Wood Load Combinations
Wood Load Combinations
(OP)
Does anyone find the load combinations in the NDS considerably more involved than load combinations for say.. steel or concrete?
It's not like you can even just find out the worst load combination and design for it, becuase each load combination has a different asociated allowable stress depending on the load case with the greatest load duration factor, Cd.
Therefore, it seems to me that every load combination must be checked against every allowable stress.
It's not like you can even just find out the worst load combination and design for it, becuase each load combination has a different asociated allowable stress depending on the load case with the greatest load duration factor, Cd.
Therefore, it seems to me that every load combination must be checked against every allowable stress.






RE: Wood Load Combinations
RE: Wood Load Combinations
D + L + W Cd = 1.6
D + L Cd = 1.0
D Cd = 0.9
RE: Wood Load Combinations
There is no confusion there, but you restated my exact concern. For steel, you take the worst load combination and design the member based on that loading.
For wood, as you noted above, you have 3 different combinations and a different allowable stress associated with each one (based on the different Cd values).
Cd get applied to the allowable stress, not the load.
For example,
D+L+W = 500 # tension
D+L = 400 # tension
D = 280 # tension
You can't automatically design for teh D+L+W case because teh Cd is much higher and will give you a much higher allowable stress.
As I said above, it seems like each load combination needs to be checked against the corresponding allowable stress (i.e. each combination must be checked, not just the worst case).
RE: Wood Load Combinations
I use RISA on a lot of my wood designs so that each load combo can have it's Cd input. Then I use "solve envelope" to run each load combo simultaneously and check the output for the worst case.
RE: Wood Load Combinations
RE: Wood Load Combinations
The duration of load factors (Cd) is for use with ASD. My interpretation of IBC(2006) 1605.3.1 and ASCE 7-05, 2.4 is that D+W; .6D+W; and D+.75W+.75L are some of the combinations that are to be checked, and the 1.6 factor is applicable to all these cases (IBC 1605.3.1.1).
Any differing thoughts?
RE: Wood Load Combinations
The load case with the highest result is the critical case and I design for that one only.
RE: Wood Load Combinations
Yes, 1.6 is applicable for all of those load combinations, but you still have to check D + L and D because they have lower Cd values
RE: Wood Load Combinations