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1997 UBC Section 1634 - Non-building Structures question

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Title: 1997 UBC Section 1634 - Non-building Structures question

Section 1634.5 of the 1997 UBC specifies increased minimum seismic loads for "Other Nonbuilding Structures" relative to those for buildings.  It says that nonbuilding structures not covered by Sections 1634.3 & 1634.4 shall be so designed.  Section 1634.3 covers rigid structures and Section 1634.4 covers tanks with supported bottoms.  Section 1630.2 (non-building structures with structural systems similar to buildings) appears not to be exempted from the increased minimum.

In contrast, in the 1994 NEHRP Recommended Provisions and in ASCE 7-95, the exception applies to all three categories (rigid structures, tanks with supported bottoms, and non-building structures with structural systems similar to buildings).   See NEHRP section 2.7.5 and ASCE 7-95 section 9.2.7.5.

It seems that logically the exception should apply non-building structures with structural systems similar to buildings since the R factor is typically coming from UBC Table 16-N rather than Table 16-P.   I'm inclined to assume this is a typo in the code (although I don't find it corrected in the Errata).  Does anyone know of any other reason why the 1997 UBC deviates from the 1994 NEHRP Recommended Provisions in this regard?

Ed Marshall, PE