Laterally supported top flange

[IFRs]

I have a group of parallel and relatively long 4" tall aluminum I-beams whose bottom flanges are continuously connected together by a thin membrane that effectively braces the bottom flange. The beams are simply supported at their quarter points and loaded with point loads between the supports. I've been using the quarter length as the unbraced length of the top flange to calculate the allowable compressive strength of the top flange. Is that overly conservative?

 

[msquared48]

Strictly speaking, the loading situation you describe makes me nervous regarding the lateral stability of the top flange. Unless you consider the inflection points in the moment diagram as lateral restraint points, the top flange is technically unsupported along it's entire length. In some cases, consideration of the inflection points is used, but usually for negative moment (bottom flange) stability, not the positive moment seen in the top flange.

In my opinion, my guts tell me that your assumption is not conservative enough. If there were transverse members at the 1/4 point support locations, I would concur, but not as you describe it. I think roll-over will be a problem.

Mike McCann
MMC Engineering

 

[IFRs]

Sorry - I forgot to mention that at the quarter points where the load is applied, there are transverse beams. I was actually thinking that since the beam is so short, the top flange might be considered braced on closer intervals. I mean, at some point, that has to occur, doesn't it? What if the beams were only 3" tall?

 

[msquared48]

If that's the case - 3" transverse beams, detail the connection so the tops of the flanges of all the beams align, then your assumption will be OK.

Mike McCann
MMC Engineering

 

[haynewp]

"The beams are simply supported at their quarter points and loaded with point loads between the supports."

"I forgot to mention that at the quarter points where the load is applied, there are transverse beams"


If the beams are broken at the simple supports and not continuous over simple supports, then the beam top flange unbraced length is likely to be the length between the support and the loaded location. Can you post a picture of the elevation of the beam system and section where the transverse beams are framing into the girder so I can be sure what is going on?

 

[IFRs]

I will figure out how to post a picture. I have a continuous beam over six supports (4 equal spans) with loads between the supports. The bottoms of the I-beams are continuously welded to a membrane. The load being considered here is applied on the beam top flange. At both the load and support points there are transverse members that effectively brace the compression flange. However, my real question boils down to: At what point if any does the top flange no longer need it's own bracing because the I-beam is short enough? A 24" I-beam no, but what about a 4" or 3" or 2" I-beam?

 

[haynewp]

Oh you mean short in terms of depth and not length. I don't think that you can assume that because an I beam is 2" deep that it doesn't need something more than bottom flange bracing to effectively brace the top flange. If transverse members are coming in on top of the I beam, then I would consider the top flange braced at the connection intervals.