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RE: Rigid vs. Flexible Diaphragm
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Rigid vs. Flexible Diaphragm
- From: "Bill Allen" <T.W.Allen(--nospam--at)cox.net>
- Date: Thu, 6 May 2004 07:00:29 -0700
I agree; I would use a linear
interpolation for hold down deflection as long as the uplift force was greater
than zero. The problem with the bolt holes is the very reason why I try not to
use these types of hold downs anymore.
Now, my questions:
- You don’t force
deflection compatibility along a grid line? In other words, if you have a
short shear wall in the same line as a long shear wall, you let them
deflect independently? If so, how do you rationalize the difference in the
deflections? Of course, it probably doesn’t make a lot of
(practical) difference either way.
- What do you use for the
deflection of floor to floor hold downs like metal straps?
- What do you use for the
deflection of hold down systems like Simpson’s ATS, Earthbound
Impasse system and Quake-Tie cable system?
From: Mlcse(--nospam--at)aol.com [mailto:Mlcse(--nospam--at)aol.com]
Sent: Thursday, May 06, 2004 12:01
Subject: Re: Rigid vs. Flexible
Question about calculation of the holdown deflection.
If the holdown is rated for 2000 pounds of uplift that
results in 1/8" deflection, why not consider a linear relationship for
uplift. If the uplift force is 1000 pounds than the holdown deflection is
(1000)(0.125")/(2000) = 0.0625" uplift. The relationship most
likely isn't perfectly linear, but its probably not a bad approach. If the
shear wall is a 2:1 aspect ratio, than the horizontal deflection is
(2/1)(0.0625") = 0.125" deflection.
We have written our program using this approach which
self iterates to a solution where all walls and frames must have the same
deflection (tolerance cutoff of 0.005" between maximum and minimum shear
wall deflection). Forces to individual walls are based upon all walls
having the same deflection (pure translation). Holdown sizes
are checked on each iteration, and the sizes automatically increased to the
appropriate size for the given uplift size. The holdown contributes
greatly to the shearwall drift unless you have a very stiff holdown. But
if you assume 1/16" oversized holes for a bolted holdown, and the shear
wall is 2:1 aspect ratio, than you have 1/8" of shearwall horizontal
deflection just due to the bolt holes slip in the wood post which is about
20% of your allowable drift for an 8 foot tall wall, and you haven't put any
load on the wall yet (assumption is that any uplift on post must move
1/16" to engage the bolts to resist the uplift). I have found that
for the short walls, the nail slip and holdown movement are the major
contributor to shear wall deflection.