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RE: Steel Moment Frames per ASCE 7-05

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Casey,
 
The ordinance I have referenced applies to ALL types of frames, (OMF, IMF, SMF), and I would assume any type of lateral resistive system if you are required to design that system to comply with Sec. 12.4.3.2 of the ASCE. Why do we even have the load combinations of Section 1605 of the '06 IBC, ('07 CBC), or those in chapter 2 of the ASCE, if we have to design to these extremely conservative formulas, (I am coming up with the frame members of a moment frame as about 100% overstressed combared to designing to the IBC/ASCe chapter 2 load combinations)?



Date: Tue, 7 Oct 2008 16:09:14 -0700
From: khemmatyar(--nospam--at)gmail.com
To: seaint(--nospam--at)seaint.org
Subject: RE: Steel Moment Frames per ASCE 7-05


Larry:
I've seen similar amendments but have not come across a jurisdiction requiring the "Load Comb. with Overstrength Factor".
I believe with the old Code we did have some similar penalties when using OMFs.
Keep in mind that in Load Comb. Equations of 12.4.2.3 only ρ applies to QE.
But in Load Comb. Equations of 12.4.3.2, there is no ρ (which could be up to 1.3) and only Ωo applies to QE.
So in case of OMF, the columns and beams may be designed only for 3.0/1.3=2.3 times the Qforces (depending on ρ).
 

Casey (Khashayar) Hemmatyar, SE
khemmatyarAThotmail.com>
 
------------------------------------------------------------------------------------------------------------------------------------------------
From: Larry Hauer [mailto:lhauer(--nospam--at)live.com]
Sent: Tuesday, October 07, 2008 2:50 PM
To: Struct. Eng. Assoc.
Subject: Steel Moment Frames per ASCE 7-05
There is a jurisdiction in southern California, (Ventura County), that has issued an ordinance requiring that "Steel Frames in Light-Framed, (Flexible Diaphragm), Low Rise Buildings" be designed for the following load combinations, (if ASD), in the ASCE 7-05 manual:
 
Page 127, formulas 5, 6, and 8 on left side of page, AND
Page 127 formulas 5, 6, and 8 on right side of page, (overstrength factor applied)
 
I thought you only used the formulas with overstrength factors for support of discontinuous elements, collectors, etc., not for the design of actual frame beams and columns, (OMF, IMF, and SMF).
 
Can anyone shed light on whether the above requirements for the Eh of frames is also required in the jurisdictions they work with?
 
Thanks in advance,
 
Larry Hauer, S.E.
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