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Overturning stability

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

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.

RE: Overturning stability

(OP)
Thanks for the post.  I also see that this has already been kicked around here, going to review those responses as well.

RE: Overturning stability

Yes, 0.6D + W does satisfy the requirement for overturning stability.

DaveAtkins

RE: Overturning stability

agree with Dave.

RE: Overturning stability

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.

RE: Overturning stability

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.

 

RE: Overturning stability

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

 

RE: Overturning stability

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.

 

RE: Overturning stability

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.

RE: Overturning stability

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.

 

RE: Overturning stability

The 0.6DL + WL / EL load combination was developed to provided the same OTM safety factor as the 1.5 facor applied to the 0.9 DL + WL/EL did.  

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

Sorry for the delay in getting back to everyone. I work an early day. Based on the overwhelming evidence....and great responses, I have to recant and say that, *gulp* I was wrong!!

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!

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