Reinforcement for mass concrete

[rlflower]

There was a thread here regarding reinforcement of mass concrete a few months ago. I would like to open the subject again with hopes to get some input on a specific design scenario.

I am submitting a proposal for a contractor's bid on the installation of a pair of large (and tall) industrial tanks on a new foundation. Initial analysis included review of calculations done by another engineer. According to ASCE 7-10, with a seismic R factor of 2.0 (tank on legs) the result is a foundation with plan dimensions of 15 feet by 28 feet and a depth of 15 feet! The excessive depth is to answer to the significant overturning load due to seismic.

This is definitely mass concrete. My question is, how to reinforce this mass. It would qualify as Structural Plain Concrete (ACI318 Ch. 22) if we do not anticipate tensile stresses within the mass; however, this is not the case since we are depending upon the mass to withstand uplift forces due to overturning.

Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.

 

[SlideRuleEra]

rlflower - The foundations for our electric generating stations have to withstand heavy loads, high overturning moments, and are located in high wind / seismic areas. At one plant in particular, we have a 600' tall chimney and several silos and tanks up to 150' high - the foundations for each of these structures are 15' thick. Foundation reinforcement was designed with closely spaced, double layer, top and bottom mats of #18 rebar. The Contractor's design of supports for the rebar until concrete placement was a significant effort by itself.

Designing the foundations as structural plain concrete was intentionally not even considered - not a time to look for short cuts. Pay special attention to anchor bolt locations, diameter, length and anchorage.

FWIW - Five years after initial operation, the plant took a direct hit from a Category 4 hurricane, with winds approaching design speed - no structural damage.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[BUGGAR]

For industrial tanks, my engineering judgement would be to reinforce the foundation. I would consider how much reinforcement you may need to counteract thermal stresses of mass concrete? I've never used reinforcing to resist thermal but it could reduce the need to cool the concrete or make multiple pours.
I presume you will have tank hold down anchors which will have to be reinforced. How can you justify justifying reinforcing around the anchors and nowhere else? My opinion.

 

[WARose]

I would certainly reinforce it. If you are placing it all in one pour, you may have no choice due to the differential temperatures. I would suggest checking ACI 207 for guidance as far as reinforcing goes. IIRC, they have some worked out examples.

 

[LannyBudd]

The intent of following the provisions of Mass Concrete is to monitor the temperature differential from the core with respect to the surface. Typically for large foundations that you have described, the minimum area of steel is what controls...which will limit cracking due to temperature. However, no amount of steel will counter act the thermal effects if the core exceeds the maximum allowable temperature. I have seen many Hamon and Pullman stack drawings with associated foundations and none of the have any special reinforcing for Mass Concrete.

Just read up on mass concrete in ACI 207 and 301.

I have work on many structures within Power Plants and I have never seen anything designed as Structural Plain Concrete.

 

[bowlingdanish]

Punting:

I would just make sure your structure can't pull out of the mass concrete. If the loads are so large that you need steel from a strut-tie perspective then OK, but surely there is no way a layer of steel at the top and bottom of a 15 foot mass of concrete is doing anything. I would also get advice from a concrete specialist (Ron?) to make sure the mix is spot on. Aggregates, binder, w/c ratio, admixtures, lifts, monitoring internal temperatures, SCMs, etc. That's what's important. Not a mat of reo.

 

[SlideRuleEra]

...there is no way a layer of steel at the top and bottom of a 15 foot mass of concrete is doing anything.

For the 15' x 28' x 15' being asked about, I agree - my mistake. I should have clarified the example I gave. The free-standing 600' chimney foundation is an octagon, about 190' across the flats, 15' thick. My calling the rebar in the foundation a "top and bottom mat" is a gross simplification - it is a full rebar cage and it is essential. The foundation was made 15' thick to minimize deflection so that 200, or so, 4' diameter drill piers supporting it are equally loaded under high moment conditions. What I had hoped to point out is that this foundation is in a high seismic (and wind) area, and that reinforcing should be considered mandatory. Sorry for the confusion.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[JoshPlumSE]

This is going back a number of years (mid 90's?). But, one of my former colleagues had a similar situation. Large vertical vessel. 4 or 5 meter thick mat foundation. Thickness was to help control overturning as well as to limit punching shear.

If I recall correctly, they added in a lot of temp / shrinkage reinforcing (i.e. total area of steel = 0.0018 * gross area in each directly) spread out through the thickness of the mat in 3 or 4 layers. I don't recall them talking about any problems with their project when it was built or the ensuing few years. So, I assumed that all went well.

ACI 2014 would, unfortunately, require rho = 0.0018 each for the top and bottom reinforcement. So, the new code more than doubles the amount of reinforcing required for those types of foundations.

 

[JStephen]

More commonly, for a large overturning moment, you'd make the foundation wider and thinner. Or if it needed to be founded at that depth, make it a pedestal-on-slab design.
Forming could be a challenge as well.

 

[BigH]

Just a thought - if your foundation has to be such to resist foundation overturning, why would you not consider some tie-down anchors to counter uplift/overturning forces?

 

[rlflower]

Thanks to all for the input. Upon discussing this with the client, I have suggested they look into a design of three or four piles with a pile cap in lieu of the aforementioned massive footing. The other thing I suggested was a modification of the legs supporting the tank; if the legs are braced, then we can use R = 3 instead of 2, thus greatly reducing the overturning load. So, I gave them food for thought, and they will let me know how they desire to proceed.

Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.