6. Outline of the dimensioning and constructing of the walls and bottoms of cylindrical tanks

Cylindrical tanks might be one or multi-chambered. Multi-chambered are difficult to execute and take more space than rectangular tanks with the same capacity but benefits at the static schemes.

Walls of cylindrical tanks usually are monolithic. Tanks with a large diameters are exposed to large tensile stresses because of that often are constructed as prestressed with strains or cables placed in a one-layer way in a half through a wall thickness or in two-layer way. It is important in case of high tanks which walls have the same thickness along height. Prestressing of tank walls is the most efficient method of providing their leak tightness. If tank is not prestressed and walls are high then walls are executed with linearly changing thickness, wider section is at the bottom and inside surfaces are vertical. Position of reinforcement depends on wall thickness- to 150 mm one-layer way, more than 150 mm two-layer way.

Not prestressed cylindrical walls are connected with the bottom in monolithical way, which is treated as an elastic support. In case of dilatation between wall and the bottom, wall is usually supported by ring foundation beam which gives us also elastic joint. Only the foundation on non-deformable ground provides complete attachment. The walls can also be based on the structure of the bottom in a sliding manner, so that no additional bending moments were formed.

Most of the tank bottoms have slab form. The slope inside is performed using lean concrete. In case of ground water we should pay attention at large bending moments which might appear in the bottom slab, therefore we should avoid the flat bottom slab with a big span, unsupported by columns in the middle (max spacing between columns should be about 4-5m). The range can be reduced by use of side walls, consisting of two parts: cylindrical and conical.

When the hydrostatic pressure of the groundwater is high, then the bottom can be made in the shape of vault trough or cylindrical reversed or inverted dome-shaped. When the buoyant force of water from the outside is greater than the empty weight of the tank,then the wall or bottom thickness is increased, either through designing construction which gives ectra loads, e.g., the use of cantilevers which are loaded by the ground.

Calculations

We divided mentally the tank on the surfaces in kinks .

The walls are dimensioned to the meridional forces acting axially and meridional bending moments .

Ring reinforcement is calculated by one of the cases :

• When the load comes from the outside, we dimensioned to axial compressive forces (as renovation and construction state ) - should be compared to the strength of the critical buckling force.

• For loads from the inside to the stretch ( leaking test ) – whole force is taken by ring reinforcement and the wall thickness is determined from the condition of tightness .

Vertical bars take bending moments , calculated on the compression eccentric , from the meridional forces . These bars usually have a smaller diameter than the ring bars. They are placed on the inner side of the wall along the range of boundary moment . In the middle of the wall there is a opposite moment for the sign that can be taken through the concrete .

The bottoms are calculated in terms of their shape .

• For flat bottoms reinforcement is calculated according to the values ​​of the bending moments

• The case of the convex dome-shaped working on tenisle in latitudinal and meridional cross-section – calculating on the condition of tightness , reinforcement takes tensile .

• concave dome works mostly on compressive stresses – boundary condition of thickness to the radius.

• If the bottom slab is using expansion joints from the foundation wall , the minimum reinforcement is calculated on the shrink and thermal conditions, slab thickness min 8cm , and the use of lean concrete base .

The tank must be calculated for leaks , it depends on used aggregate, the type and amount of cement , water , use of plasticizers or sealing admixtures, vibration of concrete and its care .

Tanks can be considered as leakproof if in a cracks state a width of the cracks is not larger than 0,1 mm and simultanously tank is inside fully sealed with liquid proofing or insulating sprayer

Additional figures

Scheme of work of mechanical whirling wind-up reel (usage: tanks witha diameter of 5 - 100 m and height of up to 30 m): a) vertical projection; b) horizontal projection; “1”- circular working trolley; “2”- tight wire; “3”- drive mechanism; “4”- lift device; “5”- flexible closed tension member stressing a trolley with a force N/2;“6” - drive circle; “7”- gear circle of a working trolley moving with a inactive run

System of one-chambered tanks with circular section: a) underground tanks (“1- 8”, situated under the ground); b) on-the-ground tanks (“1 - 10”, situated on the ground) and over-ground tanks (“11 – 12”, situated over the ground), substitutionally called „water towers” or „elevated tanks”) [9

Styki pionowe w elementach prefabrykowanych: