Top Compression Flange Bracing

[engpes]

I have a unique situation where I have a C12x25 channel girder with later W8x35 beams tying in with steel floor plate on top (all welded / full MR connection). Typically this floor plate and W8x35 would tie in at the top flange (compression) giving the top compression flange effective lateral support.

In this particular case, the W8x35 and the floor plate are offset downward a few inches and do not tie in directly to the top (compression flange). I have always used the rule of thumb that as long as the entire C12x25 girder is prevented from twisting and that the girder is supported at least 2/3 of the way up its depth (W8 lateral beam), the top (compression flange) can be considered supported at that location.

My question is:

1.) Can this C12x25 girder compression flange be considered supported if the W8x35 lateral beam does not exactly line up with the top compression flange?

2.) Is there any section in AISC that addresses these actual requirements of compression flange bracing?

Thank you in advance.

 

[KootK]

1) Yes. I use the 2/3 rule as well.

2) Not sure.

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.

 

[canwesteng]

Are you not given the option of providing lateral or torsional restraint? From my understanding you have torsional and lateral restraint here so it's quite a robust point of LTB restraint.

 

[JAE]

Agree with KootK on number 1.

On number 2 - look at the definition of Lb in the AISC manual - [blue]"Length between points that are either braced against lateral displacement of compression flange or braced against twist of the cross section...[/blue]



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[wannabeSE]

Appendix 6 of AISC 360-10, can be used to evaluate the bracing. Does the brace satisfy the torsional beam brace requirement in section 6.3.2?