AMERICAN FOREST & PAPER ASSOCIATION
American Wood Council
Engineered and Traditional Wood Products
American Wood Council
Engineered and Traditional Wood Products
STD302:
Wood Frame Construction
Manual 2001
National Edition
A F & P A®
®
Copyright © 2004-2007 American Forest & Paper Association, Inc. All rights reserved.
Welcome to the eCourse on the Wood Frame Construction Manual (WFCM) for One- and
Two-Family Dwellings, 2001 National Edition.
Copyright © 2004-2007 American Forest & Paper Association, Inc.
All rights reserved.
1
Copyright of Materials
This presentation is protected by US and International copyright
laws. Reproduction, distribution, display and use of the
presentation without written permission of the American Forest &
Paper Association / American Wood Council is prohibited.
Copyright © 2004-2007 American Forest & Paper
Association, Inc. All rights reserved.
3
STD302: Learning Outcomes
By the end of this eCourse, you will be knowledgeable about:
1. The purpose of the 2001 WFCM and its development
process
2. Code acceptance and references
3. 2001 WFCM document layout
4. Design provisions, including:
•
•
•
•
5.
Shear walls – “Standard” Wall concept
Wind load resistance
Snow load resistance
Seismic load resistance
Design Examples for each load type
In this eCourse, you will see a brief historical background to the WFCM and its acceptance
in the model building codes. The layout of the WFCM documents is described in detail and
pertinent provisions for wind snow and seismic loads are discussed. Finally, design
examples illustrate how to use various features of the WFCM as well as new design
technologies for lateral load resisting wood systems. This edition of the WFCM is a dramatic
enhancement of the 1995 WFCM and is applicable nationwide.
4
Outline
Purpose & Background
Development process
Code Acceptance
Document layout
Design provisions:
• Snow
• Wind
• Seismic
Design Examples
• Snow Design
• Wind shearwall design
• Seismic shearwall design
Here’s the outline for the course. Let’s start with the top three topics for background.
5
Purpose
provide a rational
engineering basis and
design tools for
residential wood
frame construction
serve applied need for:
• high wind initially
In response to increasing scrutiny of building design and construction in the United States,
AF&PA formed a Subcommittee on Conventional Construction (SCC) in 1991, to evaluate
emerging state-of-the-art design methods for wood-frame buildings. In areas where existing
reference standards and accepted engineering practices were too conservative, or did not
exist, research was used to establish new design methods.
The first WFCM was developed by AF&PA’s Subcommittee on Conventional Construction
whose goals were to:
- Provide guidelines on residential construction,
- Provide technical recommendations on the proper design of residential structures,
- Provide technical justification for structural systems,
- Prepare residential construction document for reference in model codes and use in design
and construction.
Hurricane Andrew reinforced the need for the work being done in the high wind area.
Northridge earthquake contributed to the need for high seismic provisions.
6
WFCM 1995 High Wind Edition
guidelines on residential
construction
proper design of residential
structures
technical justification for
structural systems
reference in model codes
based on 1991 NDS®
• high wind to SBC
In 1996, the SCC completed work on a high wind version of the Wood Frame Construction
Manual for One- and Two-Family Dwellings (WFCM). The WFCM contains comprehensive
design and construction guidelines for wind-resistant residential wood-frame construction.
The document has three distinct parts: general scope and limitations, engineered
requirements, and prescriptive requirements. Dead, live and wind loads were all calculated
based on the provisions of the 1994 Standard Building Code. Resistance of members,
connections, and structural systems were calculated from two sources; the 1994 edition of
the Standard Building Code with 1996 Amendments (SBC-96) and AF&PA's 1991 National
Design Specification® for Wood Construction (NDS®).
The SCC examined existing industry recommendations and found areas in design to
improve overly restrictive limitations. As a result, two new procedures were proposed and
adopted in the SBC-96 and incorporated in the WFCM. These design procedures provide
new repetitive member factors for wall studs sheathed with structural sheathing and a
perforated shearwall method. A detailed explanation of these changes is available in related
literature.
Pacific Rim Conference of Building Officials, Wood Frame Construction Manual for
Residential Structures, 1994
7
Cost of Compliance using WFCM is
favorable
Cost of compliance comparison of existing construction
methodologies for residential wood structures
Prescriptive Analysis
One Story
Two Story
1995 CABO One- and Two-Family Dwelling Code
(Limited high-wind prescriptive requirements)
$4,655
$11,144
1994 Standard Building Code
(Low wind prescriptive requirements)
$4,500
$9,123
1994 Uniform Building Code
(Low wind prescriptive requirements)
$4,584
$9,256
Wood Frame Construction Manual
(High wind engineered prescriptive requirements)
$4,929
$10,260
One Story
Two Story
ASCE 7-95
$7,849
$14,297
1994 Standard Building Code
$5,454
$11,492
Engineered Analysis
The National Association of Home Builders Research Center analyzed the associated costs
of compliance with the WFCM compared to other national recognized codes and standards.
The study evaluated the costs of constructing structural elements for two generic homes,
containing typical construction characteristics, using prescriptive provisions of the 1995
CABO One- and Two-Family Dwelling Code; the 1994 Uniform Building Code (UBC); the
1994 Standard Building Code (SBC 94); and the WFCM. The report also compared the cost
of these four prescriptive design methods to engineered designs using load requirements of
ASCE 7-95, Minimum Design Loads for Buildings and Other Structures (ASCE 7), and the
SBC 94.
The National Association of Home Builders Research Center, The State_of_the_Art
of Building Codes and Engineering Methods for Single_Family Detached Homes: An
Evaluation of Design Issues and Construction Costs, June 1997
The report concluded that, “The WFCM appears to embody the most economical,
engineering-based prescriptive construction requirements for residential construction in high
wind conditions.” This report also confirmed that the WFCM offers engineering-based
solutions that are cost competitive with prescriptive code requirements and offers substantial
savings over pure engineering analysis, particularly as required by ASCE 7.
8
WFCM 2001
The New National Edition
based on 1997 NDS®
updated for:
• all wind
• snow
• seismic
for one- and two-family
wood frame dwellings
…nationally
AF&PA’s American Wood Council is a nationally-recognized Standards Writing Organization
which follows the standard-development guidelines of the American National Standards
Institute (ANSI). AF&PA maintains and publishes the NDS, the nationally-recognized design
standard for wood construction. The NDS is referenced by the three U.S. model building
codes and numerous other codes and standards currently enforced throughout the United
States. NDS provisions provide designers with the state-of-the-art practice on structural
wood and connection design.
WFCM - The New National Version
AF&PA has currently released the WFCM 2001 edition. An ANSI Canvass Committee was
established with representation by manufacturers, regulators and users. Consensus on
WFCM 2001 provisions was prior to publication. The goals for publishing the WFCM 2001
were to:
- develop national consensus on design of wood frame one and two-family dwellings in high
wind, seismic and snow zones;
- provide a forum for the introduction of new and innovative design methodologies for one
and two-family wood-frame dwellings;
- provide national education on design of one and two-family wood frame structures;
- create a national forum to introduce design changes resulting from analysis of natural
disaster surveys.
9
Model Code Acceptance
2003, 2006 IBC
2003, 2006 IRC
2003 NFPA
The WFCM 2001 is recognized by the following model building codes in the U.S.:
•2003 and 2006 IBC International Building Code
•2003 and 2006 IRC International Residential Code
•2003 NFPA 5000 Building Construction and Safety Code
10
ASCE 7-98 Load Reference
2001 WFCM
calibrated to work
with ASCE 7-98
Loads
The WFCM 2001 is calibrated to work with ASCE 7-98 loads and maps. This is important –
no other load document should be used with the 2001 WFCM.
11
Outline
Purpose & Background
Development process
Code Acceptance
Document layout
Design provisions:
• Snow
• Wind
• Seismic
Design Examples
• Snow Design
• Wind shearwall design
• Seismic shearwall design
Now let’s explore the WFCM a little to get familiar with it.
12
WFCM 2001
WFCM 2001 is a 2 part document: the Manual contains the standard and the WFCM
Commentary contains background information and example calculations.
13
Manual Overview/Contents
Chapter One
• General Information
Chapter Two
• Engineered Design
Chapter Three
• Prescriptive Design
Supplement
Commentary
The primary WFCM volume comprises three chapters with an accompanying supplement
containing design values and other useful information. The second volume Commentary
provides complimentary explanation and detailed calculation examples for the WFCM
provisions.
14
WFCM 2001 - Layout
1 General Information
2 Engineered Design
3 Prescriptive Design
Supplement
The manual is laid out in three chapters: general information, engineering design, and
prescriptive design; and the supplement. But the WFCM is worked as follows: Chapter 1 to
see if your project will qualify under the WFCM provisions, then Chapter 3 to design
approximately 80% of the structural elements in your house, then Chapter 2 to design the
next 10 to 15% of the elements, and lastly anything left (typically about 5%) will need to be
designed with current NDS provisions. Organizing yourself in this fashion will optimize the
time needed to design the house and dramatically increase your efficiency.
Now let’s see what’s in each chapter.
15
1 General Information
Background material
for manual
• definitions
• terminology
• symbols
what?
Chapter 1 General Information gives definitions, terminology explanations, and symbolic
representations that apply to the remaining chapters and the manual.
16
1 General Information
Scope
Definitions
Referenced standards
Applicability
limitations
The scope and limitations of the applicability of the manual to your project is described.
Here are some of them:
17
1 General Information - Limitations
DETERMINING NUMBER OF STORIES ABOVE THEFOUNDATION
Stories / Mean Roof
Height (MRH)
O
N
E
T
W
O
≤6
12
S
T
O
R
Y
hR/2
MRH
G
G/2
T
H
R
E
E
12
>6
MRH
S
T
O
R
Y
• 3 Stories or less
• MRH = 33'
hR
12
≤6
MRH
12
>6
MRH
MRH
≤6
12
12
>6
MRH
MRH
S
T
O
R
Y
- Foundation per section 1.1.4
MRH - Measured from average grade
Limitations on building dimensions including mean-roof-height (MRH), # of Stories, and
aspect ratio that must be met in order to use the WFCM. Applications outside the prescribed
limits require design per AF&PA’s NDS or LRFD standards, or other code approved means.
Figures such as this one are included to help the user interpret design limitations. The
Figure illustrates the definition of MRH and # of Stories as used in the WFCM. Roof Slopes
> 6:12 are considered as another story above the foundation.
18
1 General Information - Limitations
Vertical floor offsets
Shearwall plan offsets
CONNECT AS NEEDED
TO TRANSFER SHEAR
OFFSET NOT > d
d
SHEARWALL
SHEARWALL OFFSET
BLOCKING
SHEARWALL
WFCM Figures are included to help the user interpret limitations on vertical floor offsets and
shearwall plan offsets.
19
1 General Information - Limitations
LATERAL LOAD
DIRECTION
Irregular
Structures
Option #1
STRUCTURE WITH
OFFSET GREATER
THAN 4 FEET
SEPARATE
STRUCTURES
Option #2
INSCRIBED
STRUCTURE
Building 2
> 4'
Building 1
Building 1
> 4'
Building 3
Building 1
> 4'
Building 2
Building 1
> 4'
> 4'
Building 3
Guidance for designing irregular structures as separate structures or inscribed is provided.
The inscribed structure option is used for wind loads only, while the separate structures
option can be used for both wind and seismic loads.
20
1 General Information - Limitations
Inscribed Structure Method
• for Wind loads only
• inscribe overall building into
one rectangle
The inscribed structure method, applicable for wind loads, is a means of facilitating the
calculation of shear wall sheathing lengths where building geometry is non-rectangular - by
inscribing the overall building into one rectangle.
21
1 General Information - Limitations
Separate Structure Method
• for Wind and Seismic loads
• in-story wall offsets > 4 ft
• split into rectangular
structures and determine
sheathing requirements for
walls in each
• shared wall sheathing
amount is the sum of shear
wall sheathing lengths for
each “separate” structure at
that location (Wall A)
Where wall offsets exceed 4 feet within a single story, rectangular portions of the structure
are to be considered as separate structures in accordance with Section 3.1.3.3c. Sidewall
and endwall shear wall sheathing lengths are determined based on the geometry of each
“separate” structure comprising the overall building. Where wall lines are shared, such as
Wall Line A, the shear wall sheathing amount for the wall line is taken as the sum of shear
wall sheathing lengths for each “separate” structure at that location.
22
1 General Information - Limitations
Inscribed Method - in-story wall offsets
> 4 ft
• use inscribed tributary area within dotted
lines to provide a balanced shear wall
layout (as per separate colors shown)
• shear wall sheathing lengths are based on
MWFRS loads
Bracing lengths be determined in accordance with the inscribed method where wall offsets
are greater than 4 feet within a single story. As noted in Section 3.1.3.3c, determination of
shear wall sheathing length based on the overall “inscribed” building dimension is permitted
for wind. Shear wall sheathing lengths can be determined using the inscribed area as shown
(in dotted lines). Distribution of required shear wall sheathing lengths, as shown in Figure 3,
based on tributary area for each wall line provide a balanced shear wall lay-out. For
example, the thicker purple line shows the tributary wall area to be used (in plan) for
designing the shear wall shown in purple. Similarly the same holds true for the tributary
areas and shear walls shown in green and orange.
Note that shear wall sheathing lengths are based on MWFRS loads. Localized components
and cladding (C&C) loads on roof surfaces, rafters, dormers, etc. are handled in other tables
of the WFCM.
23
1 General Information - Limitations
In-story wall offsets < 4 ft
• use either “inscribed method” or “separate structures”
method
• for such small offsets, distribution of shear wall sheathing
lengths based on tributary area is not required where the
inscribed method is used
• WFCM assumption: diaphragms in accordance with WFCM
provisions are capable of maintaining load path to supporting
elements below where offsets are small
Both the “inscribed method” and “separate structures” method are permitted where walls are
offset by less than 4 feet within a single story. For such small offsets, distribution of shear
wall sheathing lengths based on tributary area is not required where the inscribed method is
used. In the WFCM, it is assumed that diaphragms in accordance with WFCM provisions are
capable of maintaining load path to supporting elements below where offsets are small.
24
1 General Information - Limitations
Inscribed Method - in-story
wall offsets > 4 ft and ridge
lines not parallel
• use inscribed structure method
and conservatively determine
sheathing lengths assuming
wind is perpendicular to ridge
(WFCM Table 3.17A)
The inscribed structure method can be used but required shear wall sheathing lengths
should be conservatively determined assuming wind is perpendicular to the ridge (see
WFCM Table 3.17A).
25
1 General Information - Limitations
For more information, see:
• DA5: Inscribed versus
Separate Structures in the
WFCM
Free download from
www.awc.org
For more information, see Design Aid 5 which was developed as supplemental information
for the 2001 WFCM to better the understanding of the design of inscribed versus separate
structures.
26
1 General Information - Limitations
Aspect Ratios
L2
L
L 2 / L1 < 4
L1
O1 < Lesser of 12 ft
or L 1 / 2
O2 < Lesser of 12 ft
or L 2 / 2
H
L2
H / L < 3 1/2
L1
Exterior walls adjacent to the opening shall be
framed using full height studs, where the
opening is less than 2ft. from the exterior wall.
Here are some more limitations found in Chapter 1: Floor and roof diaphragm aspect ratios,
shearwall segment aspect ratios, and floor diaphragm opening limits. We’ll talk about these
more later.
27
2 Engineered Design
minimum loads for
establishing specific
resistance requirements
results of engineering
calculations for specific
elements, in specific
configurations, under
specific loads
tabulated information a
significant time saver
for the busy design
professional
Chapter 2 contains a set of engineered requirements, reiteration of importance of
maintaining load path, applicability limits that apply to use of Chapter 2, and enabling
language for design values in the WFCM Supplement.
28
2 Engineered Design - Limitations
Specifically outlines
applicability of
engineering provisions
Here are the limitations as found in the WFCM that apply to the use of Chapter 2, and
guidance once limitations are exceeded.
29
2 Engineered Design
General Provisions
Connections
Floor Systems
Wall Systems
Roof Systems
Tables
Figures
Chapter 2 is presented in terms of assembly systems, beginning first with connections. The
manual is profusely illustrated with helpful diagrams and 3D detail drawings for common
standard assemblies.
30
2 Engineered Design
Lateral Loads
FLOOR JOIST
LATERAL
FRAMING
LOADS
STUD
FLOOR JOIST
WFCM Table 2.1: Calculated loads to be resisted with lateral framing connections.
31
2 Engineered Design
2001 WFCM wind load tables have been enhanced from the 1995 SBC edition to include
broader range of wind speeds and a better understanding of wind pressures acting on
various building surfaces. This example shows the table for Roof and Wall Sheathing
Suction Loads for 3 second gust speeds from 85 to 150 mph. The building surface is zoned
into 6 areas, with a corresponding pressure for a given wind speed.
32
2 Engineered Design
Suction Loads
Designing for suction loads is critical. Hip and gable roof examples show panels lost at
edges in high wind roof zones. Nailing patterns are critical in high wind zones. This nailing
pattern didn't meet code minimums, which led to breach of the structure during a hurricane
(Andrew).
33
2 Engineered Design
Results of Sheathing Loss
Once sheathing is lost, progressive failures of other components follow. Content damage is
the most expensive result.
34
2 Engineered Design
LOADBEARING WALL
or SHEARWALL
Cantilever limits
JOISTS SHALL BE LOCATED DIRECTLY
OVER STUD UNLESS TOP PLATE IS
DESIGNED TO CARRY THE LOAD
REQUIRED
BLOCKING
d
BAND JOIST
*
MAX. d
L
* See 3.3.1.6.1 Exception
NON-LOADBEARING NON-SHEARWALL
REQUIRED
BLOCKING
BAND JOIST
JOISTS SHALL BE LOCATED DIRECTLY
OVER STUD UNLESS TOP PLATE IS
DESIGNED TO CARRY THE LOAD
L
MAX. L / 4
WFCM has limitations on cantilevers as shown in these figures. WFCM span table values
will apply up to these limits.
35
2 Engineered Design
LOADBEARING WALL OR SHEARWALL
REQUIRED
BLOCKING
d
BAND JOIST
JOISTS SHALL BE LOCATED DIRECTLY
OVER STUD UNLESS TOP PLATE IS
DESIGNED TO CARRY THE LOAD
L
Setback limits
MAX 4d
WHEN DESIGNED FOR
ADDITIONAL LOAD
LOADBEARING WALL
or SHEARWALL
MAX. d
REQUIRED BLOCKING
d
JOISTS SHALL BE LOCATED DIRECTLY
OVER STUD UNLESS TOP PLATE IS
DESIGNED TO CARRY LOAD
Similarly, WFCM has limitations on setbacks, to which span table values are limited to. This
is a simple way of dealing with specific loading for specific configurations.
36
2 Engineered Design
JOISTS
BLOCKING
SECTION A-A
ENDWALL
Endwall blocking
A
A
BLOCKING
Endwall blocking details are very critical to ensure load transfer from endwalls into a
horizontal ceiling diaphragm by providing a nailing surface.
37
2 Engineered Design
V
Hold down
calculations
V
h
Rotation
v
L
T
v = V/L
T = v*h
V = Lateral Load (lbs)
v = Required Unit Shear Capacity (plf)
T = Required Holddown Capacity (lbs)
Overturning details and holddown calculations are also provided for shearwalls. More on
this, later.
38
2 Engineered Design
4" o.c. Nail Spacing
Connection per Section 2.2.2.1 & 2.2.6.5
4' Perimeter Zone
L
panel field nailing
4' Perimeter Zone
panel edge nailing
Lesser of
L/2 or 2'
2x6 Outlooker (2x4 minimum)
Blocking
A
Gable Endwall
A
Section A-A
Rake details - outlookers
End framing of gable roofs can be addressed using outlookers and WFCM has rake
overhang limits.
39
2 Engineered Design
4" o.c. Nail Spacing
4' Perimeter Zone
panel field nailing
4' Perimeter Zone
panel edge nailing
< 1'
Lookout Block
Gable End Truss or Endwall
Section A-A
A
A
Rake details - ladder
Another alternative is the popular lookout blocks (ladder frame), and WFCM has rake
overhang limits.
40
2 Engineered Design
Consequences of inadequate rake details
If rake overhangs are not properly detailed, the building may be breached.
41
2 Engineered Design
Enhancements from 1995 WFCM
• covers lower wind speeds
• new snow loads
• new seismic loads
SBC
loads
2000 IBC
loads
(ASCE 7-98
with
amendments)
The chief enhancements in Chapter 2 from the 1995 to the 2001 WFCM are as a result of
the national scope of the document:
•wind speeds across a broad spectrum
•snow loads
•seismic loads
all consistent with 2000 IBC (ASCE 7-98 with amendments). For example, the WFCM 2001
assumes an enclosed structure; other enclosures are not tabulated.
42
3 Prescriptive Design
specific set of resistance
requirements for
residential buildings
Chapter 3 contains prescriptive design information intended for use by builders and lay
people. Similar in information content to Chapter 2, but conveyed in a different way.
43
3 Prescriptive Design
Prescriptive Design
General Provisions
Connections
Floor Systems
Wall Systems
Roof Systems
Tables
Figures
Chapter 3 contains prescriptive solutions based on Chapter 2 engineered provisions and
follows the same format as Chapter 2 including connections, floors, walls, and roofs.
44
3 Prescriptive Design - Limitations
Specifically outlines
applicability of
prescriptive provisions
Based on minimum
prescriptive design
limitations in model
building code provisions
Once again, limitations are imposed on the use of the provisions within this chapter and are
described right at the very beginning. Guidance is given if limitations are exceeded - often
reference is made back to Chapter 2.
45
3 Prescriptive Design
Sidewall
Sheathing
AMENDED
SIDEWALL
ENDWALL
WFCM - Table 3.6B shows one table for sidewall sheathing length requirements for
traditional shearwalls in wind exposure B.
46
3 Prescriptive Design
Full height sheathing length for
end walls and sidewalls, Tables
3.17A and B, pages 169-170
Additional tables cover endwalls, segmented shearwalls (Type I), and perforated shearwalls
(Type II).
47
3 Prescriptive Design
Framing to Foundation
Details
WFCM - Figures 3.2a-c: Sill and Bottom Plate anchorage to the foundation. Foundation
design is beyond the scope of WFCM, but detailing anchorage requirements is not.
48
3 Prescriptive Design
Outer third of
Notching Limits
Notch
depth < 1/4d
span only
1/4 Joist
depth, Max.
d
2" Min.
1/3 Joist
Hole edge
distance > 5/8"
Outer 1/3 of
span only
depth, Max.
1/3 Joist
depth, Max.
Hole diameter
< 2/5d
Single Stud
Stud
1/6 Joist
depth, Max.
Hole diameter
< 3/5d
Double Stud
Plate
Hole edge
distance > 5/8"
WFCM - Figures give notching and boring limitations for joists, rafters, and studs.
49
3 Prescriptive Design
Stud
Moisture-Resistant
Drywall
Floor Framing
Details
Stud
Bottom
Plate
Subflooring
Band
Joist
Joist
Bathtub
Double
Plate
Block
Stud
Stud
Subflooring
Double Trimmer
Plate
Joist
Subflooring
Joist
Blocking
Stud
Double Plate
Band
Joist
Joists shall be located
directly over studs
…as well as common framing details for things such as joists framing on a stud wall,
cantilevered joists, and framing for concentrated loads like a bath tub.
50
3 Prescriptive Design
Rafter/Collar Ties
Ridge Board
Collar Tie (Collar Beam)
(Located in upper third of
attic space)(see 3.2.5.1)
Rafter
Ridge Board
Rafter Tie
(Located in lower third
of attic space)
Collar Tie (Collar Beam)
(Located in upper third of
attic space) (see 3.2.5.1)
Rafter
Ceiling joist
parallel to
rafters
WFCM - Figures give clarifications of Ridge Board vs. Ridge Beam and Rafter Tie vs. Collar
Tie.
51
3 Prescriptive Design
Gable End Bracing
2x4 Continuous
lateral brace at 6' o.c.
Truss or Ceiling Joist
2-10d Nails
Gable End Truss
10-8d Nails
5d Cooler Nails at 10" o.c.
2"x4" Block nailed to each
brace with 4-10d nails
Gypsum Board
5d Cooler Nails at 7" o.c.
20 Gage Strap
10-8d Nails
Endwall Studs
Ceiling bracing gable endwall - very important detail to ensure that a hinge does not form
between gable end trusses and gable end stud walls. This is a typical failure seen in
hurricanes.
52
3 Prescriptive Design
Hold down Details
Blocking at 24" o.c. allows
holddown installation
Sidewall
Holddown
1/2" Spacing
allows
holddown
installation
Holddown
Endwall
Corner stud
connected to
transfer shear
2-16d Common
nails at 10" o.c.
Corner stud
connected to
transfer shear
2-16d Common
nails at 6" o.c.
Some typical holddown details. Note the nailing detail for where the wall end studs meet.
This is often the most overlooked connection on a house – and one of the most critical to the
structure’s integrity.
53
3 Prescriptive Design
Connections
• nails - minimum limits
Chart
values
invalid!
Minimum prescribed nailing provisions are tabulated….but then, if the nails miss the studs
as in this picture, you can forget about the table.
54
3 Prescriptive Design
Connections
• bolted
anchorage
FRONT DOOR STEP
Guidance is given on structural anchorage in prescriptive form to hopefully prevent homes
from becoming roller-skates under wind loads.
55
3 Prescriptive Design
Enhancements from 1995 WFCM
• updated with information from 1997 NDS
• tables based on Wind Exposure B, with
Appendix for Wind Exposure C
• national version in accordance with 2000
IBC/IRC
• ANSI approved
The 2001 WFCM prescriptive provisions have been updated with design information from
the 1997 NDS. Wind tables are based on exposure B, however and Appendix for wind
exposure C information has been included. All of the information in the prescriptive
provisions is national in scope and ANSI approved (consensus basis).
56
Supplement
1 General Information
2 Engineered Design
3 Prescriptive Design
Supplement
The supplement is the last segment in the WFCM.
57
Supplement
1A
1B & 1C
2A & 2B
2C
3A
3B
4A - 5C
6 &7
Nominal and Dressed Sizes of Sawn Lumber
Section Properties of Sawn Lumber and Glulam
Sheathing Spans for Floor and Roof Sheathing
Shear Capacities for Horizontal Diaphragms
Sheathing Spans for Wall Sheathing
Design Criteria and Capacities for Shearwall
Materials
Framing Member Design Values
Nail Connection Capacities
The Supplement contains design values needed for engineered design. It is located at the
end of the WFCM manual, not in the Commentary volume. It’s based on 1997 NDS
Supplement with the addition of Tables 2A-C, 3A&B, 6, and 7.
58
WFCM 2001 Commentary - Layout
1 General Information
2 Engineered Design
3 Prescriptive Design
Supplements
The WFCM Commentary contains background information and example calculations.
59
WFCM 2001 Commentary
Background information
and calculation procedures
used in development of
WFCM
• General Information
• Engineered Design
• Prescriptive Design
It provides the user with assumptions used in development of each table, derivations, and
sample calculations for each table.
60
WFCM 2001 Commentary
Detailed explanation
Example calculations with
graphics
Here is an example page from the commentary highlighting detailed information, calculation
procedures, and graphics to facilitate understanding.
61
Outline
Purpose & Background
Development process
Code Acceptance
Document layout
Design provisions:
•
•
•
•
Snow
Shear walls – “Standard” Shear Wall
Wind
Seismic
Design Examples
We have seen some of what is in the WFCM and how the manual is presented in terms of
the engineering, and prescriptive approaches. Next we’ll take a quick look at some of the
2001 provisions categorized by load type. First, we’ll begin snow load (a gravity load) and
how the WFCM address snow. The we’ll address lateral loads and resistance with a
discussion on shearwalls, then a combined discussion of load resistance behavior with
WFCM provisions for each of wind and seismic loads.
62
WFCM 2001 Provisions - Snow
per 2000 IBC provisions
30 - 70 psf ground snow load
unbalanced snow loads
considered in tables
Let’s turn our attention first to snow issues. It snows in various regions of the country, and 30
to 70 psf ground snow load provisions are included in the 2001 WFCM. Snow load span
tables automatically reflect the consideration of unbalanced snow loads.
63
WFCM 2001 Provisions - Snow
Ground snow
load contours
• Snow loads are
shown in psf
• Snow loads at
higher elevations
are shown in
parentheses
IBC Figure 1608
IBC Figure 1608 presents ground snow loads in the US. Some states and municipalities
may have their own snow load maps and provisions that the designer should be aware of.
64
Snow Issues
If rafters are not properly designed and constructed, the weight of snow can cause damage.
•
Rafters can deflect excessively
•
Rafters can spread, causing excessive loads on exterior walls
•
Rafters can fail and collapse.
65
WFCM 2001 Provisions - Snow
found in
Chapter 2 by
wood strength
and stiffness
properties
As an example, span tables for rafters subjected to snow loads in chapter 2 (engineering)
are ordered by wood strength and stiffness parameters familiar to technical wood
designers...
66
WFCM 2001 Provisions - Snow
and in Chapter 3 by wood grade and species group
…while the same information is presented in Chapter 3 (prescriptive) by the more userfriendly wood grade and species group that easily be correlated to wood grade stamps found
on each piece of construction lumber.
67
WFCM 2001 Provisions - Snow
and in Chapter 3 for panel selection for snow
Panel information is provided in Table 3.12B for Rood Live and Snow loads...
68
WFCM 2001 Provisions - Snow
and in Chapter 3 for panel selection for floor live
…and Table 3.14 for Floor Live Loads.
69
Outline
Purpose & Background
Development process
Code Acceptance
Document layout
Design provisions:
•
•
•
•
Snow
Shear walls – “Standard” Wall
Wind
Seismic
Design Examples
Now let’s look at the topic of shearwalls. The WFCM emphasizes completeness of load
path, and shearwalls are one of the key elements used to do this.
70
Shearwalls – a lateral resistance device
Load Path
This is a graphical illustration of how wind load moves through various building assemblies
and gets transferred into the ground: first, the windward wall; which transfers in-plane load
to the roof diaphragm; which transfers reaction loads to the tops of the side shearwalls;
which transfer load into the ground. Note the important components: the windward wall, the
roof diaphragm, and the side shearwalls with associated shear and hold-down anchorage
devices.
71
Resisting Elements - Shearwalls
racking resistance
perimeter nailing
sliding
resistance
Shearwalls are a vertical building element that can resist lateral forces applied at the top of
the wall. In a wood shearwall, the panel perimeter nails provide the bulk of the racking
resistance through wood bearing and nail deformation when the lateral external force is
applied. Horizontal wall sliding is resisted by nailing or other anchorage installed along the
bottom of the shearwall sufficient to resist the external lateral force.
72
Resisting Elements - Shearwalls
panel aspect ratio
≤ 3.5
1
racking action
cantilever
beam action
In order to make this concept work, panels must have a height-to-width aspect ratio of less
than 3.5 to 1. This ratio is sufficient to develop “racking action” in the shearwall panel.
Aspect ratio’s greater than this produce cantilever beam action - a completely different
behavior that is much less effective in resisting lateral forces.
73
Resisting Elements - Shearwalls
Five parts of a shearwall
wood
structural
panels
wood frame
nails
hold downs
plate
anchors
The shearwall is a device which has five essential components to make it complete:
-- a wood frame,
-- wood structural panels: the brace,
-- nails: perimeter nails to resist the racking moment, and field nails to resist wind suction
and panel separation from the frame,
-- plate anchors: to resist sliding due to applied horizontal force, and
-- hold downs: to stop the wall panel from in-plane overturning.
The perimeter nails are key to making this device work to design lateral load. Their ability to
repeatedly distort under load over many cycles to dissipate energy makes them a vital
component.
74
Center-Point Bending Test
Load
Wall Sheathing Fasteners
Nails – low / medium carbon steel
ASTM F1667
NDS Table I1 Fyb values
Roofing Nails
(WFCM Table3.1 note 3)
Corrosion resistant 11 gage
Check IBC for additional
requirements
Staples
(WFCM Table3.1 note 3)
Corrosion resistant 16 gage
Check IBC for additional
requirements
Since the nails repeatedly distort so much while dissipating the racking energy over a
significant period of time; it is very important that they be made of a ductile, but strong, metal
so they can do their job without failing. The WFCM requires nails of low to medium carbon
steel for this purpose – a steel that is known to be ductile. High carbon steels as found in
some fasteners fail in a sudden brittle manner, often with very little repetitive distortion. Note
that roofing nails and staples referenced in the WFCM need to be of the indicated gage, as
well as be corrosion-resistant. The IBC has more information on this requirement.
75
Resisting Elements - Hold Downs
overturning
Finally, overturning may be a problem, especially for high aspect ratios. These skinny
panels usually develop high overturning forces at the bottom corners of the walls that need
to be resisted with the installation of special hold-down hardware.
76
“Standard” Shear Wall
Assembly details for these panels as described in 2001 WFCM 3.4.4.2
8d common nails
@ 6” OC on panel
perimeter
8d common
nails @ 12” OC
in field
7/16” wood
structural panel
continuous
height over wall
plates
panel exterior
5d cooler nails
@ 7” OC on
panel perimeter
5d cooler nails
@ 10” OC in
field
1/2” gypsum
wallboard on
interior
panel interior
Here are the wall assembly assumptions used for the development of the preceding
methodology. A cooler nail is also known as a drywall nail. For discussion purposes, we’ll
call this assembly “the standard shearwall”. It’s actually the base case shearwall that the
WFCM is created around.
77
“Length of Wall” Concept
More perimeter nails = shorter wall length
for same applied lateral load
p
p
6:12 perimeter:field
nailing pattern
Length of wall
=
3:12 perimeter:field
nailing pattern
Length of wall
The “length of wall” concept is used in the WFCM as a measure of the unit shear capacity of
the shearwall. By decreasing the perimeter nail spacing, more wall racking resistance is
provided. Thus for the same load “p”, the required wall length would shorten because of the
increased number of perimeter nails.
78
“Length of Wall” Concept
More perimeter nails = more load resistance
for same wall length
P++
p
6:12 perimeter:field
nailing pattern
Length of wall
=
3:12 perimeter:field
nailing pattern
Length of wall
Another way of expressing this is as follows. If the wall length remains constant, and more
load capacity is required (P++), simply increase the number of perimeter nails in the
shearwall (decrease the panel perimeter nail spacing).
79
Wall Design Modification
Alternative assembly details for these panels are available from Table 3.17D
which accordingly modify the wall lengths with factors.
8d common
nails @ 6” OC
on panel
perimeter
8d common
nails @ 12”
OC in field
7/16” wood
structural
panel
continuous
height over
wall plates
panel exterior
Modified wall length =
standard wall length x
Table 3.17D factor
(wind or seismic)
The WFCM makes this concept work very easily in Table 3.17D. The Table takes the
“standard shearwall” as the base case, with seismic and wind modification factors as 1.0.
Depending on the desired assembly listed in the table, seismic and wind modification factors
respectively can be used to modify the required base case wall length. The Table also lists
the unit shear capacities of the various wall assemblies. We’ll see a numerical example of
this implementation a little later.
80
Shearwall Design Methodologies
Safety Factor
Adjustments
Summing Dissimilar
Shearwall Materials
Perforated Shearwall
Method
System Factors for
Stud Walls
Much of the shearwall design methodologies presented in this eCourse are as adopted by
the SBC, and based on AF&PA’s National Design Specification® (NDS®) for Wood
Construction 1997, national and international research, and reevaluation of existing practice.
81
Safety Factor Adjustments
Issues:
• Requires higher safety
factor shear walls and
diaphragms
Solution:
• Adjusted safety factor
to generally acceptable
level
AF&PA’s Subcommittee on Conventional Construction (SCC) looked at safety factors
associated with wood structural panel (WSP) shearwall & diaphragms design and felt they
were more conservative than for other materials, therefore they were adjusted to a more
generally acceptable level
82
Safety Factor Adjustments
Shear Capacities for Shear
walls & Diaphragms:
• increased by 40% for wind
design
• based on an adjustment of the
minimum load factor from
2.8 to 2.0
• IBC 2306.3.1 (diaphragms)
• IBC 2306.4.1 (shearwalls)
Effect of adjustment was 40% increase in shear capacity of Wood Structural Panels (WSP)
for wind design.
83
Shearwall Design Methodologies
Safety Factor
Adjustments
Summing Dissimilar
Shearwall Materials
Perforated Shearwall
Method
System Factors for
Stud Walls
84
Summing Dissimilar Shearwall Materials
Issues:
• Ignores the
contribution of interior
wall sheathing
Solution:
• Develop procedure
which recognizes the
contribution of interior
sheathing materials
A second concern was that current practice ignored contribution of interior sheathing
(gypsum), therefore the SCC developed a procedure to recognize added shear capacity of
interior sheathing.
85
Summing Dissimilar Shearwall Materials
The allowable shear
capacity of a shear wall
segment sheathed on both
sides, with similar or
dissimilar materials, shall
equal the sum of the
individual shear capacity
of each side
• IBC 2305.3.8
wind applications only!
The shear capacity of a shearwall segment sheathed on both sides to resist wind loads only
is additive.
86
Summing Dissimilar Shearwall Materials
Substantiating Test Data
• FPL - Predicting Racking Performance of Walls Sheathed
on Both Sides
• FPL - Racking Performance of Light-Framed Walls
Sheathed on Two Sides
• FPL - Contribution of Gypsum Wallboard to Racking
Resistance of Light Frame Walls
• Forintek - Lateral Resistance of Nailed Shear Walls
Subjected to Static and Cyclic Displacement
• APA - Report 157 - Wood Structural Panel Shear Walls
with Gypsum Wallboard and Window/Door Openings
Various sources used in development and substantiation of this procedure.
87
Summing Dissimilar Shearwall Materials
APA Wall Tests
• Wall #1 - 15/32" plywood only applied vertically
– Blocked with 8d nails @ 3" (edge) and 12" (field)
• Wall #2 - 1/2" gypsum only applied horizontally
– Unblocked with 5d nails @ 7" (edge) and 7" (field)
• Wall #3 - Sheathing from Wall #1 and Wall #2 on
opposite sides
APA tested 3 walls: WSP only, gypsum only, and a combination of the two.
88
Summing Dissimilar Shearwall Materials
APA Wall Tests
Wall
Existing Tabulated
1.4 x Tabulated
Tested
Number Shear Capacity (plf) Shear Capacity (plf) Shear Capacity (plf)
#1
#2
#3
450
100
450
630 (a)
100 (b)
730 (c)
1680
254
2040
(a) Using safety factor adjustment of 1.4. (450 * 1.4) = 630
(b) No adjustment was taken on gypsum wallboard shear capacities.
(c) Proposed allowable design capacity. (450 * 1.4) + 100 = 730
Results show shear capacity of each wall using traditional shear capacities and procedure,
compared to adjusted shear capacities and addition of interior sheathing. Walls tested show
2-3 times greater capacity than these methodologies allow for. Note that Wall #3 exceeds
the sum of Walls #1 and #2.
89
Shearwall Design Methodologies
Safety Factor
Adjustments
Summing Dissimilar
Materials
Perforated Shearwall
Method
System Factors for
Stud Walls
90
Perforated Shearwall Method
Issues:
• Requires numerous
interior hold downs for
shear walls
Solution:
• Eliminate need for
interior hold downs by
reducing effective
shear capacity of wall
assembly
The third design methodology was developed because of numerous holddowns required on
shearwall segments, which creates a challenge both in installation and in maintaining load
path. Current practice is valid, however a method was needed to eliminate some of the
interior holddowns.
91
Segmented Shearwall Method
An example of the traditional shearwall method shows that 6 holddowns and enough mass
in the foundation would be required to prevent overturning of these typical wall segments.
92
Perforated Shearwall Method
In this example, the perforated shearwall has a reduced shear capacity from the traditional,
but interior holddowns have been eliminated.
93
Perforated Shearwall Method
Substantiating Test Data:
• Professor Hideo Sugiyama
– 12 studies in Japan over a period of 20 years
• APA - Report 157
– Wood Structural Panel Shear Walls with Gypsum
Wallboard and Window/Door Openings
• Virginia Tech
– Monotonic and Cyclic Tests of Shear Walls with Openings
• NAHB - RC
– Monotonic and Cyclic Tests of Shear Walls with Openings
Three sources were used in development and substantiation of perforated shearwalls,
including Sugiyama, APA-The Engineered Wood Association, Virginia Tech and NAHB
Research Center tests.
94
Perforated Shearwall Method
Test data closely
matches design
procedure
Correlation of test data from these studies to the perforated shearwall method is very good
and conservative.
95
Perforated Shearwall Method
Maximum Unrestrained Opening
Height (Window or Door Height)
8' Wall
10' Wall
Percent
Full-Height
Sheathing
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
2'-8"
3'-4"
4'-0"
5'-0"
5'-4"
6'-8"
6'-8"
8'-4"
8'-0"
10'-0"
Effective Shear Capacity Factor
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.67
0.69
0.71
0.74
0.77
0.80
0.83
0.87
0.91
0.95
1.00
0.50
0.53
0.56
0.59
0.63
0.67
0.71
0.77
0.83
0.91
1.00
0.40
0.43
0.45
0.49
0.53
0.57
0.63
0.69
0.77
0.87
1.00
0.33
0.36
0.38
0.42
0.45
0.50
0.56
0.63
0.71
0.83
1.00
IBC 2305.3.7.2
In the Perforated Shearwall Method, shear capacity is reduced using these effective shear
capacity factors, adopted into the Standard Building Code. Critical factors in this table are
percent of full height sheathing, and maximum unrestrained opening height.
96
Traditional (Segmented) Shearwall
Method
Required Capacity
5:12 Roof Slope
36'
2884#
4'
2-1/2' 2-1/2' 2-1/2'
5-1/4'
4'
3360#
4680 lbs
2-1/2'
5-1/4'
5'
2-1/2' 2-1/2' 2-1/2'
4'
15/32" WSP (8d at 6/12)
SPF Framing (G = 0.42)
Tabulated Capacity
= 230 plf
Shearwall Capacity
= 230 plf * 13 ft
= 2990 lbs
Holddown Capacity
= 230 plf * 8 ft
= 1840 lbs
Required Capacity
= (2884lbs + 3360 lbs)/21ft
= 297 plf
4'
2840 lbs
Base Shear = 6244 lbs
= 2884lbs/13ft
= 222 plf
15/32" WSP (8d at 4/12)
SPF Framing (G = 0.42)
Tabulated Capacity
= 355 plf
4680 lbs
Shearwall Capacity
= 355 plf * 21 ft
= 7455 lbs
Holddown Capacity
= 355 plf * 8 ft
= 2840 lbs
unadjusted shear capacities
To demonstrate the effects of each of these three shearwall design methodologies. First
look at an example of the traditional shearwall using the unadjusted shear capacities. Note
that a dozen holddowns are required along the length of the first floor.
The WFCM uses 60% of the dead load to resist wind uplift. In most cases, the wind uplift is
much greater than 60% of the dead load, and will require more than the dead load alone to
offset the wind force component. For seismic loads, the holddowns are conservatively sized
in the WFCM to meet the shear capacity of the shearwall.
97
Traditional (Segmented) Shearwall
Method
T AB L E 3B - WOOD STR UCT URAL PAN EL S HEAR CAPACIT IES FOR S HEA RW ALL ASS EMB LI ES
Framing Species
G $ 0.49
Framing Species
0.49 > G $ 0.42
Framing Species
G < 0.42
Panel Edge Nail Spacing (in.)
6
4
3
2
6
4
3
6
4
3
2
4202
130
195
255
3302
6002
180
280
360
4752
450
6002
180
280
360
4752
355
420
450
545 2
6002
7152
180
220
280
330
360
430 2
4752
5652
Sheathing
Thickness
(in.)
Structural I
5/16
6d
200
300
390
5102
165
245
320
3/8
8d
2303
3603
460 3
6102,3
230
355
450
7/16
8d
2553
3953
505 3
6702,3
230
355
15/32
8d
10d
280
340
430
510
550
665 2
7302
8702
230
280
Structural
Sheathing
Plywood
Siding
2
Recommended Shear Capacity (plf)1
Sheathing
Material
Nail
Size
5/16
6d
180
270
350
4502
165
245
320
4202
130
195
255
3302
3/8
6d
8d
200
2203
300
3203
390
410 3
5102
5302,3
165
230
245
355
320
450
4202
6002
130
180
195
280
255
360
3302
4752
7/16
8d
2403
3503
450 3
5852,3
230
355
450
6002
180
280
360
4752
15/32
8d
10d
260
310
380
460
490
600 2
6402
7702
230
280
355
420
450
545 2
6002
7152
180
220
280
330
360
430 2
4752
5652
19/32
10d
340
510
665 2
8702
280
420
545 2
7152
220
330
430 2
5652
2
2
5/16
6d
140
210
275
360
115
175
225
295
90
135
180
2352
3/8
8d
160
240
310
4102
130
200
255
3402
105
155
200
2652
WFCM 1997 Supplement Table 3B is used to determine shearwall capacity. Assuming SPF
framing with G=0.42, 8d nails, 15/32” Structural sheathing, and nailing of 6”/12” and 4”/12”
along the panel edges.
98
Traditional (Segmented) Shearwall
Method
Required Capacity
= 2884lbs/8ft
= 361 plf
15/32" WSP (8d at 6/12)
SPF Framing (G = 0.42)
Tabulated Capacity
= 230(1.4) + 100
= 422 plf
2884#
3360#
6752 lbs
3376 lbs
6752 lbs
Shearwall Capacity
= 422 plf * 8 ft
= 3376 lbs
Holddown Capacity
= 422 plf * 8 ft
= 3376 lbs
Required Capacity
= (2884lbs + 3360 lbs)/16ft
= 390 plf
15/32" WSP (8d at 6/12)
SPF Framing (G = 0.42)
Tabulated Capacity
= 230(1.4) + 100
= 422 plf
Shearwall Capacity
= 422 plf * 16 ft
= 6752 lbs
Holddown Capacity
= 422 plf * 8 ft
= 3376 lbs
Base Shear = 6244 lbs
summing dissimilar materials
With increased shear capacities (40% increase) and summation of dissimilar materials (100
plf for gypsum) the required sheathing has been reduced, however there are still numerous
interior holddowns required.
99
Perforated Shearwall Method
Required Capacity
= 2884lbs/23.5 ft
= 123 plf
Percent FH Sheathing = 23.5'/36' = 65%
Shear Capacity Adjustment (H/2) = 0.85
15/32" WSP (8d at 6/12)
SPF Framing (G = 0.42)
Tabulated Capacity
= (230(1.4) + 100) * 0.85
= 359 plf
2884#
3360#
Shearwall Capacity
= 359 plf * 23.5 ft
= 8430 lbs
Holddown Capacity
= 422 plf * 8 ft
= 3376 lbs
Required Capacity
= (2884lbs + 3360 lbs)/21ft
= 297 plf
Percent FH Sheathing = 21'/36' = 58%
Shear Capacity Adjustment (5H/6) = 0.62
8152 lbs
8152 lbs
Base Shear = 6244 lbs
15/32" WSP (8d at 4/12)
SPF Framing (G = 0.42)
Tabulated Capacity
= (355(1.4) + 100) * 0.62
= 370 plf
Shearwall Capacity
= 370 plf * 21 ft
= 7770 lbs
Holddown Capacity
= 597 plf * 8 ft
= 4776 lbs
perforated shearwall capacity
By using the perforated shearwall method the amount of sheathing has been increased,
however holddowns are only required at each end of the shearwall.
The WFCM uses 60% of the dead load to resist wind uplift. In most cases, the wind uplift is
much greater than 60% of the dead load, and will require more than the dead load alone to
offset the wind force component. For seismic loads, the holddowns are conservatively sized
in the WFCM to meet the shear capacity of the shearwall.
100
Copyright © 2001 American Forest & Paper Association, Inc.
Perforated Shearwall Method
Maximum Unrestrained Opening
Height (Window or Door Height)
8' Wall
10' Wall
Percent
Full-Height
Sheathing
221
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
2'-8"
3'-4"
4'-0"
5'-0"
5'-4"
6'-8"
6'-8"
8'-4"
8'-0"
10'-0"
Effective Shear Capacity Factor
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.67
0.69
0.71
0.74
0.77
0.80
0.83
0.87
0.91
0.95
1.00
0.50
0.53
0.56
0.59
0.63
0.67
0.71
0.77
0.83
0.91
1.00
0.40
0.43
0.45
0.49
0.53
0.57
0.63
0.69
0.77
0.87
1.00
0.33
0.36
0.38
0.42
0.45
0.50
0.56
0.63
0.71
0.83
1.00
IBC 2305.3.7.2
Using the perforated shearwall method, shear capacity is reduced using effective shear
capacity factors. Assuming an 8’ wall height, window openings are H/2 or 4’ and door
openings are 5H/6 or 6’-8”. Interpolation is permitted based on percent full-height sheathing
in the wall.
101
Shearwall Design Methodologies
Safety Factor Adjustments
Summing Dissimilar
Materials
Perforated Shearwall
Method
System Factors for Stud
Walls
102
System Factors for Stud Walls
Issues:
•
•
•
•
Repetitive member factor, Cr
sheathed with minimum materials
studs act alone
Cr underestimates system effects
Solution:
• Develop factors based on structural
design procedures which account
for system effects
The fourth design methodology was the incorporation of system factors for stud walls.
Problems with current practice were: assumptions that walls were sheathed with minimum
materials, that studs act alone as bare members, and the current repetitive member factor,
Cr, underestimates system effects.
103
System Factors for Stud Walls
Components in the System Effect
• Partial Composite Action
– Interaction between structural elements providing
increased stiffness and strength
• Load Sharing
– A characteristic of repetitive member assemblies where
load is distributed according to relative member stiffness
• Structural Redundancy
– The reserve strength available in an assembly after
failure of an individual member
Textbook methods tend to over-predict composite action for wood assemblies because they
assume full composite action. Wood assemblies demonstrate partial composite action due
to nail slip.
In general load sharing can be simply explained that the stiffer member will carry more load.
If you had an infinitely stiff load sharing element, all joists would share load equally. Since
this is not the case, the stiffer member picks up load first until it reaches its capacity and
then "shares" that load with adjacent members.
104
System Factors for Stud Walls
Bending design values,
Fb , for wood studs
shall be permitted to be
multiplied by the
following factors in lieu
of the 1.15 repetitive
member factor
2x4
2x6
2x8
2x10
2x12
1.50
1.35
1.25
1.20
1.15
IBC 2306.2.1
The system factors are used in lieu of the 1.15 repetitive member factor, Cr. Note that a
value of 1.15 is retained for 2x12’s.
105
System Factors for Stud Walls
Requirements for system factors
for wall studs (IBC 2306.2.1):
• Resisting wind loads
• Exterior
– min 3/8" wood structural sheathing
– min 8d common nails
– max 6" o.c. edges and 12" o.c. field
• Interior
– min 1/2" gypsum wallboard sheathing
– min 5d cooler nails
– max 7" o.c. edges and 10" o.c. field
L
W
Example: Gable
end walls
Gable endwalls are good example of type of wood system where use of new system factors
for stud walls is applicable.
106
Outline
Purpose & Background
Development process
Code Acceptance
Document layout
Design provisions:
•
•
•
•
Snow
Shear walls – “Standard” Wall
Wind
Seismic
Design Examples
Now with this background on shearwalls, let’s look at load resistance behavior with WFCM
provisions for each of wind and seismic loads.
107
Wind Issues
Let’s first look at wind issues.
108
High Winds & Consequences
More damage is caused annually by wind in
the United States than by fire.
Wind damage is caused by:
•
•
•
•
Hurricanes,
Tropical Storms,
Tornadoes, and
Intense Thunderstorms.
More damage is caused annually by wind in the United States than by fire.
Wind damage is caused by:
•Hurricanes,
•Tropical Storms,
•Tornadoes, and
•Intense Thunderstorms.
109
Consequences
This is an example of the consequences of poor design and construction.
110
High Winds & Consequences
Each hurricane has a “personality”
• Some destroy though wind
• Some primarily through flooding
No matter the “personality” of a storm,
damage does not have to be significant!
The application of sound engineering and
diligence in construction will reduce losses
dramatically!
Each hurricane has a “personality”
•Some destroy though wind
•Some primarily through flooding
No matter the “personality” of a storm, damage does not have to be significant!
111
Consequences
F-3 Tornado
OK!
This is an example of damage in Central Florida resulting from an F-3 tornado. Neighboring
buildings designed and constructed in accordance with the Florida Building Code suffered
very little damage.
112
Consequences
There is little evidence that the building
material system matters.
• Concrete masonry, wood-frame, and steelframe buildings all suffer catastrophic damage.
But, properly designed and constructed
buildings of each material system fare quite
well.
Concrete-masonry construction, light-gauge steel construction and wood-frame construction
are all good if properly designed and constructed.
There is little evidence that the building material system matters in resisting damage.
•Concrete masonry, wood-frame, and steel-frame buildings suffer catastrophic
damage.
But, properly designed and constructed buildings of each material system fare quite well.
113
Frequently Observed Damage
Damage common to many buildings:
• Loss of roof shingles and other roofing
membranes resulting in water damage
• Loss of roof sheathing, especially at gable ends
and along edge zones
• Loss of roof rafter systems
• Loss of windows and doors - resulting in
pressurization of building
Damage common to many buildings:
•Loss of roof shingles and other roofing membranes resulting in water damage
•Loss of roof sheathing, especially at gable ends and along edge zones
•Loss of roof rafter systems
•Loss of windows and doors - resulting in pressurization of building
114
Dispelled Myths
Continuing pattern of damage dispels
several myths:
• Concrete-masonry and steel construction are
superior to wood-frame construction in high
wind regions.
• Wood-frame construction cannot withstand
hurricane wind loads.
• Wind resistive construction is cost prohibitive.
Continuing pattern of damage dispels several myths:
•Concrete-masonry and steel construction are superior to wood-frame construction
in high wind regions.
•Wood-frame construction cannot withstand hurricane wind loads.
•Wind resistive construction is cost prohibitive.
115
Dispelled Myths
Concretemasonry
construction
is superior?
This type of damage should not have occurred! This type of damage dispelled the myth of
the superiority of concrete-masonry construction.
116
Common Causes of Damage
Masonry Buildings:
• Inadequate vertical and horizontal reinforcing
• Improper placement of bars in load bearing
elements
• Inadequate coverage of bars by concrete grout
• Inadequate bond of mortar to masonry units
– Lack of mortar - end joints, bed joints
– Dehydration of mortar
Masonry Buildings:
•Inadequate vertical and horizontal reinforcing
•Improper placement of bars in load bearing elements
•Inadequate coverage of bars by concrete grout
•Inadequate bond of mortar to masonry units
•Lack of mortar - end joints, bed joints
•Dehydration of mortar
117
Dispelled Myths
Bad bond from
dehydrated mortar
on block laylay-up
This photo illustrates the problem with dehydrated mortar. The mortar stuck to the top of the
block, but not to the block laid on top of it.
This masonry was probably laid up on a hot Florida day. The block was probably very hot
and dry and once the mortar bed was laid, the free moisture was sucked out – preventing a
good bond.
Also, it is obvious that this wall had inadequate vertical reinforcing.
118
Dispelled Myths
Inadequate
reinforcing of
masonry units
This photo illustrates inadequate reinforcing of masonry units.
119
Dispelled Myths
Inadequate
consolidation
of grout in
reinforced cells
Inadequate consolidation of the grout in the reinforced cells is also an all too common
problem. Note the gap!
Much of the reinforcing steel did not show evidence of a grout bond. This kind of problem
can be easily prevented.
120
Common Causes of Damage
Wood-frame Buildings:
•
•
•
•
•
•
Inadequate nailing of built-up corners
Inadequate nailing of wall sheathing
Connections!
Inadequate nailing of roof sheathing
Inadequate fastening of roof to walls
Inadequate fastening to foundation
Lack of shear diaphragms to transfer loads
What kind of problems do wood-frame buildings all too frequently have?
Wood-frame Buildings:
•Inadequate nailing of built-up corners
•Inadequate nailing of wall sheathing
•Inadequate nailing of roof sheathing
•Inadequate fastening of roof to walls
•Inadequate fastening to foundation
•Lack of shear diaphragms to transfer loads
121
Hurricane
Andrew 1992
Here are some examples of unnecessary damage caused by Hurricane Andrew in 1992.
Similar damage was observed in Hurricane Hugo in 1988 and the hurricanes that struck
Florida in 2004.
122
Hurricanes East
Florida 2004
This damage occurred along the East coast of Florida in 2004.
123
Common Causes of Damage
Steel-frame Buildings:
•
•
•
•
•
•
•
Inadequate stud gauge (strength)
Inadequate construction & fastening of corners
Inadequate fastening of wall sheathing
Inadequate fastening for roof sheathing
Inadequate fastening of roof to walls
Inadequate fastening to foundation
Lack of shear diaphragms to transfer loads
What about steel-frame buildings? Their problems are very similar to that of wood-frame:
•Inadequate stud gauge (strength)
•Inadequate construction & fastening of corners
•Inadequate fastening of wall sheathing
•Inadequate fastening for roof sheathing
•Inadequate fastening of roof to walls
•Inadequate fastening to foundation
•Lack of shear diaphragms to transfer loads
124
Hurricane Charley –
West Florida 2004
This is a photograph of a commercial building along Florida’s Gulf Coast in 2004 following
the passage of Hurricane Charley.
125
Hurricane Charley –
West Florida 2004
This is another picture from Hurricane Charley. Yet, the same type of damage was observed
following the other three hurricanes that struck Florida in 2004
126
Damage is Preventable
Most damage is preventable through proper
design and construction!
Proper design and construction requires:
• Basic understanding of wind loads;
• Basic understanding of wind-resistive construction;
• Commitment to properly design and construct by each
of the players in the construction process!
Most damage is preventable through proper design and construction!
Proper design and construction requires:
•Basic understanding of wind loads;
•Basic understanding of wind-resistive construction;
•Commitment to properly design and construct by each of the players in the
construction process!
127
Hurricane Charley Eye –
northern tip Pine Island
Florida 2004
This photo illustrates this point. This house on the Northern tip of Pine Island is virtually
unscathed, yet the eye of Hurricane Charley went right through this area.
128
Hurricane Charley Eye –
northern tip Pine Island
Florida 2004
This is another house that is virtually unscathed following Hurricane Charley.
129
Hurricane Charley Eye –
northern tip Pine Island
Florida 2004
And, yet another example. This is one of several wood-frame apartment buildings that
suffered very little, if any, damage.
130
Wind Design
Proper design of wood structures to resist high wind loads requires the correct use of wind
load provisions and member design properties. A thorough understanding of the interaction
between wind loads and material properties is important in the design process.
131
Wind Basics
LIFT
WIND
PRESSURE or SUCTION
Reactions
UPLIFT
OVERTURNING
SLIDING
SHEAR
Wind-structure interaction is highly complex. Wind can induce a variety of structural
responses as a whole building, and on individual components and assemblies, as seen
here. Each of these responses needs to be checked for structural integrity as part of the
wind design process.
132
Building Code Requirements
Generally, building codes require:
• Buildings to be designed for the 50-year wind
event - that is - a 2% probability that design
winds will be met or exceeded in any one year.
– For hurricane prone regions, the probability is based
on a Class III storm.
– Wind speeds are measured at 33 ft. (10 meters)
above ground for open terrain.
Generally, building codes require buildings to be designed for the 50-year wind event - that
is - a 2% probability that design winds will be met or exceeded in any one year. For
hurricane prone regions, the probability is based on a Class III storm. Wind speeds are
measured at 33 ft. (10 meters) above ground for open terrain.
133
Wind Design
For 1- and 2-family dwellings, wind
resistive design is usually based on:
• Simplified or analytical procedures of ASCE
7-98, or
• Low rise wind load provisions of the IBC (2000
IBC and 1997 NDS) or
• ANSI/AF&PA 2001 WFCM – engineered and
prescriptive.
The NDS provides factors to adjust design values for wood
members and connections for specific conditions frequently
encountered in service. It does not set forth general requirements
for adjusting design values for all possible applications and
related conditions of use, particularly those involving extreme
loading and service exposures (see 1991 Commentary 2.1.2 for
example). To include all conditions in this manner would require
use of overly conservative and economically prohibitive
adjustment factors not required for most applications. However, it
is the designer's responsibility to determine the design value
adjustment factors that are appropriate for each application.
134
Wind Design
Analysis of low rise wood frame buildings
review wind load provisions in accordance
with the IBC
• reference: 2000 IBC and ASCE 7-98
design examples for designing a low rise
wood structure to resist IBC wind loads
• reference: AF&PA 2001 WFCM
The NDS provides factors to adjust design values for wood members and connections for
specific conditions frequently encountered in service. It does not set forth general
requirements for adjusting design values for all possible applications and related conditions
of use, particularly those involving extreme loading and service exposures (see 1991
Commentary 2.1.2 for example). To include all conditions in this manner would require use
of overly conservative and economically prohibitive adjustment factors not required for most
applications. However, it is the designer's responsibility to determine the design value
adjustment factors that are appropriate for each application.
135
ASCE 7-98 Wind
based on 3 Second Gust measurement.
hurricane wind speed contours based on 500year recurrence on Atlantic/Gulf coast.
hurricane wind speeds adjusted from 500-year
to 50-year recurrence for use with ASD.
• divide 500 year values by 1.6 to get 50 year value
Hurricane Importance Factor built into map.
There are varying wind load provisions in local, state and model building codes currently
used in the United States. Most of these provisions are based on wind engineering research
conducted over the last 50 years. Proposals to change current code provisions are the result
of interpretations of new state-of-the-art wind engineering research.
The wind load provisions of the national load standard ASCE 7-98 Minimum Design Loads
for Buildings and Other Structures include general wind load provisions which, in turn, are
used as the basis for wind load requirements in most U.S. building codes. For the purposes
of this paper, the references to wind loads in this article have been limited to the provisions
found in ASCE 7-98.
Wind Load Provisions
Design wind load provisions in ASCE 7-98 are based on wind speed data collected during
severe wind events in the United States. The wind speed contours provided in ASCE 7-98
are presented in terms of three second gust. Three second gust wind speed is based on the
peak wind speed at a given height and exposure averaged over 3 seconds. The three
second gust wind speed data has been statistically adjusted to a 50-year recurrence interval
with an average annual probability of occurrence of 2 percent. The data has also been
adjusted to a reference height of 33 feet and Exposure Category B, which assumes a flat,
open terrain with scattered obstructions. The wind load provisions of ASCE 7-98 provide
adjustments for variations from reference conditions such as increased wind speeds during
hurricane events, different exposure conditions, different elevations, and localized peak
gusts.
136
Design Gust Wind Speeds (mph)
ASCE 7-98 /
IBC
3-second
Gust Wind
Speeds
Batts, M. E., Cordes, M. R., Russell, L. R., Shaver, J. R. and Simiu, E. (1980). “Hurricane Wind Speeds in the United States”,
National Bureau of Standards, Report Number BSS-124, U.S. Department of Commerce.
Durst, C. S. (1960) “Wind Speeds over Short Periods of Time”, Metorological Magazine, Vol. 89, pp. 181-187
ESDU (1983). “Strong Winds in the Atmospheric Boundary Layer, Part 2: Discrete Gust Speeds”, Engineering Sciences Data Unit
Item No. 83045, London, England.
Georgiou, P. N., Davenport, A. G. and Vickery, B. J. (1983). “Design Wind Speeds in Regions Dominated by Tropical Cyclones”, 6th
International Conference on Wind Engineering, Gold Coast, Australia, 21-25 March and Auckland, New Zealand, 6-7 April.
Georgiou, P. N. (1985). “Design Windspeeds in Tropical Cyclone-Prone Regions”, Ph.D. Thesis, Faculty of Engineering Science,
University of Western Ontario, London, Ontario, Canada.
Krayer, W. R., and Marshall, R. D. (1992). “Gust Factors Applied to Hurricane Winds”, Bulletin of the American Meteorological
Society, Vol. 73, No. 5, pp. 613-617.
Vickery, P.J., and Twisdale, L. A. (1995a). “Wind-Field and Filling Models for Hurricane Wind-Speed Predictions”, Journal of
Structural Engineering, Vol. 121, No. 11, pp. 1700-1709.
Vickery, P.J., and Twisdale, L. A. (1995b). “Prediction of Hurricane Wind Speeds in The United States”, Journal of Structural
Engineering, Vol. 121, No. 11, pp. 1691-1699.
Vickery, P.J., and Skerlj, P. F. (1999). “Hurricane Gust Factors Re-Visited”, Submitted to Journal of Structural Engineering
Vickery, P.J., Skerlj, P. F. and Twisdale, L. A.. (1999). “Simulation of Hurricane in the United States Using an Empirical Storm Track
Modeling Technique”, Accepted for publication in Journal of Structural Engineering
137
ASCE 7-98 Overview of Wind Loads
Three design method options
Ch 3
• 6.4 - Method 1 - Simplified Procedure WFCM
Prescriptive
Ch 2
• 6.5 - Method 2 - Analytical Procedure WFCM
Engineered
• 6.6 - Method 3 Wind Tunnel Procedure
ASCE 7 contains three methods for determining wind loads for design. Prescriptive design
requirements in the 2001 WFCM make use of the Simplified Procedure. However,
engineered design typically uses Method 2, the Analytical Procedure.
138
Wind Speeds
Historically, wind speed data was in fastestmile wind speeds – 1-minute mean.
• Inherent flaw due to variable averaging time
Beginning in the 1970’s the collection of
fastest-mile data was phased out in favor of
3-second peak gust data.
• Constant averaging time of 3-seconds
Historically, wind speed data was in fastest-mile wind speeds – 1-minute mean. This is an
inherent flaw due to variable averaging time.
Beginning in the 1970’s the collection of fastest-mile data was phased out in favor of 3second peak gust data which is a constant averaging of wind speed over a time interval of 3seconds.
139
Wind Speeds Vary with Time
This printout of a anemometer shows the variance that occurs in a wind event.
140
Wind Speeds
Different wind speed methodologies based on
different duration of measurements.
• Fastest-mile wind speeds include gust factors.
New codes include conversion tables for
determining equivalencies
V-3s 110 120 125 130 150
V-fm 90 100 110 120 130
The different wind speed methodologies that have been used in the building codes over the
years are based on different duration of measurements. Current code provisions are based
on 3-second gust wind speeds. Fastest-mile wind speeds included gust factors.
Codes commonly include conversion tables for determining equivalencies of the two
methods as a means by which to continue the use of various prescriptive design and
construction standards.
141
ASCE 7-98 Exposures
A Center of large cities (not for Simplified Method)
B Suburban, use as DEFAULT
>60% to 80% of all buildings are in this category
C Open country, 1500 ft transition zone
D Over water, EXCEPT on hurricane coast due to surface
roughness of the water surface
Here are the exposure categories as identified in ASCE 7-98. Suburban exposure B is the
DEFAULT. Other categories are used where special condition warrant. WFCM 2001 tables
are based on exposure B. However, tables are provided for exposure C in an Appendix.
142
ASCE 7-98 Exposures
Exposure A
Exposure C
(<1500’ of B)
Exposure B
Urban
Exposure D
Exposure A is rare and should be used with extreme caution. Consideration is being given
to elimination of this category completely. Exposure B is the default for the WFCM 2001,
and an appendix provides tables for Exposure C conditions.
143
ASCE 7-98 Exposures
Exposure A
Large city center with at
least 50% of the buildings
having a height in excess of
70 ft. The subject building
must have this terrain
upwind for at least 1/2 mile
or 10 times the height of the
building, whichever is
greater.
Exposure A is rare and should be used with extreme caution. Consideration is being given
to elimination of this category completely.
144
Exposure B Urban
Exposure B
Urban
Urban area with numerous closely
spaced obstructions having the size of
single family dwellings or larger. For all
structures shown Exposure B extends
more than 1500 ft. or 10 times the height
of the structure, whichever is greater, in
any wind direction.
145
Exposure B Suburban
Suburban residential area with
mostly single family dwellings.
Structures in the center of
photograph have an Exposure B
terrain greater than 1500 ft. or
10 times the height of the
structure, whichever is greater,
in any wind direction.
Exposure B
Suburban
146
Exposure B Urban w/ Hole
Exposure B
with a Hole
Structures in the foreground are subjected to an
Exposure B terrain. Structures in the center top
adjacent to the clearing on the left, which is greater than
1500 ft. in length, are subjected to an Exposure C when
wind comes from the left over the clearing.
147
Exposure C
Exposure C
(<1500’ of B)
Open terrain with scattered
obstructions having heights
generally less than 30 ft. For
most wind directions, all
structures in the photo are less
than 1500 ft. or 10 times the
height of the structure from an
open field, preventing the use
of Exposure B.
148
Exposure C Classic
Flat open grassland with scattered
obstructions having heights generally less
than 30 ft.
Exposure C
149
Exposure D
Exposure D
A building at the shoreline
(excluding shorelines in
hurricane prone regions) with
wind flowing over open water
for a distance of at least one
mile. Included in Exposure D
are inland waterways, lakes
and coastal areas in CA, OR,
WA, AK.
150
WFCM 2001 Provisions - Wind
per 2000 IBC provisions
85 - 150 mph wind speed (3 second gust)
Exposure B & C
Exceptions:
• Main Wind Force Resisting System (MWFRS) not
checked for 10 psf minimum load on building
vertical projected area (under review by ASCE 7)
• shearwall and diaphragm designs shall not be less
than tabulated requirements for 100 mph wind
speeds
Wind provisions for the 2001 WFCM are per IBC 2000 provisions: 85 - 150 mph based on 3
second gust.
Note that in the WFCM 2001, the main wind force resisting system (MWFRS) in not checked
for a 10 psf minimum load on a building vertical projected area as required by IBC 2000.
This is currently under review by ASCE 7 Committee with the expectation that the 10 psf
requirement will drop to a much lower magnitude.
Further, shearwall and diaphragm designs must resist 100 mph wind speeds according to
the WFCM 2001.
151
Wind Load Resolution
Load Path
Remember this? This is a graphical illustration of how wind load moves through various
building assemblies and gets transferred into the ground. Note the important components:
the windward wall, the roof diaphragm, and the side shear walls with associated shear and
hold-down anchorage devices.
152
WFCM 2001 Provisions - Wind
Components and Cladding (C&C)
Main Wind Force Resisting Systems (MWFRS)
Found in:
• lateral provision tables in Chapters 2 (engineered) and 3
(prescriptive)
There are two systems that are considered in wind design: components and cladding
(C&C), and Main Wind Force Resisting System (MWFRS), with correspondingly different
design approaches. Applicable provisions for each are found in both prescriptive and
engineering provisions of the WFCM.
153
WFCM 2001 Provisions - Wind
components and cladding
• Table 2.4
Example:
roof and wall sheathing
(out of plane)
Under C&C design, Table 2.4 governs for components such as roof and wall sheathing
subjected to out-of-plane wind loading.
154
WFCM 2001 Provisions - Wind
Main Wind Force Resisting systems
• Table 2.5 a, b
Example:
studs, beams, sheathing
(primary structural skeleton)
However, MFWRS provisions are manifested in Tables 2.5 A and B for much larger primary
structural skeleton assemblies such as roof or floor diaphragms.
155
WFCM 2001 Provisions - Wind
Roof and Wall Sheathing /
Cladding for Panels are
found in Chapter 3 – Tables
3.12A, 3.13A & B
For panels, wall sheathing and cladding data is found in Chapter 3.
156
WFCM 2001 Provisions - Wind
From 1.3 Definitions:
Cladding
Exterior structural elements that receive loads
normal to their surface. (i.e. exterior finish
materials)
Sheathing
The structural covering used directly over framing
members such as studs, joists, or rafters, which
transfers perpendicular loads to the framing
material.
The difference between cladding and wall sheathing is subtle – here are the WFCM 2001
definitions.
157
WFCM & Wind Behavior
LIFT
WIND
PRESSURE or SUCTION
Reactions
UPLIFT
OVER
TURN
I
NG
SLIDING
SHEAR
The Wood Frame Construction Manual takes these wind basics fully into consideration to
provide an engineered design of wood-frame buildings for wind speeds up to 150 mph.
158
Concrete-masonry residence
Hurricane Charley
Burnt Shore Road, south of Punta Gorda, FL
Properly designed and constructed
This is a concrete-masonry building under construction along Burnt Store Road south of
Punta Gorda where Hurricane Charley caused extensive damage. Except for severely
twisted interior steel studs and the loss of dry-in felt, no damage occurred. This building is
properly designed and constructed.
159
Wood frame residence
Hurricane Charley
Bokeelia, FL
Properly designed and constructed using WFCM
This wood-frame building in Bokeelia under construction only suffered loss of building paper
and dry-in felt. This building complies with the design provisions of our Wood Frame
Construction Manual.
160
Outline
Purpose & Background
Development process
Code Acceptance
Document layout
Design provisions:
•
•
•
•
Snow
Shear walls – “Standard” Wall
Wind
Seismic
Design Examples
Now let’s discuss load resistance behavior with WFCM provisions for seismic loads.
161
Seismic Issues
Properly designed buildings can also withstand most seismic forces. Seismic loads arise
from ground shaking which can be a result of many causes. Earthquakes by far are the
most serious and unpredictable seismic load type. Other more predictable seismic loads
arise from human-induced activities.
162
Seismic Loads
ground shaking:
•
•
•
•
earthquake
railway tracks/trains
heavy machinery
manufacturing
(punch presses)
• foot steps
Seismic loads arise from ground shaking which can be a result of many causes.
Earthquakes by far are the most serious and unpredictable seismic load type. Other more
predictable seismic loads arise from human-induced activities.
163
Seismic Loads
time-varying
ground motion
• distance
• time
f(x,t)
how does this
translate to a load?
The primary feature of seismic motion is that its magnitude varies with time. The time
variation can be very short, such as a sharp jolt, or longer, such as a slow rumble.
Moreover, the motion direction is typically random and constantly changing. Such a
behavior can be described in terms of a wave.
Using recorded seismic data, we can describe a seismic motion mathematically in terms of a
wave function of distance and time.
164
Wave Characteristics
amplitude
frequency / period
acceleration
duration
amplitude
time
Period (1 cycle)
Waves have three primary characteristics:
•amplitude (the magnitude of the wave),
•frequency (the number of complete wave cycles per second) or inversely its period (the
number of seconds per complete wave cycle), and
•duration (the time lapse of the wave).
165
Wave Characteristics
damping (wave decay)
acceleration
damping ratio (%)
time
Damping describes the decay rate of the wave amplitude as the wave “dies” out. Friction in
the wave generating system is an example that causes waves to damp.
166
Building Characteristics
mass
stiffness
damping
natural frequency of vibration
x
mass
stiffness
displacement
time
damping
m
∂x
∂2 x
+ c + kx
2
∂t
∂t
With respect to dynamic response, buildings offer three primary characteristics:
•mass of the building or sub-assemblies,
•stiffness of the building structural system,
•and damping inherent in the building construction.
These can be simply modeled as the “lollypop” shown here. If the stick of the lollypop is
sufficiently thin (low stiffness), and the mass is pulled back and released, the mass will
swing back and forth in free motion. The free sway motion can be described by the mass
displacement wave shown here, with measurable frequency. This simple sway mode is
known as the natural frequency of vibration. Mathematically, the sway motion equation
takes the form of a second order differential equation with respect to time.
167
Building Response
Equation of motion
response
demand
m&x& + cx& + kx = f ( x, t )
mass inertia
system
damping
system
stiffness
disturbance
In the motion equation, all the components of the dynamic structural behavior are evident.
The equation here is written in terms of linear displacement, x, although angular
displacement terms (not shown) and other directional displacements may be present.
Solution techniques for this equation exist mathematically.
168
Building Response
Motion Modes
fundamental / natural / 1st mode
2nd mode
3rd mode
etc
Solution of the motion equation can lead to a modal result: a series of frequencies at which
the structure will freely vibrate if disturbed. An example of this solution can be heard when a
guitarist uses fret harmonics (octave pitches) to tune a guitar. In a building, the sway shape
takes different forms that correspond to the modal frequencies in the solution. Thus, a
structure can have a number of sway modes with associated frequencies of vibration.
169
Force Demand (Load)
inertial force = mass x acceleration
..
F=ma=mx
x
mass
stiffness
damping
..
acceleration a = x
Let’s put the whole seismic problem together now. Ground shaking occurs with a certain
acceleration, a, moving the soil under the building. As the soil moves, the building mass
wants to stay put due to its inertia, putting a force on the structure equal to the mass times
the exciting acceleration. Eventually the mass moves, lagging the exciting acceleration,
causing further inertial forces to develop on the structure. This gets even more problematic
when the exciting acceleration changes direction, as the mass wants to keep moving
(through inertia) in the the original direction of mass movement. This is sometimes referred
to as the whipping force.
170
Resonance
seismic frequency = building frequency
D
T
S
E
C
U
R
O
TI
N
=
If the exciting wave characteristics match any of the building’s modal wave characteristics,
then resonance results when the exciting and response systems vibrate in unison.
Resonance is very dangerous since the response system normally self-destructs due to its
inability to cope materially with the exciting wave. Hence, it is very desirable from a building
design perspective to separate building modal response frequencies from any potential
exciting frequencies.
171
Building Response Motions
planar sway (racking)
torsional twist
racking
center of stiffness
twist
Buildings move under dynamic conditions. Two principle movements are: racking and twist.
We’ll talk more on twist in a minute.
172
Resisting Elements - Diaphragms
horizontal force transfer
• in-plane shear
Vmax at edges
Structures have horizontal surfaces that can be used to transfer loads applied laterally to the
structure. These surfaces, transferring in-plane loads are commonly called diaphragms, and
can be characterized as a wide, flat deep beam. An inertial mass load can originate in the
surface/diaphragm and transfer the same way. Resulting shear forces develop across the
surface, with maximum values occurring at the supported edges of the surface. These
maximum “reaction” forces are the lateral forces that are transferred into the vertical building
elements below, causing them to rack..
173
Resisting Elements - Shearwalls
racking resistance
perimeter nailing
sliding
resistance
These lateral “racking” forces applied at the top of the wall can be picked up by shearwalls.
In a wood shearwall, the panel perimeter nails provide the bulk of the racking resistance
through wood bearing and nail deformation when the lateral external force is applied.
Horizontal wall sliding is resisted by nailing or other anchorage installed along the bottom of
the shearwall sufficient to resist the external lateral force.
174
Resisting Elements - Shearwalls
panel aspect ratio
≤ 3.5
1
racking action
cantilever
beam action
In order to make this concept work, panels must have a height-to-width aspect ratio of less
than 3.5 to 1. This ratio is sufficient to develop “racking action” in the shearwall panel.
Aspect ratio’s greater than this produce cantilever beam action - a completely different
behavior that is much less effective in resisting lateral forces, as mentioned previously.
175
Resisting Elements - Hold Downs
overturning
Finally, overturning may be a problem, especially for high aspect ratios. These skinny
panels usually develop high overturning forces at the bottom corners of the walls that need
to be resisted with the installation of special hold-down hardware.
176
Resisting Elements - Diaphragms
twist
mass
center
mass center ≠ stiffness center
stiffness
center
eccentricity
Building forms impact how lateral forces get transferred into the vertical supporting
elements. Here’s an example. A floor has a center of mass located somewhere in it. The
structural system below provides a torsional stiffness that can also be centered somewhere
within the floor plane. If the stiffness and mass centers coincide, then the building will simply
rack in the direction of the applied lateral load. If however, the mass and stiffness centers
are displaced, the building frame will twist. The greater the displacement, the greater the
twist. The diaphragm reactions transferred to the top of the shearwalls can also become
very large. Thus, good design for lateral performance would suggest that centers of mass
and stiffness be kept in as close proximity to each other as possible.
This subject is important for a rigid analysis where the stiffnesses of the system components
are known. The WFCM 2001 assumes a flexible analysis: flexible components that lend to a
tributary area approach for the loads.
177
Building Response
The theory of light and strong...
There is another way to resist lateral forces - a technique that dates back to early human
inhabitation some 10,000 years ago. Archaeological findings prove the theory of light and
strong in known seismically active areas of the earth. The theory holds that humans
discovered early that heavy things fall down easily when disturbed with catastrophic results.
Light things are not disturbed nearly as easily, and are much easier to support and be made
strong. Thus, the simple tent has become a common domestic structure to many peoples of
the earth in regions that are seismically active, even to this day. Wood frame structures tend
to fit this philosophy, mainly because of wood’s very high strength-to-weight ratio.
178
Load Provisions - ASCE 7-98
The seismic problem statement:
seismic event
site conditions
building type and use
ASCE 7-98 provides the data and methodology to determine loads for buildings - seismic
loads in this case. The parameters to do this include: data on the design seismicity, site
sub-surface conditions, and building type and intended use.
179
Load Provisions
seismic maps &
ground
accelerations
• examine for short
periods Ss
and
• 1 second period S1
ASCE 7-98 provides seismic maps of the US contoured in terms of %g (% gravity at 32.3
ft/s2) ground acceleration. Two such maps provide for short period seismicity, and 1 second
period seismicity, respectively. Both will be needed to determine which governs later on.
180
Load Provisions
site conditions
Sub-terrainean site conditions are presented in terms of a site class letter. Characteristics of
each site class are given in terms of soil bearing capacity, penetration number, or sonic
velocity criteria - basically to assess underlying soil density.
181
Load Provisions
site conditions coefficients Fa and Fv
The acceleration-based site coefficient (at 0.3s period) , Fa, and velocity-based site
coefficient (at 1.0s period) are determined from these ASCE 7-98 tables.
182
Load Provisions
building type and use
These ASCE 7-98 tables arrive at the determination of the building class for seismic design
purposes. The building class sets out which method is to be used for determining the
respective loading.
183
Load Provisions
seismic design category
S DS =
2
Fa S s
3
S D1 =
2
Fv S1
3
The design 5% damped spectral response acceleration at short periods, and the
corresponding one for 1 second periods is determined from these formulae, using site
accelerations and site conditions coefficients as input. Using the spectral acceleration
numbers with the seismic use group for the building in the tables, yields the seismic design
category for our structure, on our site. The seismic design category is important as it is used
widely in the AF&PA 2001 Wood Frame Construction Manual.
184
Load Provisions
seismic design
category
http://earthquake.usgs.gov/research/hazmaps/design/
…or you can go to the USGS website and use their calculator to find a seismic design
category for any building by longitude/latitude or by zip code.
185
WFCM 2001 Provisions - Seismic
per 2000 IBC provisions
seismic design categories
A-D
WFCM considers seismic loads for design categories A to D.
186
WFCM 2001 Seismic Provisions
Chapter 2 Engineered
• use with seismic design categories A to E
Chapter 3 Prescriptive
• use with seismic design categories A to D
The 2001 WFCM Engineering and Prescriptive provisions make use of the seismic design
categories from ASCE 7-98. Note that prescriptive provisions only apply up to category D,
and engineered provisions up to category E. Categories above E are beyond the scope of
the WFCM.
187
Load Provisions
Story shears distribution (Equivalent Lateral Force)
ASCE 7-98
roof
floor1
WRD
FRD
WFD1
FFD1
Vwall1
Vwall2
floor2
WTOT
building masses
V base shear
story forces
story shears
WFCM Chapter 2 Engineering provisions are based on the ASCE 7-98 Equivalent Lateral
Force procedure. Building masses/weights are calculated and collected at floor plane levels.
The seismic event base shear is calculated from the seismic loads and distributed on the
basis of weights at each story as story forces. Finally, story shears are determined for each
floor by adding all the story forces above the floor of interest. The story shears are the
forces applied to the top of the lateral force resisting system at each floor level.
188
WFCM 2001 Provisions - Seismic
Engineered seismic
provisions
• includes: vertical
distribution of shear,
consistent with 2000
IBC requirements
WFCM engineering provisions include vertical distribution of shear that is consistent with
IBC 2000 requirements.
189
WFCM 2001 Provisions - Seismic
Prescriptive seismic
provisions
• based on IBC Simplified
Design Method
• seismic loads are increased by
factor of 1.2
– vertical distribution of shear,
redundancy, etc. included
through 1.2 factor
The prescriptive provisions are based on the IBC Simplified Design Method. Seismic loads
are increased by a factor of 1.2 to include the effects of vertical shear distribution,
redundancy, etc.
190
WFCM Provisions - Seismic
Here’s how the 2001 WFCM
does this in detail:
Chapter 2 Engineered
Chapter 3 Prescriptive
Let’s take a look at how the WFCM provisions for seismic are presented, both for
engineered and prescriptive design processes.
191
WFCM 2001 - Chapter 2 Engineered
obtain acceleration data from maps, enter this chart…
extract Cs
Chapter 2 facilitates the determination of story shears. First, enter Table 2.5C at the bottom
with the seismic design category, and the spectral response accelerations SDS and SD1.
Extract the pertinent value of the seismic response coefficient, CS. Consider the notes in the
bottom of the table if they apply.
192
WFCM 2001 - Chapter 2 Engineered
extract weights and
adjustments for
vertical distribution
of forces for walls
and horizontal
assemblies
On the same table in the cells above, calculate heights hi and weights Wi at each level using
the formulae provided, and the vertical distribution factor Cv for each level.
193
WFCM 2001 - Chapter 2 Engineered
determine total
shear capacity
required at each
level for direction 1
Table 2.5-1
ASCE 7-98 instructs the designer to consider seismic forces from two orthogonal directions.
Table 2.5-1 helps the designer determine the story shears for loads applied perpendicular to
the ridge from the data just previously calculated from Table 2.5c.
194
WFCM 2001 - Chapter 2 Engineered
determine total shear capacity
required at each level for the
orthogonal direction 2
Table 2.5-2
Table 2.5-2 calculates the story shears in the other direction parallel to the ridge. Observe
the notes in the bottom of each table. Proceed now to select assemblies to meet these load
demands.
195
WFCM 2001 - Chapter 3 Prescriptive
applies to Chapter 3
buildings only
table comes up with
minimum length of
sheathing required
for a given wall for
seismic load
based on ASCE/IBC
Simplified Design
Procedure
This Chapter 3 table applies to Chapter 3 buildings and gives the minimum length of
sheathing required for a given wall for seismic load in a particular seismic design category.
The C2 is a geometry coefficient that accounts for non-square buildings. The seismic base
shear is increased by 20% to account for vertical distribution of seismic forces, according to
the ASCE/IBC Simplified Design Procedure.
196
WFCM 2001 - Chapter 3 Prescriptive
Prescriptive
seismic provisions
• tables based on
assumed structure
dead load
– adjustments
provided for roof,
wall, and floor
loads alternative
to dead load
assumptions
WFCM tables are based on a presumed dead load, but if this varies, then adjustment
equations are included to adapt the tabled values. The footnotes amend table data for
masses other that those assumed in the Table. The roof and floor loads must be higher
than 15 psf for dead load and the adjustment factors only increase this value. The wall
loads begin at 15 psf and only go down in adjusted value.
Knowing now the percent of full height sheathing, simply allocate the same amount to the
shearwall lines in each direction of the building.
197
Outline
Purpose & Background
Development process
Code Acceptance
Document layout
Design provisions:
• Wind
• Snow
• Seismic
Design Examples
• Snow Design
• Wind shearwall design
• Seismic shearwall design
Let’s look now at a few examples in depth for snow, wind and seismic design using the
provisions of the 2001 WFCM.
198
Design Example 1
Snow Design
Given:
Geometry:
2x8 RAFTERS @ 16” OC
Materials:
SPF #2
Loads:
Dead
10 psf
Live
30 psf
Performance:
ceiling not attached to rafter
ΔLL ≤ L / 180
Find the maximum permissible span, L.
Find the maximum permissible span for these 2x8 rafters on 16” OC. Loads and response
criteria are given above.
We’ll resolve this design two ways: by direct calculation, and by using WFCM tables.
199
Design Example 1
Snow Design
Solution (by calculation):
Design Values - 2001 NDS Supplement or 2001 WFCM Table 4A
SPF #2
– Fb = 875 psi
– E = 1.4 million psi
Adjustment Factors
2x8 Rafters
– CD = 1.15 (snow)
– CF = 1.2
– Cr = 1.15
A first step is to determine the design values for #2 SPF rafters. Using Table 4A of the
Supplement chapter, the base values for Fb and E are determined.
Also referencing Table 4A, the adjustment factors for 2x8 rafters is determined.
Table 4A of the Supplement chapter is the same as Table 4A of the NDS Supplement.
200
Design Example 1
Snow Design
Solution (by calculation - ASCE 7-98):
wtotal = wdead + wsnow
wsnow = Cs pf = (1.5)[(0.7) Ce Ct I pg]
where
Cs = roof slope factor = 1.5
pf = flat roof snow load
Ce = exposure factor = 1.0
Ct = thermal factor = 1.1
I = importance factor = 1.0
pg = ground snow load
Now determine the tributary load for a rafter due to the pressure loads. Here is the basic
snow load equation from ASCE 7-98, that converts ground snow load to roof snow load.
Note that this equation is used for balanced snow loads. Other unbalanced cases may need
to be considered if drifting is an issue.
201
Design Example 1
Snow Design
Solution (by calculation - ASCE 7-98):
Loads:
wdead = 10 psf (16in. / 12) = 13.33 plf
wsnow = Cs pf = (1.5)[(0.7) Ce Ct I pg]
= (1.5)(0.7)(1.0)(1.1)(1.0)(30 psf)(16 in. / 12)
= 46.2 plf
wtotal = 13.33 + 46.2
= 59.5 plf
Placing the numbers in the equation and adding in the dead load leaves us with 59.5 plf total
load on a rafter.
202
Design Example 1
Snow Design
Solution (by calculation):
Strength:
Fb’
= Fb CD CF Cr
where
Fb' ≥
CD = load duration factor = 1.15
(NDS 2.3.2)
CF = size factor = 1.2
(NDS Tables 4A & 4B)
Cr = repetitive member factor = 1.15 (NDS 4.3.9)
w totalL2
8S
where
S = section modulus = 13.14
L = length of span
(NDS Table1B)
Check rafter strength first. Here are the basic equations we use to do this. The first
equation calculates the design material stress in bending for the case we have at hand. The
second equation is the capacity-demand relation in terms of material stress in bending. In
the next slide, we’ll re-arrange this equation to solve for the unknown length, L.
Note that tables from the NDS Supplement are included in the Supplement chapter of the
Wood Frame Construction Manual. Table 1B of the NDS Supplement presents Section
properties of standard dressed (S4S) sawn lumber; while Table 4A of the NDS Supplement
presents adjustment factors and base design values for bending design value, Fb.
203
Design Example 1
Snow Design
Solution (by calculation):
Strength: calculate the moment-limited span:
Fb’
= Fb CD CF Cr
= 875 psi (1.15)(1.2)(1.15) = 1,389 psi
w totalL2
F ≥
8S
'
b
8SFb'
8(13.14)(1389)
L=
=
= 171.6 in.
wtotal
(59.5 / 12)
= 14 ft. 4 in.
Setting in the numbers, we adjust the wood bending stress for on-site conditions. Now, work
the strength capacity-demand relation to isolate and solve for span, L. This is the maximum
span limited by member strength.
204
Design Example 1
Snow Design
Solution (by calculation):
Stiffness: calculate the deflection-limited span:
Δ LL ≤
5 w liveL4
384EI
L=4
384EIΔ LL
384(1.4 x10 6 )(47.63)
=3
= 194 in.
5 w live
5(180)(46.2 / 12)
= 16 ft. 3 in.
Strength governs at 14 ft. 4 in.
Now check stiffness. Again, isolate span L in the displacement relation. This answer is
higher than the strength result, so the problem is strength-constrained: use the lower of the
two values.
Because these spans are long, and loading relatively light, shear should not constrain the
design further.
In almost all circumstances, lumber beam deflections are calculated using the conventional
deflection formulas found in general engineering handbooks. For spans with uniformly
distributed loads the deflection equation is δ = 5wL4/384EI, where w = uniformly distributed
load in pounds per inch of span, L = span in inches, E = modulus of elasticity, I = moment of
inertia in inches (Table 1B or bd3/12 for rectangular sections), and δ = deflection in inches.
205
Design Example 1
Snow Design
Solution (by WFCM Table):
L = 14 ft. 4 in.
Here’s another approach that you can use without a calculator or inverting equations. Go to
WFCM 2001 Table 3.26c for Rafter Spans for 10 psf DL and 30 psf LL. Enter the table from
the top with the cross-section size at 2x8 (green), and the left side of the table with the rafter
spacing at 16 inches. After rafter spacing, select the appropriate species and grade: SPF
#2. Read across the table row (yellow) intersecting the 2x8 column (green) and read a span
of 14 ft. 4 in.
206
Design Example 1
Snow Design
Solution (by WFCM Table 3.12B) - Panel:
Any of these panel
products will work
since the actual rafter
spacing is 16” OC
Roof panel selection is most easily obtained from Table 3.12B for snow, and it is easily seen
that with the loads in this example, any of the listed panel products will work since the actual
rafter spacing is 16 inches OC.
207
Outline
Purpose & Background
Development process
Code Acceptance
Document layout
Design provisions:
• Wind
• Snow
• Seismic
Design Examples
• Snow Design
• Wind shearwall design
• Seismic shearwall design
Let’s continue with a final example on seismic and shearwall design using the provisions of
the 2001 WFCM. First, a little background on seismic design methodology.
208
Design Example 2 - Prescriptive
2nd Level Wall of this home:
Seismic
SDC D1
Wind
120 mph
Here’s a design example right out of the WFCM Workbook - the second level wall of this
house that is in Seismic Design Category D1 and a 120 mph wind load. We’ll design the
north (back) level 2 wall for seismic and wind respectively, then compare results. We’ll
consider both segmented and perforated shearwall variations in the solution. And we’ll use
the Chapter 3 Prescriptive provisions of the 2001 Wood Frame Construction Manual.
209
Design Example 2 - Level 2 Plan
4’
4’
4’
4’
Walls on this
floor are 9 ft tall
Roof slope
12:12 so roof
space is
considered an
additional story
(front of
Chapter 3)
Here is the dimensioned second floor plan….
210
Design Example 2 - Level 1 Plan
…and the first floor plan. We’ll be needing these dimensions. The second level wall is 9
feet tall.
211
Design Example 2 - Wall
40’
9’
4’
4’
6’
6’
4’
4’
north elevation
Length of perpendicular wall
Perimeter edge nail spacing
Length of full height sheathing (Lfull height)
Exterior Type I
Shearwalls
(segmented)
(WFCM 3.4.4.2)
shaded areas
show potential
eligible shearwall
panels
32 ft
6”
28 ft
The roof has a steep pitch - steep enough to warrant it being considered a story unto itself.
The shaded areas shown on this level 2 north wall elevation represent the potentially
available shearwall panels since their aspect ratios are less than 3.5 to 1. Their lengths
along the bottom of the wall sum to 28 ft.
212
Design Example 2a - Seismic D1
Two stories
braced (roof
and level 2)
Building Level 2
Lmin = 32’
Lmax = 40’
In seismic design, these overall level 2 floor dimensions are important. We assign the the
biggest dimension to Lmax, and the smaller to Lmin.
213
Design Example 2a - Seismic D1
Two stories
braced (roof
and level 2)
Table 3.17C
C1 = 57
C2 = 15
Using Table 3.17C from the 2001 WFCM, we extract the coefficients C1 and C2. Do this by
first locating the wall in the drawings in the left side of the table. Since the roof in our
example is a story, we choose the diagram corresponding to Roof plus 1 story. The
intersection of Lmax (left) and the seismic design class (top) yields C1. C2 is found at the
bottom in the SDC column.
214
Design Example 2a - Seismic D1
⎡
⎞ ⎤L
⎛L
SheathingRequired = ⎢C1 + ⎜⎜ MAX − 1⎟⎟C2 ⎥ MIN
⎠ ⎦⎥ 100
⎝ LMIN
⎣⎢
⎡
⎛ 40 ⎞ ⎤ 32
= ⎢57 + ⎜ − 1⎟15⎥
= 19.4 ft
⎝ 32 ⎠ ⎦ 100
⎣
< shearwall panels provided (28 ft) OK
Using the extracted coefficient values from the table, perform the calculation of the formula
at the top of the table to get the length of required full-height sheathing for a Type I
segmented shearwall design. Our wall is sufficiently long, thanks to all the shearwall panels
between the windows.
215
Design Example 2b - Wind
9’
4’
4’
6’
6’
4’
4’
north elevation
Exterior Type I
Shearwalls
(segmented)
(WFCM 3.4.4.2)
shaded areas
show potential
eligible shearwall
panels
Length of perpendicular wall
Perimeter edge nail spacing
32 ft
6”
Length of full height sheathing (Lfull height)
28 ft
Wall height adjustment factor (CWH)
9’/8’ = 1.125
Now let’s consider the same segmented wall, but subject to the 120 mph wind load. The
house is situated in Exposure B site conditions. Since our wall height is 9ft, we must adjust
our solution from the standard 8 ft height by using the CWH factor.
216
Design Example 2b - Wind
Two stories
braced (roof
and level 2)
Building Level 2
W = 32’
120 mph
Exposure B
parallel to ridge
L = 40’
The wind table assigns the overall floor dimensions to different variables: L for the length of
the wall of interest, and W to the width of the building perpendicular to it. Our wall is parallel
to the ridge line of the roof. This criteria helps us select the right wind table….
217
Design Example 2b - Wind
Two stories
braced (roof
and level 2)
Table 3.17B
L = 10.6 ft
…Table 3.17B in this case. Locate on the left hand drawings of the table where the wall of
interest with respect to the building. The intersection of the building width W and the design
wind speed gives the required length of shearwall, L. This L is for a shearwall 8 ft high.
218
Design Example 2b - Wind
Minimum Length Full Height Sheathing - Type I Wall
LType1 = L CWH
= (10.6) (1.125)
= 11.93 ft
< 28 ft shearwall panels provided
OK
Now we adjust L by multiplying it by CWH for the 9ft wall height, to get the length of shearwall
required.
219
Design Example 2a & b - Type II Walls
6’
9’
4’
4’
6’
6’
4’
4’
north elevation
Length of wall (Lwall)
Length of full height sheathing (Lfull height)
Exterior Type II
Shearwalls
(perforated)
(WFCM 3.4.4.2)
shaded areas
show potential
eligible shearwall
panels
40 ft
28 ft
% Full Height Sheathing (Lwall/Lfull height)
70%
Maximum unrestrained opening height
6 ft
If it is desired to use perforated shearwalls as a solution, the additional design steps are
minimal. You must determine Type I length requirements first. From the wall elevation,
determine the % of full wall sheathing, and the maximum unrestrained opening height
(window or door). This data is needed to enter...
220
Design Example 2a & b - Type II Walls
Table 3.17E
Type II
increase
factor
= 1.18
….Table 3.17E to calculate the Type II wall increase factor. At the top left of the table, from
the intersection of the shearwall height and maximum unrestrained opening, read down to
the intersection of the % full height sheathing value (blue). Read off the increase factor.
221
Design Example 2a & b - Type II Wall
Minimum Length Full Height Sheathing (ft) - Type II Wall
LTypeII = LTypeI CL
Wind
Seismic
LTypeI
11.93
19.44
CL
1.18
1.18
LTypeII
14.08 ft
22.94 ft
Table 3.17E
Multiply the increase factor by the required Type I wall lengths from before to get required
wall lengths for Type II perforated shearwalls.
222
Design Example 2a & b - Summary
Minimum Length Full Height Sheathing
Level 2 Wall North Elevation
Type I
segmented
Type II
perforated
Seismic
Wind
Seismic
Wind
19.4 ft
11.93 ft
22.94 ft
14.08 ft
The longest wall here will drive the lateral design which in this
case is caused by seismic forces.
Once we collect all this data, we see that we have sufficient wall length (28 ft) to
accommodate all these possibilities. This design appears to be seismically-driven (largest
required length) with Type II length requirements slightly higher than Type I.
223
Design Example 2a & b - Solution
Assembly details for these panels as described in 2001 WFCM 3.4.4.2
8d common nails
@ 6” OC on panel
perimeter
8d common
nails @ 12” OC
in field
7/16” wood
structural panel
continuous
height over wall
plates
panel exterior
5d cooler nails
@ 7” OC on
panel perimeter
5d cooler nails
@ 10” OC in
field
1/2” gypsum
wallboard on
interior
panel interior
Remember, here are the wall assembly assumptions used for the development of our
solution so far based on the “standard shear wall” – the default wall design. A cooler nail is
also known as a drywall nail. It’s quite unlikely this wall will ever be built this way in the field.
224
Wall Design Modification
Want a different shearwall design? Choose one from Table 3.17 D
and modify the capacity accordingly…
If you don’t want the default wall, other walls are available with associated wall length
modification factors for wind and seismic loads from Table 3.17D. Application of this
information is as easy as multiplying the calculated required default wall length by the length
modification factor for your wall of interest from the Table.
225
Wall Design Modification
Alternative assembly details for these panels are available from Table 3.17D
which modify the wall lengths with factors accordingly.
8d common
nails @ 6” OC
on panel
perimeter
8d common
nails @ 12”
OC in field
7/16” wood
structural
panel
continuous
height over
wall plates
panel exterior
Modified wall length =
standard wall length x
Table 3.17D factor
(wind or seismic)
….and we can modify them through the use of Table 3.17D (see WFCM Workbook for a
detailed example) ….
226
Design Example 2a & b - Solution
…and don’t forget the holdowns from 2001 WFCM 3.4.4.2.3
…and don’t forget the holdowns. You’ll find a handy table in 2001 WFCM Table 3.17F.
227
Design Example 2a & b - Solution
A selection of sliding/uplift anchorage design aids are found
here:
Wind (Exposure B)
Table 3.2 Sill or Bottom Plate Connections (plf capacity)
Table 3.2A Wind Shear Loads - 1/2” & 5/8” Anchor Bolts
(numbers of bolts in shearwall line)
Table 3.2B Wind Shear Loads - 1/2” & 5/8” Anchor Bolts
(bolt spacing)
Table 3.2C Wind Uplift Loads (bolt spacing)
Table 3.4 Rafter/Truss Connections (lbs. capacity)
Seismic
Table 3.3 Sill or Bottom Plate Connections (lbs. capacity)
Table 3.3A Seismic Shear Loads - 1/2” & 5/8” Anchor Bolts
(bolt spacing)
Here’s a handy list for referencing design aids for anchorage against wall sliding and/or
uplift. Roof/truss anchorage are also included in the Table 3.x list, among other assembly
connections. Many of these tables will give a connector capacity, or a connector spacing as
a result.
228
Presentation Summary
Purpose & Background
Development process
Code Acceptance
Document layout
Design provisions:
• Wind
• Snow
• Seismic
Design Examples
• Snow Design
• Wind shearwall design
• Seismic shearwall design
This concludes all the topics presented here.
229
More information...
Wood Frame
Construction Manual
2001 Edition
• detailed calculations
• design examples
• graphics
available now!
The new 2001 WFCM Provisions and Commentary is available now. If you tend to design
one- and two-family housing, this is a design resource that should be on your reference
desk.
230
More information...
Design of Wood Frame
Buildings for High
Wind, Snow and Seismic
Loadings (workbook)
•
•
•
•
real design example
detailed calculations
checklists
blank worksheets for
your use
download free!
This Wood Frame Construction Manual Workbook (WFCM Workbook) provides a design
example, typical checklist, and background information related to design of a wood-frame
structure in accordance with AF&PA’s Wood Frame Construction Manual (WFCM) for One
and Two-Family Dwellings, 2001 Edition. The design example uses plans from a 2-story
residence designed to resist wind, seismic and snow loads. Typically, these load conditions
do not all apply to the same structure (e.g., usually only 2 of these conditions are evaluated
depending on the geographic location and local building code requirements). However, all
three load conditions are evaluated in this example to show the broader range of
applicability of the WFCM. The authority having jurisdiction should be consulted for
applicable load conditions.
The design example is based primarily on prescriptive provisions found in Chapter 3 of the
WFCM. References to page numbers, tables and section numbers are for those found in the
2001 WFCM, unless noted otherwise. Additional engineering provisions or alternate
solutions are provided where necessary.
231
3 Prescriptive Design
Here is an isometric view of the house designed in the WFCM Workbook.
232
3 Prescriptive Design
The WFCM Workbook is filled with very useful tables, checklists, and instructional aids. All
tables are fully referenced to the standard. Blank worksheets are provided for future use.
233
3 Prescriptive Design
The checklists are complete so that nothing is left out of the design…
234
3 Prescriptive Design
… and are arranged by system…
235
3 Prescriptive Design
…the same way the WFCM is laid out.
236
3 Prescriptive Design
Full documentation aids are included, for loads…
237
3 Prescriptive Design
… and displacement limits.
238
3 Prescriptive Design
Assembly plan and elevation views are shown.
239
3 Prescriptive Design
Blank Tables are included for rafter design…
240
3 Prescriptive Design
… as well as a completed table example.
241
3 Prescriptive Design
Same for ceiling/floor framing – blank…
242
3 Prescriptive Design
… and completed.
243
3 Prescriptive Design
Table for floor sheathing design.
244
3 Prescriptive Design
Table for load-bearing wall stud design – blank...
245
3 Prescriptive Design
… and completed.
246
3 Prescriptive Design
Table for top plate design, including splicing points – blank…
247
3 Prescriptive Design
… and completed.
248
WFCM-2001
• Header Example:
• 60 psf ground snow load
• 90 mph wind speed (3-second gust)
• bldg width 36 feet
• exterior load bearing wall header, supporting roof,
ceiling, and one center-bearing wall
• header span of 6 feet
Here is a short header design example taken out of the WFCM Workbook. Design info is as
displayed in the slide.
249
WFCM-2001
• Section 3.4.1.4.1 Headers, p.113• Maximum spans for common species of lumber
headers. . . shall not exceed the lesser of. . . spans
in Tables 3.22A-E and Table 3.23A
• The number of jack studs at each end given in
Table 3.22F
• The number of full height studs at each end of
header given in Table 3.23C
Here is the WFCM verbage on header design, and how to proceed.
250
3 Prescriptive Design
The WFCM Workbook makes it easy by consolidated the design process into a handy table.
251
WFCM-2001, p.187
First, we determine the gravity load resistance of the header from Table 3.22B. Using the
blue inputs, we come out of the Table with the solution in orange: 3 - 2x12’s.
252
WFCM-2001, p.192
Now we check out-of-plane loading (wind) in Table 3.23A, again using the blue inputs to the
table. The solution (in orange) is: 3 – 2x12’s, same as before. If a results are different
between the two tables, the larger result will govern the design.
253
WFCM-2001
• Section 3.4.1.4.1 Headers• Maximum spans for common species of lumber
headers. . . shall not exceed the lesser of. . . spans
in Tables 3.22A-E and Table 3.23A
• The number of jack studs at each end given in
Table 3.22F
• The number of full height studs at each end of
header given in Table 3.23C
Now we need to determine the framing needed to support the header. WFCM shows us
where to go…
254
WFCM-2001
… and illustrations clearly identify all the needed parts.
255
WFCM-2001 Gravity
The Jack Studs support the header carrying gravity load, and these studs are continuous
from the underside of the header to the bottom plate of the wall. Using the blue inputs to
Table 3.22F, the solution in orange requires: 3 Jack Studs at each end of the header.
256
WFCM-2001 Lateral Out-of-Plane
Out-of-plane loads on the header (from wind) need to transfer from the header into the wall
framing. Full height (King) studs next to the header ends, and Jack Studs, are used to do
this. The King Studs act like a point loaded (from the header) beam. Using the inputs in
blue to the King Stud Table 3.23C gives: 6 King Studs required at each end of the header.
While this might seem excessive, consider the load: 120 MPH wind!!! The design gets even
more interesting if the header is dropped in height from the wall top plate!
257
Wall Header Solution
All of this information is summarized neatly in the WFCM Workbook header table. Additions
to this table include sill plate design due to out-of-plane loading (wind).
258
New! - Hot off the press!!!
Guide(s) to Wood Construction
in High Wind Areas
AF&PA’s newest design
standard has just been developed
(2006).
By request, AF&PA developed five handy Guides to Wood Construction in High Wind Areas, one for
each wind speed from 90 to 130 mph. These Guides fully illustrated in full color provide simplified
extractions of design information right out of the 2001 WFCM.
259
Engineering Standards
Guide(s) to Wood Construction in High Wind Areas
• Individual, easy-to-use Guides addressing wind design in 90,
100, 110, 120, and 130 mph wind zones.
• Based on provisions contained in AF&PA's Wood Frame
Construction Manual (WFCM) for One- and Two-Family
Dwellings, 2001 Edition.
– Use of the high wind provisions of these Guides will result in designs
that meet the requirements of the WFCM and the IRC.
• Guides are free for downloading from
www.awc.org
These Guides fully illustrated in full color provide simplified extractions of design information right out
of the 2001 WFCM. They are completely free of charge and downloadable from www.awc.org.
260
Guide - Layout
1.
2.
3.
4.
5.
General Provisions
Foundation Anchorage
Floors
Walls
Roofs
Glossary of Terms
Checklist
Each Guide is laid out in in the same fashion as the 2001 WFCM: by system assembly.
261
Guide – General Provisions
Verbal and graphical presentations
Color illustrations and large text add to the user-friendly nature of these design tools.
262
Guide – General Provisions
Tables delineate building aspect ratios and
widths.
Tables are easy to read and interpret.
263
Guide – Foundation Anchorage
In some cases, tables are tied to explanatory graphics.
264
Guide - Floors
Figures illustrate
cantilever limits
and floor bracing.
Another example.
265
Guide - Walls
Illustrations and
tables explain
wall
construction….
Wall design tables are straight forward…
266
Guide - Walls
…and sheathing
attachment for
lateral resistance.
… following in the same fashion as the 2001 WFCM.
267
Guide - Roofs
Illustration
explains forces
and spans
The emphasis on complete load path is apparent.
268
Guide - Checklist
Checklist is
included to
assist in code
compliance.
Handy checklists complete each Guide.
269
More information...
www.awc.org
For more information on this and other subjects, and how to order WFCM, see the AWC
website at www.awc.org, or call the AWC publications department toll-free at 1 800 890
7732.
270
Questions?
www.awc.org
• Online eCourses
• FAQ’s
HelpDesk
• AWCinfo@afandpa.org
• (202) 463-4713 or (800) 292-2372
Comments
• AWC_education@afandpa.org
This concludes this approved
continuing education program.
271