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Re: Rigid vs. Flexible Diaphragm

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My responses are included below.
Mike Cochran
In a message dated 5/6/2004 7:14:45 AM Pacific Standard Time, T.W.Allen(--nospam--at) writes:

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.
I do force deflection compatibility along a girdline, all walls along a gridline must have the same deflection (the forces are redistributed between walls along the gird line till the walls all have the same deflection), in fact, all girdlines in the same direction must have the same deflection, otherwise you don't have relative rigidities between gridlines that are meaningful.
  1. What do you use for the deflection of floor to floor hold downs like metal straps?
I don't use straps for holdowns, we typically use Zone4 holdowns for floor to floor appications.  Since wood shrinks, I don't believe in using straps for holdowns, and we do include shrinkage devices with the zone4 holdowns
  1. What do you use for the deflection of hold down systems like Simpson’s ATS, Earthbound Impasse system and Quake-Tie cable system?
We will calculate the elongation of the tie rods and the flexural movements of the holdowns based upon the loading applied to that specific shear wall in determining shear wall stiffness. We typically use Zone4 holdowns, but have used Simpson and Earthbound when the framer has asked for a substition (Substitution requrests are generally generated based upon the best price the framer can get from the three tie-rod manufacturers).  Holdown bracket deflection is based upon a linear explolation of the published values for the zone4 brackets.  If the contractor is substituting tie rods for conventional zone4 holdown connections to studs above and below the floors than the design capacity of the tie rod system must equal or exceed the capacity of the holdowns shown on the structural drawings and deflections of the tie rod are generally much less than 1/8".  And for simplicity of installation, the tie -rod supplier may opt to use all the same size rods at that floor level instead of using the minimum size required by design.




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