Large Footing without Any Reinforcement? 1

[JohnyD]

I am a fairly new engineer (just graduated and started back in August) and am having some concerns about a current design I am working on.

We have a segmental retaining wall that a contractor wants to replace the bottom portion of with a 6' tall concrete footing instead due to some watermains they do not want to have to worry about.

After initial design and everything, I found that the footing would be a 6' tall by 7.5' deep footing that went for 150'. After this I went to design the reinforcement like I normally would for a strip footing and when I brought my findings to a senior engineer, they said that the contractor wanted to not use any reinforcement. Having done most of my classwork solely on reinforced concrete, I am having trouble seeing how a 6'x7.5'x150' footing will require no reinforcement? The engineer pointed to how the calcs from the software we use show that no reinforcement is required.

I have read through chapter 14 of ACI 318-14 on plain concrete multiple times and I am just having concerns as I have not worked with plain concrete before.

I apologize if this is a weird question but I was just wondering if footings/concrete of this size are typically designed as plain concrete?

Thank you for any and all discussion

 

[JAE]

I suppose you could technically meet ACI's provisions for plain concrete but for something that size I'd be very reluctant to do so.
At 150 ft. in length it would be expected that there would be some significant shrinkage cracks opening very wide unless the concrete was placed in sections limited in length with extreme care taken with regard to the concrete mix, water/cement ratio, etc.



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[RickyTickyTavi]

I've never run across it in modern structure. Seen it a few times in pre-1960's era concrete footings. At the very minimum to me, I'd be looking at Minimum area of steel for shrinkage. 150' long strip footing is going to crack as JAE stated. In my opinion, unless you know this contractor well, I would be skeptical of their position regarding no reinforcement.

 

[oldestguy]

It would help to show where that water main is located and any other details, such as ground elev off to the lower area. That wall above appears pretty skimpy. I suspect there is a better way.

 

[SlideRuleEra]

...contractor wants to replace the bottom portion of with a 6' tall concrete footing instead due to some watermains they do not want to have to worry about.

6' x 7.5' x 150' is 250 yd[sup]3[/sup] of mass concrete. How much concrete was required for the original design? Has the contractor volunteered to pay for the difference... which makes his (contractor's) work "worry-free"? Suggest stepping back and taking a good look at the entire design. The contractor's proposal looks amateurish to me, for example deep excavation appears to still be required. Don't just "buy-into" the contractor's request, assuming the contractor knows what he is doing.

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[BridgeSmith]

That's alot of concrete that's basically ballast. My first thought was, if it was my project I would want to do it as an MSE wall. Geogrid goes around all sorts of obstacles, including pipes, very easily. There are also many possibilities for the facing as well - from precast concrete panels, to concrete blocks in several readily available colors, timber, even vegetation. The block facing is the most common, since it's the most straightforward to construct.

 

[msquared48]

If this footing is just providing a better bearing surface than the original soil for the mse wall, there may not be a need for reinforcing.

Is the concrete a CDF material?

Mike McCann, PE, SE (WA)

 

[charliealphabravo]

Do you have any geotechnical design recommendations?

A length of mass concrete that long could have enough differential settlement to tear the wall apart. If I went that route I'd be thinking about "segmenting" the mass concrete portion into sections short enough to limit internal stresses and keyed to adjacent units to ensure compatibility (say max 2:1 aspect ratio). You may also need a special mix design to limit internal stresses during curing. The concept is the same as ballast concrete but I don't know offhand if those are reinforced.

I agree that there is probably a better way.

Edit: I should add that dimensioning to limit flexure, internal stresses, and cracking is the principle of unreinforced concrete design (ACI 318 22.3).

 

[KootK]

This will be a minor point relative to other things but another consideration may be the joint between the wall, which presumably will be reinforced, and the footing. Particularly at the edge like that, there's more to the design of the joint than just developing the bars. And what exactly that should entail can be a bit tricky.

Still, within reason, it's good business to say yes to your clients whenever you can. And it's not as though there isn't plenty of precedence out there for un-reinforced gravity retaining walls which are not all that different in principle. In fact, that might be a good place for you to start with respect to researching the issues.

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