Plywood Shear Wall Design
Plywood Shear Wall Design
Plywood Shear Wall Design
Warwick Banks1
Plywood has been used for bracing in residential buildings for many years using the Bracing unit methodology adopted
by NZS3604.
This paper is based on a presentation that was given at a Timber Design Society seminar and is aimed at enabling
structural engineers to carry out specific design of plywood bracing elements in low rise multi-storey buildings of up to 5
or 6 storeys.
Plywood shear walls consist of:
Plywood, to transfer shear forces
Chords, to resist tension/compression generated by the over turning moments
Base connections to transfer shear to foundations.
Capacity design:
Energy is absorbed by the yielding of a ductile link in the structural load path. This ductile link in the plywood shear walls
is the nails.
Everything other than the nails are designed for the actions associated with the shear force over-strength capacity of the
nailed connections, or alternatively everything other than the nails can be designed for an elastic level load.
The non ductile components of a plywood shear wall are the plywood, chords, chord connections, base shear connection
and foundations.
Over strength value for nailed connections = 2.0 (NZS3603)
Use ductility level, = 3.0 (coincides with the ductility of Gib bracing)
Design Tip:
Try to avoid mixing plywood and Gib board bracing elements acting in the same direction within a building as they may
not be deflection compatible.
O
O
C
=
O
O
C
C
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V3
G
V2
G
V1
T
V base
Stress Distribution
Short Walls
H<<d
Long Walls
H>>d
d.
V3
= (v3 h3) + (v2 h2 ) + (v1 h1)
V2
h3
V1
h1
h2
T =C =
Mo
Tensile
stress
Compressive
stress
jd
Mo
z=
bd 2
6
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10
M o
V3 h3) + ( V2 h2) + ( v1 h1 )
=(
Mo
h3
T
V2
T =C =
jd
h1
V Base
h2
V1
V * base
d
(n/mm)
Plywood
V*<V=k1k8fpstd
Design tipDesign nailed connection first as this is likely to be the most difficult design component to achieve.
P
H
**t
stretch
**sc
squash
Chord effects
H
(S c + S t )
B
Sc & St
Tor CL
AE
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2(1+a) men
PH
GBT
L = original length
A= area of component
E= Youngs Modulus of component
Wall flexural
deformation
Nail Slip
Plywood
shear
deformation
I wall = I chords @ NA
Refer cl.4.2.2.3
NZS3603
Wall
st
= 2 I chord + areachordxh 2
itself
11
@ ULS
Nail Slip
In the absence of specific test data, slip in nailed joints may be determined from NZS3603 clause 4.2.2.3 which has the following
information:
(a) A load equal to 1.25 times the nominal short term strength of a single nail gives an average slip of 2.5 mm.
(b) From 0 to 0.5 mm slip, the slip can be calculated from
k 37 (0.8) p 2
Qn
. (Eq. 4.4)
Where :
K37 = is given in table E1 in Appendix E
P
Qn = nominal strength for a single nail with short term loading (k1=1)
(c) From 0.5 mm to 2.5 mm slip, interpolate linearly between (a) and (b)
(d) Above 2.5 mm slip, the load may increase 20% to 40% to give maximum load at a slip between 6 mm and 10 mm
Each of these componenets is illustrated below for 2.8mm nails.
NZS3603 cl.4.2.2.3 references
10 mm
(d)
{
6 mm
(mm)
en
(a) - 2.5 mm
1.4 mm
- (c)
0.5 mm
(b)
725
Back calc
from (b)
917
Interpolate
F. (n)
1150
1.25
Qn
(a)
1376
(1.25 Qn) 1.2
(d)
1604
(1.25 Qn) 1.4
(d)
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Foundation Design
Foundations hold down shear walls by providing resistance to overturning. This can be by pure mass or by using the geometry to
increase resisting moment lever arms.
Design foundations for elastic loads or for the forces generated by the over strength capacity of the nails in the shear wall.
Foundations spread load to the ground limiting the loads to below the bearing capacity of the founding ground by the means of the
foundation type and geometry.
Mass
- Bearing pressure
Mass footing
Piled foundation
Shear forces
transferred via
diaphragm to
lower walls
ii.
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Beam
stiffness
effects shear
wall
deflections
Chord forces
transferred to
posts
Shear
load
Shear load paths
from diaphragms
Paths
down
wall
Single stud
Double Lining
Platform framing
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Single stud
Double Lining
Balloon framing
14
Double stud
Single lining
Platform framing
(Most common in greater than 4
storeys multi unit developments)
Steel
Chord
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