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B31.3 & FEMA 450 - seismic design of process piping

B31.3 & FEMA 450 - seismic design of process piping

B31.3 & FEMA 450 - seismic design of process piping

(OP)
FEMA 450 Section 6.2.6.1 "Allowable Stress Design" gives an 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 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.


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