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AISC truss unbraced length

AISC truss unbraced length

AISC truss unbraced length

(OP)
Hi all,

For truss top chords under compression, does AISC-360 allow that the purlins be considered as a braced point for out of plane buckling? so that the out of plane buckling length is equal to the spacing of the purlins? or AISC leaves it for the engineer's judgement?

I used to design using AISC-ASD 89 and I remember that AISC leaves a lot for the engineer's judgement (which caused alot of argument among engineers).

thanks for the help

RE: AISC truss unbraced length

Look up "stability bracing" in the 13th edition manual index. There are several sections that cover it. I don't think it specifically mentions trusses anywhere, but the principles are the same.

RE: AISC truss unbraced length

Why would you think that purlins may not constitute brace points for out of plane buckling of the top chord if they are attached to the deck?

BA

RE: AISC truss unbraced length

Check Yura's Lecture

RE: AISC truss unbraced length

Purlins = brace if you have a deck diaphragm

If you have no properly secured deck diaphragm but only a light metal roofing and the building diaphragm is developed via rod X braces or similar, then you probably might not want to depend on the purlins as brace points.

RE: AISC truss unbraced length

In my opinion, and I think all PEMB manufacturers and most of us who purpose design steel buildings would agree, purlins which support steel roofing do brace the top flange of beams and top chords of trusses, even if the roofing is not considered to be a diaphragm.

RE: AISC truss unbraced length

(OP)
Spats - I will read on this. I hope i see what I am looking for.

BA - Actually I am using it as a braced point of the top chord.

The term "braced" is at times too wide or too narrow for some people. Problem is some engineers wont accept that purlins can act as a braced point to the top chord of a roof truss, even after explaining the reasons you consider it as a braced point. My encounter with one such engineer will only accept that purlins can be used as a braced point if the code says "purlins are braced point". Its really a pain if that engineer is in a position of power to approve your plans & calculations.

RE: AISC truss unbraced length

I have never been in the position where I had to submit calculations or justify my assumptions to the authority having jurisdiction and I sympathize with your position. In my area, the authority having jurisdiction is happy to accept any design from a structural point of view provided some engineer has applied his professional seal to the drawings. I am not sure that is correct, but that's the way it is.

BA

RE: AISC truss unbraced length

Why not just analyze the situation. Assume some conservative value for the required buckling resisting force. This is the value that you will get argument with as I have seen anywhere between 2% and 5% of the cord force. Use this as an axial load in the purlin. Make sure that you have enough connection between the purlin and the roof deck to dissipate the force into the diaphragm.

RE: AISC truss unbraced length

This isn't really close to your situation, but in Civil Engineering Magazine there's a new article on the Cowboys Facility Failure. One of the root causes was an incorrect assumption on the truss bracing. Apparently using a fabric roof as a bracing system is a no-no.
I hope the link is able to be opened by non ASCE members.

RE: AISC truss unbraced length

Well, Jed, that is exactly the concept I would worry about with a PEMB metal roofing being depended on to allow purlins to be braces.

For a brace to work you need BOTH strength and stiffness. The PEMB metal roofing may not have the adequate stiffness and thus, the purlins would be too "LOOSE" to be effective braces for a large truss.

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