Wall Footing Design for Lateral Seismic Forces

[Delmarva_Struct]

Please see the wall footing snip attached. I have a 450 kip lateral force at the top of the wall which is about 23 ft high and 10 inches thick and 39ft long.

This is a non-load-bearing wall and the wall self is the only thing I have to resist overturning as well as sliding. The footing is about 2 ft below ground. I am considering the dead load due to soil overburden as well. When performing the calculation I am getting E>L/2 footing instability and high bearing pressures. In theory, I would need a 20 ft wide wall strip footing to make this work. This seems preposterous. Even then, the wall footing fails in sliding..however considering that it is connected to 2 other perpendicular wall footings, I think I can choose to neglect sliding.
I am not sure if I can consider a certain portion of the dead load of the connected walls to resist overturning and thus reduce the e. If yes, then what proportion of those walls can I use?

Does anyone have any sample calculations, suggestions, or references that I can use to design these types of footings? I do have bowles but am looking for a more straightforward sample calc or reference for the seismic design of shear wall foundations.

 

[lexpatrie]

This sounds more like a retaining wall than the way you are approaching the design, while it's perhaps possible to pull out shear forces at the connections to the perpendicular walls, that sounds either unrealistic due to the high forces you'll need to transfer or unnecessary as you'd rather use the soil pressure to counteract it without trying to transfer the forces through the perpendicular walls. The wall is quite high and the load is also quite high, so that length of toe, perhaps, is legit.

A cross-section view would be helpful. Both to us and to you.

 

[Delmarva_Struct]

Please see the attached cross-section. I have considered the passive soil pressure as well. It does not help much considering the small cross-sectional area of the footing. Please note that the shear force is in-plane to the wall and that is a precast concrete shear wall.

I have one more question...Does this type of wall footing require top and bottom transverse rebar? considering that there is an interchangeable uplift at one end and a downward force at the other end...

 

[Delmarva_Struct]

The cross-section shown is of a similar adjacent wall which is load-bearing..So the double T stems are shown resting on the wall..The wall under discussion does not have this...

 

[bones206]

I am not sure if I can consider a certain portion of the dead load of the connected walls to resist overturning and thus reduce the e. If yes, then what proportion of those walls can I use?

I think you could start by looking at one orthogonal wall panel segment providing tributary dead load to the shear wall ends. Since the orthogonal walls are bearing, you should be able to mobilize the dead load from the double tees on that one panel as well. If that doesn't reduce the overturning sufficiently, I don't think I would take the tributary weight much further around the corner than that one panel, but that's a judgment call - others might disagree one way or the other. I think the PCI manual touches on this in one of the examples.

Another consideration - the panel-to-panel connections at the corner would have to be able to handle that vertical shear from the uplift in order to mobilize the dead load.

 

[driftLimiter]

I have an approach I call L or I shaped footing for this.

I Include some area of the ftng perpendicular at each end. Be sure to include any other vertical loads on this region as well the foundation weight of these regions.

I treat the entire footing as a 1 way rigid body. You can get the reaction bearing pressure as a line load. Then divide it over the respective widths for each region.
Where there is a transition from your parallel ftng to your perp footing, you want to calculate the bearing pressure at both widths. Then again at the peak of the UDL.


I don't get to crazy assuming perpendicular lengths are acting as rigid. Maybe a few feet up to maybe 6ft. My assumption is that a locked in corner with walls above is going to behave mostly rigid.

 

[Delmarva_Struct]

Thanks! Dritflimiter..Ill give that a shot..

 

[DETstru]

I'd definitely use maybe 10' of the return wall, and it's associated roof DL, to help resist overturning.