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Re: Dynamic base shears, is it considered?[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaoc(--nospam--at)seaoc.org
- Subject: Re: Dynamic base shears, is it considered?
- From: "Bill Sherman" <SHERMANWC(--nospam--at)cdm.com>
- Date: 25 Feb 98 14:17:09 -0500
(This message is being re-sent since it did not appear to go thru the first time.) There have been several excellent detailed descriptions of design practice in response to this question. This reply is intended to be more of a "brief summary" to clarify normal practice. UBC wind and seismic forces must both be considered - often times a rough estimate of each type of load will show which will govern design, so a full analysis of each condition is not always required. In most cases the "static lateral force procedure" is used for seismic design, unless the structure is complex or operability after an earthquake is critical (such as power plants or other utilities). (This method used to be referred to as the "pseudostatic method", recognizing that the forces are not really static.) Where a dynamic analysis is required for critical structures, the base shear must also be determined by the static force method as the minimum allowable base shear - but the full structure need not be analyzed in detail by the static force procedure. Thus most designs require comparison of more than one lateral force method but may only require detailed load development in all members for one design lateral force procedure. For critical structures, the response spectrum is often determined by a site specific seismic review by the geotechnical engineer. Where resulting base shears exceed the code minimums, the UBC allows scaling the results down to the code design level - however, I have designed critical utility structures for higher than code levels where the client wants improved probability of operability after a seismic event.
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