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Load Combinations using Strength Design

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Can anyone elaborate on the f2 factor in load combination (12-5) of  Section 1612.2.1 of the 1997 UBC?  

The factor is defined as 0.7 for roof configurations that do not shed snow, or 0.2 for roof configurations that do shed snow.  I am assuming that the two numbers represent some combination of Load Factor (1.6 for live load) times some portion of the full design live load.

For the case where roofs do not shed snow (fs=0.7)

Assuming the factor for live loads is 1.6, then the "S" is 44% of the full roof design load.  

     1.6 * (0.44)Ls = 0.7Ls

Historically, UBC has allowed up to a 75% reduction of the roof design snow load included in the seismic load, or "S" = 25% of full design load. 

     1.6 * (0.25)Ls = 0.4Ls

For the case where roofs shed snow (fs=0.2)

If the live load factor is 1.6, then "S" is 13% of the full design snow load.  It is reasonable to assume that where snow loads are possible, there will be some snow present during a design seismic event.

Has the 75% reduction been replaced with a 56% reduction, or is there something else included in the load factor to produce the 0.7?

Further, what constitutes a "roof that sheds snow"?

Bruce Hopper, P.E.

Life in Alaska isn't always easy, but it has its rewards!

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