R/f detailing-Anular concrete base 1

[struct19]

Hi
I am designing a 12'-0" thick reinforced concrete anular concrete base to carry the sanitary structure on that. The anular base is supported on a shaft at its inner diameter. the shaft inner diameter and my concrete base inner diameter is 52'-0" and the outer diameter of the concrete base disc is 88'-0". As there is water pressure on the disc and line load along the outer phirepharal due to the structure it carries, I am geeting very high moment tangentially and a small moment radially. I provide top and bottorm r/f in both tangential and radial directions. But I don't see a need for any intermediate r/f for this 12'-0" thick. I am little nervous whether my approach is correct. Is anybody experienced with similar structure. I forget to mention that i hav eprovided skin r/f to control crack along the vertical faces.
Hoping to hear more from you all and my advance thanks to everybody.

 

[ishvaaag]

At these 12 ft depth and following your sketch say 8 ft width you have a massive member where thermal stresses could have a say. So at least a moderate quantity of rebar in the mass (and more being a sanitary structure) wouldn't be bad, without any analysis.

 

[struct19]

Thanks. I am in the same page with you. But I am seeking for some structural backup to justfy as it will be a considerable cost to r/f the whole 12' thick disc.

 

[ishvaaag]

An article in ACI SP-139 Durable COncrete in Hot CLimates, SP 139-9 Concreting of Thick sections in the Tropics describes more or less one approach to make such calculation for a beam just over 1m in x y section dimensions. It draws the section of the beam as a set of FEM plate rectangles, some of which are steel, the others concrete. Then a layer of insulation, also in FEM plates, then applies to the (I imagine nodes) some differential temperature as measured or known to happen, then gets the strains, and compares it to the early tensile strength of the concrete (that you may find I think to remember in the Bazant-Pakula formulation). Since exposed as a 2D problem any FEM package able to set temperatures at points and having plates may approximate the problem mechanically; better of course those that make thermal analysis.

Another ACI special publication, SP-152, Design and Performance of Mat Foundations has a section, SP 152-7 Mass concrete Pour Techniques for the Mat Foundation at 1100 Alakea Plaza, where a mat oe 2300 m2 and thickness between 1.5 and 3.4 m was poured continuously. Quoting, is the opinion,...

"the real problem in massive concrete pours is not the total amount of heat generated from the hydration of cement, but rather the temperature gradient within the concrete mass. It is suggested that once the temperature gradient exceeds 19.5ºC cracking of concrete ensues."

So again here this is seen as a thermal conductivity problem very alike to the other article approach.

By the way, the insulation in the first case was 10 cm and reduced the gradient from 20ºC between insulation to 8ºC with. Since likely the thickness was enough to develop already at the innermost point the whole attainable gradient, bigger masses of concrete must not behave very differently and insulation must be almost as effective.

Of course using the lower hydration cement available compatible with the wanted strength will also help.

Now I quote some ACI codes that you may find in its Manual of Concrete Practice that may have a say in what you want:

ACI 305R-9 Hot Weather Concreting

ACI 207.4R-93 Cooling and Insulating Systems for Mass Concrete

ACI 209R-92 Prediction of Creep, Shrinkage, and Temperature Effects in Concrete Structures

ACI 224R-01 Control of Cracking in Concrete Structures

that by the way says...
"The minimum amount and spacing of reinforcement to be used in structural floors, roof slabs, and walls for control of temperature and shrinkage cracking is given in ACI 318 or in ACI 350R. The minimum-reinforcement percentage, which is between 0.18 and 0.20%, does not normally control cracks to within generally acceptable design limits. To control cracks to a more acceptable level, the percentage requirement needs to exceed about 0.60%." ,

that is a very interesting quote for us all. Was not the minimum steel to control rheological effects? Well, yes, it seems, but not to a "generally acceptable design limits" for generic crack control.

ACI 349.1R-91 Reinforced Concrete Design for Thermal Effects on Nuclear Power Plant Structures

ACI 304R-00 Guide for Measuring, Mixing, Transporting, and Placing Concrete

ACI 435.7R-85 State-of-the-Art Report on Temperature-Induced Deflections of Reinforced Concrete Members

ACI 122R-02 Guide to Thermal Properties of Concrete and
Masonry Systems

ACI 224.1R-93 Causes, Evaluation and Repair of Cracks in Concrete Structures