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RE: Rigid vs. Flexible Diaphragm

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Michael –


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:


  1. 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.
  2. What do you use for the deflection of floor to floor hold downs like metal straps?
  3. What do you use for the deflection of hold down systems like Simpson’s ATS, Earthbound Impasse system and Quake-Tie cable system?




T. William (Bill) Allen, S.E. (CA #2607)

San Juan Capistrano, CA

-----Original Message-----
Mlcse(--nospam--at) [mailto:Mlcse(--nospam--at)]
Sent: Thursday, May 06, 2004 12:01 AM
Subject: Re: Rigid vs. Flexible Diaphragm


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.


Mike Cochran