Do you consider that your Purlins restrain unbraced length of top chord?

[atakancengiz]

Hello guys,

I have a questions for you about unbraced length of a top chord. I have a supervisor and we are trying to design a big and long mall.(Span = 45m/147' and Length = 20@10 m / 20@33') To make it economical, he is thinking "We should assume that purlins restrain top chord beacuse of 5 mm Cold-Formed Profiles."

Though we are using bridging trusses, he insists that we should accept that purlins restrain in order to make it official. I made a research but cannot find anything that can be referred. I am telling him that it might restrain but we should not assume it does.

So, what is your opinion about this assumption. If you tell me a reference (such as book or a Standardt/Specification) I would be appreciated.

Thank you in advance.

 

[hokie66]

Yes, they should restrain the top chord.

 

[KootK]

The latest editions of the AISC manual have specific requirements (appendix 6) for the strength and stiffness of members serving as bracing. I'd seek to apply those provisions to verify the adequacy of the purlins to brace the top chord.

The purlins likely will brace the top chord for out of plane buckling. Depending on the nature of the connections, however, they may not brace the top chord torsionally. In that case, the top chords would need to span between truss panel points for lateral torsional buckling checks.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[atakancengiz]

KootK thanks for your answer, I will look it up AISC Appendix 6. But when you say "The purlins likely will brace the chord for out of plane buckling" Did you take into account reversal effects such as wind or EQ?