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Re: WT Bracing for OCBRs

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As usual, you are a fountain of knowledge, Mr. Sprague.  

On Wed, Dec 15, 2010 at 3:33 PM, Harold Sprague <spraguehope(--nospam--at)hotmail.com> wrote:
I responded off line to Tom.  This is an issue that I have on a daily basis at work.  I took it to those that wrote the AISC 341 section in question.  I took it off line because the authors prefer anonymity and do not want to be mobbed by autograph seekers, paparazzi and stalkers;>) 

 

It is believed that the 0.30sqrtE/Fy is the correct interpretation for AISC 341-2005.  The 2005 footnotes try to cite most common applications for different members, and WT braces were not included since it was felt not to be that common.

 

Note that in 2010 this was relaxed for OCBF from 0.3 to 0.38.   Also modified was the footnotes in the table to avoid the confusion.   This change was brought up by others. The 2010 table has a footnote that says the ratio can be relaxed to 0.38 under certain conditions for braces in SCBF.  

 
Basically for WT braces as part of a OCBF there are no rules currently regarding compact element other than what is contained in the AISC 360.  I looked at the AISC 341 commentary and saw that the provision was driven by a concern for nonlinear behavior.  I have no problem with designing structures for a linear response.  That way I address the concern and I am able to maintain a reasonable structural design. 
 
There are many of us who design nonbuilding structures and we need a bit more latitude. 

Regards, Harold Sprague


 

Date: Wed, 15 Dec 2010 14:55:56 -0800
Subject: Re: WT Bracing for OCBRs
From: d.topete73(--nospam--at)gmail.com
To: seaint(--nospam--at)seaint.org


have you gotten an answer?  I agree with you that it would be the 4th item of table I-8-1.  I also agree that the WT 6x95 being incredibly conservative based on the expected demand...

On Tue, Dec 14, 2010 at 3:21 PM, <Tom.Hunt(--nospam--at)fluor.com> wrote:
I have a non-building steel piperack (similar to buildings) in SDC D where we are using an Ordinary Concentrically Braced Frame System (R = 3.25) with WT double story cross bracing.  For this, ASCE Table 15.4-1 requires AISC 341 detailing that gets you to Section 14.2 which says the braces must meet the requirements of Section 8.2b (i.e. the braces must be seismically compact) per TABLE I-8-1.

Now for the question.  Which item(s) in TABLE I-8-1 applies to WT flanges and stems in an OCBF system?  The way I read the table, it would be the fourth item down per the last part that says "....and braces [c]...." where footnote [c] second part is for "...braces in OCBF, Section 14."

Others read the table as the last item that specifically addresses stems of tees but footnote [g] only applies to STMF systems.

The reason for the questions is that with my first interpretation, the smallest WT that has a bf/2tf AND h/tw less than 7.22 (i.e. 0.30 x sqrt[E./Fy]) is a WT6 x 95 which would be rather ridiculous for a small structure.

Thanks,

Thomas Hunt, S.E.
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David Topete, SE



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David Topete, SE