Spread Footing with Trench to Frost Depth?

[JR55]

I am designing spread footing for a single column that will see relatively high moments at the base. The client wants the footing to start at grade, and so we're left trying to meet frost depth.

My thought would be to use an approximately 42" thick footing and put enough steel top and bottom to meet minimum steel, which could be quite a lot of rebar. We also want a replicable solution, so I'm not convinced we should rely on a shallow footing with a thick granular base to meet the frost requirements of different locations.

My co-worker suggested designing a standard 20" thick isolated footing at grade and add essentially a grade beam down to frost depth. He suggested designing the dimensions as if it were just 20" thick, and then arbitrarily add the trench on all 4 sides (sketch attached). However, I feel the grade beam on all sides would significantly change the soil interaction and we wouldn't see a triangular soil pressure distribution like we would typically expect. Because the footing is stand alone and only about 8'x13', this doesn't seem like a solution where the behavior will be predictable, but I've also never seen it done before.

Am I missing something rejecting his assumption of designing it as a stand along 20" thick footing?

 

[JLNJ]

The turn-down-edge detail looks way too fussy for concrete footing work. You are not building a watch here.

I would try the full-depth footing, using the additional mass from the extra thickness to reduce the eccentricity and therefore decrease the footing size. You probably don't need top steel at all in the footing - check it as an unreinforced member. Reinforce just the bottom (use straight bars only).

To wit: dig the minimum size hole, set the bottom bars each way, fill the hole up.

 

[canwesteng]

The turn down can be formed with dirt so it really isn't that bad, as long as you give yourself a nicer slope than the sketch.

 

[azcats]

Over-excavate to frost depth and fill with flowable fill to B.O. actual footing?

 

[SwinnyGG]

The turn down doesn't look too bad to me, we do similar details all the time. Although your contractor will appreciate it if you make the sloped portion roughly 45 degrees or so as opposed to the steeper angle shown.

I wouldn't use flow fill for a deep trench under a footing... it's water permeable, and it will heave just like dirt will under the right (wrong?) conditions. I personally don't see why the grade beam detail is an issue - you could always dim the depth of the 'grade beam' below your 1'9" footing as 'to 6" below frost depth' or whatever you want, the contractor is very likely going to want to pour everything monolithically anyway. If it were me, and the attached sketch was my detail with an added grade beam below the footing, the first constructability RFI I'd be submitting would be to make any vertical bar continuous between the two and pour them together.

 

[HTURKAK]

Can you provide more info.? ( the ftg design loads, size and location of the column...)

In past i have designed this type of foundation ( inverted bowl ) but not for single column with high moments ..

If the footing is 8'x13', the thickness should be more than 20in.

I will suggest you the use of solid spread ftg with thickness around 40"..,







I cannot give you the formula for success, but I can give you the formula for failure..It is: Try to please everybody.

 

[JR55]

The column is 3' diameter round and centered on the footing with loads around 300 ft*kips.

 

[MotorCity]

We use that detail all the time, no issues or complaints.

 

[TLHS]

If you want to do a surface footing but also don't mind excavating, I would just excavate to frost depth and replace with a low fines material that isn't susceptible to frost lensing.

 

[JR55]

It sounds like the detail is constructable and a good option. I guess my only remaining question is then what would the pressure distribution look like? Can I assume it will be triangular and design it as I would for a uniform thickness?

If not, are there any resources you could point to for the design of the inverted bowl?

 

[XR250]

For one footing, why not just fill the whole mess with concrete?

 

[BAretired]

I agree with XR250. In the long run, that is probably the most economical option. Concrete is cheap; labor is not.