Subject: Re: 1920's 9 story concrete frame building
Date: Tue, 11 May 1999 22:43:47 EDT
In a message dated 5/11/99 5:50:43 PM EST, smthengr(--nospam--at)sirius.com 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.
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.