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Course Description
This course is intended for structural engineers and building
designers seeking an overview of design steps, considerations
and detailing best practices related to the wind- and seismic-
resistive design of wood-frame diaphragms and shear walls. It
provides an overview of relevant 2015 International Building
Code (IBC) provisions and American Wood Council (AWC)-
referenced standards, a discussion of common design errors,
and guidance related to load path continuity. Discussion will
cover diaphragm load paths, chords, collectors and openings,
as well as shear wall components, construction options,
overturning restraint systems and detailing considerations.
Design examples will be used to illustrate key principles and
code provisions.
Learning Objectives
1. Review seismic and wind load paths in wood-frame
structures.
2. Discuss relevant code and referenced standard
provisions related to the design of shear walls and
diaphragms.
3. Highlight methods for designing wood-frame
diaphragms and related components including
chords and collectors.
4. Explore shear wall design principles and highlight
three code-compliant configuration options for
solid walls vs. walls with openings.
Overview
• Diaphragms
• Shear Walls
Diaphragm Design
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WSP Diaphragm Capacity
• Capacities listed in AWC’s Special
Design Provisions for Wind and
Seismic (SDPWS)
• WSP diaphragms most common.
Can also use single-layer horizontal
and diagonal lumber sheathing,
and double-layer diagonal
sheathing.
• Note that capacities are given as
nominal: must be adjusted by a
reduction or resistance factor to
determine allowable unit shear
capacity (ASD) or factored unit
shear resistance (LRFD)
Diaphragm Boundary
All edges of a diaphragm shall be supported by a boundary element.
(ASCE 7-10 Section 11.2)
• Diaphragm Boundary Elements:
• Chords, drag struts, collectors, Shear walls, frames
• Boundary member locations:
• Diaphragm and shear wall perimeters
• Interior openings
• Areas of discontinuity
• Re-entrant corners.
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Diaphragm Capacities in AWC SDPWS
Unblocked
Blocked
SDPWS Table 4.2A
SDPWS Table 4.2C
• Capacities in SDPWS are Nominal values. Not ASD
Divide Nominal Values by 2.0 for ASD Capacity
Multiply Nominal Values by 0.8 for LRFD Capacity
• Capacity is reduced for
species with Specific
Gravity < 0.5
• For Spruce Pine Fir
multiply by 0.92
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Assume Basic Wind Speed = 115 mph Ultimate
Exposure B
Diaphragm Design
• Capacity
Shearwall Design
(SDPWS 4.3.5)
• Conventional
• Force Transfer Around Opening
• Perforated Shearwall
Example: Retail Restaurant
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Diaphragm Aspect Ratios
SDPWS TABLE 4.2.4
TYPE - MAXIMUM LENGTH/WIDTH RATIO
For an 84 x 34 diaphragm the aspect ratio is 2.5 < 3.
Diaphragm aspect ratio is OK.
Wood structural
panel, unblocked 3:1
Wood structural
panel, blocked 4:1
Single
-layer straight lumber sheathing 2:1
Single
-layer diagonal lumber sheathing 3:1
Double
-layer diagonal lumber sheathing 4:1
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Retail Restaurant – Diaphragm Design
84’
34’
10’6’
8’5’
6’
6’
6’
6’
6’
21’
29’
24’
10’
3’
3’
W = (9.6psf*(5’+3’)+(24.6)*3’) = 150.6 plf
V = (150.6 plf)*(84’/2) = 6,325 lb
M = (150.6 plf)*(84’
2
)/8 = 132,829 lb*ft
T = C = (132,829 lb*ft)/(34 ft) = 3,907 lb
n
diaphragm
= 6,325 lb/34’ = 186 plf
P
Wall
Height
Parapet
Roof
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Diaphragm Chords
Wall Top Plates Typically Function as Both Diaphragm
Chords and Drag Struts/Collectors
Diaphragm Design – Chords
84’
34’
V = 6,325 lb
C = 3,907 lb
w = 150.6 plf
T = 3,907 lb
Chord
Chord
• T = C = 3,907 lbs
• F’
t
= F
t
C
d
C
M
C
t
C
F
C
i
• F’
t
= (450 psi) 1.6 = 720 psi
• f
t
= T/A = 3,907/(1.5”x5.5”) = 474 psi < 720 psi chord ok
• Note only 1 top plate
required for chord force
Diaphragm Design – Deflection
From SDPWS commentary:
The total mid-span deflection of a blocked, uniformly nailed (e.g. same
panel edge nailing) wood structural panel diaphragm can be
calculated by summing the effects of four sources of deflection:
• Framing bending deflection
• Panel shear deflection
• Deflection from nail slip
• Deflection due to chord splice slip
SDPWS equation C4.2.2-1:
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Light Frame Wood Diaphragms often default to Flexible Diaphragms
Code Basis: ASCE 7-10 26.2 Definitions (Wind)
Diaphragms constructed of wood structural panels are permitted to be idealized as
flexible
Code Basis: ASCE 7-10 12.3.1.1 (Seismic)
Diaphragms constructed of untopped steel decking or wood structural panels are
permitted to be idealized as flexible if any of the following conditions exist:
[…]
c. In structures of light-frame construction where all of the following conditions are
met:
1. Topping of concrete or similar materials is not placed over wood structural
panel diaphragms except for nonstructural topping no greater than 1 1/2 in.
thick.
2. Each line of vertical elements of the seismic force resisting system complies
with the allowable story drift of Table 12.12-1..
Rigid or Flexible Diaphragm?
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ASCE 7-10 12.3.1.3 (Seismic)
[Diaphragms] are permitted to be idealized as flexible where the computed
maximum in-plane deflection of the diaphragm under lateral load is more
than two times the average story drift of adjoining vertical elements of
the seismic force-resisting system of the associated story under equivalent
tributary lateral load as shown in Fig. 12.3-1.
IBC 2012 Chapter 2 Definition (Wind & Seismic)
A diaphragm is rigid for the purpose of distribution of story shear
and torsional moment when the lateral deformation of the
diaphragm is less than or equal to two times the average story
drift.
Can a Rigid Diaphragm be Justified?
Average drift
of walls
Maximum
diaphragm
deflection
Some Advantages of Rigid Diaphragm
• More load (plf) to longer interior/corridor walls
• Less load (plf) to narrow walls where overturning restraint is tougher
• Can tune loads to walls and wall lines by changing stiffness of walls
Some Disadvantages of Rigid Diaphragm
• Considerations of torsional loading necessary
• More complicated calculations to distribute load to shear walls
• May underestimate “Real” loads to narrow exterior walls
• Justification of rigid assumption
Rigid Diaphragm Analysis
Semi-Rigid Diaphragm Analysis
• Neither idealized flexible nor idealized rigid
• Explicit modeling of diaphragm deformations with shear wall
deformations to distribute lateral loads
• Not easy
Enveloping Method
• Idealized as BOTH flexible and rigid.
• Individual components designed for worst case from each approach
• Been around a while, officially recognized in the 2015 SDPWS
Two More Diaphragm Approaches
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Overview
• Diaphragms
• Shear Walls
Lateral Loads create shear (sliding) and racking
forces on a structure
Sliding resisted by shearwall base anchorage
Racking resisted by shear panel & fasteners
Shearwall Functions
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WSP Shear Wall Capacity
• Capacities listed in AWC’s Special Design
Provisions for Wind and Seismic (SDPWS)
• Sheathed shear walls most common. Can also
use horizontal and diagonal board sheathing,
gypsum panels, fiberboard, lath and plaster, and
others
• Blocked shear walls most common. SDPWS has
reduction factors for unblocked shear walls
• Note that capacities are given as nominal: must
be adjusted by a reduction or resistance factor
to determine allowable unit shear capacity
(ASD) or factored unit shear resistance (LRFD)
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Multi-Story Continuous
Rod Tiedown System
Source: Simpson Strong Tie
Source: MiTek
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Shear Wall Anchorage
4.3.6.4.3 Anchor Bolts:
• Foundation anchor bolts shall have a steel plate washer under each nut.
• Minimum size-0.229”x3”x3” in.
• The hole in the plate washer - Diagonally slotted, width of up to 3/16” larger
than the bolt diameter, and a slot length not to exceed 1-3/4” is permitted if
standard cut washer is provided between the nut and the plate.
• The plate washer shall extend to within 1/2" of the edge of the bottom plate
on the side(s) with sheathing.
• Required where sheathing nominal unit shear capacity is greater than 400 plf
for wind or seismic. (i.e. 200 plf ASD, 320 plf LRFD)
Square Plate
Washers
Shear Wall Anchorage
Standard cut washers
• Permitted to be used where anchor bolts are designed to resist shear only and
the following requirements are met:
a) The shear wall is designed segmented wall with required uplift anchorage at
shear wall ends sized to resist overturning neglecting
DL stabilizing moment.
b) Shear wall aspect ratio, h:b, does not exceed 2:1.
c) The nominal unit shear capacity of the shear wall does not exceed 980 plf for
seismic or 1370 plf for wind.
Cut Washers
Retail Restaurant – Shear Wall Design
84’
34’
10’6’
8’5’
21’
29’
24’
V = 6,325 lb
• Shear wall capacity: wall sections not equal in width
• Assume 15/32”, Wood Structural Panels - Sheathing attached
with 8d nails @ 3” o.c to 2x6 Spruce Pine Fir framing spaced 16”
o.c.
C = 3,907 lb
w = 150.6 plf
M
max
= 132,829 lb-ft
T = 3,907 lb
n
diaphragm
= 186 plf
4’
9’
Shear Wall Aspect Ratios
34’
9’
21’
4’
10’
Check Aspect Ratios: Assume blocked WSP shear wall
Shear Wall aspect ratios: SW1 = 10’/4’ = 2.5 < 3.5 OK
SW2 = 10’/9’ = 1.1 < 3.5 OK
Note that aspect ratio of SW1 is greater than 2, so it’s capacity
will need to be adjusted per SDPWS 4.3.4.2
Rear Wall Elevation
SW1 SW2
Shear Walls in a Line
34’
9’
21’4’
10’
SW1 SW2
d
d
SDPWS 4.3.3.4.1
Shear distribution to individual shear walls in a shear wall line
shall provide the same calculated deflection, d
sw
, in each shear
wall.
d
SW1
= d
SW2
= Equal Deflection Method
Given the same
load, which
shear wall will
deflect less?
Shear Walls in a Line
34’
9’
21’
4’
10’
Equal Deflection Method
SW1
h/b
s
= 2.5 > 2
Aspect Ratio Factor = 1.25-0.125(h/b
s
) = 0.938 (SDPWS 4.3.4.2)
Nominal Unit Shear Capacity = 1,370 lb/ft (SDPWS Table 4.3A)
Adjusted ASD Capacity = [(1,370 plf)(0.92)/2]*0.938 = 591 lb/ft
SW1 SW2
Shear Walls in a Line
34’
9’
21’
4’
10’
SW1 SW2
Equal Deflection Method
SW2
h/b
s
= 1.1 < 2
Nominal Unit Shear Capacity = 1,370 lb/ft (SDPWS Table 4.3A)
Adjusted ASD Capacity = (1,370 plf)(0.92)/2 = 630 lb/ft
Shear Walls in a Line
Determine the deflection of SW2 at its ASD unit shear capacity
v = 630 lb/ft
E = 1,400,000 psi (NDS Supplement Table 4A)
A = 2(1.5”x5.5”) = 16.5 in
2
(2-2x6 stud end post)
b = 9’
h = 10’
G
a
= 14 k/in (SDPWS Table 4.3A)
Δ
a
= vertical elongation of wall anchorage
Shear Walls in a Line
Determine the deflection of SW2 at its ASD unit shear capacity
*From the holdown manufacturer, the deflection of the anchor at its
capacity of 6,560 lbs = 0.091”
SW2 anchorage force = (630 lb/ft)(10’) = 6,300 lb
*Assuming vertical elongation is linear, we can calculate elongation for
our load of 6,300 lbs.
Δ
a
= 6,300 * 0.091” / 6,560 lb = 0.087”
d
SW2
= 0.571”
k = stiffness of the anchorage = F / d (deflection / elongation)
k = 6,560 lbs / 0.091” = 72,087 lb/in
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Shear Walls in a Line
34’
9’
21’
4’
10’
SW1 SW2
Shear Wall Line Capacity
V = (497 lb/ft)*4’ + (630 lb/ft)*9’
V = 1,988 lbs + 5,670 lbs
V = 7,658 lb > 6,325 lb
1,988 lbs 5,670 lbs
Shear Walls in a Line
34’
9’
21’
4’
10’
SW1 SW2
Simplified Method
For Wood Structural Panels, distribution of shear in proportion
to shear strength of each shear wall is permitted provided that
shear walls with aspect ratio greater than 2:1 have strength
adjusted by the 2b
s
/h factor.
2015 SDPWS 4.3.3.4.1, Exception 1.
Shear Wall Aspect Ratios
34’
9’
21’
4’
10’
Simplified Method
Aspect Ratios: Assume blocked WSP shear wall
Shear Wall aspect ratios: SW1 = 10’/4’ = 2.5 < 3.5 OK
(SDPWS Table 4.3.4) SW2 = 10’/9’ = 1.1 < 3.5 OK
SW1 = 2.5 > 2 : nominal shear capacity will need to be
adjusted by 2b
s
/h per SDPWS 4.3.3.4.1 Exception 1.
Rear Wall Elevation
SW1 SW2
Shear Walls in a Line
34’
9’
21’
4’
10’
Simplified Method
SW1
h/b
s
= 2.5 > 2
Aspect Ratio Factor = 1.25-0.125(h/b
s
) = 0.938 (SDPWS 4.3.4.2)
2b
s
/h = 2( 4’/10’) = 0.8
Nominal Unit Shear Capacity = 1,370 lb/ft (SDPWS Table 4.3A)
Adjusted ASD Capacity = [(1,370 plf)(0.92)/2]*0.8 = 504 lb/ft
SW1 SW2
Shear Walls in a Line
34’
9’
21’
4’
10’
SW1 SW2
Simplified Method
SW2
h/b
s
= 1.1 < 2
Nominal Unit Shear Capacity = 1,370 lb/ft (SDPWS Table 4.3A)
Adjusted ASD Capacity = (1,370 plf)(0.92)/2 = 630 lb/ft
Shear Walls in a Line
34’
9’
21’
4’
10’
SW1 SW2
Simplified Method
Shear Wall Line Capacity
V = (504 lb/ft)*4’ + (630 lb/ft)*9’
V = 2,016 lbs + 5,670 lbs
V = 7,686 lb > 6,325 lb
2,016 lbs 5,670 lbs
** Note that the
capacity of the wall is
quite a bit higher
than needed.
Designer could look at
increasing sheathing
nail spacing.
Shear Walls in a Line
34’
9’
21’
4’
10’
SW1 SW2
Equal Deflection Method
Simplified Method
2,016 lbs 5,670 lbs
1,988 lbs 5,670 lbs
SW1 SW2
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Perforated Shearwall Capacity
n
shearwall
= 527 plf
Try reducing nail spacing to 2” with 8d nails – will require 3x framing
Nominal Tabulated Capacity = 1790 plf
Adjusted ASD Capacity = (1790 plf)(0.92)(0.75)/2 = 618 plf
618 plf > 527 plf, OK
8d nails at 2”
o.c. acceptable for perforated wall
PANEL GRADE
FASTENER
TYPE &
SIZE
MINIMUM
PANEL
THIICKNESS
MINIMUM
FASTENER
PENETRATION
IN FRAMING
NAIL
SPACING
AT ALL PANEL
EDGES
PANEL
EDGE
FASTENER SPACING
Wood Structural
Panels
–
Sheathing
8d (2½ “ x
0.131”)
15/32” 1 3/8” 2 IN. 1280 (Seismic)
1790 (Wind)
SDPWS Table 4.3A
Perforated Shearwall Overturning
34’
6’
6’ 6’ 6’ 6’2’
2’
10’
n
shearwall
= 527 plf
Hold downs required at ends of perforated wall
T = nh/C
o
(similar to SDPWS equation 4.3-8)
T = 527 plf*10’/0.75 = 7,027 lb
Hold down capacity from segmented wall
option = 7,045 lb: could use same hold down
Perforated Shearwall Uplift
34’
6’
6’ 6’ 6’ 6’2’
2’
10’
n
shearwall
= 527 plf/0.75 = 703 plf, use same magnitude for uniform uplift
at full height segments
One option is to use anchor bolts with large washers to resist uplift in
bearing
If net washer area = 8 in
2
, can resist (425 psi)(8 in
2
) = 3,400 lb in uplift
• Max. anchor bolt spacing = 3,400 lb/703 plf = 4’-10” o.c.
• Will also need to check shear loads on anchor bolts for controlling
case
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Double-Sided Shear Walls
High-strength wood shear walls can be double-sided with
WSP sheathing on each side:
SDPWS 4.3.3.3 Summing Shear Capacities: For shear walls
sheathed with the same construction and materials on
opposite sides of the same wall, the combined nominal
unit shear capacity shall be permitted to be taken as twice
the nominal unit shear capacity for an equivalent shear
wall sheathed on one side (4.3.5.3 has max capacities for
double-sided perforated walls)
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Gypsum Wallboard and Shear Walls
2015 SDPWS 4.3.7.2 Shear Walls using Wood Structural Panels over
Gypsum Wallboard or Gypsum Sheathing Board:
Shear walls sheathed with wood structural panel sheathing over gypsum
wallboard or gypsum sheathing board shall be permitted to be used to resist
seismic and wind forces. The size and spacing of fasteners at shear wall
boundaries and panel edges shall be as provided in Table 4.3B. The shear
wall shall be constructed in accordance with Section 4.3.7.1
Further requirements in 2015 SDPWS 4.3.7.5
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