Does a steel beam framing into a girder prevent twist enough to be considered a brace point?

[abusementpark]

In general for structural steel, does a beam framing into a girder prevent twist enough to be considered a brace point for the purposes of lateral torsional bucking?

If it is a W18 framing into a W21, then I would definitely say yes. But what about more questionable situations like a W12 framing into a W24?

 

[Ron]

abp...you're on the right track. If the beam is shallow compared to the girder, the answer might be no; however, you probably only need the top flange to be braced so it should be fine. If stress reversal (uplift) is a sufficient issue, then you have to look at the depth comparison or adding a kicker to the bottom flange of the girder.

 

[saad73joss]

Hi
I do the same thing as Ron said. It s a question of the type of connection which would let the main girder twistor not.

 

[abusementpark]

In this case, I am talking about whether the beam is sufficient to the brace the bottom flange of the girder for purposes of lateral torsional buckling.

 

[JAE]

Well for LTB the key is to either prevent rotation of the overall section or prevent lateral translation of the compression flange.

A small beam, framed into the larger beam, and fixed at its far end to something else that is independently rigid, can certainly be a brace to the larger beam.

 

[ishvaaag]

I think the case is covered in Galambos IV or V, and may have a worksheet dealing with that. Will try to find later.

 

[DCBII]

AISC has provisions (Appendix 6 in the '05 specification) regarding what constitutes a braced point. JAE is correct: you must brace it against either twist of the overall section or translation of the compression flange. A smaller beam framing into a larger beam will brace the compression flange against translation, assuming the smaller beam provides a resisting force and stiffness in accordance with Appendix 6.

This requires that the smaller beam be fixed against lateral translation (nodal brace point) or be a strut in a horizontal truss system (relative brace point), or have some other means of preventing translation of the smaller beam. Otherwise when the larger beam's compression flange buckles laterally, it will just take the smaller beam along for the ride. The commentary to Appendix 6 shows a couple of examples of nodal and relative bracing.

 

[ToadJones]

"In this case, I am talking about whether the beam is sufficient to the brace the bottom flange of the girder for purposes of lateral torsional buckling"

Can you explain this?

Is your smaller beam framing in with a lower Top-of-Steel elevation?

If so, I have had the same issue sever times in the past where I had floor beams framing into a large girder and the bottom flange elevations were the same.

I used clip angles from the smaller beams into the web of the girder and I used a full depth stiffener on the backside of the girder. I dont think this alone was sufficient to prevent twist/translation of the top flange, but there was also a heavy slab that was on the lower beams.

I this case the floor beams were something like 14" deep and the girder something like 30" deep and the slab on the 14" beams was 8" thick. I felt comfortable that the whole system was sufficient to call the top flange unb length = floor beam spacing.

 

[structSU10]

There are ways to check the web distorsional stiffness to determine if a brace is adequate to resisting twisting.

I tried attaching a paper "fundamentals of beam bracing" by joseph yura, but it wouldn't take. I believe its from AISC, but might have luck on google.

 

[ishvaaag]

Attached printout of torsional bracing following Galambos V at p. 475

The bracing beam can be set at any height along the web of the braced girders.

If I would have to proceed now on the matter I would try to follow the info in the standing AISC 360 code since this info is along a more older publication, and not a code.

 

[abusementpark]

"In this case, I am talking about whether the beam is sufficient to the brace the bottom flange of the girder for purposes of lateral torsional buckling"

Can you explain this?

Is your smaller beam framing in with a lower Top-of-Steel elevation?

No, the top of the steel of the beams is the same.

If so, I have had the same issue sever times in the past where I had floor beams framing into a large girder and the bottom flange elevations were the same.

I used clip angles from the smaller beams into the web of the girder and I used a full depth stiffener on the backside of the girder. I dont think this alone was sufficient to prevent twist/translation of the top flange, but there was also a heavy slab that was on the lower beams.

I this case the floor beams were something like 14" deep and the girder something like 30" deep and the slab on the 14" beams was 8" thick. I felt comfortable that the whole system was sufficient to call the top flange unb length = floor beam spacing.

Interesting scenario there.

 

[abusementpark]

A smaller beam framing into a larger beam will brace the compression flange against translation, assuming the smaller beam provides a resisting force and stiffness in accordance with Appendix 6.

This requires that the smaller beam be fixed against lateral translation (nodal brace point) or be a strut in a horizontal truss system (relative brace point), or have some other means of preventing translation of the smaller beam.

What if the bottom flange is in compression?

Maybe, I should have been clearer about the actual scenario. I am thinking about situations where you have shallower beams framing into a deeper girder and the girder is subject negative moment (compression in the bottom flange). There is a judgement to be made about whether or not the shallower beam is adequate to brace the bottom flange of the deeper girder. For example, if I have a W12 framing into a W21 with the same top of steel elevation, then I don't think I can say that the W12 will prevent displacement of the bottom flange since its connection to W21 web will be a significant distance from the bottom flange. Perhaps, I can argue that the W12's connection might have enough fixity to prevent twist of cross-section and thus, brace the bottom flange, but I don't know.

I'm know about the AISC bracing provisions. But those calcs aren't always quick or convenient, especially for torsional bracing. I guess I thought by now that someone would have developed some general rules for this type of scenario.

 

[wannabeSE]

For the beam to girder connection, why not use a single plate shear connection with a full depth plate and normal size holes (no horizontal short slots)? The shear plate is welded to the top and bottom flanges and the web.

 

[BAretired]

The girder compression flange must be laterally supported by a system which resists 2% of the compression in the flange without exceeding the permissible initial displacement between brace points.

The full depth shear plate suggested above will usually be adequate to provide the necessary resistance to LTB, but this can be determined for each case.

BA

 

[structSU10]

Applying the equations are all that can be done in this situation. Beam buckling is a complicated phenomenon; when a beam fails in LTB its a 3D buckling problem, which requires some simplification and assumptions just to be able to get a solution for in many situations. A good text on the subject is "Structural Stability Theory and Implementation", although it is somewhat dated. It does go through derivation of some of the rules for bracing equations within AISC.

Have you ever tried MASTAN2? It is free, although not the best user interface wise, but it can perform buckling analysis, which can be a way to quickly verify if you have sufficient brace strength / stiffness in order to consider the beam braces at that point.

 

[abusementpark]

For the beam to girder connection, why not use a single plate shear connection with a full depth plate and normal size holes (no horizontal short slots)? The shear plate is welded to the top and bottom flanges and the web.

That should do the trick. But, it is more costly than a regular beam to girder connection. If I have this condition in a lot of places on a project, I wouldn't want to hastily go there without being sure I thought it was necessary.

 

[abusementpark]

Have you ever tried MASTAN2? It is free, although not the best user interface wise, but it can perform buckling analysis, which can be a way to quickly verify if you have sufficient brace strength / stiffness in order to consider the beam braces at that point.

Yes, I used that program in graduate school. Although, wouldn't that require the use of shell elements? How would you set-up the model and idealize the beam to girder connection? I only used the program for pushover analysis of frames.