Truss with inflection point

[SteelMover]

We are analyzing a 100’ long truss that is part of a moment frame, and as such there is an inflection point towards the end of the truss under heavy snow loads. The end fixity causes a fairly large compression force to be generated in the bottom chord over the last two panel points, and then it changes to tension. If we consider the bottom chord a column from the end to the first bridging, the effective length is too long (40’ ) for us to support the compression force, whereas if we consider the buckling length to be only to the inflection point (8’) we are fine. The latter seems to make more sense. The question is this, does anyone with experience with this know of any resources that address the issue of column effective length with non-uniform axial load?

 

[JAE]

I believe that the common understanding for continuous beams is that inflection points are not brace points - I would say this is true of your truss too.

 

[WillisV]

Agree with JAE -

For backup see AISC 13th edition Commentary to Appendix 6 - 6.3 "For beams with double curvature, the inflection point can not be considered a brace piont because twist [can occur] at that point..."

 

[SteelMover]

Thanks for the response. I agree indeed, but at least with beams we get the Cb factor for modifying the LTB equation in a very analogous situation.

 

[SteelMover]

Upon further analysis it looks like we only need to brace from the wall to the first panel point 40" out and the compression load drops to acceptable levels for the 40' unbraced length. So we'll probably do that but it still leaves me suspect...

 

[csd72]

Steelmover,

I agree with you in theory. I cannot see why the bottom chord should be worse than an 8' cantilever column (which gives an effective length of approx 2.2x8' say 18'.

You will be hard pressed to find any reference for this though.

If you think about standard design for an unbraced compression chord, you design it for full length compression even though the full axial load is only on the middle section.

Best to stick to standard practice.

 

[SteelMover]

We are bracing inflection point as it is fairly cheap, but when I have time I will run a buckling analysis of it and let you guys know what the upper bound safety factor was without it. If anyone comes across some research though still let me know.