shear wall design

CE 479: DESIGN OF BUILDING

COMPONENTS AND SYSTEMS

FALL 2012 – J. LIU

Shearwalls

 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”)

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

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

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.

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

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

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

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 2b_s/h (SDPWS 4.3.4.1)

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,

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

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

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)

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)

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

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

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

Anchorage for Perpendicular Force

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

Deflection

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