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Re: RE: Seismic Zones in Texas[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: RE: Seismic Zones in Texas
- From: Eddie Gonzalez <Eagonzal(--nospam--at)ENG.CI.LA.CA.US>
- Date: Fri, 08 Jan 1999 11:33:11 -0800
Gentlemen, it seems relisting a FAQ draft from Mr. Perkins, who is part of the Golden Colorado Project (Frankel's Project) may be appropriate: Mr. Dave Perkins of USGS was also kind enough to send me a copy of another proposed FAQ on the above subject which they may post in their website ( http://geohazards.cr.usgs.gov/eq/faq ). I though those on this list may find it interesting. Also, perhaps, some of you may share some insights on this issue which I hear has going on for some time on capped vs. uncapped maps ********** Edward There are two sets of maps: 1. The national probabilistic ground motion hazard maps. These are straiaght probabilistic ground motion and HAVE NOT BEEN CAPPED. 2. The maps prepared by the USGS Golden GIS staff, supervised by members of Frankel's group, according to the recipe given by FEMA-BSSC subcommittees, to be used as design maps in the FEMA Recommendations for Seismic Standards for New Construction. The FEMA-BSSC recipe includes capping the ground motions on the maps in such a way that the resulting design ground motions do not exceed the largest values in the previous code, EXCEPT that when deterministic ground motions from faults exceed the cap values, the deterministic ground motions appear on the map. The maps used as a basis for the code maps are the 2 percent probability of exceedance spectral acceleration maps for 0.2 and 1.0 sec period, but recontoured so that certain contours correspond to break points in the code. Design values are 2/3 of the map values. The result (in uncapped areas) is that design values are approximately 500-yr return period ground motions in the west and approximately 1000-yr return period ground motions in the east. I hope this clears up your question. Maybe we should be distinguishing between national hazard maps and national design maps. At the site we have put up ONLY the uncapped maps. FEMA is publishing the capped maps. The California/Nevada maps at the site represent only a "zoom in" on that part of the maps. (bold added by ed gonzalez) >>> Roger Turk <73527.1356(--nospam--at)compuserve.com> 01/08/99 07:49AM >>> No, Ken, you are not getting old and afraid of change. You are getting wiser in your older age. What is happening is what Henry Petroski describes as the 30 year (or 35 year) syndrome. Since 30 to 35 years is the approximate working life of an individual, what he/she has learned *not* to do in that time, and the reasons for it, has not been adequately passed down to the younger members of the profession, they *don't* know what experience has shown to be inadequate. Therefore, we start having a series of failures every 30 - 35 years. Compound this with educators that have had little or no practical experience who rely on computers that spit out answers in accordance with the instructions that *they* put into the computer and we are in for some *big* problems. In the early 1960's it was a real chore to get masons to put reinforcing in masonry, at least here in Arizona. ("All we need is a header every 5 courses and the wall is not going to go anywhere.") Finally we were able to get masons to accept the need for reinforcing, however, the new masonry codes erode this requirement for seismic zone 2 locations by extrapolating tests that were intended only for small, single story, lightly loaded masonry construction to all masonry construction in seismic zone 2. The USGS Seismic maps are not gospel. Depending on how the head of the USGS seismic branch wants to have the maps look will depend on what we see. Algermissen's first map (1971), based on historical seismicity, was quite a bit different than his second map (ca. 1985), based strictly on probability (and politically modified), which is quite a bit different than Arthur Frankel's map (ca.1990's), which is based on a combination of probability and historical seismicity, which is quite a bit different from his map that doesn't have the accelerations divided by 4. Have you seen the USGS map that shows California accelerations of 3.9g? I would venture to say that very few people have. A. Roger Turk, P.E.(Structural) Tucson, Arizona Ken Tarlow wrote: . > Is anyone else thinking this will be a disaster? or am I just getting . > old and am afraid of change.
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