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Re: UBC97 question[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: UBC97 question
- From: "Bill Sherman" <SHERMANWC(--nospam--at)cdm.com>
- Date: 02 Mar 99 20:29:07 -0500
Regarding the following question raised: >The governing code for the project is UBC 97. Our current frame design software is capable of Static Force Procedure and Dynamic Analysis (utilizing UBC Normalized Response Spectra, NEHRP Normalized Response Spectra or El-Centro Response Spectra). I am having trouble understanding why there is such a huge difference in member forces and displacements/drifts between the results obtained using the Static Force Procedure and results obtained using the Dynamic Analysis. The Dynamic Analysis results are much higher than those obtained through the Static Force Procedure (typically 3 times larger). < The above procedure refers to "Normalized Response Spectra" used in the software - this terminology is consistent with the 1994 UBC but the 1997 UBC refers to "Design Response Spectra". At zero period, the 1994 "normalized" spectra had a value of 1.00 whereas the 1997 UBC has a value of "Ca". In effect, the 1994 curve is "normalized" to Ca = 1.00. Thus it is possible that the software is using a value of Ca=1.00 for the normalized dynamic analyses vs a much lower Ca for the static procedure - which could easily cause the magnitude of difference in results noted. I suggest that the software methodology with respect to response spectra values be reviewed.
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