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RE: VERTICAL SEISMIC COMPONENT

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After Northridge, I observed several situations where roof framing members
(wood) failed at existing roof top units. 

	
T. William (Bill) Allen, S.E. (CA #2607)	
ALLEN DESIGNS	
Consulting Structural Engineers	
http://www.AllenDesigns.com	
V (949) 248-8588	 .	 F (949) 209-2509	


-----Original Message-----
From: Mark Gilligan [mailto:MarkKGilligan(--nospam--at)compuserve.com] 
Sent: Tuesday, November 09, 2004 10:59 PM
To: INTERNET:seaint(--nospam--at)seaint.org
Subject: RE: VERTICAL SEISMIC COMPONENT


Contrary to previous postings the 1988 UBC considered the impact of vertical
accelerations.  It was the opinion of the SEAOC Seismology committee that
special provisions were needed only for cantilever members and post
tensioned members.

While vertical accelerations are real there have not been significant
problems as a result of the failure to specifically account for such
members.  Typical members have enough capacity to resist expected upward
accelerations.  The load resulting from increase in downward accelerations
is typically accomodated by the normal factors of safety for member design.
Before we spend too much time worrying about vertical accelerations I would
suggest that wefirst identify evidences of failures resulting from vertical
accelerations.

Mark Gilligan

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