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RE: AISC Seismic for ASD

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The factors applied to allowable stress in the AISC Part III seismic
provisions are intended to be used with the load combinations given in that
section - load combinations with E/1.4 would not apply to those provisions. 

IBC 2000 Section 1617.1.2 has a similar multiplier for allowable stresses
when using the "special load combinations of 1605.4". However, there is a
discrepancy between AISC and IBC. IBC states to use a resistance factor of
1.0 but AISC uses resistance factors less than 1.0 for this application. I
am currently corresponding with the AISC Steel Solutions Center regarding
this discrepancy, but it appears that this is a conflict which has not yet
been addressed in the codes or addenda/supplements. 

William C. Sherman, PE
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com


> -----Original Message-----
> From: Hood, Matthew O. [mailto:HoodMO(--nospam--at)ci.anchorage.ak.us] 
> Sent: Friday, December 05, 2003 2:23 PM
> To: seaint(--nospam--at)seaint.org
> Subject: AISC Seismic for ASD
> 
> 
> Seaint,
> 
> I have a question about the ASD alternative (Part III) of the 
> 1997 AISC seismic provisions. Section 4.2 allows the nominal 
> strength of a member to be calculated by multiplying the 
> allowable stresses by 1.7. According to section 4.3 the 
> design strength is calculated my multiplying this nominal 
> strength by the appropriate Resistance Factor.
> 
> Ok, so now I've got the design strength of my member. To me 
> this indicates I should be comparing the values calculated to 
> the required strength (LRFD) load combinations of the 
> building code, in my case the 2000 IBC. 
> 
> My question is: where is this called for in the code? I'm 
> using the allowable stress alternative in the seismic 
> provisions so could I actually use the ASD load combinations 
> (divide E by 1.4)!? This doesn't seem right to me, but I 
> can't figure out why it's wrong. Any thoughts?
> 
> Thanks,
> Matthew O. Hood, E.I.T.
> Anchorage, AK
> 
>  
> 
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