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Load Combinations

Load Combinations

Load Combinations

I'm confused with the load combination's.  The minimum live load is 20 psf. If designing a roof and taking a reduction due to area I get a roof Lr =12 psf.  I have a snow load of 15.4 psf, DL = 20 psf, and WL = 15 psf.

In the load combination's is there still a LL or is the Lr the only live load?

D + .75W + .75L? +.75(Lr or SL)


RE: Load Combinations

The "L" live load in this case would be any floor live loads, crane live loads, i.e., live loads other than on the roof area.  For most single-story buildings you would only be concerned with roof live load, Lr.
Note that for low slope roofs your tributary reduction down to 12 psf may not apply because you are subject to minimum uniform loads, typically equal to ground snow load, and/or rain on snow loads of 5 psf.  
Design for the larger of the appropriately adjusted live load or the snow load.

RE: Load Combinations

Make sure that you have not reduced your snow load for area supported.  Only any live load.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Load Combinations

And if you use a reduced roof live load of less than 20 psf, the IBC code (i.e. ASCE 7) requires you to alternate the live loads (odd, even, adjacent loads, etc.) if you have continuous members.


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