In this section, an analysis will be performed on a simple rectangular, one-story wood building, illustrating the load propagation and transfer that occurs in a roof diaphragm and shearwall resistive system. Wood-framed roofs and walls are stiffened considerably by the use of plywood or OSB/waferbo

Question

In this section, an analysis will be performed on a simple rectangular, one-story wood building, illustrating the load propagation and transfer that occurs in a roof diaphragm and shearwall resistive system.

Wood-framed roofs and walls are stiffened considerably by the use of plywood or [latex]OSB[/latex]/waferboard sheathing acting as diaphragms and shearwalls.

Wind force (assumed as a uniform pressure on the windward front wall) is initially distributed to the roof and floor diaphragm (or foundation). Conventional wood stud framing in the walls functions as vertical beams and distributes
half of the wind load to the roof and the other half to the floor construction.

Pressures on the windward wall and suction on the leeward wall convert into uniformly distributed loads [latex]\omega [/latex] along the windward and leeward boundary edges of the roof diaphragm. Often, the loads on the windward and leeward edges are combined as one load distribution along the windward edge.

Accepted Answer

The [latex] \frac{1}{2} [/latex] wall height dimension represents the tributary wall dimension that loads the roof diaphragm. Uniform load [latex]\omega [/latex] carries the units of pounds per lineal foot, the same as a uniform load on a beam.

[latex]\omega [/latex]=wind pressure [latex]p imes \frac{1}{2} [/latex] the wall height

In fact, roof diaphragms are essentially treated as flat, deep beams spanning from wall support to wall support.

Shearwalls represent supports for the roof diaphragm with resulting reactions [latex]V[/latex], where

[latex]V=\frac{\omega L }{2}[/latex] (pounds)

The intensity of the shear reaction is expressed as lowercase [latex] u[/latex] , where

[latex] u=\frac{V }{d}[/latex] (pounds/ft.)

Shear load [latex]V[/latex] is applied to the top edge of the shearwall. Wall equilibrium is established by developing an equal and opposite shear reaction[latex]V^\prime[/latex] at the foundation, accompanied by a tension [latex](T)[/latex] and compression [latex](C)[/latex] couple at the wall edges to counteract the overturning moment created by [latex]V[/latex]. The tension [latex]T[/latex] is normally referred to as the

tie-down force.

Because wind cannot be assumed to act in a prescribed direction, another analysis is required with the wind pressure applied to the end walls, perpendicular in direction to the earlier analysis. Each wall is designed with the requisite wall sheathing thickness, nail size, and spacing to reflect the results of the wind analysis. Details of this design procedure are covered more thoroughly in subsequent courses on timber design. An excellent discussion, with examples of shearwalls in actual structures, can be found on the Internet at http://www.mcvicker.com/vwall/dti.htm.

Question 4.9: In this section, an analysis will be performed on a simple r...

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