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RE: Steel strsss increase for seismic loads

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Wood technically is not really the same as the 1/3 stress is a load duration factor...even if it produces a similar "effect".
I will further note that some local jurisdictions further restrict the load duration factors.
Adrian, MI

From: Larry Hauer [mailto:lhauer(--nospam--at)]
Sent: Tuesday, April 21, 2009 5:36 PM
To: Struct. Eng. Assoc.
Subject: RE: Steel strsss increase for seismic loads

Thanks Tom, that is what I thought with WOOD and masonry the only materials being allowed the 1/3 increase. (Oh well!).

To: seaint(--nospam--at)
Subject: Re: Steel strsss increase for seismic loads
From: Tom.Hunt(--nospam--at)
Date: Tue, 21 Apr 2009 14:22:48 -0700


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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