Bracing a Beam using the Tension Flange

[dgboley]

Does anyone have any information on the bracing of a member, assuming its braced on the tension flange but not on the compression flange. Treating the cross section as a cantilevered column, calculating that the diaphragm bracing the tension flange has enough stiffness to treat the member as braced.

Thank you for your time,
Dennis

 

[JAE]

Sketch?

 

[BAretired]

The compression flange can be braced using stiffeners laterally and rotationally braced at the tension flange, similar to a half-through bridge truss. The strength of the bracing material and its deformation at the compression flange would be required to meet code requirements.

BA

 

[TLycan]

Refer to the AISC appendix 6 and the beautiful paper by Yura "fundamentals of beam bracing". If you Don't have it, tell me and I will email it to you. Also according to my understanding from point 6.3 in the AISC appendix 6 Commentary that attaching of the horizontal bracing at one flange, compression or tension, will be effective except at the inflection point, but you have to recheck this.

 

[BAretired]

I don't agree with the last point, i.e. that bracing attached to the tension flange is effective except at the inflection point. That is simply wrong.

BA

 

[TLycan]

BAretired, the following is the first paragraph from point 6.3 in the AISC appendix 6 Commentary
"Beam bracing must control twist of the section, but need not prevent lateral displacement.
Both lateral bracing, such as a steel joists attached to the compression
flange of a simply supported beam, and torsional bracing, such as a cross-frame or
diaphragm between adjacent girders, can be used to control twist. Note, however,
that lateral bracing systems that are attached only near the beam centroid are generally
ineffective in controlling twist."

I get from the last sentence, that bracing near the tension or compression flange will be effective.

secondly suppose I have the upper flange of an I-Beam is in compression and my horizontal bracing is attached to the bottom flange, I see that the beam will never twist especially if it is a shallow beam (i.e. small depth). the problem is with large depth beam., where I feel that the compressive flange will take part of the web and twist.

 

[RFreund]

TLycan - If the upper flange of your I-beam is free to rotate how are you preventing twist by bracing the tension flange?

EIT

http://www.HowToEngineer.com

 

[Hokie93]

Per AISC Specification (2005) Appendix 6, section 6.3.2: "It is permitted to attach the (torsional) 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". The Yura paper ("Fundamentals of Beam Bracing") cited by TLycan discusses torsional bracing in great detail and provides the equations necessary to consider cross-section distortion and web flexibility. The Yura paper was published in AISC Engineering Journal, first quarter, 2001.

Chapter 12 of "Guide to Stability Design Criteria for Metal Structures" by Galambos provides an alternate Cb equation for the case of a uniformly loaded beam/girder subject to net uplift and braced only at the top flange (e.g., the compression flange is unbraced). Use of this equation can provide a significant increase in capacity compared to Specification Equation F1-1.

 

[RFreund]

ah, I see. So the bracing connection is a 'fixed' connection and the beam is checked for distortion (as stated).



EIT

http://www.HowToEngineer.com

 

[TLycan]

RFreund- Twisting is the rotation of the I-beam around its centroid and it is the rotation of the whole section around it longitudinal axis. If I added a lateral restraint any point near the top or bottom flange, it will prevent twisting.
that is: if I restrained the lower part of the I section, the compression flange will have to twist along with part of the web (type of deformation or distortion rather than a whole rotation) while the lower part of the I-beam still the same. I think this is very difficult in case of shallow beams.



The problem with the deep web beam that the compression flange will take part of the web and twist even if the lower part of the beam is restrained from movement; its like a deformation or distortion of part of the section not a total rotation or movement of the section.

this is how I FEEL it

 

[RFreund]

Right, right that makes sense. Thanks.

EIT

http://www.HowToEngineer.com

 

[TLycan]

Another thought, I think the beam might twist not at the centroid but at the location of the lateral bracing restraint if it was at the tension flange.

o.K. then the lateral bracing as you and BAretired implied should be at the compression flange.

and I think I might send a question to the AISC about that but after checking "Guide to Stability Design Criteria for Metal Structures"

 

[TLycan]

While I was sleeping at night yesterday I was thinking of the bracing at the tension flange.

I apologize and take back my words defending that the bracing at the tension flange will prevent LTB.

I imagine that the beam will still twist around point of intersection of bracing and beam (not at the centroid as I previously mentioned)

Regards

 

[Guest262653]

Just to add a few thoughts to the discussion.

There is a beneficial effect of the bracing if it can prevent lateral sway of the beam being restrained. Full LTB behaviour does not consist only of twisting but also of a horizontal displacement of the beam. That said, I wouldn't rely on it for design and I'd consider it unbraced.

An interesting article by Galéa can be found if you google "DÉVERSEMENT ÉLASTIQUE DES PROFILS LAMINES AVEC MAINTIEN LATÉRAL RIGIDE ET CONTINU AU NIVEAU D ’UNE SEMELLE".
The author has a couple of charts for the calculation of the elastic critical moment when one of the flanges is continuously restrained. An example is also presented.

 

[charliealphabravo]

Here is a technical bulletin on modernsteel.com that interacts briefly with the Yura paper and others. Unfortunately, not much quantifiable direction is given...mainly that different types of bracing have different degrees of effectiveness.

http://www.modernsteel.com/steelinterchange_details.php?id=596