Unbraced Bottom Flange Length 1

[chriskrol]

Dear All,

Is there min beam to beam girder depth ratio that allows you to assume that beam girders bottom flange is braced in case of negative bending?
I have a case that my beam girders are 33in deep and the beams are only 16in.
Thanks

 

[DaveAtkins]

My engineering judgment tells me that the bottom flange is not braced. If you are in a bind, and can't add diagonal bracing from the bottom flange to the beam framing in, then...
You could check the web of the beam bending like a plate (weak axis bending). The force applied at the bottom flange would be the bottom flange buckling force (2% of the bottom flange axial force would be some engineers' estimate). The moment in the web would be this force times the distance between the bottom flange and the bottom of the connection angles at the beam framing in.

DaveAtkins

 

[lkjh345]

I agree with DaveAtkins, I would not consider the bottom flange braced without some further investigation in this situation.

FWIW: Another idea I have used, is to use a full height stiffener plate as both a single shear connection plate for the beam coming in, and to brace the bottom flange of the girder.

 

[JAE]

In a lot of stability articles I've read, and from a seminar by Dr. Joe Yura at Texas, I know that if you prohibit beam rotation, that is actually more effective in bracing the beam against LTB than by simply laterally bracing the compression flange.

The connection between the intersecting beams and the girder may or may not help in resisting rotation of the supporting girder. One method of achieving this is to extend the connection through to the bottom flange such that any rotation would be effectively resisted by the connection. Now this would also probably require pretensioned bolts in the connection but even with pure bearing connections, the small slip might be allowed and still offer enough stiffness and strength to resist LTB.

 

[UcfSE]

You can check the girder with the beam providing torsional bracing. This is in Appendix 6.3 of the new AISC 360-05 code.