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Re: 1920's 9 story concrete frame building

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In a message dated 5/11/99 5:50:43 PM EST, smthengr(--nospam--at) writes:

<< A past client of mine is putting in a bid for a condo flat in Pacific
 Heights S.F. Million dollar views, very exclusive. It has a concrete frame
 in the short direction and property line infill walls in the long direction.
 It could be on bedrock. The floors are concrete pan joists. The building is
 9 stories with single units per floor. My initial reaction without any
 analysis is....not a good idea. Is there any possibility that a building
 like this could have any margin of safety during a near fault 7.0
 earthquake? Building plan records are available, but not until after the bid
 date is due. My only comment will be that this building is not expected to
 do well during a credible seismic event.
 Jeff Smith


I would think that there would be extensive damage, possible collapse 
depending on the number of openings at the first floor.  Our office has 
completed several seismic retrofits of concrete frame buildings of this 
vintage and story height.  The solution was to add new interior and perimeter 
concrete shear walls and brace the infill (URM or hollow clay tiles) where 
required.  Foundation work was usually extensive.  The design criteria was to 
limit the drift to 1.5% max of the building height using an unreduced 
response spectra (R=1) for maximum capable.  The biggest problem is trying to 
determine the stiffness of the existing building using actual concrete 
properties (f'c = 1500 to 3000 psi, rebar fy < 40 ksi, cracked wall sections, 
I effective <<< I gross) in combination with the new concrete walls.  Based 
on our experience, the structural seismic retrofit costs for the new shear 
walls only are probably in the range of $25-$30 dollars per square foot (use 
total square footage of building, all floors).  You can add to this any new 
Architectural and MEP costs.  

If you can limit the building drift to less than 1%, the building will 
probably perform fairly well during a moderate earthquake.

Hope this helps. 

Michael Cochran