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Re: Load Capacity of a Holddown Post as a Compression Member

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Thor,
 
This problem is discussed  in Timoshenko's "Theory of Elastic Stability."  With remarkable yet usual for him elegancy and thoroughness, Timoshenko shows that the critical force for a column loaded with concentrated compressive forces at the ends is more than 3 times higher than for a similar column loaded by the equivalent distributed force.  Of course, this is an analytical elastic-buckling solution...
 
I did not follow this thread from the very beginning, so the following may not exactly address the nature of the original post.  Still, I do not believe that the compressive force in the shear wall is resisted by the end post.
 
Indeed, we - more often then not - assume the horizontal diaphragms (glued and nailed) to be "flexible"; then why not do the same for vertical - nailed only - diaphragms?  If so, the rocking-related compression is resisted by almost all studs of the shear wall, with some sort of distribution (say, triangular or similar) of force away from the tension post (holdown).  
 
BTW, this may shed some light on the effect of a uniformly distributed moment due to one-sided plywood application to the end post.  A combined action of such distributed moment and the aforementioned quite unfavorable loading by the distributed compressive force should most likely result in spectacular failures of compressive posts during earthquakes.  AFAIK, that rarely - if ever - happens.  This means that both "m" and "qc" are indeed distributed along the substantial portion of the length of the shear wall.  
 
Steve Gordin, SE
 
----- Original Message -----
From: Mattesons
Sent: Thursday, August 05, 2004 10:06 PM
Subject: Re: Load Capacity of a Holddown Post as a Compression Member

Nels,

My former employer had a text that showed an iterative method for
determining the capacity of a post loaded incrementally along its length.
Sorry, I don't remember the title or the author.  What I  DO remember is the
answer:   You got about a 10-percent increase over what the post could carry
if loaded concentrically at its ends.  NOTE--this does not take into account
the eccentricity of the shear panels exerting loads along the face of the
post rather than its centerline.  It also does not account for all the bad
things that happen to shear walls during construction.  Unless you have
symmetrical shear panel installation on both sides of the post (and build
the shear wall yourself), err on the side of caution.

Best wishes,

Thor

www.shearwalls.com


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