AWC NDS Nominal Fastener Capacities vs ASD/LRFD

[Mike L]

Hello,

I am working with some data calculated via the 2018 AWC-NDS. In looking lateral capacities for wood screws (my question applies to other scenarios as well), the NDS calculates the ASD capacities, and the LRFD capacities using the basic equation in Table 11.3.1

Unfortunately, for the work I am doing I really need to know the nominal capacities, not the ASD or LRFD capacities. Is there an appropriate way to back out the Nominal capacities?

Thanks much,

 

[phamENG]

What data do you have? It should indicate the calculation method used. So you just rearrange the equation to solve for Z.

 

[Eng16080]

Perhaps I don't understand what you're asking, but per LRFD, you compare nominal loads to the calculated capacity. If you look at NDS 2018, Appendix N, Section N.1.2, it clearly states that nominal loads are to be used with the LRFD methodology. If you mean something else by the term "Nominal", then I have no idea.

 

[Eng16080]

Based on a quick look at "Design of Wood Structures" by Breyer, I'll try to give a better response than what I wrote above. They describe the format conversion factor, KF, as being used to "convert tabulated reference design values to nominal design values for LRFD."

I believe that Zn (Z nominal) is therefore:
Zn = Z x CM x Ct x Cg x Cdelta x Ceg x Cdi x Ctn x KF x lambda

Note that you don't use the CD factor for LRFD. You use lambda instead. Also, per LRFD, the resistance factor, phi, is typically not included in the nominal capacity.

You should look at Breyer, as they explain it better than I am here. Hope that helps.

 

[Mike L]

Thanks. It appears the entire connection calculation process for NDS is to provide a result in ASD (safety factor already 'applied'). I don't see a way to get back to a nominal capacity. I'm not sure when it occurred, but looks like LRFD adjustment factors were added to AWC-NDS when the LRFD approach became more mainstream. That said, don't see a way to get back to just a nominal value. This differs from steel where the AISI standards run calculations to define nominal values, then the safety and resistance factors are applied AFTER. NDS just spits out a number with unknown safety factor built in.

 

[bones206]

I believe the Rd term is essentially the safety factor. That factor reduces the limit state capacity from the 5% yield offset strength, which is what the yield mode equations predict. See AWC Technical Report 12 for more in depth discussion.