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Finite Element Analysis of Fully BuriedUnderground Water Tank by SAP2000

WORKING PAPER DECEMBER 2015

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Debojit Sarker

Bangladesh University of Engineering and

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1Mohammad Ishtiaque Iqbal, Civil Engineering, Bangladesh University of Engineering andTechnology, +8801912956402, [email protected]

2Mohammed Raihanul Alam Chowdhury, Civil Engineering, Bangladesh University of

Engineering and Technology, +8801713369186, [email protected]

3Debojit Sarker, Civil Engineering, Bangladesh University of Engineering and Technology,+8801712499987, [email protected]

4Dr. A.M.M. Taufiqul Anwar, Professor, Civil Engineering, Bangladesh University of Engineering

and Technology, +8801711703308,[email protected]

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11th Global Engineering, Science and Technology Conference

18-19 December 2015, BIAM Foundation,63 Eskaton, Dhaka, Bangladesh

Finite Element Analysis of Fully Buried UndergroundWater Tank by SAP2000

Mohammad Ishtiaque Iqbal1, Mohammed Raihanul Alam Chowdhury

2,

Debojit Sarker3, and Dr. A.M.M. Taufiqul Anwar4

Abstract

Underground water tank is defined as any one or a combination of tanksthat have 10% or more of their volume below the surface of the ground inwhich they are installed. There are usually many environmental regulationsapplied to the design and operation of storage tanks, often depending onthe nature of the fluid contained within. From PCA regulation it is knownthat underground water tank has different support conditions, as like hingedtop and hinged base or free top and hinged base or free top and fixed baseor hinged top and fixed base. Storage tank has two types of loadingconditions, triangular and rectangular. The study is an analysis for free topand fixed base support condition and triangular loading from soil outside thetank and water inside the tank. This study also investigates the influence ofdesign parameters on response of underground water tank. Structuralparameters are taken as length-height ratio, width-height ratio, wallthickness of water tank and soil density. The response characteristics aremaximum horizontal and vertical moments for long wall and short wall.Underground water tank response has been evaluated by Finite ElementMethod (FEM) using powerful package software SAP2000 analysis. Series

of underground water tank with different parameters varying over a rangewere selected for analysis. Analyses of the model were conducted byvarying only one parameter at a time within specific range as well as

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keeping other parameters at their reference values. The responsecharacteristics are evaluated to understand the structural behavior of thesystem. The results are plotted against the varying structural parametersand presented in graphical form.

Field of Research: Underground Water Tank; Parametric Study; FEM;SAP2000; Length to Height Ratio; Width to Height Ratio; Wall Thickness;Saturated Soil Density.

1. Introduction

The purpose of the present research is a parametric study of underground water

tank. Few researches are done on this topic. So, this topic deserves properattention. In treatment plants concrete made underground reservoirs are inextensive practice, as it provides large capacity. Different models are analyzedfor values shifting over a specified region of required variables. Structuralengineers who use spread sheet to do their own calculations or design manuallyneed a complete design aid for scrupulous estimation. Computer program aideddesign can be checked using design charts and graphs to eliminate error thusensuring safety of structure. Structural engineers interested in this type of topiccan make further analysis for their models and develop graphs for a wide rangeof variables. The problems created by latest and modern design have to be facedby us. The topic undertaken for research here in relatively new in this area is ofpractical significance and deserves due attention.

2. Objective

The primary objective is to develop graphs for underground water tank inaccordance with the variation of different parameters. The detailed objective ofthe study is to develop models of underground water tank (fully buried) foranalysis using SAP2000, and to conduct an extensive parametric study.

3. Methodology

The influence of various parameters on underground water tank is investigated inthis study. Analyses of different models with respect to different structuralparameters are done. The analysis is carried out by Finite Element Method usingSAP2000v14.2.0. When the effect of a structural parameter is to be studied,different models are made with different values of that parameter. All otherparameters are kept at their reference values to analyze the model. In the same

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way, study on the effect of other parameters is also conducted. Moments andshear calculated at different location of the model for different values ofparameters. Effects of Moment and shear for different parameter on the modelsare presented graphically.

4. Selection of Underground Water Tank Design Parameterfor Analysis

A simple model of underground water tank is selected for analysis. Supportcondition of this model is selected fixed base and free top according to case-7 ofRectangular Concrete Tanks, Revised fifth edition by Javeed A. Munshi, PCApublication. This model will be analyzed for different parameters inSAP2000v14.2.0. For all the models that will be analyzed for varying parameters,maximum moment along X-axis and Z-axis for long wall, maximum moment

along X-axis and Z-axis for short wall will be calculated and finally the effect ofthe parameters on the mentioned responses of the underground water tank willbe set out.

Since completely general and variable structure cannot be used for analysis, areference structure is used. For each structural parameter to be analyzed,different models are made with different values of that parameter varying within aselected range while all other parameters of the structure are kept fixed at theirreference values. The reference values of different parameters for the referenceunderground water tank to be analyzed are in Table 01.

Table 01: Reference values of selected parameters for Underground Water Tank.

Parameters Reference Value

Tank height (a), ft 10

Tank length (b), ft 30

Tank width (c), ft 20

b/a ratio 3

c/a ratio 2

Wall thickness, inch 18

Concrete Grade, ksi 4

Reinforcement Grade, ksi 60

Saturated Soil density, psf 100

Uniform Live Load, psf 100

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Water density, psf 70

The loading and support conditions are given below (ACI Committee 350, 1995):

Dead load: Self weight of the walls and top and bottom slab. Live load: Uniformly distributed load on top slab.

Leakage test prior to backfilling (D1).

Backfill prior to adding tank cover (D2).

Tank full with cover in place. Resistance provided by the soil is ignored.

Buoyancy forces.

Fixed base, free top.

Triangular lateral load.

Material Properties are given in Table 02. The 3D models have been analyzed forvariable parameters. For each parameter, variation within a range is done, whilekeeping other parameters fixed at their reference values. The ranges ofvariations of different parameters are shown in Table 03.

Table 02: Material properties (ACI Committee 318, 1995)

Modulus of elasticity 5.191 * 10 psf

Poisons ratio 0.2

Unit weight of concrete 150 psf

Coefficient of thermalexpansion

5.5 * 10-

Shear modulus 2.163 * 10 psf

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Table 03: Ranges of parameters for Underground Water Tank.

Parameters Variable value Reference valueTank height (a), ft 10 10

Tank length (b), ft 30, 40, 50, 60 30

Tank width (c), ft 5, 10, 15, 20 20

b/a ratio 3, 4, 5, 6 3

c/a ratio 0.5, 1, 1.5, 2 2

Wall thickness, inch 12, 15, 18, 21, 24 18

Concrete Grade, ksi 3, 3.5, 4, 4.5, 5 4

ReinforcementGrade, ksi

60 60

Saturated Soildensity, pcf

100, 115, 128, 134,140

100

Uniform Live Load,psf

100 100

Water density, psf 70 70

Table 04: Saturated soil density of different soil (Bowles, 1988)

Type ofsoil

Ysat(KN/m3)

Ysat(lb/ft3)

Gravel 21 134

Sand 18 115

Silt 20 128Clay 22 140

For the purpose of the study selected response characteristics are shown below:

Maximum vertical and horizontal moment of long wall

Maximum vertical and horizontal moment of short wall

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Maximum shear along X-axis and Z-axis of long wall

Maximum shear along X-axis and Z-axis of short wall

5. Influence of Structural Parameters on Behavior ofUnderground Water Tank

A series of curves are plotted using the results of the analysis. Variation of slabbehavior is plotted along Y-axis and variation of structure parameters is plottedalong X-axis.

The effects of the following parameters are included in the study:

Effect of wall thickness

Effect of Saturated Soil Density

Effect of Width(c)/Height(a) ratio

Effect of Length(b)/Height(a) ratio

The notations used in the graphical representations are:

M11= Maximum Horizontal Bending Moment of wall of model (lb-ft/ft)

M22= Maximum Vertical Bending Moment of wall of model (lb-ft/ft)

V13= Maximum Shear along Y-axis of model (lb-/ft)

V23= Maximum Shear along Z-axis of model (lb-/ft).

5.1 Effect of Thickness Variation on Walls

Figure 1. Effect of thickness variation

on bending moment at long wall for

D1 and D2

Figure 2. Effect of thickness variation

on bending moment at short wall for

D1 and D2

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5.2 Effect of Saturated Soil Density on Walls

Figure 3. Effect of thickness

variation on shear at long wall for

D1 and D2

Figure 4. Effect of thickness

variation on shear at short wall

for D1 and D2

Figure 5. Effect of saturated soil

density variation on bending

moment at long and short wall for

D2

Figure 6. Effect of saturated soil

density variation on shear at long and

short wall for D2

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5.3 Effect of Width(c)/Height(a) ratio on Walls

5.4 Effect of Length (b)/Height (a) ratio on Walls

Figure 7. Effect of c/a ratio

variation on bending moment at

lon wall for D1 and D2

Figure 8. Effect of c/a ratio variation


D1 and D2


on shear at short wall for D1 and D2

Figure 12. Effect of b/a ratio variation


D1 and D2

Figure 11. Effect of b/a ratio

variation on bending moment at

long wall for D1 and D2


on shear at lon wall for D1 and D2

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6. Result and Discussion:

Maximum moments and shear increase with thickness of wall exceptmaximum horizontal bending moment at short wall. For larger surface area,long walls are subjected to bigger load than short wall and the load istransferred to short wall through fixed connection between them. Thus shortwall experiences tension from both side and bending moment for lateralpressure. As a result, short wall behaves differently.

Maximum moment and shear of all axes increase at the same ratio as soildensity increases. As the density increases from sand to clay, it may berequired more reinforcement in critical sections to resist the momentproduced.

Maximum vertical moment and shear at short wall decreases as c/a ratiodecreases.

Maximum moment and shear decreases as b/a ratio decreases to a certainpoint except maximum horizontal bending moment at short wall continuesto decrease.

Only one support condition (fixed base, free top) was considered. Furtherstudies may cover other seven possible support condition containingvarious combination of fixed and hinged base with free and hinged top.Applying spring characteristics to the supports can explore differentsimulation of practical situation.

Soil height was selected conservatively same as tank height. Effect of soilheight variation with the height of tank must be considered which has thepotential for providing more economical design guideline.

Material properties were kept constant.Different type of newly developed

Figure 13. Effect of b/a ratio

variation on shear at long wall for

D1 and D2

Figure 14. Effect of b/a ratio variation

on shear at short wall for D1 and D2

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concrete such as- high density concrete could be selected as material fortechnical advancement

Graphical representation was done only for the selected range. Soilparameters, loads and different grades for reinforcement should beconsidered for more accuracy for the preparation of the design aid.

7. Conclusion

Maximum moments and shear increase with thickness of wall exceptmaximum horizontal bending moment at short wall.

Maximum moment and shear of all axes increase at the same ratio as soildensity increases

Maximum vertical moment and shear at short wall decreases as c/a ratiodecreases.

Values of shear decreases sharply as c/a ratio decreases.

Maximum design horizontal bending moment at short wall is developed forc/a ratio 1.5.

Rates of change of both shear with the variation of c/a ratio to the referencevalue are almost same.

Minimum design vertical moment at long wall and minimum design shear atshort wall is developed for b/a ratio 4.5 for both loading condition.

The results found by SAP analysis vary slightly than the values given atPCA manual.

8. References

[1] PCA Publication, Rectangular Concrete Tank, Revised Fifth Edition byJaveed A. Munshi

[2] ACI Committee 350, Environmental Engineering Concrete Structures (ACI

350R -89),American Concrete Institute, Detroit, 1995.[3] ACI Committee 318, Building Code Requirements for Structural Concrete

(ACI 318-95),American Concrete Institute, Farmiigton Hills, MI, 1995.

[4] Bowles, J.E., Foundation Analysis and Design,4th Ed., McGraw-Hill, Inc.,NY, 1988.

[5] Gupta, A.K., and Sen, S., Design of Flexural Reinforcement in ConcreteSlabs, Journal of the Structural Division,ASCE, Vol. 103, ST4, 1977, pp.793-804.

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[6] Hengst, R., Concrete Watertight Structures and Hazardous LiquidContainment,ASCE Press, American Society of Civil Engineers, NY, 1994.

[7] Iqbal, M. I., Chowdhury, R. A., A Parametric Study on Underground WaterTank, UG Thesis, Bangladesh University of Engineering and Technology(BUET), Dhaka, 2014.

[8] Moody, W.T., Moments and Reactions for Rectangular Plates, UnitedStates Department of the Interior, Bureau of Reclamation, Denver; 1960, 74pages.

[9] Szilard, R., Theory and Analysis of Plates-Classical and NumericalMethods,Civil Engg. and Engg. Mechanics Series, Prentice Hall, Inc., NJ,1974.

[10] SAP90 - A Series of Computer Programs for the Finite Element Analysis ofStructures, Computers and Structures, Inc., Berkeley, CA, I992.

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