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CE 479: DESIGN OF BUILDING
COMPONENTS AND SYSTEMS
FALL 2012 – J. LIU
Wood: Shear walls
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Shearwalls
The vertical elements in the lateral force-resisting
system (LFRS)
Support the roof/floor diaphragms and transfer the
lateral forces into the foundations
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http://www.jlconline.com/Images/LateralForce%20Collectors%20for%20Seismic%20and%20
Wind-Resistant%20Framing_tcm96-1095375.pdf
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Materials
Most typically used to develop shear wall action:
Wood structural panels (e.g. plywood and oriented
strand board (OSB))
Lumber sheathing (diagonal or horizontal “strips”)
http://www.woodworking-online.com/images/image120.jpg
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Materials
Might be adequate if design forces relatively small
Gypsum wallboard (drywall)
Interior and exterior plaster (stucco)
Fiberboard (including fiber-cement panels)
Note: generally, interior partition walls neglected in
lateral force design
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Basic Shear wall Action
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Design Considerations
Sheathing thickness
Shear wall nailing
Chord design (tension and compression)
Collector (strut) design
Anchorage requirements (hold-downs and shear)
Shear panel proportions
Deflection
http://www.ehow.com/list_7192716_ubc-
wood-shear-wall-spacing.html
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http://www.finehomebuilding.com/design/articles/how-it-works-shear-walls.aspx
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Sheathing Thickness
Sheathing-type loads and spacing of wall studs
may determine thickness
Unit shear often controls
May also be governed by the required fire rating
of a wall
e.g., 1-hour fire rating for 2x4 wall studs @16”o.c.,
with 5/8” gypsum on the interior, and 5/8” Type X
gypsum sheathing & minimum 3/8” plywood siding
together on the outside.
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Nailing, Chords, Collectors
Nailing
Function of unit shear in the wall and
materials
Chords
As with diaphragms, these are
designed to carry the moment
Required at both ends of a shear wall
Collector (strut)
Same collector we discussed with
diaphragm design
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Shearwall Proportions
Measured by height-to-width ratio, h/b
In buildings with two or more stories, the height, h, is
the vertical clear distance between diaphragms
IBC sets upper limits on h/b for various wall
sheathing materials used as shear walls
Shear walls satisfying h/b limits considered to be
better with regards to deflection control
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Height-to-Width Ratios
IBC sets h/b limit for
wood structural panel
shear walls to 3.5 for
wind
(SDPWS uses this limit,
blocked shear walls;
Table 4.3.4)
IBC sets h/b limit to
2.0 for seismic
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Height-to-Width Ratios
For tables, may increase
h/b for seismic up to 3.5
provided that tabulated
allowable unit shears
reduced by multiplier
2bs/h (SDPWS 4.3.4.1)
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Allowable Unit Shears
Tabulated values assume framing members are
Douglas Fir-Larch or Southern Pine
Adjustment factors for other species given in footnote
Panels resisting wind loads are permitted to use unit
shear capacities 40% higher than for seismic
Note that tables are for short-term forces (wind and
seismic)
If panels used to support loads of longer duration,
tabulated unit shear must be reduced
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Allowable Unit Shears
LRFD – use resistance factor D = 0.80
(SDPWS Section 4.3.3)
Tabulated values apply to panels installed
vertically or horizontally
Assumed that all panel edges are supported by
and are edge nailed to wall studs or blocking
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Unit Shears
Can be obtained with more than one layer on other
side of the wall
If same nail size and spacing is used, second layer
can double shear capacity of the wall
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Unit Shears
Generally wall covering (gypsum wallboard,
plaster, stucco) capacity not additive to shear
capacity of wood structural panel sheathing
One exception is gypsum wallboard under wood
structural panel exterior (as in Table 4.3B)
Plywood siding can also be used
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Unit Shears
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Three Methods
Segmented shearwall (SDPWS 4.3.5.1)
Most common
Each segment designed separately
Design for force transfer around openings (SDPWS 4.3.5.2)
above and below openings designed as coupling beams
Requires special detailing around openings
Perforated shear wall method (SDPWS 4.3.5.3)
Semi-empirical method, like 2nd method but less detailing requirements and with capacity adjustment factor for openings (Table 4.3.3.5)
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Segmented Shearwall
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Force Transfer Around Openings
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Perforated Shearwall
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Shearwall Chord Members
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Shearwall Chord Members
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Shearwall Chord Members
Some designers will include overturning resistance
due to dead load; others neglect
Force in compression chord can be underestimated
when dead load neglected
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Anchorage Considerations
Critical locations are where diaphragms connect to
shearwalls and where shearwalls tie into the
foundation
Commonly use „tie-downs‟ or „hold-downs‟
(engineered prefabricated metal brackets)
Must consider:
Vertical (gravity) loads
Lateral forces parallel to the wall
Lateral forces perpendicular to the wall
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Anchorage
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Shear Anchorage
Attachment of sheathing to bottom wall plate will
transfer shear to base of the wall
Anchor bolts are designed to transfer shear to
foundation
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Anchorage for Perpendicular Force
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Anchorage for Perpendicular Force
Must rely on nail connection between stud and
bottom wall plate, etc.
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Deflection SDPWS eq. C4.3.2-1
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Deflection
Can also use SDPWS eq. 4.3-1 with Ga – apparent
shear stiffness (including nail slip)