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[SEAOC] Seismic Rehabilitation of Buildings using FEMA 178, 172 or 273 Provisions.

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December 2, 1996

Dear Colleagues:

Frank McClure in response to Dave Evans' inquiry regarding the use of the
above mentioned documents provided a very informative and fairly complete
historical background on all these documents.  However, reading through his
comments, one could discern that he was advocating FEMA 178 over 273.  

It may be worth reiterating that despite being a "consensus standard", the
existing FEMA 178 Handbook has certain limitations.  For example, this
procedure is based on a single performance level which is less than the
life-safety standards, implicitly adopted by the current model codes.  The
obvious drawback here is the lack of explicit options for retrofit skemes
beyond marginal life-safety or collapse prevention.  

On the other hand, such options are available under FEMA 273 Provisions in
terms of immediate occupancy, or fully operational states at various ground
motion levels (say design GM or maximum considered GM).  Note that despite
slight differences in the  terminology, general concepts parallel those
outlined in the Vision 2000 document.

Additionally, the FEMA 178 uses the 1988 NEHRP Provisions as the bases for
assessing the seismic demand versus the FEMA 273 which similar to the
proposed 1997 NEHRP utilizes the Project 97 -- USGS Design Value maps -- for
determination of seismic hazards.  As a result, evaluation of the seismic
demands and the member (strength or deformation) capacities should not be
expected to render identical results when these two procedures are employed.

Lastly, FEMA 178 Provisions use a single global R (force reduction) factor to
modify the seismic demands which are then compared to the elastic capacity of
the lateral force resisting elements to determine their vulnerability.
 Again, FEMA 273 takes a quantum leap by introducing the "m" (ductility
modification) factors which constitute the framework of the component based
evaluation, considering truly expected forces and deformations; entirely
different from the FEMA 178 global system evaluation (masked by R-factors).
 This will go a long way to help make the basis of our seismic design
assumptions more transparent.

The preceding summary does not conclude that FEMA 273 is a panacea for all
rehabilitation work; on the contrary, it will be many years before all its
tabulated values and assumptions are fully tested, not just in the labs, but
through  retrofitted structures living-up to their designated levels of
performance under various levels of earthquakes.

Many issues remain to be addressed in the future editions of FEMA 273.  A few
are itemized below: 

1.   Various damage indices:  How to explicitly account for cumulative damage
due to    
      repeated excursions.  Note that drift as used in FEMA 273 is a
      damage index.  Also the back-bone curve is as implicit measure of this
      based on limited available component testing particularly on archaic