Precast Concrete Shear Wall Foundations 1

[Delmarva_Struct]

I am designing the foundations for a large precast concrete structures that is Seismic Design Category D. I have received the lateral loads from the precast designers from a separate analysis of the superstructure considering fixed based approach. The lateral loads are large and in designing the wall strip footing, these loads are causing sliding of the foundations. I am having to increase the size of footing to a very large degree to avoid sliding. Is there any other way of resisting lateral sliding other than friction between soil and footing and the lateral bearing capacity of soil? Please note that all shear forces are in-plane of the wall. I did consider shear keys similar to retaining walls going across the footing but i have never seen this done before. Structure is a single story shear wall structure aka the shear walls are the gravity and lateral load bearing members. Walls are connected to other adjacent walls and the footings would be connected as well. I assume the connection to adjacent footing would offer some sliding resistance but I'm not sure how much...

 

[JStructsteel]

Are you looking at passive pressure too, perhaps some walls perpendicular to the stem wall every so often. Seems unusual due to the amount of weight of the precast wall.

 

[HTURKAK]

Yes ! You can add the passive thrust of the leading face to the total thrust..

Tim was so learned that he could name a
horse in nine languages: so ignorant that he bought a cow to ride on.
(BENJAMIN FRANKLIN )

 

[Brad805]

Any floor slab connection permitted?

 

[driftLimiter]

Passive thrust (don't count to the layer of soil oft 12" or more). ALso you can consider some volume of soil that is on top of your footing to increase the friction resistance to sliding. Use all the weight you can justify getting down to those footings for friction.

 

[Delmarva_Struct]

Thank you for your replies. When you say passive thrust, i assume you mean passive earth pressure on the front face of the footing? Also driftlimiter i think you meant that i should not count the first 12" of soil? is that correct?

 

[Delmarva_Struct]

Brad in looking at the precast shop drawings I do not believe there is a connection between the slab and the wall. We could recommend connections if required.

 

[JStructsteel]

Can you post a sketch and the parameters, something seems unusual

 

[Brad805]

What is your role on the project? In our area there is a EOR. That person defines how the precast is to be arranged. There are many options how the forces are resolved. If left to the precaster, they tend to pick what is advantageous to them and do not consider other factors.

 

[Delmarva_Struct]

Brad, we are the EOR. Precast wall is connected to foundations via Angles and embedded studs.

 

[Brad805]

Ok. I see details like the one pictured below frequently. Some use threaded rods, others use weld plates that are patched after welding. I do not like that detail mixed with other footing connections if your footing is shallow, but many EOR's have insisted we use both over the years. You could look at something like that before adding odd details to your footing to deal with sliding forces. I have neve encountered a case where sliding was a problem, but we do not work in very high seismic regions.

 

[JLNJ]

If the forces are in-plane, can they be dragged along the length of the wall by the foundation so that more of the foundation's mass is mobilized?

I picture uplift being more of an issue than sliding, but without any geometry it's hard to guess what's going on. A sketch or better explanation of the details would probably help us provide better answers.