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Re: 97 UBC 1921.3.3 for Plywood Shearwalls on Concrete Slab Found

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From: "Swingle, Mark" <Mark.Swingle(--nospam--at)dcp.state.ma.us>
To: "seaint (E-mail)" <seaint(--nospam--at)seaint.org>
Subject: Re: 97 UBC 1921.3.3 for Plywood Shearwalls on Concrete Slab Found
Mark,
Thank you for the correction. When I re-read my comment I realized that what
I stated is not the point I was trying to make and rather I screwed up:o) I
did not mean to imply that the wall does not induce a moment into the
foundation, however, what I was trying make the point that the capacity of
the strap connection and it's connection to a double stud (this is a nailed
connection) would fail before the concrete yields - there is not sufficient
force at the maximum capacity of the strap to fail the foundation in bending
when adequately reinforced with at least the minimum #4 continuous bars top
and bottom.
You are absolutely correct as the vertical components to the uplift and
compression on the wall creates a couple that places the foundation in
bending.
I forwarded your response to the engineer for reference and do appreciate
the help.

regards,
Dennis


Dennis,

I agree with you that the plan checker is incorrect in asking for hoops in
the foundation, but for a different reason than you.

A strap (or any other holdown) certainly will induce a moment on the
footing.  It is a point load, and point loads induce moments on beams all
the time.  The net forces on the thickened edge foundation are due to the
overturning of the shear wall, and the foundation must be designed for this
moment in combination with other forces per the code.

The detailing, however, is a different story.  UBC 1921.3.3 is for elements
of a concrete frame when the structural system is a SMRF of concrete, using
R=8.5, NOT for a light-framed wall with shear panels, using R=5.5, as in
your case.  The reason for the specific detailing in 1921.3.3 is to maintain
a confined core in the beam after repeated inelastic cycles and spalling of
the cover concrete.  This is not required for foundations.  Trying to claim
that the foundation is a portion of a SMRF is stretching the intent of the
code to absurdity.

I agree that the language, taken out of context, of UBC 1921 can be
misleading, since it states that it is for "Reinforced Concrete Structures
Resisting Forces Induced by Earthquake Motions."  However, taken as a whole,
the code clearly intended this section for structures whose PRIMARY (above
the ground, that is) LFRS is of concrete.

This particular provision does not apply to your building, however, I will
repeat that the foundation must be designed to resist the induced forces.

Mark Swingle