ASCE41 Overturning Effects

[structeng2]

Hello All,

I have a question regarding ASCE41-13 7.2.8.1. I am checking overturning on pre-cast panels of a steel frame building. The panels are not positively attached to the columns for in-plane/overturning - only out-of-plane. All in-plane and overturning forces are resisted at the foundation level.

As I understand the code - equation 7-5 and 7-6 can both be used to determine the net overturing on an element and whether positive attachments are required (i.e. if the overturning exceeds the stabilizing moment from dead load alone).

However, the two equations are vastly different. Assume my C1C2 = 1.4, J = 1.0 and performance level = Life Safety.

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Eq 7-5 yields: Mst > [Mot/1.4]

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Eq 7-6 yields: 0.9*Mst > [Mot/11.2]

I feel like I must be missing something? Thanks for your help.

 

[chris3eb]

The two equations are, in fact very far apart. Eq 7-5 uses the methodology of ASCE 41. That methodology produces insane overturning moments because it is assuming the entire building remains elastic. Eq 7-6 is reverse engineered to give general agreement with ASCE 7 overturning - see below from the commentary section:

 

[structeng2]

Thanks for the reply. I read the commentary for C7.2.8, but not that one. Now it is clear.

However, considering this is a precast panel that is not positively attached to the structure for in-plane loading (basically a free-standing precast wall), would eq 7-5 be more accurate? I wouldn't expect the panel to exhibit much yielding or ductility.

 

[TLHS]

Yeah, I wouldn't be using that for a free standing wall. A lot of gravity based stability against seismic failure comes from the ability to rock. To do that, an object needs to be able to rotate horizontally a reasonable distance before the center of gravity moves out over the toe of the foundation or support point. Once that happens, you've got overturning.

With a pure wall, your base is very thin in relation to the height and I can't see there being appreciable rocking happening in a stable fashion. Definitely not to the extent allowed by that provision.

 

[structeng2]

The ratio of the wall is ~2:1. (35ft tall, 15.5ft wide).

With eq. 7-5, there is significant overturning forces. Considering there won't be any ductility in the wall, I think these forces are more accurate than eq 7-6.

 

[TLHS]

Oh, you said in-plane, sorry. I was thinking you were trying to do something wacky out of plane.

 

[chris3eb]

ASCE 7-10 lists an R factor of 3 for ordinary precast concrete bearing walls and 4 for ordinary precast concrete nonbearing walls. So there may be more ductility than you're thinking. Also, overturning has a very nonlinear nonproportional relationship with your lateral load (if the stabilizing weight stays the same). So don't think of the Mu factor of 8 or 10 as meaning that your building needs that much ductility.

As an example, let's assume your ASCE 41 lateral load is twice as much as your stabilizing force, Fp = 2Wp, and for ASCE 7, your lateral load (reduced by R) is half your stabilizing force, Fp = Wp/2. For overturning on you wall with ASCE 41, you would have: T = FpH - WpL/2 = 2WpH-WpL/2 = 2Wp(35)-Wp(15.5)/2 = 62.25 Wp. And for ASCE 7, you would have: T = FpH - WpL/2 = (0.5)WpH-Wp(15.5)/2 = (0.5)Wp(30)-Wp(15.5)/2 = 7.25 Wp. So reducing your lateral load by a factor of four reduced your overturning force by a factor of 8.6.

Of course you can consider whatever overturning force you feel comfortable with, but that number doesn't have to be as high as the result you would get from Eq 7-5 because you just need to show that it's higher than what 7-6 gives.

 

[structeng2]

Thanks, chris3eb. We discussed this today - just using ASCE7 w/ R factor reduction since it puts us in between eq 7-5 and eq 7-6. I think that is a reasonable and defendable approach to the situation we have.