Wood Load Combinations

[Lion06]

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.

 

[csd72]

You will get a feel for it eventually and learn which loads cases govern for certain members.

 

[PMR06]

I don't understand the source of confusion. Each load case (in ASCE/IBC) has one Cd associated with it thus one allowable stress. For example:

D + L + W Cd = 1.6
D + L Cd = 1.0
D Cd = 0.9

 

[Lion06]

PMR-
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).

 

[PMR06]

I see... Yes it is frustrating. I had one case where a beam was loaded with a lot of dead load, but barely any live. It "worked" under the D+L load case (Cd=1.0) but not under dead only (Cd=0.9).

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.

 

[CJJS]

Yes it is a pain. But a good computer program is the solution.

 

[JKW05]

I always sigh when a client or architect says "It's just a wood building". Proper wood design can be sooooo tedious; connections in particular. Spreadsheets can help. Having said that, maybe this will complicate the issue

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?

 

[apsix]

I've created a simple spreadsheet. Inputs are the different loads; dead, floor live, roof live etc. The loads are then combined as required and divided by the appropriate Cd.
The load case with the highest result is the critical case and I design for that one only.

 

[Lion06]

JKW05-
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

 

[Lion06]

Thanks everyone for the input.