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Z-purlin question

Z-purlin question

Z-purlin question

(OP)
Roofer wants to retro fit a new roof over an existing roof system attached to 14 Ga. z-purlins (8x3). Span is 25 feet spaced at 5' o.c. I don't know what is going on but I can't seem to get the purlins to check out. New roof system is only adding about 2.5 psf and the existing roof is about the same. I am using a standard LL of 20 psf but I can't wrap my head around how the original designer would of sized these members for such a small dead load. Steel frame building was built back in 1980...am I missing something here?

RE: Z-purlin question

Could there be multi-span continuity that you're neglecting?

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.

RE: Z-purlin question

Are you using load tables, or trying to calculate the capacity? Purlin capacities are normally based on full scale testing by the manufacturers. But if these purlins don't match a tabulated system, you may be out of luck.

RE: Z-purlin question

I agree with KootK. Zee purlins are almost always built as a continuous system--they lap with each other over every support frame.

DaveAtkins

RE: Z-purlin question

And you also get the benefit of twice the moment capacity over every support frame, because there are TWO (lapped) purlins at that location.

DaveAtkins

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