huangyhg

fema 450 and allowable stress design factor for seismic forc
fema 450, in its section on the design of architectural, mechanical and electrical components, chapter 6, has a method of converting the calculated seismic forces to those that can be used in codes that still use working stress methods for their acceptance criteria.
section 6.2.6.1 "allowable stress design" gives this expression for converting the strength design seismic forces that are calculated using eqn 6.2-1 to those that can be used in acceptance criteria that are in terms of allowable stresses:
"the earthquake loads determined in accordance with the provisions shall be multiplied by a factor of 0.7."
i am dealing with the seismic design of asme b31.3 process piping at elevations substantially above grade and am interested in reducing the design forces that are determined from eqn 6.2-1 to a minimum. i am keen to find out the theoretical background for this factor of 0.7 so i feel comfortable using it.
the commentary to fema 450 does not refer to or add any elucidation to how they came to use this figure. in the past i have seen structural codes that were in transition between working stress and strength methods and so have quoted load combination equations for both analysis methods. the figure they used tended to be a factor of 0.8 rather than 0.7.
i would be interested to receive any thoughts on how fema came up with this conversion.
in structural codes earthquake loads are given at the ultimate level (factored) with the equations used. to convert from the ultimate level to a working stress level load combinations that i have seen, namely in the asce 7 i believe, multiply the earthquake load e by a coefficient of 0.7. it's not a "stress increase" nor is it related to duration of loading, it's just keeping things consistent with lrfd or asd instead of mixing the two. i am pretty sure this is what i read and will look it up in the asce 7 when i get to work.

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