Soil bearing pressure and seismic overturning

[apaulos15]

Hey guys,

I was wondering if any of you could help me out on a problem I've been stuck with. Say you have a continuous wall footing with two shearwall segments. Say the footing is 20' long, and you have a 5' long x 8' high shearwall on each end. The seismic force on this wall line is 5 kips. The overturning moment for each wall segment would be 20 kip-ft.

Is there any reference or equation that I can refer to to help me find the soil pressures? I have two overturning moments on each side of the footing. Normally, for a single wall segment, I would find my pressure from the single overturning moment and the weight of the footing. However, when multiple moments are introduced, I don't really know how to proceed. I know that for this example, I could just treat it as one overturning moment, but for my problem, the shear walls aren't the same lengths and aren't placed at each end of the footing (i.e. one is in the middle, and one is on the end).

Any help would be appreciated!

-Ant

 

[msquared48]

Sounds like a beam on an elastic foundation problem to me.

Mike McCann, PE, SE (WA)

 

[KootK]

If the footing can be considered rigid, then you can always treat the multiple moments as one moment for OT, no matter where they're located or however moments there are. The real question, per Mike's comment, is whether or not you can consider your footing rigid. Depending on the answer to that question, you may even decide to assume that you have two independent footings.

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.

 

[dhengr]

Apaulos15:
You have a cont. conc. wall ftg. below the wall segments. What kind of structure do you have over the top of these 8' high walls, particularly immediately over the plane of the walls and over the opening btwn. the two wall segments? This structure above and the conc. wall ftg. will tend to work (want to work) as coupling beams btwn. the two wall segments, with nasty beam shears and moments right at the opening jambs of the wall opening. This works in conjunction with Mike’s beam on an elastic foundation thinking. And, as Koot suggests you might be better off de-coupling the two wall sections and their ftgs.

 

[apaulos15]

Thanks for the input, guys.

Mike:

I was thinking that this was a beam on elastic foundation problem. I guess I was hoping to find some example problems that had this condition. I'm limited to hand calculations at the moment, since I won't be able to do a computer model. I would model it as a beam on compression springs, but my plan checker requires me to get a soils report for those values.

Koot:

Yeah, I usually treat problems such as these as one OT moment. However, my plan checker wants me to go a bit more in-depth. I'm required to show the soil pressures, moments, and shears for the footing as well.

Dhengr:

To elaborate on this problem, it is part of a seismic retrofit. The wall in question supports three wood-framed stories. However, I also have similar walls that support no deadloads and are only intended to resist seismic forces.

 

[BAretired]

Where dead loads are very light or non existent, separate footings may present a problem of having insufficient dead load to ensure the entire length of each footing is engaged to resist seismic forces. If that is the case, it may be preferable to consider a single footing full length (or perhaps a couple of feet longer).

Before getting into a complicated elastic analysis, it may be worthwhile to investigate single versus separate footings assuming the footing is a rigid body.

BA