Flange brace force

[ajnweb]

I have a question regarding the flange brace force on a beam. Lets say I have a beam in bending due to gravity loading, the flange brace force on the compression flange is 1% to 2% of the flange force (moment/depth). So I have applied a gravity load to a beam and now have a lateral load to resist, and since we all know that we cannot apply a gravity load and get a net lateral load, where does the equal and opposite force to the flange brace force come from? Is it taken by the flange brace back up to a lateral beam that is attached to the tension flange, and is it the tension flange that then provides the equal and opposite force? Thank you for any and all responses.

 

[swearingen]

On the issue of all of the beams going one way or canceling:

From Yura's stability conference, when beams are tied together and they all laterally-torsionally buckle at the same time (they are all loaded to this point and LTB is the limit state), they will ALWAYS all go the same direction. If you have 9 beams and five of them want to go one way and 4 of them want to go the other, there will be a slight tilt towards the direction of the 5 and the 4 will "change their minds" and go along with the rest.

 

[UcfSE]

I would definitely check out the beam bracing provisions. The 9th ed. ASD doesn't have these, as you know. The 2% rule is a rule of thumb with no real theoretical base to my knowledge. It tends to be conservative which is why it's so popular along with the ease of use. Going with a more rigorous analysis with the LRFD code may save you some material at the expense of a headache.

 

[ajnweb]

Thank you to everyone for responding, all of the answers have been a great help.

 

[WillisV]

As a follow up to UcfSE's post - the 2% loading "rule of thumb" is generaly conservative for the strength provisions of the LRFD code (which generally run in the 1% range) however it is the STIFFNESS considerations and requirements that typically govern bracing design - in which case ONLY looking at a 2% strength condition can grossly underestimate bracing requirements.