Z-purlin question

[kmart30]

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?

 

[KootK]

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.

 

[hokie66]

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.

 

[DaveAtkins]

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

DaveAtkins

 

[DaveAtkins]

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