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Structural Analysis of a Water Tank

Prepared by

Emre Turkoz, BSME

Can Ozcan, MSME

emre.turkoz@akroengineering.com

can.ozcan@akroengineering.com

AKRO R&D Ltd.

Phone: +90 (262) 678-7215

KEMAL NEHROZOGLU CAD. GOSB TEKNOPARK

HIGH TECH BINA 3.KAT B5

GEBZE/KOCAELI/TURKEY - 41480

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1. Introduction Structural analysis of a water tank is a common task in manufacturing industry as it is necessary to keep

the design within standards, keep costs down and provide a robust & reliable design. It is also an

interesting finite element analysis problem, since it requires the utilization of the shell meshing

technology for accurate stress calculation; where a common solid meshing strategy turns out to be

harder to implement and impractical regarding the computational cost for matching the accuracy levels

of the calculations.

The purpose of this analysis is to investigate the displacement and stress distributions which exist due to

tank’s own weight and fluid contained resulting in hydrodynamic pressure within. Effects of changing

sheet metal thickness and the number of equally spaced supports beneath the tank are studied.

Displacements and safety factors against yielding are also documented.

Autodesk Simulation Mechanical 2012 is used to perform this analysis.

2. Problem Setup The tank under consideration is 6m high and has the

capacity of containing 50,000 liters of water. It has 5

supports beneath the bottom surface of the tank for the

original configuration. The tank is made of alloy steel,

which has yield strength of 250 MPa.

The thickness of the sheet metal panel which build up

the tank is 5 mm. Since the plates are too thin compared

to their length, shell mesh type is preffered. For shell

meshing, Autodesk Simulation Mechanical software

employs “Midplane” mesh type. Midplane meshing is

easy-to-use, which does not require any other geometry

operation like extracting the mid-surface of the object

to be meshed. Like solid meshing, midplane meshing

also only needs the element size input from the user.

The mesh is generated automatically.

40 mm is assigned for the element size, which would allow mesh to be fine enough to cover the results.

This results in total number of 55,928 nodes and 58,062 elements.

The interior surfaces of the tank are loaded with hydrodynamic where the point on the surface of water

is given as the reference point to pass the information of the height of the water in the tank.

Figure 1: The bottom part of the water tank

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3. Design Scenarios and Results Four design scenarios are studied to address the effect of bottom plate thickness and number of

supports beneath the bottom surface of the tank on yielding. The critical section came out to be the

bottom part of the tank, especially around the region where bottom surface and neighboring cylindrical

surfaces meet. Two versions of support structure are tested: equally spaced 5 and 6 supports. Sheet

metal thickness values tested are 5 mm and 7.5 mm. Table 1. shows maximum Von Mises stresses in the

structure for 4 design variations.

Von Mises Stress [MPa] 5 supports 6 supports

5 mm thick. 1603.12 1547.5

7.5 mm thick. 873.08 688.13 Table 1: Maximum Von Mises Stress values for different design scenarios

Figure 2: Von Mises Stress contours, scaled from 0 to 250 MPa, where 250 MPa is the yield strength

of the material used. Stress values in red areas exceed or approach to the yield point. (a) 5 supports

with 5 mm plate thickness, (b) 5 supports with 7.5 mm plate thickness, (c) 6 supports with 5 mm

plate thickness, (d) 6 supports with 7.5 mm plate thickness.

(a) (b)

(c) (d)

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From Figure 2, it can be deduced that increasing plate thickness and increasing number of supports

results in lower stresses. It is also seen that plate thickness has more influence on the stress results than

number of supports.

Displacement contour plots for 5-support and 6-support tanks are given below.

Figure 3: Displacement Contour Plot Patterns of 5-support and 6-support Design Scenarios

Maximum displacement values are given in the table below for the four design scenarios:

Maximum Displacement [mm] 5 supports 6 supports

5 mm thick. 23.45 22.14

7.5 mm thick. 14.04 10.92 Table 2: Maximum displacement values of 4 different design scenarios

4. Conclusion From the results given above, it’s found out that both increasing bottom plate thickness and the number

of supports result in decreasing Von Mises stress.

A further interpretation would state that increasing the bottom plate thickness has much stronger effect

on stress than increasing the number of supports. A %50 percent increase in bottom plate thickness

results in approximately halving the maximum stress and displacement.

Mid-plane meshing is the appropriate meshing type for problems which deal with long and thin plate

elements. Autodesk Simulation has an easy-to-use mid-plane mesher which allows this kind of studies to

be completed with less effort and in a shorter time span.

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Figure 4: Safety Factor of the bottom plate of the tank for the 6-support 7.5 mm bottom plate thickness design

scenario. Red denotes critical areas with 0<SF<1 whereas other colors denote safer regions.