General design requirements for members of a tank
General Design for Requirements (I.S.I)
1. Permissible Stresses in Concrete.
(a) For resistance to cracking. For calculations relating to the resistance of members to cracking, the permissible stresses in tension (direct and due to bending) and shear shall confirm to the values specified in Table 1. In members less than 225 mm. thick and in contact with liquid on one side these permissible stresses in bending apply also to the face remote from the liquid.
(b) For strength calculations. In strength calculations the permissible concrete stresses shall be in accordance with Table 1. Where the calculated shear stress in concrete alone exceeds the permissible value, reinforcement acting in conjunction with diagonal compression in the concrete shall be provided to take the whole of the shear.
3. Permissible Stresses in Steel
(a) For resistance to cracking. When steel and concrete are assumed to act together for checking the tensile stress in concrete for avoidance of crack, the tensile stress in steel will be limited by the requirement that the permissible tensile stress in the concrete is not exceeded so the tensile stress in steel shall be equal to the product of modular ratio of steel and concrete, and the corresponding allowable tensile stress in concrete.
(b) For strength calculations. In strength calculations the permissible stress shall be as follows
(i) Tensile stress in member in direct tension 1000 kg/cm2
(ii) Tensile stress in member in bending on liquid retaining face of members or face away from liquid for members less than 225 mm thick. 1000 kg/cm2 . On face away from liquid for members 225 mm. or more in thickness. 1250 kg/cm2
(iii) Tensile stress in shear reinforcement, For members less than 225 mm thickness 1000 kg/cm2. For members 225 mm or more in thickness 1250 kg/m2
(iv) Compressive stress in columns subjected to direct load. 1250 kg/cm2
5. Floors
(i) Provision of movement joints.
(ii) Floors of tanks resting on ground. If the tank is resting directly over ground, floor may be constructed of concrete with norminal percentage of reinforcement provided that it is certain that the ground will carry the load without appreciable subsidence in any part and that the concrete floor is cast in panels with sides not more than 4.5 m. with contraction or expansion joints between. In such cases a screed or concrete layer less than 75 mm thick shall first be placed on the ground and covered with a sliding layer of bitumen paper or other suitable material to destroy the bond between the screed and floor concrete.
In normal circumstances the screed layer shall be of grade not weaker than M 100, where injurious soils or aggressive water are expected, the screed layer shall be of grade not weaker than M 150 and if necessary a sulphate resisting or other special cement should be used.
(iii) Floor of tanks resting on supports
(a) If the tank is supported on walls or other similar supports the floor slab shall be designed as floor in buildings for bending moments due to water load and self weight.
(b) When the floor is rigidly connected to the walls (as is generally the case) the bending moments at the junction between the walls and floors shall be taken into account in the design of floor together with any direct forces transferred to the floor from the walls or from the floor to the wall due to suspension of the floor from the wall.
(c) In continuous T-beams and L-beams with ribs on the side remote from the liquid, the tension in concrete on the liquid side at the face of the supports shall not exceed the permissible stresses for controlling cracks in concrete. The width of the slab shall be determined in usual manner for calculation of the resistance to cracking of T-beam, L-beam sections at supports.