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RE: Single or Double angle bending member

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I think it is a single angle problem. The angles can't buckle in lateral torsional mode together like they can in a back-to-back double angle section. How does one angle know there is another angle on the other side of the wall?

I see these angles a lot in cmu walls, with the tips of the angles facing each other. A guy told me the following method before and I suspect this is probably what a lot of engineers use. You figure out the section modulus of only the vertical legs of the angles, add these together, then limit the stress to 0.6 Fy based on this section modulus only. I don't know how accurate this technique actually is, probably not very.

To begin doing it correctly, I think sections 5.1 and 5.2 of the LRFD single angle spec would apply, specifically section 5.1.1. I don't think checking the LTB equation in section 5.1.3 is  correct, since each angle is forced to rotate about it's shear center (leg intersection) and this rotation is being resisted by the wall. So neither angle can buckle as a unit in lateral torsional mode. I am going to think about this some more tomorrow, I would really like to know if this is the correct approach.

Will

 



 

>From: "Joe Grill" <jgrill(--nospam--at)swiaz.com>
>Reply-To: <seaint(--nospam--at)seaint.org>
>To: <seaint(--nospam--at)seaint.org>
>Subject: RE: Single or Double angle bending member
>Date: Mon, 25 Apr 2005 14:32:37 -0700
>
>Yes, it is an interesting problem, one that I didn't need tody.  Anyway, the
>channel idea won't work (as you guessed) because of the width that would be
>required.
>Joe
>
>
>
>
>-----Original Message-----
>From: Charley Hamilton [mailto:chamilto(--nospam--at)uci.edu]
>Sent: Monday, April 25, 2005 11:28 AM
>To: seaint(--nospam--at)seaint.org
>Subject: Re: Single or Double angle bending member
>
>Joe -
>
>When you say "toe to toe", do you mean configured like a channel?
>Why not use a channel then?  From your description, I see a
>"channel-with-a-gap" header carrying a chunk of masonry wall above
>it.  Is this the correct picture?  Is a standard channel just too
>wide, then?
>
>The biggest issue with treating a pair of angles like a "double angle"
>is in relation to whether or not the angles will act as a unit,
>especially regarding buckling (either flexural or compressive).
>
>Things to consider:
>1) How are the two angles tied together?
>2) Are the ties frequent enough to cause the members to act as
> a unit?
>3) What failure modes can occur that might negate the "double angle"
> assumption?
>
>Sounds like an interesting problem.
>
>Charley
>
>--
>Charles Hamilton, PhD EIT               PGR and Lecturer
>Department of Civil and                 Phone: 949.824.3752
>      Environmental Engineering           FAX:   949.824.2117
>University of California, Irvine        Email: chamilto(--nospam--at)uci.edu
>
>
>
>
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