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

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>December 3, 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 schemes
>beyond marginal life-safety or collapse prevention.  

>....<<


Frank, Dave Evans, Ali Sadre and other Colleagues:

There has been a recent discussion on the applicability of 
FEMA 178 vs 273 for the retrofit of existing structures.  
One thing that should not be lost site of, is that the 
premise for FEMA 178 is as an evaluation handbook.  
Specifically, it is intended to function as a means to 
quickly SCREEN FOR GOOD BUILDINGS, using criteria 
developed to offer a consistent rational basis of 
evaluation between engineers, buildings and building 
types. 

"Good" buildings by FEMA 178 standards are those that 
provide for "substantial life safety."  This is 
intentionally a lower standard than is implied for new 
buildings or buildings being retrofitted to comply with 
the performance intent for new buildings as covered by the 
Code (UBC).  The "substantial life safety" criteria is 
achieved by using a mean eq response, as opposed to the 
mean plus one std. deviation used by the Code.

Once, the task of identifying the "good" buildings is 
done, only the "bad" ones, by definiton remain.  The "bad" 
ones, those not meeting the FEMA 178 screening criteria, 
could be further evaluated using the more refined and 
demanding FEMA 273 "zero retrofit" procedures to see if 
compliance is met or if a structural seismic strengthening 
is required to meet the appropriate performance standard.

If FEMA 178 criteria is used to retrofit a building, then 
the engineer must understand the limitations implied and 
intended in its development.  FEMA 178 and 273, by 
definition are different standards.

One critical question to ask is:  "Is it appropriate to 
have an evaluation (screening) document, that represents a 
lower standard than is required in a design document?"



Edwin T. Dean, S.E.

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