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Re: Questions about Residential design and 1997 UBC

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1. I also use Rw=6 on all of my wood framed jobs. First of all, at the start
of my lateral analysis, I do not know if all my shear walls will be plywood.
If any one of my shears are 180plf or less, then I specify stucco, then my Rw
is OK. If it turned up all plywood, then I'm conservative Still ok with me.
I'm not going to recalcualte all my shear walls.

I checked the SEAOSC Blue Book and found the following:

C104.6.1.............Rw=8 is...............for buildings which have
demonstrated excellent resistance to strong ground shaking when designed
,detailed and constructed to good engineering standards..................Also,
Rw=8 is based on the condition that the diaphragm does not have excessive
openings and/or discontinuities and the wall system is reasonably continuous
and complete down to the foundation level...............Structures with large
openings, such as lower level garage spaces  having only column and narrow
wall piers supporting the upper stories , should use the Rw=6 or a lower
value.

So, unless I am designing a very simple shaped building with no vertical or
horizontal irregularity, all plywood shearwalls, no narrow shearwalls, no
large openings in the diaphragms, and I provide structural observation making
sure that the contractor follows my structural plans and details without
having to do a compromise fix on a mis-built item in the interest of
practicality, then and only then will I use Rw=8.

2. Having to do a rigid diaphragm and shearwall relative stiffness analysis
to correctly distribute lateral loads is OK with me. I posted on this
listserve my agreement with this concept before. If you want see my post,
check the Seaint e-mail archive for ErnieNSE as author and I have two posts
dated  31Aug 1998 and 1Sep 1998. 

As a summary, I said that....... the 1997UBC Chap16 Div IV 1630.6 defines
flexible diaphragm and states that ............story shear shall be
distributed  to the various elements of the vertical lateral resisting system
in proportion to their rigidities, considering the rigidity of the
diaphragm.....I did a sample calculation for a one story wood framed garage
and based on UBC97 provisions and formulas for diaphragm and shear wall
deflection, I came up with the conclusion that the diaphragm is rigid. 

Regarding the complexity of calculations, we can develop simplified or
approximate formulas for plywood shearwall rigidity and make a table similar
to the one at the back of Armhein's masonry book. This will make it a little
bit easier. But we still have to develop approximate formulas for determining
the relative rigidity of  diaphragms in relation to the relative rigidities of
the various shear wall lines. Or do a 100% rigid diaphragm and a 100% flexible
diaphragm analyses and pick a number in between using judgement.

4. My stand on UBC Conventional Framing Provisions has been to leave it alone
for now since the chance of upgarding it to our(structural engineers) standard
is slim considering the politics involved. Let's just work on trying to add
additional requirements on jobs approved based on this provision on the local
building department level by issuing addendums, etc....Then let's join the
politics of upgrading these provisions on the next code revision, compromising
with other trades if neccesary (which is better than nothing). Keep on trying
at every code revision cycle, until we achieve 100% satisfaction.

5. I'm not comfortable with the perforated plywood methodology discussed at
the wood fair. I prefer a detailed structural analysis of the wall similar to
anlysing a perforated concrete wall. Using FBD and relative wall rigidity of
each section, we can determine shear at each section, loads at corners of
openings, etc..then we can detail the wall properly for straps across door and
window headers. Until testing is completed and the results accepted by the
structural engineering  community, we should develop a theoretical approach.

Ernie Natividad