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FEMA/ASCE SEEK INPUT FOR SEISMIC REHAB PRESTANDARD
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- Subject: FEMA/ASCE SEEK INPUT FOR SEISMIC REHAB PRESTANDARD
- From: FEMCCLURE(--nospam--at)aol.com
- Date: Tue, 24 Nov 1998 06:22:14 EST
- Cc: FEMCCLURE(--nospam--at)aol.com
Please find the attached email message concerning the subject matter.
<HTML><PRE>Dear Colleagues, Attached please find the following information concerning the ASCE FEMA 273/274 Prestandard process which is apparently following the same procedure as the ASCE FEMA 310 Prestandard process. What is a "Prestandard?" It sounds like the Federal Food and Drug Administration putting a new drug on the market before it has been properly tested. Considering the very preliminary results of the BSSC Case Studies Project, wherein 30 consulting structural engineering firms are applying the current FEMA 273 Guidelines to 30 "real" existing buildings, it is my considered judgment that there are some very significant unresolved issues in the current FEMA 273 that it is too premature for ASCE to be putting the current FEMA 273 through even a "Prestandard" process. One of my continuing concerns is how FEMA 273 addresses the calculation and resistance of the very large tension and compression forces resulting from the overturning moments at the top and bottom of the reinforced concrete foundations. These large tension and compression forces result from the application of the FEMA 273, Equation (3-6), V = C1*C2*C3*Sa*W to produce a "Pseudo Lateral Load" (Base Shear). This "Pseudo Lateral Load" is distributed vertically according to Section 3.3.1.B. Vertical Distribution of Seismic Forces (Note:the use of the words "seismic forces."), using Equations (3-7) and (3-8) FEMA 273, Section 2.11.4.1, Linear Procedures explains how to investigate for the stability of the structural system against overturning under the effects of these seismic forces (Note: the words "seismic forces."). I realize that the authors of FEMA 273/274 keep referring to their interpretation that the "Pseudo Lateral Load" is not a real "force", but a representation of a "displacement." However, if there is to be a complete load path, there must be some connection at the bottom of structural bracing systems (plywood shear walls, reinforced concrete and masonry shear walls, reinforced concrete and steel moment frames, braced steel frames, etc.) at the top of the reinforced concrete foundations that resist the horizontal shearing forces and the overturning forces (tension and compression) which must be resisted by the reinforced concrete foundation. These overturning forces are very large when calculated using the "Pseudo Lateral Load" equation (3-6), V = C1*C2*C3*Sa*W. I have heard the argument many times that "buildings do not or seldom overturn" when subjected to to earthquake ground motions. I agree with that statement. However, if there is to be a complete load path there must be some connection at the bottom of the structural bracing systems to the top of the reinforced concrete foundations. The horizontal and net overturning seismic forces must be resisted by the dead loads of the foundation that can be calculated on a "rational engineering basis" based on the FEMA 273 Equations (3-6), (3-7) and (3-8) with certain reduction factors that are clearly defined so that different engineers can calculated these horizontal and net overturning forces at the foundation in a consistent manner. I have asked the authors of the FEMA 273/274 for an detailed explanation of how to calculate these horizontal and net overturning forces at the foundation level, based on a "rational engineering procedure." I welcome comments from others concerning the above and other problems with FEMA 273/274. Frank E. McClure FEMCCLURE(--nospam--at)aol.com November 24, 1998 </PRE></HTML>
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