PEMB Unbraced Length Question

[JoshH726]

Reading through the rafter with fly brace thread, I'm still hemming and hawwing on a condition in the attached. I may have dove too deep and the lack of oxygen is turning off my brain. Please see attached and share your thoughts on the unbraced length for flexure of rigid frame member where bottom flange bracing is present at the lowest two "x" marks. I can't imagine it would be 23'-5" but there is negative moment extending beyond the brace location. Appendix 6 prevents use of the inflection point, so do you really have to run all the way to the peak?

 

[KootK]

You appear to be practicing in the US Midwest so I'll speak to this from that perspective.

Appendix 6 prevents use of the inflection point, so do you really have to run all the way to the peak?

You'd do two LTB checks on that 23'-5" length:

1) Top chord buckling check based on an unbraced length equal to the spacing of the braces and compared to the maximum positive moment.

2) Bottom chord buckling check based on an unbraced length equal to 23'-5" and compared to the max negative moment which is quite small.

Whichever of those two checks produces the largest demand to capacity ratio would govern your design. Of course, if you're familiar with the fly brace thread, you'll know that you can likely improve your negative bending check significantly by using more sophisticated methods such as an FEM buckling model, Joseph Yura's modified Cb methods, or the lateral torsional buckling check procedures of other jurisdictions where those are more advanced in this arena.

 

[KootK]

..where bottom flange bracing is present at the lowest two "x" marks.

Based upon the explorations of the fly brace thread I suspect that, even without those two lower bottom flange braces, the bottom flange buckling mode still might not control your design. Full restraint at the columns and apex plus the intermediate top flange restraint probably gets you there. I wouldn't advocate attempting that for a real design however. When in Rome...

And, no doubt, there's an uplift case to be considered as well.

 

[BAretired]

Uplift and unbalanced snow load need to be considered.

BA