Overturning stability 1

[mijowe]

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?

 

[bigmig]

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.

 

[mijowe]

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

 

[DaveAtkins]

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

DaveAtkins

 

[archeng59]

agree with Dave.

 

[bigmig]

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.

 

[JAE]

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.

 

[ron9876]

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

 

[bigmig]

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.

 

[Lion06]

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.

 

[JAE]

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.

 

[JoshPlumSE]

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.

 

[SteelPE]

Attached is a link to an article in Structure Magazine (This article was referenced in previous discussions). Hopefully it will clear up this issue.

http://www.structuremag.org/OldArchives/2004/september/Codes and Standards.pdf

 

[JoshPlumSE]

Here's a link to the previous threads:

http://www.eng-tips.com/viewthread.cfm?qid=238589

I

 

[bigmig]

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!