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RE: Sharing Holdowns @ Corners

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Bill,

 

Always use a holddown to develop the moment produced by the shear force times the height of the wall.  A shear wall without a holddown does not have uniform shear, and the actual shear at the compression end may be as much as twice the shear resulting from an assumption of uniform distribution of shear along the full length.  Use statics to see this on a shear wall of uniform weight per foot for which the overturning moment equals the resisting moment; at a vertical section near the uplift end the vertical shear capacity approaches zero because there is little weight to resist uplift.  Near the compression end where the full weight of the wall is available to resist uplift, the shear in a vertical section equals the full weight of the wall; not half the weight of the wall as would be assumed for uniform distribution of lateral shear along the wall length.  You would need a holddown to make the shear uniform over the length of the wall.  Without a holddown, the wall will tend to lift off the sole plate – this is the condition where the square plates would tend to protect the sole plate against cracking, but using them for that purpose doesn’t seem to me to be good design because we don’t figure the load path to be working in that way.

 

You can demonstrate a similar effect with a new vinyl eraser.  Stand the eraser on a long edge on a table and, using your thumb near one end, apply on the upper edge simultaneously a lateral load toward the other end and a holddown load.  Now, while continuing to apply the load, allow your thumb to slide toward the other end, in effect moving the uplift-resisting load needed for stability toward the compression end.  Watch what happens at the uplift end.  You can see that only the portion between the holddown force and the compression end carries shear and becomes distorted to a trapezoid shape.  The portion that is not resisting shear remains rectangular and lifts off the table.  When you have moved your thumb to near the compression end, and the uplifted end is well of the table, with your other hand, you can apply the holddown force needed to produce a uniform shear in the eraser and give the whole eraser a trapezoidal shape.

 

Nels Roselund, SE

South San Gabriel, CA

njineer(--nospam--at)att.net


From: Bill Allen [mailto:T.W.Allen(--nospam--at)cox.net]
Sent: Thursday, March 02, 2006 2:02 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Sharing Holdowns @ Corners

 

Joe –

 

Just curious, but what do you do if, based on your calculations, there is no uplift even considering orthogonal effects?

 

T. William (Bill) Allen, S.E.

ALLEN DESIGNS

Consulting Structural Engineers
 
V (949) 248-8588 F(949) 209-2509

-----Original Message-----
From: Jnapd(--nospam--at)aol.com [mailto:Jnapd(--nospam--at)aol.com]
Sent: Thursday, March 02, 2006 1:05 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Sharing Holdowns @ Corners

 

And to add to Bill's comment I always upsize the corner holdown to the next larger size even if the loading that Bill mention did not require it.

 

Joe Venuti
Johnson & Nielsen Associates
Palm Springs, CA