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[SEAOC] Seismic retrofit criteria

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	It has been a little quiet, so here is a general question that might
generate some interest.  There has been some discussion about seismic
analysis of existing buildings relative to FEMA documents.  My question
is, what are the latest criteria when dealing with modifications to an
existing building that triggers a seismic upgrade?  Maybe I'm missing
something, but the FEMA documents and others don't seem to give specific
criteria which, if violated, initiate a seismic upgrade.  I'm thinking
more of specific criteria that I have run into in the past such as:
altering the stiffness of a shear wall or adding additional mass to a
building more than a certain percentage, altering the building more than
a certain percentage of the total cost of the building, etc.  To get
even more specific, does shifting an existing brace to an adjacent bay
necessitate a seismic upgrade of the whole building?  If the same brace
is simply relocated on the same column line for an interior remodel, I
would think not.  If the use of a building is altered such that new and
more mechanical units are required, does this increase in mass (and
possibly new mechanical floor area inside a building) require a upgrade
to the latest code?  Does a change in occupancy trigger a seismic
upgrade?  What is the latest thinking?  More often than not, I have seen
criteria like this in internal memos that are enforced by a given
municipality, such as the City of L.A. or OSA.  Are there intermediate
upgrades that are acceptable, such as 75% of the current base shear?
There may be two categories of responses to this issue: those based on
one's experience and judgment, and those based on specific criteria from
a given governmental agency.  Both of which may be valid in their own
way and would be interesting to discuss.  Specific criteria from
building officials would be interesting also.  

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