AISC 341 - D1.4a.2b - Column forces limited by foundation overturning uplift

[cal91]

The required axial compressive strength and tensile strength need not exceed the forces corresponding to the resistance of the foundation to overturning uplift.

At what point is there "overturning uplift". When the first corner of a footing experiences zero soil stress? Or when the entire footing overturns?

My opinion is the latter. That is why the language says "overturning" uplift. However my supervisor disagrees and thinks it's the first. This does not seem conservative to me.



Second, my supervisor has instructed me to determine bearing pressure/uplift using ASD load combinations, but for footing/column design using LRFD load combinations. He said to determine the maximum force, use the ASD load combination which results in the smallest "E" force to cause uplift. Then plug that same E into the LRFD load combinations for strength design. Again, this seems unconservative.

I would have thought that the maximum force is determined for each LRFD load combination individually. I.E. for designing a column, design to LRFD load combination X by finding the "E" force that causes uplift for that same LRFD load combination X (instead of the "E" force that causes uplift for the least conservative ASD load combo)

What are your thoughts?

 

[KootK]

My opinion is the latter.

Ditto. I see the logic chain like this:

1) Overturning in this context means the point where additional structure rotation no long produces additional internal forces. A form of plastic rotation essentially.

2) At the point of tension side footing lift off, you still need to add additional force to the system to get to #1.

3) 1 + 2 --> You gotta overturn the entire footing (and any overburden etc).

I'm afraid the load combo business confuses me to I'm gonna leave that part alone other than to say the obvious: consistent methodology --> critical design action --> good.





I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

On the uplift question I believe you are correct and your supervisor is wrong in my opinion.

The footing corner could "uplift" to zero, then the brace fracture, then the footing drop back into place. Not a good scenario.
The next question is whether the valid condition is when the uplift on the footing causes the reaction to be outside the kern (middle third) of the footing). You might be able to make a case where that condition is the limit.

On the load combinations - I think you are more correct.

The thought I had was that an unloaded building condition (such as zero live load - no one in the building) would be a possible "condition" that the building is in when an earthquake hits.
If that happens, then the brace force that "could" develop would be based on that combination - Dead Load Only.

But if the building had a lot of existing live load in it (say a huge gathering of people, furniture, etc.) then the D+L case would require a larger E force to accomplish uplift.
This would put a larger seismic force on the brace. If you had only designed for the smallest E force based on Dead Only you'd have an unsafe condition.



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[cal91]

Thanks JAE and KootK.

I forgot one additional area we disagree on is... say that the force corresponding to the footing uplift is GREATER than the E * Omega (omega from ASCE-7 table 12.2-1). My supervisor's opinion is the Omega factor is a hard upper limit to use, so use the Omega factor from ASCE-7 and ignore the force corresponding to the footing uplift. My opinion is you need to use the footing uplift, and the Omega factor form ASCE-7 doesn't apply.

So then the next question is... in the end it's his stamp. He's kinda of a stubborn guy. What does one do in my position when I think my supervisor is unconservatively wrong? I've tried respectfully arguing my point of view, but he is quick to shut me down.

The next question is whether the valid condition is when the uplift on the footing causes the reaction to be outside the kern (middle third) of the footing). You might be able to make a case where that condition is the limit.

I'm a little confused by your statement. The reaction has to be outside the middle third if there's uplift on one side (triangular stress distribution). Are you saying that if the reaction is outside the middle third (or in a 2D case, middle "ninth"? )then you can say that is the limit? That's essentially my supervisor's argument. He says it's called "racking."

 

[KootK]

My opinion is you need to use the footing uplift, and the Omega factor form ASCE-7 doesn't apply.

Rationally, I certainly agree. Omega gets your from your first yield load level to your full yield level. So if footing overturning IS first yield...

What does one do in my position when I think my supervisor is unconservatively wrong?

It sounds to me as though you've already done what I would expect you to do ethically. Sleep easy.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

I think I screwed up that statement on the kern - please neglect.

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[sandman21]

AISC Seismic Manual example 5.3.7 shows the determination of the new omega limited by foundation uplift. You compare the resisting moment for the lateral element with overturning forces including overstrength. Resisting moment is a simple FBD, taking forces acting at the tip of the foundation element. If resisting is less than the overturning you can ratio down the forces on the column. You can then use that new value for the design of the column in your case. I am not a fan of how we have implemented the allowable reduction of overstrength, as you have a lot of gray area. Do you care about cohesion, how much slab is used, etc.