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# Re: 10/lw term in calculation of rho

• To: seaint(--nospam--at)seaint.org
• Subject: Re: 10/lw term in calculation of rho
• From: Robert Shaffer <rkdn(--nospam--at)cruzio.com>
• Date: Wed, 27 Oct 1999 13:25:24 -0700

Dear Dan,

I am also aware of the fact that there was some discussion and possibly even intent to have the 10/lw not exceed 1.0 .  However,  the code is quite clear and we are penalized for having less than 10' shearwall.  As far as I know,  there will be no revisions to the 1997 UBC,  so until the new IBC is adopted it is what it is.  I do not have a copy of the IBC so I do not know what the requirements will be under that code.

Another considuration in evaluating this equation is that this is part of an attempt to quantify the redundacy of the lateral resisting elements.  Therefore, should a single 4' shearpanel  be equivelent to two 4' shearpanels or of a single 10' shearpanel.  With the current debate over whether or not to consider a plywood diaphragms as rigid relative to the supporting shearwalls then the question becomes rather complex.  ATC 7 suggests that it would be prudent ( in designing shearwalls )  to evaluate plywood diaphragms as both simple spans and with continuity when it spans over three or more supports.

So how much does the cost increase for a typical house for a few more nails and an extra bolt at the hold-down vs. the extra time the engineer spends convincing the B.O. the code is wrong.

Robert Shaffer, P.E.
Santa Cruz,  CA

Dan Morrow wrote:

In a 10/25/99 post by Robert Shaffer, he commented that if you have less
than a 10 ft shearwall you are penalized by the 10/lw term when calculating
ri.  I attended an SEAOC/ICBO seminar here in Seattle a couple of weeks ago
on the Seismic Design Manual, Vol 1.  One thing that the moderator brought
to light is that the term 10/lw was never meant to exceed 1.0.  Apparently
this fact is not documented in the 1997 UBC or commentary.
So if your structure has shearwalls less than 10 ft long there is no
penalty.
The little things they (the code folks) forget to mention!

Dan Morrow, EIT