2012 IBC Section 1605.3.2 says: “When using allowable stresses which have been increased or load combinations which have been reduced as permitted by the material chapter of this code or the referenced standards, where wind loads are calculated in accordance with Chapter 26 through 31 of ASCE 7, the coefficient ω in the following equations shall be taken as 1.3. For other wind loads, ω shall be taken as 1.”

What is the reason behind it? The wind loads from ASCE 7-10 Chapters 26 through 31 are at the strength level. Why (when allowable stresses are increased as permitted by the material chapters ) do we have to multiply the strength level wind forces by 1.3, while only multiply “other wind loads” (I assume calculated at the ASD level as we have done in the past) by 1.0?

The ω has to do with the directionality of wind. The alternative basic ASD load combinations are from the 1997 UBC, where wind did not include directionality. Wind in all editions of ASCE 7 since 1998 has included directionality. Thus, if the alternative basic ASD load combinations are to be used in conjunction with wind out of an ASCE 7 edition subsequent to 1995, directionality needs to be taken out of the wind. That’s precisely what ω does. It is one divided by the K_d factor of 0.85, which gives you 1.18. This was rounded all the way up to 1.3, confounding a lot of people. If one is designing for wind out of an ASCE 7 edition predating 1998, which is unlikely, or a foreign standard where wind does not include directionality, ω goes back to one. ω = 1 is more likely to be used where design wind forces are from wind tunnel tests and directionality was not included in the wind tunnel testing. Having said all of that, the 2012 IBC new language: “When using allowable stresses which have been increased or load combinations which have been reduced as permitted by the material chapter of this code or the referenced standards,” has never made any sense to us. I have also read it at least ten times, if not more. Susan and I have discussed it. She independently thought it does not make any sense. When we teach, we simply tell the audience to ignore it. We tried to trace the code change that placed the language in there. This was far from easy. The addition came from a public comment. We read the reason attached to the public comment. The reason did not make sense to us. Therefore, the code user is advised to continue following the direction given in the 2009 IBC, which is to use ω = 1.3 when determining wind loads in accordance with ASCE 7.