Evaluation of effects of temperature difference in storage tanks using Staad Pro

In some factories and industries, tanks are used for the storage of hot liquids which are used in production. In such scenarios, the temperature inside the tank and the temperature in the surrounding may not be the same. It is well known that internal forces are induced in statically indeterminate structures when there is temperature difference as the elements undergo differential thermal expansion/contraction. For simple frames, the internal forces due to temperature difference can be easily obtained using the force method of structural analysis. But for more complex structures like combination of beams and plates, software like Staad Pro can be used for evaluation of temperature difference.

For example, let us consider the reinforced concrete tank with the dimensions shown in Figure 1;

Tank storing hot liquid
Fig 1: Structural scheme of water tank subjected to temperature difference

Dimensions of columns = 300 x 300 mm
Dimension of beams = 300 x 500 mm
Height of column above ground level = 3 m
Length of tank = Width of tank = 3 m (centre to centre)
Height of tank = 2.5 m (centre to centre)
Thickness of tank walls and base = 250 mm
Support condition = Fixed
Temperature inside the tank = 120 oC
Temperature outside the tank = 25 oC
Maximum hydrostatic pressure from the liquid stored = 25 kPa
Modulus of elasticity of concrete = 2.8 x 107 kN/m2
Coefficient of expansion of concrete = 1.0 x 10-5 /oC

The tank has been modelled on Staad Pro (see Figure 2) using the parameters defined above.

Water tank modelled on Staad
Fig 2: Modelling of the tank on Staad Pro

The walls of the tank were subjected to a triangular hydrostatic pressure distribution of 25 kPa. You can check how apply hydrostatic loads on Staad Pro here. The temperature difference action applied to the the tank is shown below.

Temperature change for axial elongation = Average temperature = (25 + 120)/2 = 72.5 oC
Temperature difference = 25 – 120 = -95 oC

The application on Staad Pro is shown in Figure 3.

Temperature load on Staad Pro
Fig 3: Application of temperature load on Staad Pro

When analysed on Staad Pro, the results shown in Figures 4-8 were obtained for the tank shells at SLS.

Bending moment due to water pressure 1
Fig 4: Bending moment on the tank shell due to water pressure
Shear force due to water pressure 1
Fig 5: Shear stress on the tank shells due to water pressure
Bending moment due to temperature difference 1
Fig 6: Bending moment on the tank shell due to temperature difference
Shear stress due to temperature difference
Fig 7: Shear stress on the tank shell due to temperature difference
Displacement due to temperature difference 1
Fig 8: Displacement of tank shell and frame due to temperature difference

The internal stresses induced in the tank shell due to temperature difference is quite serious and requires detailed attention during design.


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