Roof Live Load Reduction

[XR250]

Doing a foundation design for a un-convential 50x100 PEMB that is to be used for a Furniture Store. It has light gage frames at 5' O.C. spanning 50 ft.
The building designer is using 12 PSF for the Roof LL and 2 PSF for the DL.
Based on a 250 sq. ft. frame trib, my numbers indicate he can use 19 psf min. for a the live load.
Apparently, he is trying to use this portion of the IBC:

"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 the following equations shall not be used unless approved by the building official. Such structures shall be designed for a minimum roof live load of 12 psf"


He has called out the note about the scaffolding. Is there ANYWAY this can be justified for a furniture store? I believe 12 psf is above the snow load for this area, but barely (central NC). Unfortunately, I don't think he can get the building to work with any larger loads so he is grasping at straws. I have never seen ANY PEMB designed with less than 20 psf for the roof load - possibly reduced for trib area.
Also, 2 psf DL? I guess there is no allowance for any PM&E except for helium balloons.
FWIW, I was not involved in spec'ing the building, nor am I the designer of record. I doubt the building official will approve this.

 

[JAE]

Is the 2 psf the "collateral" dead load in addition to the self-weight?

If so then perhaps that might fly but depends on the actual amount of lighting, ductwork, sprinkler systems, etc. that get hung on later.

For the live load - I'm not sure he can use the scaffolding "out" if the roof is an opaque "roofed" surface where someday someone (or a group of someones) will walk up on top of it
and not realize how it was designed. For glass roofed greenhouses no one in the right mind would walk on glass.

For a solid roof, how would anyone know 10 years down the line that the roof was only designed for 12 psf?

Perhaps the 12 psf would work for a few folks walking up there but is that the intent of the code?

Also - wouldn't the wind loading be more than 12 psf for the individual frames anyway?



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[XR250]

He does not specify if the DL is collateral. I am assuming it is not.

 

[Triangled]

Completely agree with JAE on future workers.... They will not walk on glass but will ”assume” the solid opaque roof will support them.... Assume it because of their confidence in the building code and the designing engineering profession.

I looked at one of these in the Midwest.... Now occupied by a national retail chain, with a ceiling hung at 16' and then an additional hung at 10'.....plus all the fire sprinklers and electrical you'd expect.
And the current tenant wanted to add HVAC equipment.

This cat was going on at least the 5th life when it came to me, and on it's trajectory, I wasn't confident it'd make it to all nine.

 

[JAE]

2 psf is not a reasonable roof dead load - for a rigid, spanning system as you describe.

You should be able to do some sort of load take-off and at least determine a feasible range.
My guess would be 8 psf on the low side to up to 15 psf for a "typical" pre-manufactured metal building system.


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[PSEPK]

I agree with JAE.

Some PEMB designers in our area use around 2 psf collateral load. That however may not be realistic in many cases. As far as roof LL is concerned, larger PEMB roof are generally subject to LL of maintenance workers. I have observed that PEMB designers mostly use 12 psf roof LL. However, IMO it is not as per code requirements in most of the caes.

In one of the designs I checked about 2 years ago, also used 12 psf. The roof was however accessible for maintenance workers. As such, this reduced LL was not in line with ASCE 7 provisions, based on roof slope or tributory area. I objected on same grounds, so the designer had to go by the code, to use a LL, higher than 12 psf as per ASCE requirements.

 

[dhengr]

ExcelEngineering:
I agree with JAE also, on both of his posts. You might also consider that the greenhouse is likely considered an ag. bldg. with their more lenient loadings and code criteria as a historic norm; and yet you say this is going to be a retail bldg. with its greater mech. needs (greater mech. DL’s) and much more public/people activity. This bldg. should be designed to the normal IBC requirements, with the possibility of unbalanced loading on the roof.

 

[XR250]

Thanks everyone. I might point the PEMB designer to this thread to give him additional perspective.