Stability bracing for flexural members
Stability bracing for flexural members
(OP)
I am designing a freestanding scoreboard with wideflange columns. The axial load on these columns is small compared to the flexural demand. I would like to design a bracing system (at 10 ft above grade) to justify an unbraced length of the compression flange of 10 ft. In order to design the bracing members (HSS braces and stringer), should I assume that the bracing is relative or nodal? I have read AISC 360 Appendix 6 and commentary as well as several other related questions on this forum but I am still unsure. See attached sketch. Thanks in advance for your recommendations.






RE: Stability bracing for flexural members
You don’t know which direction the lateral force may come from, or which col. flg. might be the buckling flg. And, you don’t want these cols. to be like drunken sailors leaning against each other for support, and hoping one of them doesn’t start to move. Why not make two identical, light symmetrical trusses, which bolt inside the flgs. on the cols. Light, but still fairly rigid in their own plane, so the cols. can’t twist. These would have two fairly strong chords and diags. as needed and bolt to the col. flgs. They could also be used in the bracing and col. setting process to help space and hold the cols. during erection.
RE: Stability bracing for flexural members
I understand your truss idea, but the concept I sketched requires very few members and connections in comparison, and as long as the braces and stringer are sized appropriately, accomplishes the same thing.
RE: Stability bracing for flexural members
I am not 100% clear on the theory of bracing cantilevers. But the code requires the tension flange to be laterally braced at the free end of the cantilever. See AISC 360-10 app 6.3.1(1). I don't know if lean-on or rotational bracing at the top of the scoreboard columns will accomplish this.
RE: Stability bracing for flexural members
RE: Stability bracing for flexural members
RE: Stability bracing for flexural members
Here is an explanation from someone who is smarter than I:
Frome thread507-194560: Relative vs. Nodal bracing
Another link:
https://www.aisc.org/store/p-2296-notes-on-the-nod...
EIT
www.HowToEngineer.com
RE: Stability bracing for flexural members
Another good link with WillisV in it:
thread172-254994: Bracing of 2-Beam System
http://www.eng-tips.com/viewthread.cfm?qid=254994
EIT
www.HowToEngineer.com
RE: Stability bracing for flexural members
When I first read this, it sounded like it was talking about beam lateral bracing (6.3.1), but perhaps it's talking about torsional bracing (6.3.2). Unless someone can convince me otherwise, my current position is that the bracing system shown in my sketch is a relative lateral brace and a nodal torsional brace. I wish there were a simpler explanation than provided by AISC that could make the concept "click" in my brain.
RE: Stability bracing for flexural members
EIT
www.HowToEngineer.com