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Re: Lateral decision -Reply -Reply

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Barry H. Welliver asks how Building Officials interpret Condition # 5 in the
acceptance criteria for retrofitting lateral-force-resisting-systems in UBC
Section  3403- " An unsafe condition as defined above is not created."
He states"There are examples of such, in Los Angeles I believe, where
the "soft" story of a damaged building was "strengthened" only to have
pushed the damage upward in the building during a later earthquake."

Barry, I do not know of any retrofitted buildings that performed this way 
but I do know of ones which did not. There were apartment buildings
where only the ground floor was strengthened and the building
performed adequately. Dwellings with raised floors (crawl spaces) also
performed well with only the underfloor level strengthened.

We considered this problem in the Division 93 Retrofit Standard when we
allowed phased construction. Our solution was to require minimum story
strength ratios.  Wood frame buildings are more tolerant of these story
strength differences that other building types may be.

"Unsafe" should simply mean that some structural element is substantially
worse off than before and  has neither the strength, stiffness or ductility
to handle this significant change. Building Officials should be certain that
the design engineer has properly analyzed the "before" and "after"
before approving the work.

Tim McCormick, P.E.
City of Los Angeles