ASD Load Combos for Wood (NDS 2005)

[bookowski]

What are the correct load combos to use for ASD wood design based on IBC '09 and 2005 NDS?

1605.3.1 lists 'Basic Load Combinations' for ASD. In 1605.3.1 it states that stress increases specified in the material chapters shall not be used with these combos 'except that increases shall be permitted in accordance with Chapter 23', where Chapter 23 is Wood.

Alternative combos are given in 1605.3.2 which state that they should be used where stress increases are allowed by the material chapter. These combos are generally all 1.0, i.e. D + L + Snow etc.

For wood I am taking the 'stress increase' to mean the duration factor. The way 1605.3.1 reads I would think that I am allowed to use the 'Basic Combinations' along with the duration factor (stress increase). However, in looking at the solved problems that come with the NDS I see that they always use 1.0 factors on all loads indicating that they are referencing the 'alternative combos' (although they don't explicitly state where the combos come from).

 

[woodman88]

Not sure what "solved problems that come with the NDS" you are looking at. Maybe if you uploaded the page here I could help you.

Now assuming this is the information you are looking for.
Technically, all the "'Basic Load Combinations for ASD" and the "Alternative combinations" are to equal 1. Some/most/all engineers take the shortcut of changing the answer of 1 in the combinations to a .9/1/1.15/1.6 for wood rather than applying the Duration of Load Factor to the other (correct) side of the equation. But the correct Duration of Load Factor may be applied in all of the Load Combinations.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[bookowski]

When you buy the NDS package it comes with several booklets (supplement, manual etc.), one of those is 'Structural Wood Design Solved Problems, 2005'

I'm not sure what you mean by all the factors are 1.0 in the basic combinations. For example, Eqn. 16-11 is 1.0DL + 0.75LL + 0.75Snow but in the 'Alternative' combos you would use DL + LL + Snow. My understanding is that the load combos reflect the statistical likelihood of concurrent loads, i.e. it's unlikely to have full LL and full Snow simultaneously. This is not the same as the load duration factor which takes into account the differences in wood strength based on duration of loading.

Maybe I'm reading the wording of the code incorrectly.

 

[woodman88]

You should be using the combinations as shown in the IBC.

The Duration of Load Factor may be used to modify the wood design values, as allowed in the NDS, for use with the "Basic Load Combinations for ASD" and the "Alternative combinations". But, the Duration of Load Factor does not change the combinations or any factor in the combinations.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[bookowski]

Right - but which combos, the basic or alternative? I have always used the basic combos from IBC along with the appropriate duration factors. I recently was looking through the NDS Solutions book and saw that they use the 'alternative' combos, i.e. 1.0 for all loads. There is a big difference in the results for some situations.

 

[DaveAtkins]

In my opinion, it is permitted to use the load combinations which include the 0.75 factor on transient loads, AND the 1.6 duration of load increase simultaneously. I read a commentary somewhere which states they are separate concepts and so can be used simultaneously.

DaveAtkins

 

[UcfSE]

I would use the basic combinations in conjunction with the duration factors, regardless of the examples. Agree with Dave.

 

[RFreund]

I agree with the above. The C factors are applied to the capacity side of the equation. Same way you apply a factor of safety of 1.67 on the bending stress of steel (AISC). It just so happens for wood, the Cd 'safety factor' is based on duration of loading and actually increasing the capacity.

EIT

http://www.HowToEngineer.com

 

[bookowski]

I agree with the last three posts - and that's what I have always done. Was just curious why the solved examples that come with NDS use the alternate combos, it got me wondering if I've been using the wrong combos. Sounds like everyone is doing the same thing.

 

[OldPaperMaker]

bookowski,
If you want more information about this subject you might go to Google and type in "history of Alternative basic load combinations in the IBC". This will lead you to a few articles that go through the path that got us to where we are now.

You have probably noticed that ASCE 7-05 & 10 do not have a Alternative basic load combination in their Chapter 2, Combinations of Loads.