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Re: ATC FEMA 273 Technical Seminar Series[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: ATC FEMA 273 Technical Seminar Series
- From: jpwykoff(--nospam--at)mmm.com
- Date: Mon, 16 Nov 1998 13:57:38 -0600
First, thank you Mr. McClure, for your many continuing contributions to our profession. In response to "...At the ATC FEMA 273 Seminar held in San Diego, California on November 5-6,1998, a Wood Frame Example was presented. This Wood Frame Example was a three story (36 feet high) by 75 foot wide by 120 foot long wood frame building braced by three story plywood shear walls. This building had a calculated weight = 864 kips and a building period = 0.88 seconds. The "Pseudo lateral load" (Base Shear), V = C1*C2*C3*Sa*W = 1.25 x 0.740 x 864 = 799 kips or 0.925 x 864 = 799 kips. (Note: 92.5% gravity) The "Pseudo lateral loads" on a single three story plywood shear wall 24 feet long are as follows: At the Roof = 50.1 kips, 3rd Floor = 57.3 kips, and 2nd Floor = 25.5 kips................................." I had the pleasure of attending the seminar in San Diego. Although I agree with all of what you have said, one speaker asked if anyone had ever seen a building overturn. (Where are the bodies?) I reminded him of the 1964 Niigata, Japan event, but that involved liquefaction. IMHO, normally, ground motion reverses direction faster than a building can overturn. Without ground failure, perhaps only a very unusual near source effect acting on a top heavy structure could create enough P-delta effect to topple it. The rock and roll may dissipate energy in soil damping if the structure survives. What do you think? One interesting reference is "Evaluation of Seismic Mitigation Measures For Art Objects", Agbabian, Ginell, Masri, and Nigbor---Proceedings 4USNCEE, 1990, Palm Springs, CA, page 3, Volume 3. It includes a figure 2 for overturning parameters of height and base width for a "representative earthquake accelerogram". Rocking stability criteria are based on Y. Ishiyama 1982, ref.  in the paper. Without foundation embedment, and having hard surface interfaces, these objects do overturn. The non-linear static "Pushover" method is another abstraction to overcome our limited knowledge of reality. I too wish codes could stay with free body equilibrium and physical meaning. I believe the seismic design of the future will have to use energy methods, but that is another thread. Nobody addressed the plan review or liability issues of "pseudo" lateral forces and capacities so much greater than current codes. Unfortunately, the structural engineering world is not as understood and valued as it should be. We have not demanded our share of research funds in relation to the uncertainties and risks to life and property. Each code and standard begins with an extensive disclaimer. It is fortunate for us that one Northridge victim owner became a benefactor in initiating testing of ONE construction detail. If all our organizations and material industries can work WITH us, we can all demand better fees and free up time to work on committees and accelerate the goal of performance based design. I like to say that without civil engineering, there is no civilization. Hopefully, we can cooperate better. Sincerely, with all the usual disclaimers, Jeff Wykoff, PE, CE, SE, depending on what state (geographical) I am in.
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