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

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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

------------------

Dennis Wish wrote:

I received a call from a local engineer yesterday who wanted my opinion on a
plan check correction he received. The plan checker asked him to provide
Hoops (#3 shear ties) around the continuous steel rebar within a thickened
edge foundation of a slab on grade for each conventional plywood shearwall
installed - or to submit an analysis showing that it was not needed.  The
project is in Seismic zone 4. The shearwalls are typically low capacity
walls (around 350-plf) and the resistance to uplift is by use of Simpson
PAHD or HPAHD Tension Straps - range of about 2200 to 4500 lbs.

The plan check agency is requiring compliance at the slab edge to the
provisions for a Flexural Members of Frames - Transverse Reinforcement. 

My opinion to the engineer was this provision was intended for use by grade
beams designed to transfer moment from the base of a frame into the grade
beam and is inappropriate for a conventional plywood shearwalls at
continuous foundations.

My argument is that the wood wall can not develop moment in the foundation
through a strap - this is not a rigid connection, therefore the foundation,
which may develop a small flexure is much stronger than the wall tension
connection which would fail much earlier than the concrete acting in
bending.

Any arguments pro or con on applying this section of code for conventional
plywood shear walls?

Regards,
Dennis S. Wish, PE