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Re: Steel strsss increase for seismic loads

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If you use the load combinations from IBC Section 1605.3.1 then a 1/3 increase in allowable stresses is not permitted.  If you use the IBC load combinations from Section 1605.3.2 then you are allowed to use the 1/3 increases but ONLY if allowed by the material chapter.  Unfortunately for steel neither the IBC nor AISC have the 1/3 increase factor anymore.  I believe the only material left is masonry and in some cases geotechnical soil bearing.

Thomas Hunt, S.E.

Larry Hauer <lhauer(--nospam--at)>
04/21/2009 02:09 PM
Please respond to seaint
"Struct. Eng. Assoc." <seaint(--nospam--at)>
Steel strsss increase for seismic loads

I am checking existing steel roof framing members for support of replaced roof top mechanical equipment. Obviously, I need to design per the '06 IBC/'07 CBC for seismic forces and will be using ASD equation 16-20 of the IBC/CBC. Based on Sec. 1605.3.1.1 it looks like a 1/3 increase in allowable stresses for the existing steel roof members is no longer allowed when considering seismic loads. Is this correct?

Thanks in advance,

Larry Hauer

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