Ribbed/Waffle Slab - Contractor Wants to Remove the Waffles

[JoelTXCive]

I have a ~50'x50' ribbed foundation for a storm water pump station. The slab will be located approximately 15 ft below grade.

The ribbed slab is 12" thick with 2 mats of reinforcing; and the beams extend an additional 16" deep, making the total section depth 28" (see below).

Our contractor has submitted an RFI requesting that they eliminate the ribs and just pour a flat mat slab that is 28" thick. As of now, they are not proposing that we alter the reinforcement setup.

If we allow the change, then I'll be in violation of minimum temperature & shrinkage steel; and probably also minimum flexural steel.

But..... ACI 318 Has the 133% exception for the flexural steel (Sect 9.6.2.1). And, ACI 350 provides provisions for only looking at the top 12" of concrete when below grade (7.12.2.1).

I'm leaning towards telling the contactor we need to add more steel if they want to do this setup; but haven't decided.

What do you think?

Thank you in advance.

 

[MotorCity]

I am guessing the contractor balked at the ribbed configuration due to the additional labor and formwork (if not earth formed).

I would allow the 28" thickened slab with the minimum amount of shrinkage and temperature steel being added(assuming that the extra concrete doesn't create any other logistical issues). I would forego adding steel to satisfy the minimum flexural steel requirement. If it works without the additional flexural steel there is no reason to add it, I would simply claim that the additional concrete is a "filler" and is non structural (i.e plain concrete which has no reinforcement or less than the minimum required). You would need to check that the additional concrete weight does not overload the 12" structural slab or the piles

 

[KootK]

1) I'd keep an eye on the additional 650 PSF that this presumably adds to the system. Make sure that you don't have a punching shear issue etc.

2) The material waste and inelegance of the proposal gets my hackles up. Any chance there'd be an appetite to have you just design them a flat slab or flat slab with drop panel setup? You'd need to get paid and have time to do the redesign of course.

It's a shame really. I'm a huge fan of one way slab and beam.

 

[KootK]

I wonder if the extra concrete could almost just be a gargantuan mud slab of sorts. I suppose that would still require forming your beam elements somehow and multiple pours.

I don't know what the limit is but, at some point, if the whole thing is structural, you might have to start getting into mass concrete considerations.

 

[Greenalleycat]

Could you refine the depth with this new setup?
Perhaps a 20" throughout slab thickness - might get you enough capacity and will mean your reinforcement works?
Also a tidier solution in terms of reducing concrete volumes

 

[Just Some Nerd]

If the contractor is willing to pour the full depth as a slab, my first instinct is that it would make more economic sense to redesign to a lower thickness. I don't know what the contractor's issue is with the current setup but its quite a bit of additional concrete to pour the full 28" all around - perhaps time constraints that are more of an issue than material cost?




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Why yes, I do in fact have no idea what I'm talking about

 

[Aesur]

I would ask why they want to pour a flat slab as a started, that's a lot of extra concrete. In my area concrete is still hard to come by in large quantities and takes quite a bit of time to get.

Is over-ex required for the soils? If so then maybe it's a case of not wanting to form, pour, remove forms and then dirt fill areas? If forming isn't the issue, maybe this additional concrete can be seen as a slurry after forming the beams? I would throw out using Geofoam, but I hear it's a year out to get for large quantities. Is there actually a requirement for minimum T&S reinforcing for "foundations"?

 

[ggcdn]

I have occasionally allowed some local overpouring for sloped soffit steps and such due to waterproofing, but with such a large area it feels like extra / min reinf should be provided.

You could probably turn this into an extra services fee.

“No, we can’t just overpour more concrete due to various code requirements, but we can redesign this for you as a mat slab at 20” thickness (or whatever you think will work). It will cost you $X.

And given that X is probably less than the cost of the concrete, they will probably oblige.

-JA
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[Brad805]

I would increase the rebar to meet minimums. Any overstressed piles as a result of the added weight?