AISC Code Unity Check (Unbraced Length)

[engpes]

I am performing an AISC unity check per section H-2. I am trying to specify my unbraced length L (KL/r) in order to determine Pc for the axial compression buckling.

I have attached a sketch that shows configuration. I have a 5x5 tubing member that is supported along its entire length in the weak and strong axis via crimped plate and flat plate (note sketch attached). Is my unsupported length L 0?

Thanks in advance.

 

[frv]

No. The plates would have to be attached to something else capable of resisting load to function as a brace.

But your Lb wouldn't be L either. You could conservatively take it as L.

 

[engpes]

Sorry. I didn't show it in the sketch, but both plates are attached (welded) to another tubing member (assumed rigid), parrallell to the one shown.

Would this act as an acceptable support to brace the 5x5 member shown?

Thanks.

 

[DaveAtkins]

Are the plates attached to something else that can resist the buckling forces? If so, the tube is braced.

If not, you should check the tube and plates as a composite section.

DaveAtkins

 

[engpes]

Yes the plates are attached to a member that can resist the buckling force.

Thanks for the help!!!

 

[JoshPlumSE]

Provided that the crimped plate meets the strength and stiffness requirements (from appendix 6) for relative bracing then I think the member can be considered continuously braced. However, I'm not entirely convinced that this is going to pass that test.

To evaluate the strength and stiffness of the plate for bracing I would probably consider it to be similar to a diagonal brace that kicks out a a 45 degree angle towards the adjacent column. The effective width of the plate would be the difficult thing to establish. I would think it would be something like 25% of the distance between the two columns. But, that's just based on eye-balling it. Not sure if there is a more rational procedure for coming up with this value.

 

[darkwing88]

A proper free body diagram needs dimensions and boundary conditions also. From what you have provided, I would consider this tube fully braced.