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RE: Sharing Holdowns @ Corners
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- Subject: RE: Sharing Holdowns @ Corners
- From: "Nels Roselund" <njineer(--nospam--at)att.net>
- Date: Thu, 2 Mar 2006 17:07:18 -0800
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. From: Bill Allen
[mailto:T.W.Allen(--nospam--at)cox.net] 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. Consulting Structural
Engineers -----Original Message----- 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 |
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- RE: Sharing Holdowns @ Corners
- From: Bill Allen
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