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RE: Wood-Framed Shearwall Compression Chords

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Dave,

Your are correct that the dead load is lumped into the chords of the
wall. In the tension side, the weight is the resisting moment to
overturning and thus reduces the tension force ... but on the
compression side, the calculation is typically lumping this forces as an
additive force to the overturning. The intermediate studs obviously have
some capacity and take load ... however, the overturning component of
the downward compression is distributed based on the distance to the
neutral axis (or pivot point) of the wall - you could do an analysis
based on this method, but it's probably not worth the effort - there are
too many assumptions in shearwall design to begin with to try to get so
exact.

The other thing to remember with the compression design of the end post
chords, is that there is a dead load from a header directly on this end
post.

-gerard


-----Original Message-----
From: Dave Adams [mailto:davea(--nospam--at)laneengineers.com] 
Sent: Tuesday, August 13, 2002 11:13 AM
To: seaint(--nospam--at)seaint.org
Subject: Wood-Framed Shearwall Compression Chords

Hello all,

I've got a copy of Donald Breyer's 3rd edition (I know there's a later
edition) for wood design and he describes the free-body diagram used to
determine the tension and compression chords of a shearwall.  If we
ignore reductions in the dead loads that the wall carries, a straight
use of the typical free-body diagram would indicate that the weight of
the wall and the dead load that it supports add a component to the
compression chord that is additional to the couple from the overturning
moment.  The FBD assumes no support below the wall between the ends,
thus simplifying to a two-reaction system.  This compression force comes
quickly during the check on the holdown, but I've always wondered if
this approach is overly-conservative for wood-framed shearwalls.  I
figure that the amount of "rocking" and realistic bearing of the wall
under a seismic event is difficult to quantify, so it's better/easier to
follow the FBD, but it still seems conservative.

Any comments?

Thanks,
Dave K. Adams, S.E.
Lane Engineers, Inc.
Tulare, Ca

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