Unbraced Length

[KarlT]

I have a quick question which I have always wondered about... Lets say you have two 12" channels spaced at 5 ft o/c and spanning 18 ft. At the third points of the main channel spans there is a 12" cross channel connecting the two main channels together.

Now is the unbraced length of the main channels 18 ft or is it 6 ft?

 

[JAE]

Simply placing a link between two channels does not necessarily make you unbraced length equal to the segment length between the braces. Its sort of like two drunks leaning against each other.

One way to design this is to still let your unbraced length = 18 feet but now you have two channels taking the full load together. So your capacity is 2 x (single channel capacity at L=18') and buckling will occur when the total load reaches twice the single channel load.

The bracing must have its own load path (and stiffness path) out of the structural elements that it is trying to brace.

 

[rowe]

For lateral torsional buckling, the unbraced length of the two main channels is effectively 6', assuming that the ends of the main channels are effective restrained against torsional rotation.

The unbraced length of the top flange of the main channels may or may not be 6', depending on the connection of the x-channel.