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FW: Seismic Overturning & Response Spectrum Analysis

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Title: FW: Seismic Overturning & Response Spectrum Analysis

We've been doing some preliminary studies on foundations for some tall non-building structures in seismic zone 4 (heights exceeding 200 feet).  The size of the mat foundations is controlled by gravity loads plus overturning.  We conducted these studies using the Static Force Procedure of the 1997 UBC (Section 1630.2).  We are now backing checking to see what the results are when a response spectrum analysis is applied.

1997 UBC Section 1631.5.4 limits the reduction in base shear compared to that computed by the static force procedure.  Is it the intent of the code to limit the reduction in the foundation overturning moment by the same amount?  While we find only a modest reduction in base shear, the reduction in overturning is quite substantial.

We've noticed that the 1997 UBC does not include the procedure to reduce foundation overturning for tall buildings when using the static force procedure that is found in the 1994 NEHRP Recommended Provisions (Section 2.3.6) and in the other national codes (ASCE 7-95 section, 1997 SBC section 1607.4.4, and 1996 BOCA section 1610.4.4).  The maximum allowed reduction in those codes is 75% of that based on all the story shears acting concurrently.   However, these provisions do limit the additional reduction in overturning moment determined by modal analysis to 10% compared to that determined using the static force procedure.

It seems to us that it is reasonable to use the response spectrum analysis overturning moment, times the ratio of the computed base shear to the base shear

as limited by UBC 1631.5 but perhaps to no lower value than allowed by the 1994 NEHRP Recommended Provisions.  Any comments would be appreciated.

Ed Marshall, PE