Lateral Torsional Support 1

• Aug 21, 2007

• #2

swearingen

Civil/Environmental

Feb 15, 2006

663

Tests have shown that full depth shear connections on beams that are of equal depth to the main beams do provide adequate rotational stiffness to be considered braced points for LTB. If you have something considerably less than that, I'd be worried...



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

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• Aug 21, 2007

• Thread starter
• #3

waldo459

Structural

Mar 9, 2005

38

(3) bolt double angle connections on 16" deep beams. Where might I find the test results of which you speak?

 

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• Aug 21, 2007

• #4

apsix

Structural

Sep 21, 2004

1,358

I would say no, unless proven otherwise, as directed by my design code AS4100 (Australian).

From the commentary;

"C5.4.2 Restraints at a cross-section
The case of a cross-section that is restrained against twist rotation but free to deflect laterally is not included because of the difficulty of providing simple guidance.
Some very stiff torsional restraints may induce buckling modes in which the cross-section acts as if fully restrained (Clause 5.4.2.1), while in other cases torsional restraints may be comparatively ineffective, and the restrained cross-section may deflect laterally."

 

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• Aug 21, 2007

• #5

SlideRuleEra

Structural

Jun 2, 2003

5,528

Perhaps you can fillet weld a diagonal brace (typical 3 places) to the underside of the W16 flanges. The welding could be simple downhand, performed from the top of the existing beams. Suitable angles, say 3x3, should be sufficient to stiffen the walkway and prevent LTB.

http://www.SlideRuleEra.net

 

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• Aug 22, 2007

• #6

swearingen

Civil/Environmental

Feb 15, 2006

663

I don't have access to the tests - the information was from a stability conference given by Yura several years back.

In your case, even though the braces don't technically provide support for lateral displacement, they provide enough torsional support to prevent LTB.

If you have any appreciable lateral load, however, I suggest SlideRuleEra's solution.



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

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• Aug 22, 2007

• #7

shin25

Structural

Jul 4, 2007

430

I have designed these kind of structures before, from my experience I have seen that as long as the bracing members are of sufficient strength and they are connected very close to the top flange of the main beam, then torsional buckling can be avoided.

 

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• Aug 22, 2007

• #8

271828

Structural

Mar 7, 2007

2,271

Get your hands on Yura's "Fundamentals of Beam Bracing" paper. The AISC seminar notes by Yura and Helwig are very helpful also and it seems like most offices have at least one copy of these floating around. The subject is more complicated than most give it credit for.

For this problem, ask yourself the question: "Does anything prevent the beam from laterally displacing *OR* twisting at the proposed braced point?" You might have a "lean-on" bracing system.

swearingen, I'd say your first statement is usually true, but surely has exceptions. I've never heard of an actual test.

I like SlideRuleEra's advice in this case.

 

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• Aug 22, 2007

• #9

civilperson

Structural

Jan 13, 2006

2,399

When the grate is removed, the loads causing the buckling are also missing. Use the grate as top flange bracing.

 

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• Aug 22, 2007

• #10

CJJS

Structural

Jan 23, 2006

154

How is the grating attached to the top flange? It may or may not provide resistance depending on this.

 

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• Aug 23, 2007

• Thread starter
• #11

waldo459

Structural

Mar 9, 2005

38

We could specify welding for the grating, say at 12" on center. But some beams would have the weak axis of the grating to provide lateral support.

 

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• Aug 23, 2007

1
• #12

miecz

Structural

Sep 30, 2004

1,400

According to Article 6.3 of AISC 13th Edition;

Lateral stability of beams shall be provided by lateral bracing, torsional bracing, or a combination of the two.

Click to expand...

Paragraph 2 under Article 6.3 states:

It is permitted to attach the bracing at any cross sectional location and it need not be attached near the compression flange. The connection between a torsional brace and the beam shall be able to support the required moment given below.

Click to expand...

I'd say you can brace one beam to the other, as long as you can prove the brace and connection can take the moment given in Paragraph 2a.

In fact, large power boilers are constructed in this way, with perimeter beams around the furnace (buckstays) bracing each other for the entire height of the furnace.

 

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• Aug 23, 2007

• Thread starter
• #13

waldo459

Structural

Mar 9, 2005

38

This makes sense to me, I was thinking that if you control the lateral OR torsional, then you control the lateral-torsional mechanism, and since thou quoteth the bible....
I'm looking at this like the beam tries to rotate, and the lever arm of the beam connected to it produces a relatively small force up/down on the beam next to it. Now if I just had a table of, or direct way of calculating, combined shear and moment capacity for shear connections....

 

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