Wood Load Duration Factor

[abusementpark]

Ok, I am unclear about how I am supposed to use this Load Duration Factor for wood.

They give you a different factor for each load case (dead, live, wind, etc...). So what do you do in each load combination? Do you prorate the factor based on how much of each load you have?

 

[frv]

Always use the factor for the shortest duration load in any combination. E.G. Roof + Wind + Dead, Use wind load duration factor; Live + Dead, use Live.

This should be in the NDS; if not, it should be in Breyer; if not, trust me

 

[Splitrings]

abusementpark,
"for the shortest duration load in a combination of loads shall apply for that load combination." If your load combination only includes dead load you will use a CD=0.9. If you load combination includes live load you will use a CD=1.0 and so on. This factor applies to the whole load combination. An example is load cases 5-7 of ASCE 7-05. They all include wind, which is the shortest duration load, so a CD of 1.6 is used to determine you allowable stresses for that combination.

 

[woodman88]

The highest value of the load duration factors that applies to that load case combination should be used. So if wind is in the formula use 1.6, or without wind and seismic but with a roof live load use 1.25, etc.

Garth Dreger PE
AZ Phoenix area

 

[Lion06]

I agree with the others about how to apply the load duration factor. This makes it even more critical to check every load combination. For other materials, you have a constant capacity and you would use the worst case loading.
For wood, you don't have constant capacity (because of the load duration factor), therefore you have to check demand<capacity for all load combinations.

 

[abusementpark]

Thanks for the responses. I heard this was what you are supposed to do, but I never saw this explicitly stated in the NDS.

 

[JAE]

Look in the appendix of the NDS as it has a listing of combinations with the applicable DLF shown for each.