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Re: Dynamic base shears, is it considered?
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- Subject: Re: Dynamic base shears, is it considered?
- From: "Bill Sherman" <SHERMANWC(--nospam--at)cdm.com>
- Date: 26 Feb 98 09:58:35 -0500
Tom Chiu wrote: >The response spectrum base shears are often higher than the Code static base >shears. I think you got confused here, Dynamic base shears should be checked at the >ultimate stress level with ductility taken into account (i.e. the R factor), while >the Code static base shears should be checked at the allowable stress level. Tom, I would disagree that the dynamic base shears are necessarily at ultimate stress level. In the code formula, Z and C are intended to represent an approximation of the response spectrum curve (with Z as the ground acceleration and C as the period dependent response). Thus a site specific response spectrum and dynamic analysis is simply a more accurate representation of the seismic response, regardless of whether you use allowable stress design or ultimate stress design. Also, note that the 1997 UBC seismic formulas represent ultimate stress levels rather than allowable stress levels. The Z-values did not change since Z represents "actual" ground acceleration, but the R-values were reduced to magnify the response to ultimate levels. Thus it is in the choice of R-values that determines the stress level being designed at, not whether it is a dynamic analysis. However, your response spectrum should be based on a seismic event with the same recurrence interval as used by UBC (i.e. 10% probability of being exceeded in 50-years) - unless you have special design requirements for the project, where a "maximum credible earthquake" may be determined. >As far as I know, when I was working for a firm designing highrises here in LA, it >is standard practice to scale the dynamic results to Code base shears and checked >v.s. allowable stresses. Sometimes we also check the dynamic results v.s. >ultimate( yield)capacity level and calculate the D/C ratios to see whether they are >acceptable. I believe the standard of practice hasn't changed that much. Since the code allows scaling down dynamic results, this method is often used as project criteria to account for the higher than code base shears. But if you have a project where operability is critical after the seismic event, such as critical utility structures, it would be more appropriate to use the site-specific response spectrum as a direct substitute for the code formulas. Then R-values and load factors should be applied as applicable to the design method (allowable stress vs ultimate stress). (R-values could be reduced even more than code-specified values if reduced damage levels are required.)
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