Residential uninhabitable attic with storage live load

[AELLC]

I understand the storage live is 20 psf applied to the bottom chord of roof trusses, but how does that combine with the roof "construction" (1.25 duration factor) live load of 20 psf?

I am using the allowable stress method of design.

Is the Code actually saying the total roof live load is 40 psf? Can the roof construction load be reduced somewhat because chances are, the construction load does not occur fully (considering re-roofing) after the house is occupied and things are stored in the attic?

 

[MiketheEngineer]

That 20 psf is usually considered dead (trusses, drywall, lights, insulation, etc). Other than a 250 man - live loads are not recommended unless designed for - and the codes are rather specific on how this should be done. Check IBC and TPI for help.

 

[woodman88]

The 20 psf live load is a live load. You have to add any additional dead loading that is being added for the support of the live load in the attic. With a seven day duration 20 psf roof live load you can use the 1.25 duration factor for the attic live load. You need to also check the 1.0 duration factor without the roof live loading. Per the 2009 IBC Table 1607.1 footnotes this live load only applies to attic heights of 42" or higher. Where less than 42" the 10 psf live load applies, which does not need to be assumed to act concurrently with other live loads.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[AELLC]

OK so there is 40 psf live and about 30-32 psf dead load at LDF = 1.25 or 20 psf live and 30-32 psf dead load at LDF = 1.0. However the 40 psf live and 30-32 psf dead due to added sheathing etc in the attic area only occurs where the headroom is 42" or more. The remaining roof area gets 20 psf live and 22 psf dead.

I may make this work after all because originally I had the 40 psf live and 30-32 psf dead on the entire roof area.

Thanks Garth

 

[MiketheEngineer]

I misunderstood your question - around here it is 20-10-10. I believe Garth has it about right.