Overturning stability
Overturning stability
(OP)
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?






RE: Overturning stability
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.
RE: Overturning stability
RE: Overturning stability
DaveAtkins
RE: Overturning stability
RE: Overturning stability
Where do I need to look? Thanks in advance.
RE: Overturning stability
RE: Overturning stability
RE: Overturning stability
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.
RE: Overturning stability
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.
RE: Overturning stability
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.
RE: Overturning stability
1.5/0.9 = 1.67 = 1.0 / 0.6
Make sense?
I've contributed to a couple of threads that discuss the issue. But, I couldn't find them in a quick search. If I find the threads, I'll post a link to them.
RE: Overturning stability
ht
RE: Overturning stability
http://www.eng-tips.com/viewthread.cfm?qid=238589
I
RE: Overturning stability
The fact is, I used a FS of 1.5 and didn't know it. Thanks
for the article steelPE. That was great.
Regarding my previous post, and the example of applying the load combo to a frame and having an OT of less than 1.5, I was referring to a problem where you would have to provide an overturning anchor (i.e. hold down or a.bolt), which is typical in the frames I have designed, since seldom do the geometry and loads create a frame with no hold down requirement.
But once I reviewed your posts, I realized that the problem of discussion is when the incoming horizontal load is set to just the right amount, so the hold down force is zero. Once I looked at this I found the hidden FS of 1.5, and learned that I had indeed been using it all along!
Thanks for the continuing education everyone. Engtips at its best!