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

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RESEND THE FINAL VERSION (MY APOLOGIES, IT IS PASSED 1:00 AM AND MY TERMINAL
IS AS TIRED AS I AM; THE LAST 6 ITEMS WERE LEFT OUT IN PREVIOUS VERSIONS):

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.  

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, the 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 target earthquakes.

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

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

2.     Other damage indices:  Modal parameter measurements (also known as 
        softening indices) is a rapidly growing area in structural damage
assessment 
        that needs to be fully addressed.  This method is often applied with
ambient 
        vibration measurements.

3.     FEMA 273 should make it very clearly that the area under the push-over
load 
        displacement curve is not a good indicator of the cumulative damage
effects.

4.     Various indicators such as ductility demand, tangential interstory
drift, or floor 
        accelerations should be utilized for assessment of damage to the
strucutral 
        elements, non-structural components and contents, respectively.
 Reliance on 
        drift as the all inclusive parameter (per FEMA 273) can be misleading
and 
        may create a false sense of protection.

5.     Quantification of damage in terms of size, type, and extent of cracks
(flexural, 
        shear and bond splitting), reinforcing strains, crushing or spalling
are among the 
        necessary tools to assess the severity of damage after an earthquake
or 
        quantify it before-hand.  Tabulated damage descriptions in FEMA 273
could be 
        made more explicit and greatly enhanced in this regard. 

6.     Despite all its shortcomings, this document is a great step forward in
the 
        systematic rehabilitation of structures and all our colleagues, who
are interested 
        in this field, should be urged to obtain a copy and provide input to
BSSC and 
        their representatives during the balloting period.  

Currently, the SEAOC Seismology Committee is in the process of using this
 document along with ATC 34, ATC 40 and Vision 2000 to lay the foundation
work for the Performance Based Engineering Guidelines for new buildings.  I
am sure we will hear much more about these documents in the near future.

Sincerely,

Ali Sadre, Chairman                                                        
SEAOC Seismology Committee

c/o Esgil Corp.
9320 Chesapeake Dr., # 208
San Diego, Ca 92123
Tel: (619) 560-1468
Fax:(619) 560-1578
 




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