huangyhg

overturning stability
i have always used a 1.5 overturning stability ratio for designing braced frames in steel buildings. however acse keeps modifying their load combinations taking out things like the old 1/3 stress increase and replacing it with a 0.75 factor for live and wind. my question is if i satisfy asd load combination 7: 0.6d + w have i satisfied the overturning stability?
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they are not really related. the overturning is a function of height, depth and load. the asd load combo is just a load factor equation.
the 1.5 fs you are talking about is not a code requirement, as far as i know. so no, the 0.6d+w really doesn't have anything to do with your overturning ratio of 1.5.
just run an imaginary problem where you take a very tall, and very narrow frame, and apply your load combination to it. obviously you can adjust the geometry to go way under an ot of 1.5.
thanks for the post. i also see that this has already been kicked around here, going to review those responses as well.
yes, 0.6d + w does satisfy the requirement for overturning stability.
daveatkins
agree with dave.
dave and archeng59, i would like to see that requirement for steel frames having an overturning restriction of 1.5.
where do i need to look? thanks in advance.
the 0.6d + w combination is meant to provide a similar overturning safety factor of about 1.5. the 1.5 overturning factor is no longer in the ibc that i'm aware of because this load combination now provides the necessary check.

i agree except the safety factor now is 1/0.6=1.67.

the 1.5 stability ratio is not the same as a 1.5 factor of safety implied in the load combo 1.0wl + 0.6dl. i think that we are all right, we are just talking about two different items. the stability ratio, as clearly listed in the opening question, is a function of geometry and applied load. the factor of safety of 1.5 applied to loads, implied by the equation just mentioned, and thoroughly reviewed in other posts, is a function of the 10% deadload overestimate and 1.5 factor of safety.
these are two different items. the stability ratio is not a factor of safety assertained from a load combo, although the factor of safety from the load combo happens to be 1.5.
a good example is that you can have a factor of safety of 1.5 applied to your loads via the 0.6dl+1.0wl, and still have a stability ratio (mresist/moverturning) well below 1.5.

bigmig-
can you give an example of your last statement? i don't believe you can have that "stability ratio" be less than 1.5 using full service loads and still satisfy the 0.6dl + 1.0wl combination.
bigmig,
i don't agree with you on this.
historically, going back to older codes (1973 ubc for example) there was a requirement for engineers to check overturning, checking the overturning moment against the resisting moment of a global structure. the safety factor required in those codes was 1.5. the allowable stress load combinations in those codes never included the 0.6d + w combo.
in recent code versions of the ibc, that 1.5 factor is no longer stated and now there is a 0.6d + w combination that has been mentioned in numerous articles and commentaries as a replacement for the overturning stability check using d and w and a 1.5 ratio margin.
the idea that load combinations are just for strength checks is not correct. load combinations are for strength, servicability and stability. the intent of asce 7 has been to consolidate all the various, loose requirements into a set of comprehensive load combinations that cover all issues.

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