Need a book? Engineering books recommendations...
[SEAOC] Seismic Rehabilitation of Buildings using FEMA 178, 172 or 273 Provisions.[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaoc(--nospam--at)seaoc.org
- Subject: [SEAOC] Seismic Rehabilitation of Buildings using FEMA 178, 172 or 273 Provisions.
- From: Sadre(--nospam--at)aol.com
- Date: Tue, 3 Dec 1996 03:06:18 -0500
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.following items are such as various damage indices (how to explicitly account for ...
- Prev by Subject: [SEAOC] Seismic Rehabilitation of Buildings
- Next by Subject: [SEAOC] Seismic Rehabilitation of Buildings using FEMA 178, 172 or 273 Provisions.
- Previous by thread: [SEAOC] Re: Proposed voluntary vendor standards
- Next by thread: [SEAOC] Seismic Rehabilitation of Buildings using FEMA 178, 172 or 273 Provisions.