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Re: Steel strsss increase for seismic loads

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I believe wood uses the duration factor Cd so it can vary from 0.9 to 1.6.

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