CE 479: DESIGN OF BUILDING

COMPONENTS AND SYSTEMS

FALL 2012 – J. LIU

Wood: Shear walls

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”)

http://www.woodworking-online.com/images/image120.jpg

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

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

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

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

diaphragm design

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

2bs/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,

tabulated unit shear must be reduced

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)

Plywood siding can also be used

Unit Shears

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)

Segmented Shearwall

Force Transfer Around Openings

Perforated Shearwall

Shearwall Chord Members

Shearwall Chord Members

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

Lateral forces perpendicular to the wall

Anchorage

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

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)