Footing Design?

[SteelPE]

I have a situation where I have a footing with a large uplift, lateral load and overturning force (not an ideal situation). I have the weight of the footing and soil above the footing resisting these forces. The top of the footing is embedded 4’-0” into the ground due to frost concerns.

When it comes to the soil weight above the footing and the design of the footing for stability, would you use the theoretical “conical” soil above the footing or just the project soil above the footing (see sketch) to resist these forces?

 

[Teguci]

If we are talking temporary loads I'd be tempted to use the friction angle of the soil to add capacity. However, concrete is cheap. Don't hold back on concrete volume for footings if it simplifies your design.

 

[SteelPE]

The loads causing the problem could be classified as temporary. I am getting a little desperate because as it stands, I have a 50x50 building that is requiring a 48'x48' footing. It just seems absurd.

 

[SteelPE]

I'm sorry, it's suppose to read (I have a 48'x48' building that requires a 10'x10' footing).

 

[JedClampett]

I agree with Teguci as far as the footing. Make it bigger, if you need to. If the Geotechnical Engineer will give you an angle that the soil acts, use it. Otherwise just use the soil over the footer.
A 4'-0" by 4'-0" footer is not that big. Step it off in your workspace. Plus the only ones who are going to see it are the contractors. Once it's buried, no one will know.

 

[ToadJones]

10' x 10' footings were the small ones on a lot of projects I have been involved with.

 

[SteelPE]

How many 48'x48' buildings have you done that required footing this large?

 

[ToadJones]

None, I guess.
I once had a monopole utility pole that had lines 90º to one another and was only about 60' tall and could not be guyed. It could not use a caisson foundation for site reasons and other utilities.
I had to use what amounted to a combination spread footing + tall pier foundation.
The spread footing was enormous.
I cant remember the exact dimensions, but I think it was something like 20' square and 4' thick.
When I watched them pour it, I felt like a dummy. But the numbers were run numerous times and it was what it was. When they back-filled ans set the pole, It looked great!

 

[ron9876]

The soil cone above for sure. You couldn't pull a plug out of the ground that is the same shape as the footing--at least that is my opinion. Also ask the geotech for allowable friction on sides of footing. While concrete in the ground is cheap it isn't free.

How thick is your 10'x10' footing? Looks like the height of your pier is making the condition much worse. Is the force in one direction only? If so a rectangular footing will be much more effective. With the values of your forces we could offer better input.

 

[SteelPE]

ron

You are spot on with the height of the pier. Someone made the brilliant decision to keep the columns 3' above the slab which is only making my problems worse. Due to geographical location, I am also required to place the footing 4' below grade.

In the end, I ended up with a 9'x9'x2' footing not taking the soil cone above. I have a spreadsheet that I wrote that designs simple footings. I did not include the soil cone above the footing because in order to engage the cone I needed the footing to "slip" a little bit.

This problem is related to the same problem I posted on this forum with regards to metal building foundations a few weeks ago. I really don't like designing foundations for these types of buildings because the mfr does not care what the reactions are at the base of the column.

 

[csd72]

have you taken the weight of the groundslab into account?

 

[SteelPE]

I have taken the weight of everything into account (footing weight, soil directly above footing, frost wall, frost wall footing, soil above frost wall footing). As far as I can tell, I have nothing left to take.

 

[ToadJones]

can you take some of the shear in the pier into the slab?
This should help your OT moment on the footing.

 

[SteelPE]

The slab is not really structural at the moment. We are looking at using a 5"-6" lightly reinforced slab. I don't think it would work really wall as a column.... plus it is in a potential flood plain.

 

[BAretired]

If it is in a flood plain, you must use 60% of the buoyant weight of the foundation and soil. Is that why the footing is so large?

BA

 

[ToadJones]

Footings on PEMB sometimes employ tie-rods connecting foundations across the entire width of the building, no ?

 

[SteelPE]

BA,

Good point, however, we are getting a written variance from the building inspector to avoid the part of the design.

Toad,

I thought about tie rods but the loads of concern are due to wind loads, not the "kick" loads associated with the dead and live loads placed on the structure.

 

[BAretired]

SteelPE,

A written variance will not alter the fact that, in the rare event of a flood, your building will be structurally unsound. I would not accept such a variance as you will likely be embroiled in litigation in the event of a failure during high water table.

Why not use drilled caissons with belled bottoms to resist uplift?

BA

 

[CTW]

How much more dead load do you need to resist the uplift?

 

[SteelPE]

CTW

Pure uplift? I have plenty to resist pure uplift. It's uplift in combination with the overturning that is killed the footing design.

With the 9x9 footing listed above I only included the soil directly above the footing. I guess this will give me a little added FOS.