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[SEAOC] Seismic Rehabilitation of Buildings using FEMA 178, 172 or 273 Provisions.[Subject Prev][Subject Next][Thread Prev][Thread Next]
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- Subject: [SEAOC] Seismic Rehabilitation of Buildings using FEMA 178, 172 or 273 Provisions.
- From: Sadre(--nospam--at)aol.com
- Date: Tue, 3 Dec 1996 05:00:15 -0500
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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