Hybrid Plain/Reinforced Concrete Strip Footing 1

[waytsh]

Hi All,

I have a situation where I am trying to resist huge lateral thrusts from a large PEMB clearspan. For reasons I won't go into we are not able to use cross ties and I am using an independent moment-resisting foundation. Due to the large lateral loads I am utilizing a continuous strip footing down the sidewalls. I supposed you could also call this a mat footing or a two-way combined footing. I am not really sure where one definition ends and another begins. Just to give you an idea of the profile, it is going to be 6' wide, 4' thick, and 172' long with a 2'-6" deep soil shear key under it. Columns will be at 25' on center and the foundation will extend beyond the endwall columns. The size is being dictated by the stability requirements.

The design will need to be in accordance with ACI 318-14 so even though my flexural stresses are very low I will still need to size the rebar on each face to 0.0018Ag. For most of the foundation this is going to be substantial overkill. In order to provide a more economical design I am considering the following:

In the areas where the stresses are low (between the columns), I plan to analyze as a plain concrete per Chapter 14 and just specify minimum T&S (0.0018Ag split between the two faces). The areas under the columns exceeds the flexural stress limits for plain concrete so I will treat as reinforced and use the minimum 0.0018Ag per face. Bars in this area will be fully developed into the plain concrete analysis sections.

My question is whether or not it is permissible to use this type of hybrid approach? Is there a better approach that I am missing? I welcome your thoughts and input.

Thank you!

 

[TehMightyEngineer]

In my opinion your approach is the acceptable to meet ACI 318-14. Obviously make sure you provide at least the minimum reinforcements for foundations prescribed by your building code for the "plain concrete" areas (even though you are designing them as plain concrete).

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[dik]

Can you use gradebeams? This will increase the deadload to help offset the horizontal thrust?

Dik

 

[waytsh]

dik, how do you mean? The foundation I designed is essentially a long wide grade beam that runs down the sidewall picking up the columns. It has been sized with enough mass to resist the horizontal and uplift forces. I will put a sketch together. Thanks.

 

[waytsh]

a picture is worth a thousand words, hope this helps.

 

[TehMightyEngineer]

Oh, I might have completely misunderstood your setup. Can you sketch the cross-section of your footing?

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[waytsh]

Here is a section from my calculations. Shear key and footing reinforcing has not been drawn in yet. Footing is slightly offset to the outside to assist with the overturning.

 

[TehMightyEngineer]

Oh, I was picturing a stem wall footing! I retract my earlier statement; I would think this should be reinforced along the whole length. At a minimum I would expect any money saved would be lost due to you and the contractor now having to keep track of transitions rather than just having a uniform reinforcement.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[waytsh]

By the way, thanks for the feedback, it is appreciated.

Just to be clear it will be reinforced over the entire length and width. If the moments fall within the allowable limit for plain concrete, which they do in the short direction, I would still use 0.0018Ag split between two faces. When you are talking about #8 bar at 9" o.c. versus 18" o.c. over a distance of 172' x 2 = 344', that is quite a bit of material and labor. I wouldn't necessarily have a problem keeping the 0.0018Ag per face in the longitudinal direction. I view the longitudinal bar as being easier to place.

Curious what you had in mind with the stem wall that would have been acceptable.

 

[Klitor]

Out of curiosity...why not large single footings tied together only at locations of longitudinal bracing?
Not enough space on site?

 

[waytsh]

Contractor preference. The footings were getting so large that they were practically touching anyway. The contractor felt it was easier to just make a long narrower excavation than to try to excavate multiple large spread footings that would have only been separated by several feet of dirt.

 

[hokie66]

waytsh,

Did you or the contractor also rule out bored piles? In similar situations, I have found that a pair of bored piles and a pile cap at each column substantially reduces the concrete volume required.

 

[waytsh]

Yes, we had discussed the option of piles but the contractor preferred the mass footings. Thank you for the suggestion.

I guess to summarize my question. Is it permissible to design the footing in the short direction as a plain footing per Chapter 14, and then as reinforced in the long direction per Chapter 13? In my mind I do not see a problem with this as long as the stresses permit it but maybe I am missing something.

 

[KootK]

I don't see an problem with this, especially with you providing the nominal reinforcement. You might be interested to hear of a related issue that I've had that I've been meaning to enquire about here. It goes like this...

- 6' square, normally reinforced footing gets formed up.
- Wild-man geotech shows up an says allowable soil stress is less than we thought.
- Footing goes to 7.5' square but getting new rebar / spliced rebar fast is a problem in remote location.
- I let the footing go at the 7.5' width with the 6' footing rebar.

I believe that you're okay so long as, at every section, you can say one of two things is true:

- You've got your flexure covered by way of plain concrete strength OR;
- You've got your flexure covered by way of RC theory with bar developed enough for the bar force either side of the section.
- You're not dealing with any concentrated loads etc near the transition that would trigger additional detailing considerations.

I think that my scenario is probably more questionable that yours.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[waytsh]

Thanks KootK, I consider myself in good company then. I agree with all three of your points.

As for your example, I had run into a similar predicament in the distant past as well. I think what I had done was to stagger the bars, alternately cheating one left then one right. In the middle of the footing ,at maximum moment, my bars were still at the required spacing and the development still checked out on the shorter rebar side. I guess it worked out because that was some years ago and the building is still standing. If you left your bars centered, and the unreinforced section around the perimeter checked out for plain concrete, I think you are OK too. If it was a typical spread footing with a concentric column your stresses should be pretty low out there anyway.

 

[KootK]

Well damn it, the stagger is a much better idea. Technical compliance aside, the potential for an abrupt transition bothers me some. I had pre-tied mats but, given that I was already bending to solve a problem, I'm sure that I had enough leverage to make a go of that.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

Just curious of your modeling as I would think that the predominant flexural stress would be longitudinal.

I threw together a quick FEA model on compression springs with an assumed soil subgrade modulus, using 50 kips per column.

If you agree with my model then the transverse flexure drops off almost immediately and is significantly lower than the longitudinal flexure. This makes sense, your transverse reinforcement is essentially acting to distribute the soil reaction to the longitudinal reinforcement. Is this the reason you're discussing where you want to stop your transverse reinforcement outside of the areas with little to no transverse flexure? I assume you're not talking about discontinuing the longitudinal reinforcement as it would appear to be required for the entire foundation.

Regarding the mixing of plain concrete and reinforced concrete, something still seems off about this to me but I'm not sure why. I guess my lack of a concrete rational (hah!) against this means I agree that it's acceptable.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[TehMightyEngineer]

I think what I had done was to stagger the bars

I've done similar to this a number of time for box culverts that have spans greater than 22 feet reinforced with rebar. Since it's longer than a single stick of rebar we use I stagger the rebar alternating so that at the walls we have 2x spacing of the interior. Hasn't failed me yet.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[waytsh]

Ian, yes your model is a close representation of what I found. That is the reason I am dropping the transverse reinforcing down to the minimum T&S requirements since the plain concrete strength is sufficient. I am now planning to leave the full longitudinal reinforcing in for the full length even though sections between the columns have sufficient plain concrete strength. Hope this makes sense.

 

[dik]

Can you do a more massive footing at the frames with a gradebeam between as shown in the attached section?

https://files.engineering.com/getfi...-477e-477b-8643-1ce79adaab0d&file=Footing.pdf

I don't see a reason that you cannot use a horizontal tie between the frames unless you have pits or some other object in the slab. It would be a lot less expensive.

Your anchor rods look way out of proportion.

Dik