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Re: Shear Wall Design

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Andrew,

If your shear wall:
1) Has no holddown, and 
2) Has dead load distributed over its length so that the dead load is exactly 
enough to prevent the wall from overturning due to the applied lateral load; 
then, the shear in the wall is not uniform.  In fact, the shear varies 
linearly from zero at the vertical edge opposite the fulcrum, to twice the 
average shear at the vertical edge at the overturning fulcrum.  If you design 
the wall as if it has uniform shear, you actually get a wall that has 
portions stressed to twice the design shear, and portions that aren't working 
very hard.  As you increase the uniform dead load, more and more of the zone  
adjacent to the fulcrum carries a uniform shear; however, no practical 
uniform dead load will eliminate the zero shear at the edge opposite the 
fulcrum, or keep the shear in the remainder of the wall from exceeding the 
design uniform shear.

You can verify this by studying the free body diagrams of finite elements 
within the wall.

An interesting effect of this arrangement is that the end of the wall 
opposite the fulcrum will lift actually off the foundation under the design 
load as the wall distorts in a curved shape.  

You can see this effect pretty well by playing with a new soft vinyl eraser: 
put its long narrow edge on the table and push laterally with your finger on 
the top edge to simulate a lateral load, while applying varying amounts of 
simultaneous downward pressure.  Place your pushing finger at different 
positions along the top edge and see what happens; try to apply the force 
along a length of you finger instead of just at the tip; when you have the 
tension edge off the table, see what sort of "holddown" force it takes to get 
it back down on the table.

Be careful who sees you doing this; there's a list circulating the Internet 
on how to tell if your husband is an engineer -- this isn't on the list, but 
it'll be a sure giveaway if someone is trying to figure you out.

Every plywood shear wall I design has a holddown at each end designed to 
resist the total overturning force.

Nels Roselund
Structural Engineer