Reduced roof live load <200 SF

[hmaskins89]

I am peer reviewing a generic set of plans for a metal building that’s risk category I.

The engineer that sealed it is in NC. They list Design loads of roof live load =12 psf. Snow load =35 psf , and wind loads up to 170 mph.

The generic drawings cover up to 24’ wide lean to structure up to 16’ tall eave height, with frames spaced at 5’-0” on center.

I don’t have their calcs, but am I missing something here? I don’t see any allowable reductions for roof live load for a trib area of 120 feet. He is claiming it’s because it’s risk category I and is trying to charge my client an additional fee for re-engineering associated with the increased live load of 20 psf, which is what I recommended they design for per code.

 

[hmaskins89]

To be clear, I know the structure strength is covered for 20psf with that snow/wind load, but still- how does he think he is getting away with this reduction? Says he does it all the time.

 

[OldDawgNewTricks]

What is the roof slope? 12 psf is applicable to steep roofs regardless of tributary area.

 

[XR250]

I have previously read that Cat. I buildings can use 12 PSF but can't seem to find the reference.
Good luck with the review. I tried that once and the liberties that were taken with the Laws of Physics were ridiculous.

 

[phamENG]

XR, could it be this:

In structures such as greenhouses, where special scaffolding is used as a work surface for workers and materials during maintenance and repair operations, a lower roof load than specified in Eq. (4.8-1) shall not be used unless approved by the Authority Having Jurisdiction. On such structures, the minimum roof live load shall be 12 psf (0.58 kN∕m2).

Don't think it quite applies here in any case, though the AHJ could turn a blind eye to it. I suspect that's more out of ignorance than desire to lower the required design loads. Both ASCE 7 and the IBC use the same equation.

Unless this thing has 12/12 slope or steeper, their code interpretation is incorrect. If you are in a position to do so, I would insist on them substantiating their claim that 12psf is acceptable. And "I get away with it all the time" isn't an adequate response. (I'm guessing they "get away with it" because they design for a snow load that meets the code requirements so nobody cares about the roof live load number they slap on there.)

 

[XR250]

Yup that is it.

 

[hmaskins89]

Thanks everyone for affirming my thoughts! Always like to check if there are some weird exclusions- the greenhouse one was a new one, but yea this is intended to be someone’s wood shop, so def not applicable. My client is going to go with a diff building supplier and ask this question from the get go.

 

[XR250]

Best to find one that is already sealed for his state.

 

[phamENG]

Or just stick frame it. He is a wood worker, after all...