Strip Footing

[Calif]

Hello everyone:

I began designing a strip footing for a pump station with the wall footing embedded 4 feet below grade. The concrete footing wall supporting the above structure is 13 inches thick. When I ran the calculations to see what bearing width I needed, my bearing width was 10 1/2 inches. I understand one of the reason why my bearing width is low is because the allowable bearing stress of the soil is about 5700 psf. It does not make since to me to create a flange width only to make my calculations easier. To me, it seems that I should just make a wall with a 13 inch thickness supported on soil or slightly more with a concrete hunch going back towards the slab. My question is, how do you design a reinforced concrete wall for bearing capacity?

Calif

The resisant virtues of the structure that we seek depend on their form; it is through their form that they are stable, not because of an awkward accumulation of material. There is nothing more noble and elegant from an intellectual viewpoint than this: to resist through form. Eladio Dieste

 

[StructuralEd]

The less obvious purpose of a strip footing is to support the wall forms. You can't just sit them in the soil.

 

[msucog]

5700 psf on soil? oddball number and perhaps reported rather high depending on the actual soil conditions. someone actually gave you "5700 psf" as an allowable bearing capacity on soil? i'm not sure i'm that good enough to give bearing capacities to the nearest 100. i would round down to the nearest 1000 or so...but that's just my opinion.

i suggest using a 2' minimum width for wall footings even if it computes smaller. i'm sure aci and ibc have an opinion on minimum width but i don't know the sections off the top of my head.

 

[Calif]

msucog:

It wasnt exactly 5700 psf but the capacity after you take soil and concrete footing into consideration. The actual soil capacity is 6000 psf (yes this is unusual) but to be fair, I took 6000 psf off of soil report done from a nearby substation that is about 30 to 50 feet away.

StruturalEd:

Why not? If the wall that you have is wide enough to support to not exceed the soil allwable bearing pressure, why wouldn't it be ok? I think this would be similar to a concrete pipe column in the soil that it used to support a wood deck? The bearing pressure is done by the tip and the skin friction. In my case, I am basing that most of the load is done by the tip and skin friction is neglible. Let me know what you think.

The resisant virtues of the structure that we seek depend on their form; it is through their form that they are stable, not because of an awkward accumulation of material. There is nothing more noble and elegant from an intellectual viewpoint than this: to resist through form. Eladio Dieste

 

[DaveAtkins]

StructuralEd's point is that it is easier for contractors to pour a footing, then use the footing as a nice, flat bearing surface for the wall forms.

DaveAtkins

 

[ron9876]

Geotech and/or building code should have a minimum width for wall footings.

 

[BigH]

If you look into literature, too, as from Tomlinson, in UK they pour strip footings all of "equal width" from bottom to top - why? Because they can excavate the strip full depth and the extra concrete (above the projections) cost is less than the costs of placing forms, etc. - and in the cost of time. As ron9876, there is usually a clause indicating that the width of the footing must extend at least 6 inches (150mm) each side of the wall - but in the first instance, that would be okay all the way to bottom. 6000 psf (300 kPa) is not unduly low for many soil types - yes, for soft to firm clays but not for compact to dense granular soils or very stiff to hard clays or clayey tills. One final point is to understand the difference between allowable bearing capacity (based on shear - or strength) and allowable net bearing pressure (based on service requirements, viz., settlement or differential settlement).

 

[Calif]

Thanks for the feedback. I will take a look at my geotech books on that BigH. I am still learning in structural engineering so my knowledge of foundation is weak.

Thanks

Calif

The resisant virtues of the structure that we seek depend on their form; it is through their form that they are stable, not because of an awkward accumulation of material. There is nothing more noble and elegant from an intellectual viewpoint than this: to resist through form. Eladio Dieste

 

[StructuralEd]

Calif;

If you want a good straight wall, those forms need to be set and held in place. You are not going to get that trying to set the wall forms in the soil because the bottom of the trench is going to undulate, not to mention what will happen if you get caught in some undesirable weather.

I did one job 35 years ago trying the "cheap" approach in detailing a grade beam. Guess what.....the contractor put in a strip footing saying it was impossible to build the wall/beam without it.

Use a footing, no matter what the numbers tell you.

 

[BigH]

StructuralEd - I agree but he is talking the footing and the pedestal on which the wall, which would be presumably formed (as a stem on retaining wall) is resting - that is, he is talking below grade (non-visible) constrution. Why would you care if the width of the pedestal is "varying" so long as it is wider than needed and the "real pedestal" is wholly within the below grade part?

 

[archeng59]

I agree with StructuralEd. I always provide a minimum 6" wide area for forms. For example, if there will be an 8" thick concrete wall on top of the footings, I will provide a minimum 1'-8" wide footing with 6" on each side of the 8" wall for formwork. The overall width of the footing may need to be wider depending on the allowable soil bearing pressure, but the footing width won't be less than 1'-8" for the condition described. A thicker wall would create a thicker footing width. The 6" allows a relatively level, stable surface for the contractor to build the forms.

I typically get a minimum strip footing width and spot footing size recommendation from the geotech engineer. Does your geotech report provide such a recommendation?

Will your strip footing be trench formed? If so, typically the minimum width is based on the commmonly used bucket size for the backhoe. In my part of the country, the common bucket width is 16" although other sizes can be obtained if needed.

 

[sundale]

How do you get 13" form ties for the wall?

 

[DTGT2002]

Sundale,

Formties are available in almost any width desireable, just takes time and minimal extra cost over stock items.

From a formwork engineer, I'd prefer a nice bearing ledge for a base to work a wallform.

Formwork in mud is not fun at all. It can be done, but it may well cost more than the concrete it would take to widen the footing.

The comment above about you can't just rest on soil is not an engineering fact, just a rather practical comment.

The next question would be constructability. Do you really want to specify a wall so narrow it can't be done with common equipment, or requires mini equipment or a rather small bucket on a regular rig? Or is it ok to have this hand excavated.

All are possible, just more practical questions.

Another question to me would be is there a benifit to the application of soil load on top of the footing.

Keep discussing.

Daniel

 

[StructuralEd]

This reminds me of the day one realizes that nothing was learned in college about construction.

Our firm once hired a professor (no offense to professors) to moonlight when things were extremely busy. He was politely dismissed when he came back with a plan that he marked up with W4 beams. "That's all that the calculations required..."

 

[csd72]

Is this wall going to be retaining (like a basement wall)?

If it is then you may want to give some width to the footing in case it is left unpropped at any stage (it does happen). A wall with a 2' wide footing has twice the resistance to overturning that a 13' wall would with no footing.

 

[Calif]

Hello everyone:

First, I would like to say thanks everyone for their reply. Many of the things talked about is what I never thought about since I am still in the beginning stages of practical design in structural engineering. Let me say that intially I posted this question because I had trouble designing a turn down slab due to not how this footing would be analysized. I intially thought it would act like a bearing wall which is why I asked the question in my second post about a wall bearing in the soil. This was the closes comparison I could make and I thought, if could design a reinforced concrete wall for bearing and make sure the bottom of the footing was above bearing capacity then I was ok. My problem came in is that I had no information in designing a concrete wall. So I went from a turned down slab to a strip footing which is easier to design. Now I think I will try to find information about designing a turn down slab. Does anyone know of any resource I could go to find out information for designing a turn down slab?

Thanks

Calif

The resisant virtues of the structure that we seek depend on their form; it is through their form that they are stable, not because of an awkward accumulation of material. There is nothing more noble and elegant from an intellectual viewpoint than this: to resist through form. Eladio Dieste

 

[StructuralEd]

Having a large integral mass at the edge of your slab will most probably be the cause of shrinkage cracks due to the restraint of the edge of the slab.

Also, it is pretty tricky to get it built and know that the soil did not sluff into the excavation having ill effects on the load bearing haunch that you designed to tightly.

Why not just put a strip footing and wall which is the most common and basic form of construction?

As a designer you have to keep in mind that taking chances at the risk of your liability is not worth saving a few dollars of construction cost. Ultimately, unless you have the time to stand there and supervise to be sure that everything is just so, you really want to make the construction as idiotproof as possible; there are idiots out there building things, and with most owners taking the lowest bids, you are likel;y to have idiots doing your job.

Don't overthink the design. Make it easily constructible and ensure your future. If it goes wrong not only will you be blamed but you will lose that client forever.

 

[archeng59]

The turned down slab is really a footing. Early in my career, my boss told me it's called a turned down slab because it's often poured at the same time the slab is poured and the width is approximately equal to the slab thickness. Regardless of the description, it is a footing bearing on soil. I typically do not use a turned down slab except along non-loadbearing walls, such as glass storefronts and sometimes along the perimeter of a metal building. The constructability issues discussed earlier still apply with regards to trenching equipment and setting up a surface for forming. Also, an 8" thickness is a recommended minimum for ease of concrete placement between formed sides.