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Re: Plywood shear wall grade beam

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-----Original Message-----
From: Cheng Ming Chu <cchu(--nospam--at)earthlink.net>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Friday, August 27, 1999 12:56 AM
Subject: Plywood shear wall grade beam


>Hi there:
>
>One of our residential wood frame project is being requested to design
>the plywood
>shear wall grade beams per ACI 318 Chapter 21/UBC Section 1921 by the
>peer review engineer.

The peer reviewer is in error. Read the section carefully (hmmm after the
wall attachment explanation or earlier posts, maybe the words aren't
intended to say what they say). Members which "deliver seismic loads to the
foundation" .... It doesn't say THE FOUNDATION must meet ductility
requirements.  It is not possible to make foundations meet all the criteria.
It is not desirable to make foundations ductile, i.e.; to dissipate energu
thru yielding, because the effects of yielding in the foundation can seldom
be accessed for examination or repair.  IT IS necessary to design,
proportion and detail foundation elements to avoid failure before seismic
elements have done all their work . This is most commonly done by designing
foundations to a strength greater than the probable strength of the members
which deliver seismic forces to them. Thus the moment capacity of grade
beams should exceed the moment capacities of moment frame columns ( and the
resulting shear forces), spread footings which support pinned base columns
should have shear and flexural capacity greater than axial load equivalent
to the probable beam forces above, or the probable axial capacity of the
column, whichever is less. And yes I do mean that the strong column-weak
beam does NOT apply to the foundation.
Similarly, a grade beam supporting a plywood shearwall need only provide
strength equal to the probable capacity of the wall, translated to moment
axial and shear forces.