Steel Deck Diaphragm stress increase

[UcfSE]

Some reference material:

In the 2001 AISI Cold-Formed Steel Specification, NASPEC 2001, we are directed to appendix A for provisions applicable to the USA. On page A3, the appendix for code section A4.1.2 "Load Combinations for ASD", the 1/3 stress increase is apparently eliminated and instead the 0.75 load factor is used, as in the ASCE 7. This section also specifically states: "When evaluating diaphragms using the provisions of Section D5, no decrease in forces is permitted for load combinations including wind or earthquake loads." Section D5 is the section with a table giving the required safety factors and resistance factors for diaphragms.

The Vulcraft steel deck catalog shows in the diaphragm notes, page 67 note 10, that the allowable diaphragm loads have already been increased by 1/3.

Because the AISI spec removed the 1/3 increase for steel diaphragms, and moved the increase to the load side of the equation only for two or more loads, do we have to multiply all of the diaphragm capacities listed in the steel deck catalogs by 0.75? It seems to me that we do.

 

[JAE]

Hmmm....yes, it would seem to look that way.

 

[UcfSE]

Did I ask a silly question? What was that about young engineers I read ...

I just never noticed the built-in 1/3 increase until recently and I don't think anyone at my company has either, hence I ask you guys.

 

[JAE]

Well, I've always been aware of the 1/3 increase built into the Vulcraft tables and then the IBC comes out with that cryptic statement "two or more transient" loads and they don't have the word "transient" ever defined so no one seems to know what to do.

I think I understand then, that with diaphragm design, you aren't really even looking at any roof live or snow loads so there would definitely be just the D+W case and in the IBC then, you can't use the 1/3. So your post sort of woke me up to the fact.