huangyhg

are asce 7-02 wind loads ultimate or service level?
are the wind loads in asce 7-02 at ultimate or service level. ibc 1605.3.2 seems to want you to multiply asce 7 wind forces by 1.3 and this requirement is in the "allowable stress" subsection of ibc 1605.3. but asce 7 indicates ultimate strength level forces for seismic load. this is confusing and i can't find anywhere in ibc or asce that says specifically that the asce 7-02 wind loads are allowable stress level.
am i correct in this assessment, that they are in fact allowable stress level loads?

wind loads in asce 7 are service level. seismic loads are ultimate.
i believe asce-7 also gives recommended load factors in the first part of the book- check if that helps any.
i believe the load combinations in 1605.3.2 are based on utilizing allowable stress increases. asce wind loads are not written to take this into effect which is why you are required to increase the wind load by 1.3. so you can either use the equations from 1605.3.1 utilizing the load reductions from 1605.3.1.1 or you can use the alternative combinations, but increasing the wind load to account for the allowable stress increase.
so you increase 1605.3.2 wind loads by 1.3 and take allowable stress increases or reduce combined loads by 25% and not take the increases?
this doesn't seem right. if the asce wind loads are too low already and you take them down another 25% (per 1605.3.1.1), aren't you lower than the 1.6 wind stress increase factor used when using 1605.3.2? (1.33 * 1.3 = 1.73)
asce-7-02, section 2.3 gives load factors used for "strength design". section 2.4 gives load factors used for "allowable stress design".
section 2.4.1 also states "increases in allowable stress shall not be used with the loads or load combinations given in this standard unless it can be demonstrated that such an increase is justified by structural behavior cause by rate or duration of load." it sounds like the intent is that you use the wind loads as calculated with the reduction factors shown in the allowable stress design load factors. or alternatively, if you have some other design code that allows stress increases for wind, you'd use the calculated loads without the reduction factor.
the 1.6 factor i believe is increased from 1.3 to take into account the fact that the asce 7 uses a directionality factor kd = 0.85 to reduce wind loads. 1.3/.85=1.53 --> rounded to 1.6.
it has to do with the calibration of the load combinations. the load combinations of 1605.3.1 are calibrated to provide maximum response by either the worst case of any two loads applied simultaneously or the combination of multiple loads reduced by 25%. the combination equations of 1605.3.2 are an alternate, which i would assume when applied correctly will give similar results to the equations of 1605.3.1. note that the asce wind loads utilize a wind directionality factor (kd) which are not to be utilized without using there load combinations. you can calculate wind loads per asce without the (kd) factor and use whatever load combinations you think are appropriate.
check out the asce 7 commentary c2.3 and c2.4.
the asce commentary and your responses have been very helpful. thank you all for your contributions.
i'm glad to see that the way i interpret the code is essentially in line with most of you.
essentially, i do use the kd = 0.85 factor, i do use 3/4 of transient loads (when both wind and live load act simultaneously), and i don't use a stress increase with any material (except i still use the 1.6 duration of load factor with wood design).
daveatkins

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