrian loading. There is a physical limit on how
much load can be applied to a bridge from the static
weight of pedestrians. It appears that this load is
around 150 psf, based on work done by Nowak (2000)
from where Figures C 1 through C3 were taken.
Although there does not appear to be any available
information relating to the probabilistic distribution of
pedestrian live loading, knowing the maximum
credible load helps to define the limits of the upper tail
of the distribution of load. The use of a 90 psf nominal
live load in combination with a load factor of 1.75
results in a loading of 158 psf, which provides a
marginal, but sufficient, reserve compared with the
maximum credible load of 150 psf.
3
4
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
Figure C3.1-1-Live Load of 50 psf
Figure C3.1-2-Live Load of 100 psf
Figure C3.1-3-Live Load of 150 psf
3.2-VEHICLE LOAD (LL)
C3.2
Where vehicular access is not prevented by
pennanent physical methods, pedestrian bridges shall
be designed for a maintenance vehicle load specified in
Figure I and Table I for the Strength I Load
Combination unless otherwise specified by the Owner.
The vehicle loading specified is equivalent to the Htrucks shown in Article 3.6.1.6 of AASHTO LRFD
2009 Interim and contained in previous versions of the
AASHTO Standard Specifications jor Highway
Bridges.
LRfD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
truck shall be placed to produce the maximum
effects and shall not be placed in combinations
with the pedestrian load. The dynamic load allowance
need not be considered for this loading.
Table 3.2-1-Design Vehicle
Clear Deck Width
7 to 10 ft
Over 10 ft
Design Vehicle
H5
HlO
14 ft 0 in.
4.0 kips
2.0 kips
Figure 3.2-1-Maintenance Vehicle Configurations
3.3-EQUESTRIAN LOAD (LL)
C3.3
Decks intended to carry equestrian loading shall be
designed for a patch load of 1.00 kip over a square area
measuring 4.0 in. on a side.
The equestrian load is a live load and intended to
ensure adequate punching shear capacity of pedestrian
bridge decks where horses are expected. The loading
was derived from hoof pressure measurements reported
in Roland et. al. (2005). The worst loading occurs
during a canter where the loading on one hoof
approaches 100 percent of the total weight of the horse.
The total factored load of 1.75 kips is approximately
the maximum credible weight of a draft horse. This
loading is expected to control only deck design.
3.4-WIND LOAD (WS)
C3.4
Pedestrian bridges shall be designed for wind loads
as specified in AASHTO Signs, Articles 3.8 and 3.9.
Unless otherwise directed by the Owner, the Wind
Importance Factor, Jr , shall be taken as l.l5. The
loading shall be applied over the exposed area in front
The wind loading is taken from AASHTO Signs
specification rather than from AASHTO LRFD due to
the potentially flexible nature of pedestrian bridges,
and also due to the potential for traffic signs to be
mounted on them.
5
-6
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
elevation including enclosures. Wind load on signs
supported by the pedestrian bridge shall be included.
In addition to the wind load specified above, a vertical
uplift line load as specified in AASHTO LRFD Article
3.8.2 and determined as the force caused by a pressure
of 0.020 ksf over the full deck width, shall be applied
concurrently. This loading shall be applied at the
windward quarter point of the deck width.
3.S-FATIGUE LOAD (LL)
C3.S
The fatigue loading used for the fatigue and fracture
limit state (Fatigue I) shall be as specified in Section 11
of AASHTO Signs. The Natural Wind Gust specified in
Article 11.7.3 and the Truck-Induced Gust specified in
Article 11.7.4 of that specification need only be
considered, as appropriate.
Wind loads are not part of the Fatigue I load
combination for vehicular bridges. This article
designates wind as a live load for pedestrian bridges,
via the designation LL. Wind should be considered a
fatigue live load for pedestrian bridges.
Neither the pedestrian live load nor the maintenance
vehicle load used for strength and serviceability is
appropriate as a fatigue design loading due to the very
infrequent nature of this loading. The fatigue loading
specified is consistent with the treatment of sign
support structures. For bridges crossing roadways, the
truck-induced gust loading should be considered. The
other loadings specified in AASHTO Signs are not
applicable to pedestrian bridges due to their decreased
susceptibility to galloping or vortex shedding
vibrations.
3.6-APPLlCATlON OF LOADS
C3.6
When determining the application of the pedestrian
live loading which maximizes or minimizes the load
effect on a given member, the least dimension of the
loaded area shall be greater than or equal to 2.0 ft.
The dimension given is meant to represent a single
line of pedestrians; any width less than this would not
represent a practical loading scenario.
3.7-COMBINATlON OF LOADS
C3.7
The types of bridges identified in Article 1.1 shall be
designed for the load combinations and load factors
specified in AASHTO LRFD Table 3.4.1-1, with the
following exceptions:
Load combination Strength II is meant for special
permit trucks, which is not applicable to pedestrian
bridges. Strength IV is for dead load dominant
structures such as long span trusses, and would not
likely apply to pedestrian bridges. Strength V addresses
the case of strong wind combined with reduced live
loading, which is not likely to occur for pedestrian
bridges. For unusual cases where the excluded load
combinations have a reasonable chance of occurring,
they should be considered in the design. The fatigue
loading specified in AASHTO Signs and referenced
herein was calibrated for a load factor of 1.0 and the
design condition of infinite life.
•
Load combinations Strength II, Strength IV, and
Strength V need not be considered.
•
The load factor for the Fatigue I load combination
shall be taken as 1.0, and the Fatigue II load
combination need not be considered.
Where main gravity load carrying elements also form
part of the railing system, railing loads as specified in
Article 13.8.2 of AASHTO LRFD shall be applied
concurrently with all other live loads for the Strength
Limit States.
LRFD GUIDE SPECIFICATIONS
7
FOR THE DESIGN OF PEDESTRIAN BRIDGES
4-FATlGUE
4.1-RESIST ANCE
C4.1
The fatigue resistance for steel components and
details shall be as specified in AASHTO LRFD, Article
6.6.1.2.5 for the Fatigue I load combination. For round
HSS components and details not covered in AASHTO
LRFD, the nominal fatigue resistance may be taken
from Table 11.3 of AASHTO Signs or Figure 2.13 of
AWS D 1.1 Structural Welding Code-Steel. For square
and rectangular HSS components and details, the
nominal fatigue resistance may be taken from the
provisions of the Design Guide 8 of the International
Committee for the Development and Study of Tubular
Structures (CIDECT).
CIDECT Design Guides are a foreign specification
available through the AISC. See Zhao et aI., 2001.
The fatigue resistance for steel reinforcement in
concrete structures shall be as specified in AASHTO
LRFD Article 5.5.3.
4.2-FRACTURE
C4.2
Except as specified herein, all of the proViSIOns
specified in Article 6.6.2 of AASHTO LRFD relating to
Charpy
V-notch
(CVN)
fracture
toughness
requirements, including Fracture Critical Member
(FCM) and Main Member designation, shall apply to
steel pedestrian bridges. Design of tubular members
shall also satisfy the provisions of Article 8.2. If
supported by the characteristics of the site and
application, the Owner may waive the FCM
requirements, including Article 8.2.3 of these
specifications.
For pedestrian bridges crossing low-volume traffic
waterways and roads, or areas not accessible to the
general public, FCM treatment may not be appropriate.
5-DEFLECTIONS
C5
Deflections should be investigated at the service
limit state using load combination Service I in Table
3.4.1-1 of AASHTO LRFD. For spans other than
cantilever arms, the deflection of the bridge due to the
unfactored pedestrian live loading shall not exceed
lI360 of the span length. Deflection in cantilever arms
due to the pedestrian live loading shall not exceed
1/220 of the cantilever length. Horizontal deflections
tmder unfactored wind loading shall not exceed 11360
of the span length.
Table 2.5.2.6.3-1 of AASHTO LRFD contains
guidance on span-to-depth ratios that may be invoked
by an Owner.
6--VIBRATIONS
C6
Unless waived by the Owner, vibrations shall be
investigated as a service limit state using load
combination Service I in Table 3.4.1-1 of AASHTO
LRFD. Vibration of the structure shall not cause
Due to the vibration problems experienced in
London on the Millennium bridge, there have been
many publications in the technical literature, primarily
in Europe, on this topic. Despite this large body of
The 11360 criteria IS consistent with the increased
pedestrian loading compared to previous versions of
AASHTO pedestrian bridge guidance. See also Article
2.5.2.6.2 of AASHTO LRFD for bridges carrying both
roadway and pedestrian traffic.
8
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
discomfort or concern to users of a pedestrian bridge.
Except as specified herein, the fundamental frequency
in a vertical mode of the pedestrian bridge without live
load shall be greater than 3.0 hertz (Hz) to avoid the
first harmonic. In the lateral direction, the fundamental
frequency of the pedestrian bridge shall be greater than
1.3 Hz. If the fundamental frequency cannot satisfy
these limitations or if the second hannonic is a
concern, an evaluation of the dynamic performance
shall be made. This evaluation shall consider:
•
The frequency and magnitude of pedestrian
footfall loadings
•
The phasing of loading from multiple pedestrians
on the bridge at the same time, including the
"lock-in" phenomena
•
Appropriate estimation of structural damping
•
Frequency-dependent limits on acceleration and/or
velocity
In lieu of such evaluation in the vertical direction, the
bridge may be proportioned such that either of the
following criteria are satisfied:
.r? 2. 861n C!O)
(6-1)
knowledge, it does not appear there has been
convergence toward one method of evaluation, or
development of any specification that adequately
covers this issue.
These provisions address the issue of vibration from
two directions: maintaining a minimum natural
vibration frequency above those induced by
pedestrians, and specifying a minimum weight to limit
vibration amplitudes if the frequency limits are not
met. Although somewhat outdated, both of these
approaches are still viable and have the great advantage
of simplicity.
The technical guide published by SETRA (Service
d'Etudes Techniques des Routes et Autoroutes) (2006)
appears to present a relatively straightforward method
for addressing vibration issues when the frequencies of
the bridge fall within the pacing frequencies of
pedestrians.
The "lock-in" phenomenon refers to the tendency of
people to synchronize their pacing frequency to the
lateral frequency of the bridge when the lateral
displacements begin to grow. In other words, instead of
random frequencies and phasing among the loading
from pedestrians on the bridge, the frequencies and
phases becomes fully correlated with the bridge
motion.
or
W ? 180e(-O.35f)
(6-2)
where:
W =
the weight of the supported
including only dead load (kips)
f
the fundamental frequency in the vertical
direction (Hz)
structure,
7-STABILlTY
7.1-HALF-THROUGH TRUSSES
7.1.1-Lateral Frame Design Force
C7.1.1
The vertical truss members, the floorbeams, and
their connections shall be proportioned to resist a
lateral force applied at the top of the truss verticals.
The lateral force shall not be less than O.OI/K times the
average factored design compressive force in the two
adjacent top chord members, where K is the design
effective length factor for the individual top chord
members supported between the truss verticals. In no
case shall the value for 0.0 11K be less than 0.003 when
This article modifies the proVIsions of AASHTO
LRFD by replacing the 300 pounds per linear foot
design requirement for truss verticals with provisions
based on research reported in Galambos (1998). These
provisions establish the minimum lateral strength of the
verticals based on the degree of lateral support
necessary for the top chord to resist the maximum
design compressive force.
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
determining the minimum lateral force, regardless of
the K-value used to determine the compressive capacity
of the top chord. The lateral frame design force shall be
applied concurrently with the loading used to
determine the average compressive force above.
End posts shall be designed as a simple cantilever to
carry their applied axial load combined with a lateral
load of 1.0 percent of the end post axial load, applied
laterally at the upper end.
7.1.2-Top Chord Stability
C7.1.2
The top chord shall be considered as a column with
elastic lateral supports at the panel points. Except as
noted herein, the contribution of the connection
stiffness between the floorbeam and the vertical
member shall be considered in determining the
stiffness of the elastic lateral supports.
The use of the 1.33 factor applied to the factored
compression load to determine Pc is in recognition that
for uniformly loaded structures there is a higher
probability of the maximum compression force
occurring simultaneously in adjacent truss panels. For
further discussion, refer to Galambos (1998). To utilize
the procedures given here for top chord stability design
without modification for cross-frame connection
stiffuess, proper detailing must be used to ensure that
the connection is "fully rigid" at service loads; if not,
the stiffness of the connections must be incorporated
into the overall stiffness of the U-frames in order to
correctly predict top chord buckling.
When the following criteria are satisfied, the
connection may be assumed rigid for the purpose of
U-frame stiffness calculation:
Matched member widths in simple unreinforced
HSS connections of the floorbeam to vertical as
applicable (no deformation allowed due to tube
wall plastification of member faces at service
loads).
..
III
The connection of the floorbeam to the vertical
shall not include the HSS chord member, i.e., the
vertical and floorbeam shall not be connected to
different sides of a HSS chord.
Fixed end moments in the floorbeams and verticals
due to floorbeam rotations in addition to the loads
derived from a U-frame analysis have been
accounted for in strength design of the
connections.
It has been seen that if a stepped connection is used,
the stiffness of U-frames in pony truss bridges are less
than that predicted when using member stiffness only.
Additionally, welding to two faces of an HSS member
that are at 90 degrees to each other causes the chord to
go into the shape of a parallelogram or "ovalize" when
trying to pass moment between the members. This too
affects the cross-frame stiffness.
See AISC guidance for the strength design of HSS
connections.
The proposed design and details for the connection of
the floorbeam demonstrating satisfaction of the criteria
specified herein shall be submitted to the Owner for
review. The Owner's decision as to acceptability shall
final.
procedure for determining the resistance of a
compression member in AASHTO LRFD may be used
determine the resistance of the compression chord
a value for the effective length factor, K, based on
a lateral U-frame and obtained from Table 1. In this
C
lateral stiffness of the U-frame made of the
truss verticals and the floorbeam taken as PIt,
(kiplin.)
Interpolation of values between those given in the table
is acceptable.
9
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN
10
P
BRIDGES
arbitrary lateral load as shown schematically
in Figure 1 (kips)
lateral deflection resulting from lateral load P
and shown schematically in Figure 1 (in.)
L
length ofthe chord between panel points (in.)
Pc
desired critical buckling load (kips) of the
truss chord member, which shall be taken as
1.33 times the factored compressive load
n
number of panels in the truss
p
p
I
I
I
\
Ir---------------------------~I
j
L
\
'~--
~--~
~
Figure 7.1.2-1-Lateral V-Frame
t
11
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
Table 7.1.2-1-Values of 11K for Various Values of CLIPc and n
11K
1.000
0.980
0.960
0.950
0.940
0.920
0.900
0.850
0.800
0.750
0.700
0.650
0.600
0.550
0.500
0.450
0.400
0.350
0.300
0.293
0.259
0.250
0.200
0.180
0.150
0.139
0.114
0.100
0.097
0.085
n =4
3.686
3.352
2.961
2.448
2.035
1.750
1.232
0.121
0
n =6
3.616
3.284
3.000
2.754
2.643
2.593
2.460
2.313
2.147
1.955
1.739
1.639
1.517
1.362
1.158
0.886
0.530
0.187
n =8
3.660
2.944
2.665
2.595
2.263
2.013
1.889
1.750
1.595
1.442
1.338
1.211
1.047
0.829
0.627
0.434
0.249
n = 10
3.714
2.806
2.542
n = 12
3.754
2.787
2.456
n = 14
3.785
2.771
2.454
n = 16
3.809
2.774
2.479
2.303
2.146
2.045
1.794
1.629
1.501
1.359
1.236
1.133
1.007
0.847
0.714
0.555
0.352
0.170
2.252
2.094
1.951
1.709
1.480
1.344
1.200
1.087
0.985
0.860
0.750
0.624
0.454
0.323
0.203
2.254
2.101
1.968
1.681
1.456
1.273
1.111
0.988
0.878
0.768
0.668
0.537
0.428
0.292
0.183
2.282
2.121
1.981
1.694
1.465
1.262
1.088
0.940
0.808
0.708
0.600
0.500
0.383
0.280
0.187
0.107
0.068
0.103
0.055
0.121
0.053
0.112
0.070
0.017
0
0.031
0.029
0.025
0.003
0
0.010
0
0.135
0.045
0
0
0
7.1.3-Alternative Analysis Procedures
C7.1.3
The use of a second-order numerical analysis
procedure to evaluate the stability of the top chord of a
half-through truss is acceptable in lieu of the procedure
above, provided the following aspects are included in
the model:
Given the increasing availability of software that is
capable of second order analyses, such an analysis is a
practical alternative to the method given in Article 7.1.2.
However, the design equations in AASHTO LRFD
account for the issues identified, and any alternative
method should also address these. One method that
might be followed would be to use the second order
elastic analysis to determine the K factor for a given
chord size and panel point frame stiffness, and then the
design equations of AASHTO LRFD to determine the
corresponding resistance.
•
Effects of initial out-of-straightness, both between
panel points and across the entire length of the
compression chord
•
Effects of residual stresses in compression
members due to fabrication and construction
•
Effects of the stiffness of vertical to floorbeam
connections
12
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
7.2-STEEL TWIN I-GIRDER AND SINGLE TUB
GIRDER SYSTEMS
7.2.1-General
C7.2.1
For potentially torsionally flexible systems such as
twin I-girder and single tub girder structural systems,
the designer shall consider:
Several incidents have highlighted the need for a
careful evaluation of the stability of pedestrian bridges,
especially during the construction stages. Structural
systems consisting of two parallel girders can exhibit
very different behavior during construction, depending
on the bracing systems used. Lateral bracing contributes
significantly to the lateral-torsional buckling capacity of
the beam. For girders without lateral bracing during
construction, lateral-torsional buckling capacity should
be carefully evaluated. After the deck is cast, the section
is effectively a "C" shape, which is singly symmetrical.
Use of the appropriate lateral-torsional buckling equation
is critical, and reference should be made to Galambos
(1998). Further information is contained in Yura and
Widianto (2005), as well as Kozy and Tunstall (2007).
•
The out-of-plane stiffness of twin I-girders, prior to
becoming composite with a concrete deck, can be
significantly smaller than the in-plane, or vertical,
stiffness. This can lead to a lateral-torsional
buckling instability during construction.
•
Single tub girders, prior to becoming composite
with a concrete deck, behave as singly symmetric
sections with a shear center below the bottom
flange. AASHTO LRFD Article 6.7.5.3 requires top
lateral bracing in tub section members to prevent
lateral torsional buckling of these sections.
•
Prior to becoming composite with a concrete deck,
twin I-girders with bottom flange bracing will
behave as a tub girder and will exhibit the same
tendencies toward lateral-torsional buckling. Top
lateral bracing shall be provided as for tub sections,
or the stability shall be checked as a singly
symmetric member.
7.2.2-Lateral-Torsional Buckling ResistanceTwin I-Girder
For evaluating the stability of twin I-girder systems
without a composite deck or lateral bracing, the
equation given by Yura and Widianto (2005) may be
used:
(7.2.2-1)
where:
Mn
=
nominal in-plane flexural resistance of one
girder (kip-in.)
Mer =
critical elastic lateral-torsional
moment of one girder (kip-in.)
buckling
s
spacing between girders (in.)
E
modulus of elasticity of steel (ksi)
L
effective buckling length for lateral-torsional
buckling (ft)
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
Iyo
=
out-of-plane moment of inertia of one girder
(in.4)
Ixo
=
in-plane moment of inertia of one girder (in.4)
13
in-plane plastic moment of one girder (kip-in.)
Where a concrete deck is used, continuous twin I-girder
systems shall be made composite with the deck for the
entire length of the bridge.
7.2.3-Lateral-Torsional Buckling ResistanceSingly Symmetric Sections
C7.2.3
The lateral-torsional stability of singly symmetric
sections not covered in Article 7.2.2 shall be
investigated usmg information available m the
literature.
Equations for the determination of the lateral-torsional
buckling moment in singly symmetric sections are given
in the Guide to Stability Design Criteria for Metal
Structures by Galambos (1998), specifically in
Chapter 5. Equation 5.10 of that chapter presents the
general formula for singly symmetric members where
bending is in the plane of symmetry. Methods for
accounting for location of loading with respect to the
shear center are provided, as well as for determining the
appropriate buckling lengths considering rotational
restraints.
8-TYPE SPECIFIC PROVISIONS
S.l-ARCHES
Arches shall be designed in accordance with the
provisions of AASHTO LRFD with guidance from
Nettleton (1977).
S.2-STEEL HSS MEMBERS
S.2.I-General
CS.2.I
The capacities or resistances of connections for steel
HSS members shall be in accordance with the Chapter
K of the specifications and commentary of AISC (2005)
or AASHTO Signs. Resistances for fatigue design shall
be in accordance with Section 2.20.6 of Structural
Welding Code-Steel ANSI!A WS D 1.1 or Section 11 of
AASHTO Signs. All loads, load factors, and resistance
factors shall be as specified by AASHTO LRFD and
these Guide Specifications. For member design other
than connections:
AISC has partnered with ClDECT to publish a set of
HSS Design Guides. These guides are published
internationally and have not been reviewed by AISC and
are not necessarily in accordance with the AISC
Specifications. However, the documents are a good
resource on HSS connections and systems.
•
Flexure resistance of steel HSS members shall be
according to AASHTO LRFD Article 6.12 as box
sections.
•
Shear resistance of steel HSS members shall be
according to AASHTO LRFD Article 6.11.9 as box
sections.
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
14
•
Tension and compression resistance shall be
according to AASHTO LRFD Articles 6.8.2 and
6.9.2, respectively.
For electric-resistance-welded HSS members, the
design wall thickness shall be taken as 0.93 times the
nominal wall thickness.
8.2.2-Detailing
C8.2.2
The minimum nominal metal thickness of closed
structural tubular members shall be 0.25 in. Corrosion
mitigation shall be considered.
Different philosophies exist on how best to protect
tubular members from corrosion. One method of
corrosion protection is to vent the interior of the tube
adequately and to provide some form of surface
treatment, often a galvanized finish, to prevent corrosion.
Issues to consider include the size of the field pieces to
be galvanized, the size of local galvanizing kettles, and
the service environment ofthe bridge.
Another method is to use a weathering steel material to
provide corrosion protection. Adequate drainage and
ventilation of the interior must be provided, and
consideration of the suitability of the operating
environment must be made. FHWA Technical Advisory
T 5140.22 (1989) provides guidance in the use of
weathering steels. An increase in the minimum thickness,
when relying on weathering steel as the corrosion
protection, should be considered.
A third, and often less successful, method is to
completely seal the interior of the member from the
atmosphere. This requires careful detailing of the
connections, as even a small opening will allow
moisture-laden air into the interior, and over time this
can result in a large accumulation of water. Box
members in a large truss that were supposedly sealed to
the atmosphere have been fOlmd to contain several feet
of water.
8.2.3-Tubular Fracture Critical Members
C8.2.3
The AASHTO/A WS Fracture Control Plan for
N onredundant Members contained in AASHTOIA WS
D1.5, Section 12, shall be applied to tubular members
(HSS members), where required by AASHTO LRFD
Articles 6.6.2 and C6.6.2, with the following
modifications:
No current specification adequately covers the use of
tubular members in a fracture critical capacity.
AASHTOIA WS D 1.5 specifically excludes tubular
members. It appears significant research is required to
address the unique aspects of both the longitudinal weld
used to create the closed shape, as well as the one-sided
groove welds without backing bars used in the
connections of HSS. Until such time as this research is
performed, the procedure specified herein represents the
best available method for addressing fracture critical
issues in HSS construction.
•
ASTM A500, A501, A847, and A618 shall be
added to those standards listed in Article 12.4.1 of
AASHTO/AWS Dl.5.
•
F or the purposes of determining preheat and
interpass temperatures, the values for AASHTO
M 270MlM 270 or ASTM A 709 Grade 50 shall be
used.
,
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
•
Steel for tubular sections (HSS) shall conform to
the Charpy V-notch requirements defined in
A709-07. Filler metal shall be treated as A709 and
conform to the requirements of A WS D 1.5
Table 12.1.
•
Welding details for cyclically loaded tubular
members specified by AASHTO/A WS D 1.1 shall
be used.
•
All welds require qualification using A WS D 1.1
Article 4.S.
15
8.3-FIBER REINFORCED POLYMER (FRP)
MEMBERS
C8.3
The minimum thickness of closed structural FRP
members (such as tubes) shall be 0.25 in. The minimum
thickness of open structural FRP members (such
as channels), including connection plates, shall be
0.375 in.
For design of FRP members in pedestrian bridges,
reference may be made to the AASHTO Guide
Specifications for Design of FRP Pedestrian Bridges
(200S). Little information is currently available regarding
resistance equations or resistance factors for this material
used in bridge structures. Several design specifications
covering FRP pultruded shapes are currently under
development by the American Society of Civil Engineers
and may be of use in the future for the design of FRP
pedestrian bridges.
16
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
9-DESIGN EXAMPLE
HALF-THROUGH TRUSS BRIDGE WITH HSS MEMBERS
ILLUSTRATIVE EXAMPLE OF KEY PROVISIONS OF GUIDE SPECIFICATIONS
LOAD AND RESISTANCE FACTOR DESIGN
GENERAL INFORMA nON
Specifications Used:
•
AASHTO LRFD Bridge Design Specifications, 2007 with 2008 Interims (AASHTO LRFD)
•
AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals,
2008 (AASHTO Signs)
•
AASHTO LRFD Guide Specifications for the Design of Pedestrian Bridges (Specification)
Geometry:
Span
=
Deck width,
Wdeck
72 ft
= 10ft
CL-CL trusses
= 10.5 ft
A500, Gr. B, Fy
= 46 ksi
!. Symm @
:
<t Span
i
12 Panels @ 6 It 0 in.
~
72 ft 0 in. Span
-
,.-,.-
,.-
10 ft 0 in. Deck Width~
.S:
.S:
<.0
0
<t=
'"
I
T
I
I
I
I
I
~Floorbeam
10ft 6 In.
if -if Trusses
J
J
--- -
J
<t=
l!)
'"
1:'
0
..c:
()
G-'
I
G-'
LRFD GUIDE SPECU'ICATlONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
TRUSS MEMBERS: ALL RECTANGULAR HSS
Top and Bottom Chords:
Section: HSS6 x 3 x 5116
A
4.68 in."
w
16.93 plf
End Posts:
Section: HSS6 x 3 x 5/16
A
4.68 in. 2
w
16.93plf
Vertical Posts:
Section: HSS5 x 3 x 5116
A
4.1 in. 2
w
14.8 plf
Ix
Ie = 12.6 in. 4
Diagonals:
Section: HSS4 x 3 x 114
A
2.91 in?
w
10.48 plf
FLOORBEAMS:
Section: W8 x 10
Ix
Ib = 30.8 in. 4
Sx
7.81 in. 3
Spacing
6 ft at each panel point
17
18
LRFD GUIDE
SPECIFICATIONS FOR THE DESIGN Of' PEDESTRIAN BRIDGES
DEAD LOAD:
Weight of each truss
62 plf per truss
Assumed deck loading
25 psf
Weight of deck and floor system
25 psfx 10.50 ft/2
132 plf per truss
62 plf + 132 plf
Total dead load
194 plf
Use 200 plf
PEDESTRIAN LIVE LOAD (SPECIFICATION, ARTICLE 3.1):
For Design of Main Truss Members:
The deck area may be used to compute design pedestrian live load for all main member components (truss
members). The deck area is the non-zero influence surface for all such components.
Use 90 psfwithout impact.
Live load per truss
pedestrian loading x deck widthl2
90 psf x 10.0 ft/2
450 plf
For Design of Secondary Members-Deck, Stringers, Floorbeams:
Use 90 psfwithout impact.
VEHICLE LOAD (SPECIFICATION, ARTICLE 3.2):
Vehicular access is not prevented by fixed physical methods; therefore, the pedestrian bridge should be designed
for an occasional single maintenance vehicle load.
Use Table 3.2-1 for Minimum Axle Loads and Spacings.
The vehicular load shall not be placed in combination with the pedestrian load. Consideration of impact is not
included with this vehicular loading.
Use the following vehicle for a clear deck width between 7 ft and 10 ft:
Front axle
2 kips
Rear axle
8 kips
Axle spacing
14 ft
19
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
Wheel spacing
6 ft
Note: For this example, the pedestrian load controls for the truss design; however, the vehicle load will control for
the floor system design.
WIND LOAD (SPECIFICATION, ARTICLE 3.4):
Assume 100 mph design wind.
Use wind load as specified in AASHTO Signs, Articles 3.8 and 3.9.
Neglect wind load on the live load vehicle.
The design life shall be taken as 50 years for the purpose of calculating the wind loading.
Horizontal Wind Loading:
Apply the design horizontal wind pressure on the truss components.
Pz
design wind pressure on superstructure using AASHTO Signs, Eq. 3-1 or Table 3-7 (pst)
(AASHTO Signs, Eq. 3-1)
where:
Kz
height and exposure factor from AASHTO Signs, Eq. C3-1 or Table 3-5
1.00 (conservatively taken from Table 3-5 for a height of 32.8 ft)
G
gust effect factor
1.14 (minimum)
V
basic wind velocity
100 mph
lr
wind importance factor from AASHTO Signs, Table 3-2
1.00
Cd
wind drag coefficient from AASHTO Signs, Table 3-6
2.00
Pz
58.4 psf (Alternatively, AASHTO Signs, Table 3-7 may be used with a Ccl value of2.0 applied.)
20
LRFD GUIDE SPECIFICAnONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
Projected vertical area per linear foot:
Chords: 2 @ 3 in.ll2 x 6 ftJ6 ft
Verticals: 3 in.l12 x 4.75 ft long/6 ft
Diagonals: 3 in.l12 x 7.81 ft long/6 ft
Total per Tmss:
0.50 ft2
0.20 ft2
0.33 ft2
1.03 ft2
Deck + Stringers: 10 in.l12
0.83 ft 2
total horizontal wind on superstmcture (plf)
(2 tmsses x 1.03
fe + 0.83 ft2) x 58.4 psf
169 plf
Note: The full lateral wind loads must be resisted by the entire superstructure. Appropriate portions of the design
wind loads must also be distributed to the tmss top chord for design lateral forces on the tmss verticals.
Vertical Wind Loading:
Apply a vertical pressure of 0.020 ksf over the full deck width concurrently with the horizontal loading. This
loading shall be applied at the windward quarter point of the deck width.
WSv
vertical wind load on the full projected area of the superstmcture applied at the
windward quarter point (plf)
Pv(Wdeck)
where:
Pv
vertical wind loading on superstmcture (ksf)
0.020 ksf
Wdeck
total deck width (ft)
10.0 ft
Therefore,
WSv
0.020 ksfx 1000 x lO.OO ft
200 plf
Verticalload on leeward tmss
200 plfx (7.5 ft + (0.5 in. + 2.5 in.)/12 in.lft)110.50 ft
147.6 plf
Verticalload on windward tmss
200 plfx (2.5 ft + (0.5 in. + 2.5 in.)/12 in.ft)/10.50 ft
52.4 plf(uplift)
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
21
TOTAL VERTICAL LOADS PER TRUSS (SPECIFICATION, ARTICLE 3.7):
DEAD LOAD (DC l +DC2 ): 200 plf
LIVE LOAD (Pedestrian, PL): 450 plf
WIND (Overturning, WS): 148 plf
Load Factors (AASHTO LRFD, Table 3.4.1-1):
Limit State:
Str I
Str III
SerI
DC l &DC2
1.25
1.25
1.00
PL
1.75
0
1.00
WS
0
1.40
0.30
TRUSS MEMBER DESIGN LOADS:
Panel point load from controlling load comb. = 1.038 kIf x 6.0 ft panel = 6.23 kip/panel
Maximum Truss Member Axial Loads (from separate truss analysis):
Chord (U05-U06)
134.57 kips
(compression)
End Post (UOO-LOO)
34.27 kips
(compression)
Diagonal (UOO-LOl)
53.52 kips
(tension)
Vertical (UOl-LOl)
28.04 kips
(compression)
TRUSS TOP CHORD LATERAL SUPPORT (SPECIFICATION, ARTICLE 7.1):
Assume the truss verticals are adequate to resist the lateral force per Specification, Article 7.1.1. (Must verify
assumption; see Article 7.1.1, Lateral Frame Design Force.)
Lateral support is provided by a transverse U-frame consisting of the floorbeam and truss verticals.
Determine the design effective length factor, K, for the individual top chord members supported between the truss
verticals using Specification, Table 7.1.2-1.
Compute CL/Pc for use in Table 7.1.2-1.
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
22
C
transverse frame spring constant, can be fOlmd from separate computer analysis, or from equation below
(from Guide to Stability Design Criteria for Metal Structures,
edited by T. V. Galambos, 1968)
E
2
h [ ( h / 3Ic ) + (b / 2Ib ) ]
where:
E
modulus of elasticity (ksi)
29,000 ksi
Floorbeam span:
b = 126.0 in.
Effective height of vertical:
h
54.0 in.
Floorbeam: W8 x lO
Ie = 12.6 in. 4
Vertical: HSS5 x 3 x 5116
C
2.863 kiplin.
Alternatively, perform a 2-D computer analysis of the U-Frame
where:
C
PI!!..
2.917 kip/in. (from a separate 2D analysis)
Use C
2.863.
L
unbraced length of the chord in compression (i.e., length between panel points) (in.)
72 in.
desired critical buckling load (i.e., factored compressive force) multiplied by
1.33 (kips)
(Specification, Article 7.1.2)
178.98 kips
CLIPe
1.15
n
number of panels
12
Therefore,
11K
0.678
K
1.47
(Specification, by interpolation of Table 7.1.2-1)
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
TOP CHORD COMPRESSIVE RESISTANCE (AASHTO LRFD, ARTICLE 6.9.2):
Check the slenderness ratio against the limiting value.
For main members: KLlr :::;; 120
For bracing members: KLlr:::;; 140
Section: HSS6 x 3 x 5/16
4.68 in?
A
radius of gyration about the x-axis (in.)
2.07 in.
radius of gyration about the y-axis (in.)
1.19 in.
K
1.47
L
72 in.
KLlrx
(1.47 x 72 in.)/2.07 in.
51.3<120
KLlry
OK
(1.00 x 72 in.)/1.l9 in.
60.5 < 120
Pr
OK
factored resistance of components in compression (kips)
(AASHTO LRFD, Eq. 6.9.2.1-1)
where:
resistance factor for compressive per AASHTO LRFD, Article 6.5.4.2
0.9
nominal compressive resistance per AASHTO LRFD, Article 6.9.4 (kips)
Determine the nominal compressive resistance, P n
If A :::;; 2.25, then:
Pn
=
0.66
A
F.vAs
If A> 2.25, then:
(AASHTOLRFD, Eq. 6.9.4.l-1)
23
G
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
24
(AASHTO LRFD, Eq. 6.9.4.1-2)
A=(KL)2 F;!
'Slt
=
(AASHTO LRFD, Eq. 6.9.4.1-3)
E
0.59
where:
As
gross cross-sectional area (in. 2)
4.68 in. 2
Fy
specified minimum yield strength (ksi)
46 ksi
E
modulus of elasticity (ksi)
29,000 ksi
KLlrs
Maximum of KLlrx,KLlry
61
Therefore, the top chord factored resistance is:
Pn
0.660.59 x 46 ksi x 4.68 in. 2
168 kips
151 kips> Pchord = 134.57 kips
OK
LATERAL FORCE TO BE RESISTED BY VERTICALS (SPECIFICATION, ARTICLE 7.1.1):
HI
minimum lateral force (kips)
O.OllK(Pavg)
where:
K
1.47
P avg
average design compressive force in adjacent chord members (kips)
134.57 kips
Verify limit 0.0111.47
0.007 > 0.003
OK
25
LRFD GUIDE SPECIFICAnONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
Therefore,
HI
0.0111.47 x 134.57 kips
0.91 kip
Apply HI as the lateral force at the top of the Truss Verticals. Apply HI concurrently with other primary forces in
the Verticals (combined compression plus bending analysis). Include lateral wind forces for AASHTO LRFD Load
Combination Strength Ill.
Length of vertical
54.0 in.
Lateral Moment in Vertical
0.91 kip x 54.0 in. = 49.27 kip-in.
Only load development is shown, check of capacity follows typical procedures.
END POSTS (SPECIFICATION, ARTICLE 7.1.1):
Apply the lateral force, C, at the top end of post and design as a cantilever combined with axial load. The lateral
force, C, is taken as 1.0 percent of the end post axial load.
Lateral Force:
C = 0.01 x 34.27 kips = 0.34 kip
Note: All other truss members are analyzed using conventional methods per AASHTO LRFD.
Only load development is shown, check of capacity follows typical procedures.
DEFLECTION (SPECIFICATION, ARTICLE 5):
Maximum pedestrian LL Deflection = 1/360 of the span length = 72.00 ft x 12/360 = 2.40 in.
From Truss Analysis, LL Deflection (WLL = 0.450 kip/ft) = 1.20 in. < L/360
OK
VIBRATIONS (SPECIFICATION, ARTICLE 6):
Vertical Direction:
Estimate the fundamental frequency in the vertical direction, f, by approximating the truss as a simply supported
tmiform beam.
The fundamental frequency in a vertical mode without consideration of live load should be greater than 3.0 Hz to
avoid the first harmonic.
f=0.18~
where:
g
t.DL
26
g
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
acceleration due to gravity (ft/s 2)
32.2 ft/s2
~DL
maximum vertical deflection ofthe truss due to the dead load (ft)
0.0444 ft (from a separate analysis with w = 0.20 kif per truss)
f = O.lS~ 32.2 = 4.S5 Hz > 3.0 Hz minimum desirable
0.0444
OK
For illustration purposes, assume higher harmonics (second, third, etc.) are a concern. The bridge should be
proportioned such that the following criteria are satisfied:
2.S6ln (lSOIW)
f
where:
w
full weight of the supported structure including dead load (kips)
2 trusses x 0.20 klfx 72.00 ft
2S.S kips (Dead Load Only)
2.S6 In (ISO/2S.S0)
5.24 Hz
f
4.S5 Hz is not greater than 5.24 Hz.
Modifications required.
Possible modifications include using a heavier deck system or increasing the depth of the truss.
Lateral Direction:
Estimate the flmdamental frequency in the lateral direction, fiat, by approximating the truss as a simply supported
uniform beam rotated 90 degrees.
The fundamental frequency in a lateral mode without consideration of live load should be greater than 1.3 Hz to
avoid the first harmonic.
Assume the lateral wind bracing is 3 x 3 x 1I4 in. structural tubing.
f=O.lS~
g
~DLlat
where:
g
acceleration due to gravity (ft/s2)
32.2 ft/s2
~DLlat
maximum lateral deflection of the truss due to the dead load (ft)
0.OS44 ft (from a separate analysis)
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LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
•
.,
d
' b Ie
j , = 0.18 ~2.2
- - - = 3 .52 Hz> 1. 3 Hz mmnllum
eSlra
27
OK
0.0844
FATIGUE (SPECIFICATION, ARTICLE 3.5):
Use AASHTO Signs, Article 11.7.3.
AASHTO Signs, Article 11.7.4-Not used as it is assumed that the pedestrian bridge is not over a highway.
Cd
wind drag coefficient per AASHTO Signs, Table 3-6
2.00
wind importance factor per AASHTO Signs, Table 3-2
1.00
10.4 psf
total horizontal wind on superstructure for fatigue (pIt)
(2 trusses x 1.03 fe + 0.83 ft) x 10.4 psf
31 plf
Maximum Member Force from Wind:
Bottom Chord, Member L05-L06
/}.f
=
5.6 kips (from a separate analysis)
Stress Range
(5.6 kips!4.68 in. 2 )
1.20 ksi
y(/}.f) ::; (M)n
(AASHTO LRFD, Eq. 6.6.1.2.2-1)
where:
y
1.0
N
1.20 ksi
(Specification, Article 3.7)
(Specification, Article 4.1)
(M)n
where:
(M)n
16 ksi
(Category B-base metal)
(AASHTO Signs, Table 11-3)
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LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
(1.0)( 1.20) :s; 16
1.20 < 16
OK
Welded member connections and fracture toughness requirements are outside the limits of this pedestrian bridge
design example. They will be the responsibility of the Designer.
LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
REFERENCES
AASHTO. 1997. Guide Specifications for Design of Pedestrian Bridges. American Association of State Highway
and Transportation Officials, Washington, DC.
AASHTO. 2001. Standard Specifications for Structural Supportsfor Highway Signs, Luminaires, and Trajfic
Signals, 4th Edition. American Association of State Highway and Transportation Officials, Washington, DC.
AASHTO. 2002. Standard Specifications for Highway Bridges, 17th Edition. American Association of State
Highway and Transportation Officials, Washington, DC.
AASHTO. 2007. AASHTO LRFD Bridge Design Specifications, 4th Edition, 2008 and 2009 Interim. American
Association of State Highway and Transportation Officials, Washington, DC.
AASHTO. 2008. AASHTO Guide Specifications for Design of FRP Pedestrian Bridges, 1st Edition. American
Association of State Highway and Transportation Officials, Washington, DC.
AISC. 2005. Specification jar Structural Steel Buildings, ANSIIAISC 360-05. American Institute of Steel
Construction, Chicago, IL.
Allen, D. E. and T. M. Murray. 1993. Design Criterion for Vibrations Due to Walking. In AISC Journal, 4th
Quarter. American Institute of Steel Construction, Chicago, IL., pp. 117-129.
A WS. 2008. Bridge Welding Code, AASHTOIA WS D 1. 5MID 1.5 :2008. American Welding Society, Miami, FL.
AWS. 2006. Stnlctural Welding Code-Steel, AASHTOIA WS D 1.1M/D 1.lM:2006. American Welding Society,
Miami, FL.
Bachmann, H. 2002. Lively footbridges-a real challenge. Proceedings of the International Conference on the
Design and Dynamic Behavior of Footbridges, November 20-22, 2002, Paris, France, pp.18-30.
Blekhennan, A. N. 2007. Autoparametric Resonance in a Pedestrian Steel Arch Bridge: Solferino Bridge, Paris. In
Journal of Bridge Engineering, Volume 12, Issue 6. American Society of Civil Engineers, Reston, VA,
pp. 669-676.
Dallard, P., T. Fitzpatrick, A. Flint, Low A., R. R. Smith, M. Willford, and M. Roche. 2001. London Millennium
Bridge: Pedestrian-Induced Lateral Vibration. In Journal of Bridge Engineering, Volume 6, Issue 6. American
Society of Civil Engineers, Reston, VA, pp. 412-417.
Dallard, P., et al. 2001. The London Millennium Footbridge. In The Structural Engineer, 79(22). The Institute of
Structural Engineers, London, pp. 17-33.
FHWA. 1989. Uncoated Weathering Steel in Structures, Technical Advisory T 5140.22. Federal Highway
Administration, US Department of Transportation, Washington, DC.
Galambos, T. V. 1998. Guide to Stability Design Criteria jar Metal Structures, 5th Edition. John Wiley & Sons,
Inc., New York, NY.
Kozy, B. and S. Tunstall. 2007. Stability Analysis and Bracing for System Buckling in Twin I-Girder Bridges. In
Bridge Structures: Assessment, Design and Construction, Volume 3, No. 3-4. Routledge Journals, Florence, KY,
pp 149-163.
Nettleton, D. A. 1977. Arch Bridges. Bridge Division, Office of Engineering, Federal Highway Administration,
U.S. Department of Transportation, Washington, DC.
;1
Nowak, A. S. and K. R. Collins. 2000. Reliability ofStnlctures, McGraw-Hill International Editions, Civil
Engineering Series. The McGraw-Hill Companies, Singapore.
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LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES
Poston, Randall W. and Jeffery S. West. 200S.Investigation of the Charlotte Motor Speedway Bridge Collapse,
Metropolis & Beyond 2005. Proceedings ofASCE's 2005 Structures Congress and the 2005 Forensic Engineering
Symposium, April 20-24, 2005, New York, NY.
Roberts, T. M. 2005. Lateral Pedestrian Excitation of Footbridges. In Journal of Bridge Engineering, Volume 10,
Issue 1. American Society of Civil Engineers, Reston, VA, pp. 107-112.
Roland, E. S., M. L.Hull, and S. M. Stover. 2005. Design and Demonstration of a Dynamometric Horseshoe for
Measuring Ground Reaction Loads of Horses during Racing Conditions. In Journal of Biomechanics,
Volume 38, No. 10. Elsevier Inc., The Nerherlands, pp. 2102-2112.
SETRA. 2006. Technical Guide: Footbridges-Assessment of Vibrational Behaviour of Footbridges under
Pedestrian Loading. Service d'Etudes Techniques des Routes et Autoroutes, Association Francaise De Genie Civil,
Paris, France.
Willford, M. 2002. Dynamic actions and reactions of pedestrians. Proceedings of the International Conference on
the Design and Dynamic Behavior ofFootbridges, November 20-22, 2002, Paris, France.
Yura, 1. A. and Widianto. 2005. Lateral Buckling and Bracing of Beam-A Re-evaluation after the Marcy Bridge
Collapse. 2005 Annual Technical Session Proceedings, April 6-9, 2005 in Monreal, Quebec, Canada, Structural
Stability Research Council. Rolla, MO.
Zhao, X.-L., S. Herion, J. A. Packer, R. S. Puthli, G. Sedlacek, J. Wardenier, K. Weynand, A. M. van Wingerde,
and N. Yoemans. 2001. Design Guide 8-for CHS and RHS Welded Joints Under Fatigue Loading. ClDECT, TOV
Verlag, Germany.
Zivanovic, S., A. Pavic, and P. Reynolds. 2005. Vibration serviceability offootbridges under human-induced
excitation: a literature review. In Journal of Sound and Vibration, 279( 1-2). Elsevier Inc., The Nerherlands,
pp. 1-74.
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