V-brace Segmenting

[eit09]

I have double angles acting as vertical X bracing in my structure. Do you consider the whole length of each double angle as the unbraced length? I have asked two coworkers one says you can segment since they are bolted together where the angles cross and the other says we must consider the entire length of the double angle unbraced. Both have valid arguments & I personally design for full length of member unbraced. What are others perspectives on the issue?

 

[swearingen]

The tension member in an X brace is generally taken to provide an out-of-plane brace point for the compression member. This means that you can design for compression for half the length.

Note that if you're using double angles, you may have to stitch them together to make them act as a unit...


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[bootlegend]

I generally consider the entire length unbraced. However, it is simple enough to check whether your member is braced using Appendix 6 of the AISC 13th edition.

 

[Stazz]

I understand Appendix 6 as a braced point should be able to withstand 1% of the load. So if there is 100k of axial compression in the angle then the bracing angle should be able to resist a 1 kip load at its midpoint.

That would not be a bad study to do: See if all the angles or double angles are able to withstand 1% of their axial compressive strength as a point load at their midpoint for all unbraced lengths.

Assume 20' Long angle braces

Uses 2L5x5x7/8 (10' braced length) resists 437k in weak axis.

1% of this is 4.37k

This load at midspan of a 20' span produces 21.85 k*ft @ midspan

2L5x5x7/8 @ 20' length and braced at midspan can take 43.83 k*ft (accounting for Cb using a point load at midspan.

So this appears to work. One thing that isn't accounted for is that the bracing angle is also in tension at this time which may or may not degrade it's capacity. I would think that tension makes the angle more stiff but it also adds tensile stress which can add to the tensile flexure stress. Tension is not codified yet in Chapter H of AISC.

 

[Stazz]

Actually, what I really meant to say was to just use HSS tube chevron braces. These are easier to detail = easier to build = cheaper.