Flexural Torsional Buckling of Built-up Column?

[Dusni]

Hi,

I'm analyzing an old truss that has built-up diagonal members composed of (2) angles with lacing (forms a channel shape, with the lacing forming the web).

AASHTO Table 6.9.4.1.1-1 says the applicable buckling modes for channels are flexural buckling (FB) and flexural torsional buckling (FTB).

Am I correct, that both of those (2) buckling modes would be applicable to the built up members I'm analyzing? The program I'm using (AASHTOWare BrR) says FTB is not applicable and only checks FB.

Side note, I'm aware AASHTO has additional provisions for built-up members (6.9.4.3).

Thanks!

 

[jdonville]

I would consider analyzing these as angles as the lacing likely doesn't provide the same torsional stiffness as a channel web the same thickness as the angle.