Lateral/Torsional Bracing 7

[BMart006]

I am trying to understand the difference between lateral and torsional bracing, and why beams framed to the centroid (or near-centroid) of a girder (2L shear connections) may not count for decreasing my Lb. I've read through Yura's article "Fundamentals of Beam Bracing" but it did not provide much of an explanation of when each type is needed. The end goal is to determine how to size lateral bracing appropriately. Any help is greatly appreciated.

 

[KootK]

lateral and torsional bracing

While commonly used, I feel that both of these terms obfuscate the truth. In my opinion, it's better to think of lateral torsional buckling (LTB) beam bracing like this:

1) Toss out the terms "lateral bracing" and "torsional bracing" in general.

2) For beam LTB, there's only one kind of bracing and it should be thought of as "rotational bracing" which restrains rotation of the entire cross section about point in space that is centered on the beam horizontally and at a critical vertical location which need not be at the beam centroid. It is a beam's tendency to rotate about said point in space that is "lateral torsional buckling".

3) Sufficiently strong and stiff "lateral bracing" may provide effective rotational bracing so long as the point of lateral bracing is not a the same location as the point in space about which the beam is trying to rotate (LTB buckle). The further the point of bracing is from the point of rotation, the more effective it will be. This is why cantilevers are often best braced at the tension flange which tends to be counter-intuitive.

4) Sufficiently strong and stiff "torsional bracing" of a beam will always provide effective rotational bracing because a beam that that is restrained from rotating about its centroidal axis will always also be restrained from rotating about any other axis (including the LTB rotation axis).

5) "Sufficiently strong and stiff" is defined in AISC manual appendix six.

Your double angle connection may well be able to serve to decrease your Lb. You just need to ensure that it's strong and stiff enough per #5.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Tomfh]

Lateral bracing - prevents lateral movement. E.g. a concrete floor sitting on a steel beam retrains the top of the beam. The top flange can no longer buckle sideways.

Torsional bracing - stops the beam from rotating. Eg. fly bracing on a roof beam. This, combined with a lateral restraint anywhere, will also stop a flange from buckling.


A lateral restraint in the centre doesn't do much. The cross section can just spin, thus the compression flange can still buckle sideways and release its energy.

 

[structSU10]

To add to the comments above, a member attached to the centroid via full depth stiffeners may meet the AISC requirements for strength and stiffness as well. Essentially the stiffener acts as a cantilever from the compression flange to the bracing.

 

[WARose]

In addition to the excellent comments above.....I would like to add: one common mistake I see is that the means of lateral bracing is not tied into anything and the net effect accounted for. (Sometimes it is significant.) In other words, your lateral bracing system typically should also be your LFRS (Lateral Force Resisting System). Something like plan bracing or a diaphragm. As per the load combinations, the net force required to brace the floor beams for an entire level can be significant and additive to the other loads. Remember: all the beams on a level will eventually buckle in one direction......so that force is not going to disappear. In commercial work......it's typically not that bad.....but in industrial design, it can control.

 

[Tomfh]

I would like to add: one common mistake I see is that the means of lateral bracing is not tied into anything and the net effect accounted for

God bless cladding

 

[Shu Jiang PEng SE]

Reduce lateral supporting distance will increase the beam lateral bending strength because short lateral support spacing means less lateral bending moment; reduce lateral supporting distance does not change the torsion applied to the beam and therefore does not affect the beam torsion capacity.

 

[StrEng007]

Seems like the answers to this question are all over the place. AISC 360-10, section 6.3.1 specifically states that lateral bracing shall be attached at or near the beam compression flange, with the exception of two cases. I too am questioning the applicability of a perpendicular beam framing into the beam center and not bracing the compression flange.

 

[KootK]

I'm certainly okay with mid-height bracing so long as it's conceived of as torsional bracing rather than lateral.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Tomfh]

I too am questioning the applicability of a perpendicular beam framing into the beam center and not bracing the compression flange.

If there's torsional restraint it works. The point of any bracing is simply to stop the beam from being lazy and flipping itself into a weaker shape. If you brace to the middle and stop the beam spinning you achieve that requirement. The beam can no longer roll over into its weak axis without doing MORE work than simply bending about its strong axis.

 

[SlideRuleEra]

Lateral deflection and lateral torsional buckling (LTB) are caused by the same thing.
Consider a simply supported, wide flange beam, that is straight, with no applied load. The beam is "long" and has no lateral support.

As vertical load increases, the first indication of problems is the entire beam's uniform lateral deflection.
The compression flange has deflected the only way it can - sideways.
Tension in the bottom flange tries to pull the beam back to it's original center line. The beam's inherent torsional resistance keep the web vertical, but cannot prevent the lateral deflection. See my sketch:

As load continues to increase, compression force (pushing one way) and tension force (pulling the other way) overcome the beam's torsional resistance. LTB begins and the beam begins to twist. How the vertical load is applied to the beam begins to matter.

When load is applied to the top of the beam, this is a destabilizing force (an eccentric load). Compression force and the destabilizing force "team up" to make LTB worse.

When load is applied to the bottom of the beam, this is a stabilizing force (still an eccentric load). However, in this case the stabilizing force tends to resist LTB. LTB is still occurring, but is not as "bad", for the same load magnitude. See this sketch:

The intent of any lateral or torsional bracing is to keep the compression flange straight, and along it's original centerline. The tension flange keeps itself on centerline.

How to accomplish adequate bracing is the subject of much technical discussion. If enough (very close together) lateral braces are provided directly to the compression flange, the compression flange stays on centerline and the beam fails in bending without LTB issues. In many cases a compromise is possible. Enough, but spaced, lateral compression flange braces can be provided to allow the beam to develop sufficient moment resistance for it's job before LTB begins. Sometimes, compression flange bracing is not an option... therefore torsional bracing.

As a former bridge contractor, I had the opportunity to design and personally use a steel beam in a way that dramatically demonstrated LTB. We needed a moveable horizontal walkway just above freshly placed 55' long concrete bridge deck spans. I used an available single 60' long HP 10x42 as a beam. Simple supports on wheels, with no lateral bracing. Walking a small beam that long puts a significant moving point load on it. Starting at one end, each step puts more and more bending moment in the beam. Soon, it is bouncing like a trampoline and laterally deflecting. As you near the middle, the bouncing worsens, and LTB begins.
This is an experience to make you take time to get an understanding of the most basic reasons for lateral deflection and LTB.

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