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






RE: Does a steel beam framing into a girder prevent twist enough to be considered a brace point?
RE: Does a steel beam framing into a girder prevent twist enough to be considered a brace point?
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.
RE: Does a steel beam framing into a girder prevent twist enough to be considered a brace point?
RE: Does a steel beam framing into a girder prevent twist enough to be considered a brace point?
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.
RE: Does a steel beam framing into a girder prevent twist enough to be considered a brace point?
RE: Does a steel beam framing into a girder prevent twist enough to be considered a brace point?
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.
RE: Does a steel beam framing into a girder prevent twist enough to be considered a brace point?
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.
RE: Does a steel beam framing into a girder prevent twist enough to be considered a brace point?
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.
RE: Does a steel beam framing into a girder prevent twist enough to be considered a brace point?
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.
RE: Does a steel beam framing into a girder prevent twist enough to be considered a brace point?
No, the top of the steel of the beams is the same.
Interesting scenario there.
RE: Does a steel beam framing into a girder prevent twist enough to be considered a brace point?
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.
RE: Does a steel beam framing into a girder prevent twist enough to be considered a brace point?
RE: Does a steel beam framing into a girder prevent twist enough to be considered a brace point?
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
RE: Does a steel beam framing into a girder prevent twist enough to be considered a brace point?
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.
RE: Does a steel beam framing into a girder prevent twist enough to be considered a brace point?
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.
RE: Does a steel beam framing into a girder prevent twist enough to be considered a brace 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.