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[SEAOC] [SEAOC] Re: Gyp. board shear walls in LA

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----------------------------- Note follows -----------------------------
Date: Sun, 23 Jun 1996 20:45:19 -0700
Message-Id: <199606240345.UAA07193(--nospam--at)scn.org>
From: yaheymoo(--nospam--at)scn.org (Greg Varney)
>Although that is _something_, it seems to me that 30 plf is practically 
>nothing...as far as I can recall, whenever there's been even a generous amount 
>of wall to utilize as gyp board shear wall, typical construction seldom results
>in stresses being much lower than about 50 plf ... so basically gyp board has 
>been practically eliminated as a shear wall option.
My opinion is that GWB is nice and pretty, but it does not have any 
post-earthquake strength.  I will try to stay as far away from that 
material as I can for my clients.

Greg

A follow-up thought on the reduction from 100 plf to 30plf for 1/2" GWB:

Quite a drastic reduction from 100 to 30plf (I doubt that any used the 
'blocked' values which are 25% higher), a factor of over 3.  All of a sudden I 
am experiencing the reaction that my wife had right after Northridge- "But I 
thought these buildings were built to meet the building code. Isn't the 
building code safe?"

My explanation to her was that the building code is intended to prevent
collapse but does not limit building damage. 

Are we now changing the building code away from that to a higher performance
standard? This implies an economic change and financial impact to construction 
in many cases. As an engineer I justify the additional cost (I have heard 
figures anywhere from $2 to $5 additional per square foot). How does overall 
society figure this?

I understand the need for post-earthquake reserve strength. I have questioned
why we use less than 1/5 of the effective peak acceleration 0.4G in the UBC,
with materials that typically have allowable loads tabulated with a factor of
safety anywhere from 2 to 3, not 5 or 6. But as it has always been explained to
me, building code is intended to provide the minimum necessary for human life
to escape the hazard, and not much more. This was always deemed the
'cost-effective' and 'prudent' solution. However, if the building 
department maintained that it was an 'essential facility' you were forced to
use 50% higher forces (later reduced to 25% higher with the revised I = 1.25).

The postulated 50% higher EPA of 0.6G was closer to the truth. The
accelerations obtained from Northridge were 0.8, 0.9, even over 1G in some
cases! Is it some surprise that we had numerous failures of buildings 'that
were built to code'?

It seems that the real problem with the current building code is not so much on
the philosophical minimum survivability side nor the material side, but on the
force and motion side. In dealing with seismic forces, there are a lot of
unknowns. Nevertheless, we can observe that the 1987 Whittier quake (5.8
magnitude I believe) generated 0.4G at epicenter. That was a small 'moderate'
earthquake...and we reached the 1985 UBC Z=0.4 value. The Northridge earthquake
(6.8 magnitude) generated twice that value in focused areas, and 0.5g over a
large affected area of the San Fernando Valley, exceeding the 1991 UBC Z=0.4
value. Northridge has been called a 'moderate' earthquake in an urban area.
Current opinion in the earthquake engineering community is that we can expect
'moderate' events every few years, and we are still waiting for 'the BIG one'.
We can expect near-fault accelerations much higher than our 'safe' building
code provides for, with accompanying catastrophic collapses.

How many years has Table 47-I been in use, and buildings constructed with the 
values tabulated? That's a lot of buidings. What do we do with them?

******************************************************************
Thomas D. Honles, S.E.
Phone: (213)367-0006   LADWP, Los Angeles, California, USA
Internet e-mail: thonle(--nospam--at)dwp.ci.la.ca.us
          (also: tdh(--nospam--at)capella.dwp.ci.la.ca.us)
******************************************************************


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