shear wall design

CE 479: DESIGN OF BUILDING COMPONENTS AND SYSTEMS FALL 2012 – J. LIU Wood: Shear walls

Shearwalls 

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The vertical elements in the lateral force-resisting system (LFRS) Support the roof/floor diaphragms and transfer the lateral forces into the foundations

http://www.jlconline.com/Images/LateralForce%20Collectors%20for%20Seismic%20and%20 Wind-Resistant%20Framing_tcm96-1095375.pdf

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”)

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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

Basic Shear wall Action

Design Considerations     

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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_ubcwood-shear-wall-spacing.html

http://www.finehomebuilding.com/design/articles/how-it-works-shear-walls.aspx

Sheathing Thickness 

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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.

Nailing, Chords, Collectors 

Nailing  Function

of unit shear in the wall and materials

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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

Shearwall Proportions  

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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

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)

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IBC sets h/b limit to 2.0 for seismic

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)

Allowable Unit Shears 

Tabulated values assume framing members are Douglas Fir-Larch or Southern Pine  Adjustment

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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

Allowable Unit Shears 

LRFD – use resistance factor  (SDPWS

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

D

= 0.80

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

Unit Shears 

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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

Unit Shears 

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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

Unit Shears

Three Methods 

Segmented shearwall (SDPWS 4.3.5.1) Most common  Each segment designed separately 

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Design for force transfer around openings (SDPWS 4.3.5.2) above and below openings designed as coupling beams  Requires special detailing around openings 

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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)

Segmented Shearwall

Force Transfer Around Openings

Perforated Shearwall

Shearwall Chord Members

Shearwall Chord Members

Shearwall Chord Members 

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Some designers will include overturning resistance due to dead load; others neglect Force in compression chord can be underestimated when dead load neglected

Anchorage Considerations 

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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

Anchorage

Shear Anchorage 

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Attachment of sheathing to bottom wall plate will transfer shear to base of the wall Anchor bolts are designed to transfer shear to foundation

Anchorage for Perpendicular Force

Anchorage for Perpendicular Force 

Must rely on nail connection between stud and bottom wall plate, etc.

Deflection

SDPWS eq. C4.3.2-1

Deflection 

Can also use SDPWS eq. 4.3-1 with Ga – apparent shear stiffness (including nail slip)