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

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I have been using all plywood shear walls ever since the initial capacity 
reductions were made as a response to the Northridge earthquake damage. As a 
result I was not aware that gyp board shear values were reduced even to 30 plf!
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.

Perhaps someone else can comment on this-

In observing the damage to multifamily residential and single family 
residential construction in the most highly accelerated areas of the Northridge
earthquake influence zone, the prime failure appeared to be highly loaded, 
eccentric plan, stucco shear walls. Over and over, three story 
(semi-subterranean garage) multi-family construction appeared to be extremely 
vulnerable. The gyp board main floor shear walls and the second floor shear 
walls in most cases appeared to be relatively intact- often the entire upper 
structure moved as a 'block' as the ground floor shear walls failed.

In practice, I have avoided using stucco as a shear element at ground floor 
under a multi-family building as such. Many reasons for this, but right off the
bat, the brittle characteristic of stucco should lead a designer away from 
using this material in a critical location, particularly where little 
redundancy in the construction is available. Ground floor garages in 3-level 
apartments clearly fit the low-redundancy description.

I think the elimination of gyp board and stucco as a shear element in highly 
redundant residential construction is a knee-jerk over-reaction. In past 
practice, I have only seen one S.E. (who will remain nameless!) state in his 
calcs "all gyp board walls are shear walls" and calc stresses based on the toal
linear footage of wall available. The typical application involves only a 
fraction of the total gyp board available. Hence, redundancy and an actual 
higher-than-calculated factor of safety exists.

Inspection is always a critical element in construction. Without a verification
that actual construction meets the design requirements stated in the 
construction documents, performance of the constructed structure cannot be 
relied upon. I think this is a strong argument for structural observation 
requirements. It is unfortunate that the economical option of using typical 
wall sheathing material as the primary lateral force resisting element has been
effectively eliminated as a result of poor follow-up in the field.


----------------------------- Note follows -----------------------------
Message-Id: <01BB5C9C.DD20F9C0(--nospam--at)dialup13.power.net>
From: Shafat Qazi <seaoc-ad(--nospam--at)power.net>
To: "'seaoc(--nospam--at)power.net'" <seaoc(--nospam--at)power.net>
Date: Mon, 17 Jun 1996 22:32:26 -0700
Encoding: 68 TEXT, 64 UUENCODE
X-Ms-Attachment: WINMAIL.DAT 0 00-00-1980 00:00
Precedence: list
X-Loop: seaoc(--nospam--at)power.net
Subject: [SEAOC] RE: [SEAOC] Gyp. board shear walls in LA
Reply-To: seaoc(--nospam--at)power.net

It is my understanding that the LA reductions are a combination of several 
things:

1. Over estimated capacity of Gyp board shear wall.
2. Lack of inspectors.
3. Poor construction quality.

The truth is that a gyp board is worth much more than 30 plf. Until LA city 
finds out what that value is, they decided to lower it to 30 plf. Actually, 
what I heard is that after Northridge earthquake, they were going to 
prevent use of gyp board shear walls. However, the gyp board industry lobby 
negotiated a 30 plf value. Something is better than nothing.

Shafat


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******************************************************************
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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