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# Re: Plywood rigid diaphragms

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Plywood rigid diaphragms
• From: ErnieNSE(--nospam--at)aol.com
• Date: Sat, 29 Aug 1998 10:42:38 EDT

```Michael Cochran says:

"Los Angeles has adopted a new voluntary ordinance called Division 93:
Voluntary Earthquake Hazard Reduction In Existing Wood Frame Residential
Buildings with Soft, Weak or Open Front Walls.   In this ordinance, you are
required to calculate shear wall deflections to show that the deflection is
less than .005H.   The shear wall deflection equation is from the UBC Volume
3."

I think this is good but they could have gone further by requiring that, for
walls in the same direction, if the relative rigidity is such that one wall
line is 50%(or any number thay want) or more rigid than any other wall line,
the distribution of lateral loads to the shear wall line should be by relative
rigidity assuming a rigid diaphragm. But they should require that the
distribution at each wall line should not be less than the value obtained
using the traditional tributary width-flexible diaphragm analysis.

You said "Using this equation, the biggest part of the calculated shear wall
deflection
is due to holdown slip (minimum 1/16" oversized hole in post for holdown
bolts,.............................."

How about requiring that all holdowns be tightened such that we finger tighten
the nuts on the bolts to the posts or studs first, then, finger tighten the
bolt on the anchor and then turn it three(or less/more) revolutions with a
wrench before finally tightening the post bolts. This should reduce the
holddown's contribution to the shear wall deflection.

You said "This becomes a nightmare when you try to distribute the load by
rigidity
though, since you do not know the rigidity of the plywood shear walls,
........."

Yes we do not have an exact formula for plywood shear wall deflection
considering the contribution of the holdown (and don't forget the slip in the
sill bolt on the sill plate hole). But if we consider the "relative" rigidity
using the inexact but approximate deflection formula to distribute the lateral
forces, since they will all be subjected to approximately the same
unpredictable and unquantifiable conditions(assuming we are analysing all
plywood shear walls), then this should be good enough, at least better than
not considering relative rigidity at all.  That is, until we have more test
and results converted to formulas which the engineering community will agree
upon and accept.

You said "I think the concerns are that if you design strictly by tributary
area, you
may end up underdesigning certain walls.  In a simple box shaped building with
perimeter shear walls only, the short wall on one side is designed for 50
percent of the tributary area, the long wall on the other side is designed for
50% of the tributary area.  The short wall deflects (never sees the full 50%)
and the long wall now sees 70-100% of the load when it was actually just
designed for 50% of the load.  Now the question is how to come up with a
reasonable solution, which can be implemented within the design fee structure
that as engineers we typically get for designing wood structures.

Any suggestions?......."

See my first suggestion about checking the relative rigidity of shear walls in
the same direction. Let's do some trial calculations of relative rigidity of
different combinations of full length plywood shear walls at the back and a
series of short walls at the "soft" front, using the only deflection formula
we have so far. Then pick a combination that we feel warrants a distribution
other than our standard "50-50"distribution, convert it to a percentage of
relative rigidity difference, round it off to a "nice" number(which I notice a
lot on our code) and use it as a criteria when relative rigidity calculations
should be required. Again, use the greater of the values obtained by using
both the relative rigidity distribution and the traditional distribution.

These are my suggestions. And I'm intersted in finding out what SEAOSC or the
presenters in the November seminar on this topic are going to say.