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UBC97 Section 1633.2.11

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Title: RE: Engineering compensation


In the UBC97 Section 1633.2.11 regarding building separations and setbacks, the exception states that "smaller separations and set backs may be permitted when justified by rational analysis based on maximum expected ground motions." This exception is vague in its reference to the "maximum expected ground motions". It is clear that the maximum expected ground motions refers to the 10% in 50 year ground motions as well as the use of maximum inelastic displacement as a part of the rational analysis. However, it is unclear as to whether any lower limits should be enforced for this motion.

For example, if one was performing a response spectrum analysis using a site specific design spectrum, a lower limit of 80% of the code computed base shear is imposed for strength design assuming the building is a regular structure (Section 1631.5.4. item 2). It is understandable that the code provides a lower limit for the strength design of structural elements in order to provide some consistency in the in the minimum code values versus site specific values. However, is this lower bound limit also imposed here for drift calculations for building separations? Often, site specific design motion may be significantly less than the UBC prescribed design spectrum and to provide a large building separation and setback based on seismic forces that will, in all probability, never be experience seems too conservative, not to mention a waste of space.

The code does "hints" that some leniency is provided when it comes to deflection calculations. Section 1630.10.3 states that for interstory drift calculations, the lower design forces limits from Formula 30-6 may be disregarded. Also, the 30 or 40 percent period cap from the Method A period calculation also be neglected.

Can anyone shed some light on this matter?


David Murphy
Murphy Burr Curry Inc
85 Second Street, Suite 101
San Francisco, California 94105
Voice: 415-546-0431x4003
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