Cross bracing unbraced length

[eit09]

I have a vert double angle cross brace that is 30' long. Being 30' long you need a min of LL6x6x5x3 to pass the code sugested l/300 values. I know you can turn this off in the tools but I would like to know how other people design with the code suggested slenderness values. Can you consider the member to be braced where the two angles cross so the unbraced length is half the length of the member. Doing this one could us a LL3x3x4x3?

 

[JoshPlumSE]

Personally, I usually consider the double angle to be braced in one direction, but not the other.

I believe there are some other standards (ASCE 10, TIA 222) which may provide some guidance on using a lower KL/r value for X bracing. I got the quote below from another eng-tips thread:

From ASCE 10-97 Design of Latticed Steel Transmission Structures
"Tension/Compression system with members connected at the crossover point. If the member in tension has a force of not less than 20% of the force in the compression member, the crossover point provides support resisting out-of-plane buckling."

 

[msquared48]

TIA 222-G allows this in three conditions under section 4.5.2.1, including the condition Josh mentioned.

The other two conditions involve other horizontal members, internal to the tower, being framed to the intersection point, which is rather obvious.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dik]

YOu may want to check the wording of the code regarding the kl/r<300 to see if there is any 'wiggle room' and exclusions... many pre-eng buildings use round bar stock for x-bracing.

I usually use 0.45*length, with a minimum of two bolts per angle connection and a bolt at the point of intersection. This info was in the kl/r material in the Aluminum Handbook published by Alcan several years ago.

On a recent project (not mine) I saw the use of 'crushed round tubes at the end and middle. The tubes were locally flattened with holes drilled through to attach to plates on the superstructure and a centre bolt. Appeared to be slick and will try these on my next project where I can use it.

Dik