dynamic behavior

DYNAMIC BEHAVIOUR OF RC INTZE ELEVATED WATER TANK USING DIFFERENT FLUID LEVEL CONDITIONS

CHAPTER 1

INTRODUCTION

1.1. General

An elevated water tank is a large water storage container constructed for the purpose of

holding water supply at certain height to get sufficient pressure required for water distribution

system. Elevated tanks are the structures frequently used in order to store fluid for not only

drinking but also for fire fighting. Supply of drinking water is essential immediately after

destructive earthquakes. Without assured water supply, the uncontrolled fires subsequent to

major earthquakes may cause damage than the earthquakes themselves.

Many new ideas and innovation has been made for the storage of water and other liquid

materials in different forms and fashions. There are many different ways for the storage of liquid

such as underground, ground supported, elevated etc. Liquid storage tanks are used extensively

by municipalities and industries for storing water, inflammable liquids and other chemicals. Thus

Water tanks are very important for public utility and for industrial structure.

Elevated water tanks consist of huge water mass at the top of a slender staging which are

expected to fail first during earthquakes. Elevated water tanks are critical and strategic structures

and damage of these structures during earthquakes may endanger drinking water supply.

Design of new tanks and safety evaluation of existing tanks should be carried out with a high

level of accuracy because the failure of such structures, particularly during an earthquake, may

be disastrous. Hydrodynamic pressures on tanks under earthquake forces play an important role

in the design of the tank. In order to make sure that the water tank design is capable of

withstanding any earthquake loads like overturning moment and base shear, needs a detailed

investigation of fluid structure interaction. The movement and response of the water towards the

wall structure may create an effect to the fundamental frequency of elevated tank.

There are many techniques to handle the dynamic response of elevated concrete water tank. The

analysis of elevated concrete water tank under dynamic load of fluid-structure interaction

problems can be investigated by using different approaches such as added mass, Lagrangian,

Eulerian, and Lagrangian Eulerian approach. These analyses can also be carried out in the finite

MVJCE, BANGALORE Page 1


element method (FEM) or by the analytical methods. Study has shown that using of analytical

model could change the results of shear forces and bending moment at the base of water tank up

to 10% compared with the FEM model, The added mass approach can be investigated by using

some of conventional FEM software such as SAP 2000, STAAD Pro and LUSAS, whilst the

other approaches of analyses needs special programs that include fluid elements such as ANSYS,

ABAQUS ADINA, ALGOR and etc

Due to the lack of knowledge of supporting system some of the water tanks were collapsed or

heavily damaged. So there is need to focus on seismic safety of lifeline structure which are safe

during earthquake and also take more design forces.

The liquid storage tanks are particularly subjected to the risk of damage due to

earthquake-induced vibrations. A large number of overhead water tanks are damaged during past

earthquakes. Majority of them were shaft staging while a few were on frame staging type.

Muzaffarabad earthquakes (2005) Kuch and Bhuj earthquakes (2001) are the recent examples, as

shown in Figure 1.2&1.3 and Figure 1.4, 1.5&1.6 respectively. It is observed from the past

earthquakes; most of the elevated water tanks undergo damage to their staging.

Circular elevated reinforced concrete water tank with framed staging support has been

studied. These tanks are analyzed for earthquake force as per Indian conditions. The seismic

analysis of these tanks has been carried out (considering the effect of sloshing) by two different

methods; first one based on Indian standard code 1893-Part 1, (2002) i.e. adopting lumped mass

modal method and second is based on draft code 1893-Part 2, (2005) considering two mass

modal (convective and impulsive mode) method for different soil conditions like, tank on rocky

or hard soil sites, tanks on medium soil sites and tanks on soft soil sites for fixed base condition.

Commercial software package SAP 2000 has been used for modeling of overhead water tank

supported on frame staging.



Figure 1.1 Representations of Elevated Tanks Supported on Shaft and Framed Staging.

Investigating the effects of earthquakes has long been recognized as a necessary step to

understand the natural hazards and its risk to the society in the long run. A rapid assessment of

general damage survey and documentation of initial important observations, besides facilitating

in emergency management and rehabilitation activities, identifies the need of follow-up areas of

research. However, long-term preparedness requires in-depth research on the identified issues

with suggestions for preparedness. At this backdrop, the damage survey conducted following few

severe Indian earthquakes such as 2001 Bhuj earthquake, 2005Kashmir earthquake etc. are

critically examined. The scrutiny of such damage histories revealed damage/failure of reinforced

concrete(R.C.) elevated water tanks of low to high capacity a very Important lifeline facility and

damage of the same often results in Significant hardships even in the post-earthquake scenario,

claiming human casualties and economic loss to build environment.

1.2 Model Provisions



Two mass model for elevated tank was proposed by Housner (1963) which is more appropriate

and is being commonly used in most of the international codes including Draft code for IS 1893

(Part-II). The pressure generated within the fluid due to the dynamic motion of the tank can be

separated into impulsive and convective parts. When a tank containing liquid with a free surface

is subjected to horizontal earthquake ground motion, tank wall and liquid are subjected to

horizontal acceleration. The liquid in the lower region of tank behaves like a mass that is rigidly

connected to tank wall. This mass is termed as impulsive liquid mass which accelerates along

with the wall and induces impulsive hydrodynamic pressure on tank wall and similarly on base

liquid mass in the upper region of tank undergoes sloshing motion. This mass is termed as

convective liquid mass and it exerts convective hydrodynamic pressure on tank wall and base.

For representing these two masses and in order to include the effect of their hydrodynamic

pressure in analysis, spring mass model is adopted for ground-supported tanks and two-mass

model for elevated tanks.

Figure 1.2: Two mass model for elevated tank.

In spring mass model convective mass (mc) is attached to the tank wall by the spring having

stiffness (Kc), where an impulsive mass (mi) is rigidly attached to tank wall. For elevated tanks

two-mass model is considered, which consists of two degrees of freedom system. Spring mass

model can also be applied on elevated tanks, but two-mass model idealization is closer to reality.



The two- mass model is shown in Fig 1.1(a). where, mi, mc, Kc, hi, hc, hs, etc. are the parameters

of spring mass model and charts as well as empirical formulae are given for finding their values.

The parameters of this model depend on geometry of the tank and its flexibility. For elevated

tanks, if the shape is other than circular or rectangular, then the values of spring mass parameters

can be obtained by considering an equivalent circular tank having same capacity with diameter

equal to that of diameter at top level of liquid in original tank. The two-mass model was first

proposed by G. M. Housner (1963) and is being commonly used in most of the international

codes. The response of the two degree of freedom system can be obtained by elementary

structural dynamics. However, for most of elevated tanks it is observed that both the time periods

are well separated. Hence, the two mass idealizations can be treated as two uncoupled single

degree of freedom system as shown in Fig.1.1 (b). The stiffness (Ks) is lateral stiffness of

staging. The mass (ms) is the structural mass and shall comprise of mass of tank container and

one-third mass of staging as staging will acts like a lateral spring. Mass of container comprises of

roof slab, container wall, gallery if any, floor slab, floor beams, ring beam, circular girder, and

domes if provided.

1.3 Fluids-Structure Interaction

The analysis of elevated tank under seismic load of fluid structure interaction problems can be

investigated by using different approaches such as added mass Westergaard or velocity potential,

Lagrangian (Wilson and Khalvati), Eulerian (Zienkiewicz and Bettes), and Lagrangian Euclidian

approach (Donea). These analyses can be carried out using FEM or by the analytical methods.

The added mass approach as shown in Fig.1.2 can be investigated by using some of conventional

FEM software such as SAP2000, STAAD Pro and LUSAS. Whilst in the other approaches, the

analysis needs special programs that include fluid elements in the elements library, such as

ANSYS, ABAQUS ADINA, ALGOR etc.

The general equation of motion for a system subjected to an earthquake excitation can be written

as,

Mu + Cu + Ku= -M ug …….. Eq. 1.1



In which M, C and K are mass, damping and stiffness matrices with, and u are the acceleration,

velocity and displacement respectively, and is the ground acceleration. In the case of added mass

approach the form of equation 1.1 become as below.

Mu + Cu + Ku= -M ug ….… Eq. 1.2

In which M is the new mass matrix after adding hydrodynamic mass to the structural mass, while

the damping and stiffness matrices are same as in equation 1.1

Fig 1.3: FEM Fluid-Structure-Interaction Mode

Westergaard Model’s method was originally developed for the dams but it can be applied to

other hydraulic structure, under earthquake loads i.e. tank. The impulsive mass has been obtained

according to GSDMA (Gujarat State Disaster Management Authority) guideline equations and is

added to the tanks walls according to Westergaard Approach as shown in Figure 1.3 using

equation 1.3.

Fig 1.4: (a) Westergaard added mass concept (b) Normal and Cartesian directions.



mai = [(7/8) √(h(h-y⍴ i)) ]Ai ………… Eq. 1.3

In the case of Intze tank where the walls having sloped and curved contact surface, the Eq. 1.3

should be compatible with the tank shape by assuming the pressure is still expressed by

Westergaard original parabolic shape. But the fact that the orientation of the pressure is normal

to the face of the structure and its magnitude is proportional to the total normal acceleration at

the recognized point. In general, the orientation of pressures in a 3-D surface varies from point to

point; and if it is expressed in Cartesian coordinate components, it would produce added-mass

terms associated with all three orthogonal axes. Following this description the generalized

Westergaard added mass at any point i on the face of a 3-D structure is expressed by the Eq.1.4.

mai =ai Ai λ it λi=ai Ai[ λx

2 λ y λx λz λx

λy λx λ y2 λz λ y

λz λx λz λ y λz2 ] ……1.4

‘Ai’ is the tributary area associated with node i, λi is the normal direction cosine (λ2y, λ2

x , λ2z )

and ai is Westergaard pressure coefficient.

The liquid storage tanks are particularly subjected to the risk of damage due to earthquake-

induced vibrations. A large number of overhead water tanks are damaged during past

earthquakes. Majority of them were shaft staging while a few were on frame staging type.

Muzaffarabad earthquakes (2005) Kuch and Bhuj earthquakes (2001) are the recent examples. It

is observed from the past earthquakes; most of the elevated water tanks undergo damage to their

staging.

Circular elevated reinforced concrete water tank with framed staging support has been

studied. These tanks are analyzed for earthquake force as per Indian conditions. The seismic

analysis of these tanks has been carried out (considering the effect of sloshing) by two different

methods; firstly based on Indian standard code 1893-Part 1, (2002) i.e. adopting lumped mass

modal method and second is based on draft code 1893-Part 2, (2005) considering two mass

modal (convective and impulsive mode) method for different soil conditions like, tank on rocky

or hard soil sites, tanks on medium soil sites and tanks on soft soil sites for fixed base condition.



(a)

(b)

Fig 1.5: (a) and (b) - Typical Example of Failure of Elevated Water Tank on Framed Staging.

A dynamic analysis of such tanks must take into account the motion of the water relative to

the tank. For certain proportions of the tank and the structure the sloshing of the water may be

the dominant factor, whereas for other proportions the sloshing may have small effect. Therefore,



an understanding of the earthquake damage, or survival, of elevated water tanks requires an

understanding of the dynamic forces associated with the sloshing water. Elevated water tanks are

generally located at high altitude locations like top of the hillock; hence they are subjected to

severe wind loads. An elevated tank consists of two parts.

They are,

1. Container, and

2. Staging (Supporting structure).

Container can be cylindrical, rectangular, conical or Intze in shape.

Similarly the supporting structure could be an axis symmetrical concrete shaft, or space

frame type. In India, elevated tanks are generally reinforced cement concrete (RCC).

1.4 Structural elements of Elevated Water Tanks

a) Circular tank with roof and bottom consist of dome

1. Top spherical dome

2. Top ring beam

3. Circular side walls

4. Bottom spherical dome

5. Bottom circular girder

6. Tower with columns and braces / shaft type

7. Foundation

b) Intze Tank

1. Top spherical dome

2. Top ring beam


4. Bottom ring beam

5. Conical dome






9. Foundation

c) Conical Tank

1. Conical dome

2. Top ring beam

3. Conical shell

4. Bottom spherical dome internal shaft

5. Bottom ring beam


7. Foundation

Elevated water tank can be simulated based on SDOF, 2DOF or FEM, which governed by

one mode, two modes, or more respectively. It is widely recognized that these analysis are not

always the appropriate approach for simulating response of structures subjected to seismic

excitation. The estimation of damages made using this approach is normally poor. The response

of elevated water tank when dynamic effects are considered is deeply dependent upon the soil

deformability and liquid characteristics. Therefore interaction between tank foundation and

liquid should be accounted in the analysis of these structures. Previous studies on the seismic

behaviour of elevated water tanks were only focusing on the linear seismic response.



CHAPTER 2

LITERATURE REVIEW

A detailed literature survey has been made on the Seismic analysis of water tanks. Here various

methods for Seismic analysis of elevated water tanks parameters such as spring mass model

parameters, time period, base shear, base moment, hydrodynamic pressure, sloshing height of

liquid was presented by various eminent persons from different parts of the world. The

observations of some the researchers are presented here.

M. V. Waghmare et., al., (2013) In the present study sloshing effect in elevated water tank is

studied by using Finite Element Method (FEM) based computer code. Various parameters have

been considered such as height of container, depth of water in tank (30%, 50%,70% and full) and

height of staging etc.

Gaikwad Madhurar V et., al., (2013)The main object of this paper is, to compare the Static and

Dynamic analysis of elevated water tank, to study the dynamic response of elevated water tank

by both the methods, to study the hydrodynamic effect on elevated water tank, to compare the

effects of Impulsive and Convective pressure results. From detail study and analysis it was found

that, for same capacity, same geometry, same height, with same staging system, with same

Importance factor & Response reduction factor, in the same Zone; response by equivalent static

method to dynamic method differ considerably. Even if we consider two cases for same capacity



of tank, change in geometric features of a container can shows the considerable change in the

response of tank .As the capacity increases difference between the response increases. Increase

in the capacity shows that difference between static and dynamic response is in increasing order.

It is al-so found that, for small capacity of tank the impulsive pressure is always greater than the

convective pressure, but it is vice- versa for tanks with large capacity. Magnitude of both the

pressure is different. The effect of water sloshing must be included in the analysis. Free should

be designed to resist the uplift pressure due to sloshing of water.

S. Bozorgmehrnia et., al., (2012) In this research, a sample of reinforced concrete elevated

water tank, with 900 cubic meters capacity, exposed to three pair of earthquake records have

been studied and analyzed in time history using mechanical and finite-element modeling

technique. The liquid mass of tank is modeled as lumped masses known as sloshing mass, or

impulsive mass. The corresponding stiffness constants associated with these lumped masses have

been worked out depending upon the properties of the tank wall and liquid mass. Tank responses

including base shear, overturning moment, tank displacement, and sloshing displacement have

been calculated. Results reveal that the system responses are highly influenced by the structural

parameters and the earthquake characteristics such as frequency content.

Ayaz hussain M. Jabar et., al., (2012): The main aim of this study is to understand the

behaviour of supporting system which is more effective under different earthquake time history

records with SAP 2000software. Here two different supporting systems such as radial bracing

and cross bracing are compared with basic supporting system for various fluid level conditions.

For later conditions water mass has been considered in two parts as impulsive and convective

suggested by GSDMA guidelines. In addition to that impulsive mass of water has been added to

the container wall using Westergaard’s added mass approach. Tank responses including base

shear, overturning moment and roof displacement have been observed, and then the results have

been compared and contrasted. The result shows that the structure responses are exceedingly

influenced by the presence of water and the earthquake characteristics



Asari Falguni P et., al., (2012): The paper presents the results of an analytical investigation of

the seismic response of elevated water tanks using fiction damper. In This paper, the behavior of

RCC elevated water tank is studied with using friction damper (FD). For FD system , the main

step is to determine the slip load. In nonlinear dynamic analysis, the response of structure for

three earthquake time history has been carried out to obtain the values of tower drift base shear

and acceleration Time Period. These values are compared with original structure. Results of the

elevated tank with FD are compared to the corresponding fixed-base tank design and indicate

that friction damper is effective in reducing the tower drift, base shear, time period, and roof

acceleration for the full range of tank capacities .The obtained results shows that performance of

Elevated water tank with FD is better than without FD.

Syed Saifuddin et., al.,(2012)An example intze shape tank is analyzed as per the Draft code Part

II of IS 1893:2002. This thesis consists of two different parts .In a first part, a theoretical point of

view & formulations for analysis. The second part focuses on the model example to determine

the seismic forces on tank.

H. Shakib et., al., (2009) In the present work, three reinforced concrete elevated water tanks,

with a capacity of 900 cubic meters and height of 25, 32 and 39 m were subjected to an ensemble

of earthquake records. The behavior of concrete material was assumed to be nonlinear. Seismic

demand of the elevated water tanks for a wide range of structural characteristics was assessed.

The obtained results revealed that scattering of responses in the mean minus standard deviation

and mean plus standard deviation are approximately 60% to 70 %. Moreover, simultaneous

effects of mass increase and stiffness decrease of tank staging led to increase in the base shear,

overturning moment, displacement and hydrodynamic pressure equal to 10 - 20 %, 13 - 32 %, 10

- 15 % and 8 - 9 %, respectively.



CHAPTER-3

PROPOSED STUDIES

3.1 Scope of the work

This dissertation work is a comparative study on the RC structural forms like Intze tank with

normal bracing, cross bracing, radial bracing . These structural forms are subjected to dynamic

analysis involving modal analysis, equivalent static method and response spectrum analysis.

Dynamic parameters such as mode period, base shear, displacement and acceleration, are obtained

and comparisons are drawn.

The scope of this study is to compare the performance of an Intze tank with different type of

bracing to conclude the effectiveness of dynamic response of RC frame Intze tank using static

and dynamic analysis method.

3.2 Parametric study

A 3D RC frame Intze tank of diameter 9m and height 23.85m has been taken for seismic

analysis. Three models are considered for comparison:

Model-1: Intze tank with normal octagonal and normal bracing with tank full and empty

conditions

Model-2: Intze tank with octagonal cross and normal bracing with tank full and empty

conditions



Model-3: Intze tank with radial octagonal and normal bracing with tank full and empty

conditions

3.3 Objective of the work

The main objectives of this project work includes the following

1) To find out the response of RC frame Intze tank with and without considering

water (i.e., mode period, displacement, acceleration, base shear by modal analysis,

equivalent static method and response spectrum analysis. Numerical modeling and

analysis are carried out using Finite Element based software SAP 2000.

2) The modal analysis is conducted to know mode period and mode shapes.

3) Equivalent static method is carried out for zone-5 as per IS 1893 (Part 1):2002 for

soil type-2 (i.e. medium soil).

4) RC frames Intze tank with and without considering water are studied using

response spectrum analysis.

5) Determination of mode period, base shear, displacement, acceleration, by using

equivalent static method and response spectrum analysis.

3.4 Methodology

Methodology includes

1. A detail introduction regarding earthquakes elevated water tank structural aspect

and dynamic behavior of elevated water tank .

2. Detail literature survey on elevated water tank.

3. The modal analysis of elevated water tank to get mode period and mode shape of

the structure.

4. The Intze water tanks for different fluid level condition (ie, full and empty) for

three different types of models.

a .Normal Octagonal and normal bracing

b . Radial Octagonal and normal bracing



c . Cross Octagonal and normal bracing

5. The response earthquake static analysis is carried to obtain mode period,

displacement, base shear, acceleration.

6. Results of the modal analysis, equivalent static method and response spectrum

analysis are tabulate and discussed.

3.5 Organization of the thesis

The thesis is presented in eight chapters. Brief overviews of the chapters are given below:

Chapter 1 Gives Introduction to water tank, model provisions, fluid-structure interaction and

structural elements of elevated water tanks.

Chapter 2 A detailed review of the literature of previous research on the RC frame Elevated

water tank.

Chapter 3 Deals with the scope of the work, parametric study, objective of the work and

methodology.

Chapter 4 Gives the Analysis of Water Tank, Defining Material Properties, Defining Frame

Sections, Local Coordinate System, Joints and Mass Source.

Chapter 5 Iintroduction to types of water tank and Description of the model.

Chapter 6 Includes detailed results and discussion of the work done.

Chapter 7 Presents the conclusions obtained from the above study. It also presents the future

scope of the work which can be extended further.

Chapter 8 Presents the references, text books and codes/standards referred during the work.



CHAPTER 4

MODELLING USING SAP

The typical plan in 3D models of elevated water tank is prepared. The models are kept

symmetric in both orthogonal directions. Container vessel for storage of water considered is

circular in shape. The height of supporting structure is varied respectively in proportions.

4.1 Study Parameter

The present study is about the behavior of elevated Intze water tank for different fluid level

conditions and various parameters are as listed below,

Shape- Intze

Capacity - 1 lakh liters

Height of supporting structure- 16m

Type of supporting structure –Fixed Base Frame type

Tank fills condition – Full and Empty

4.2 Defining Material Properties

In the property data area, name of the material, mass per unit volume, weight per unit volume,

modulus of elasticity, Poisson’s ratio should be specified for each type of material defined. The

mass per unit volume is used in the calculation of self-mass of the structure. The weight per unit

volume is used for calculating the self-weight of the structure.

The material properties are given using following commands



Select Define > Material properties > Add new material > Set, Region > India and Standrad

>India for add new material. Set material type to Concrete to define the properties of concrete

for our model. Se0lect the grade of concrete to M25. Select OK

Fig 4.1 Defining Material property for M25 grade concrete



Select Define > Material properties > Add new material > Set, Region > India and Standrad

>India for add new material. Set material type to Steel to define the properties of concrete for

our model. Select the grade of concrete to HYSD. Select OK

Fig 4.2 Defining Material property for HYSD415 steel

4.3 Defining Frame Sections

Frame sections like beams, columns and bracings are defined under this. A Frame

element is modeled as a straight line connecting two points. In the graphical user interface, you

can divide curved objects into multiple straight objects, subject to your specification. Each

element has its own local co-ordinate system for defining section properties and loads, and for

interpreting output.

Select Define > Section Properties > Frame Section > Frame Properties > Add New Properties.



Fig 4.3 Defining Section properties for Column section



Fig 4.4 Defining Section properties for Beam section

4.4 Local Coordinate System

Each Frame element has its own element local coordinate system used to define section

properties, loads and output. The axes of this local system are denoted as 1, 2 and 3. The first

axis is directed along the length of the element; the remaining two axes lie in the plane

perpendicular to the element with an orientation that you specify.



Fig 4.5 Defining local Coordinate system

4.5 Joints

Joints, also known as nodal points or nodes, are a fundamental part of every structural model. Joints perform a variety of functions:

• All elements are connected to the structure (and hence to each other) at the joints

• The structure is supported at the joints using Restraints and/or springs

• Rigid- body behavior and symmetry conditions can be specified using constraints that apply to the joints

• Concentrated loads may be applied at the joints

• Lumped (concentrated) masses and rotational inertia may be placed at the joints

• All loads and masses applied to the elements are actually transferred to the joints



4.6 Modeling Considerations

The location of the joints and elements is critical in determining the accuracy of the structural model. Some of the factors that needs to be considered when defining the elements, and hence the joints, for the structure are:

The number of elements should be sufficient to describe the geometry of the structure. For straight lines and edges, one element is adequate. Element boundaries, and hence joints should be located at points, lines, and surfaces of discontinuity:

Structural boundaries, e.g., corners and edges

Changes in material properties

Changes in thickness and other geometric properties

Support points (Restraints and springs)

Points of application of concentrated loads, except that Frame elements may have

concentrated loads applied within their spans

In regions having large stress gradients, i.e., where the stresses are changing rapidly, an

Area- or Solid-element mesh should be refined using small elements and closely- spaced joints.

This may require changing the mesh after one or more preliminary analyses.

More than one element should be used to model the length of any span for which

dynamic behaviour is important. This is required because the mass is always lumped at the

joints, even if it is contributed by the elements.

4.7 Mass Source

In seismic analysis, mass of the structure is considered rather than the weight. In SAP-

2000, by default it assumes self mass of the structure as mass for the seismic analysis. To assign

the correct mass, check the option from loads and add type of loads and its coefficients in the

drop down menu. For example in the present study mass source obtained from the load

combination DL (deal load, floor finish, wall load) +0.25 LL (live load) according to IS 1893.

Select Define > Mass source> Add New Mass Source> Set, Specified Load Patterns> Add >

OK.



Fig 4.6 Defining Mass Source

CHAPTER 5

PROBLEM DEFINITION



5.1 Introduction

An elevated Intze water tank is a large water storage container constructed for the purpose of

holding water supply at certain height to pressurization the water distribution system. Elevated

tanks are the structures frequently used in order to store fluid for not only drinking but also for

fire fighting.

Intze type tank

In the case of large diameter elevated circular tanks, thicker floor slabs are required

resulting in uneconomical designs. In such cases, Intze tank with conical and a bottom spherical

dome provides an economical solution. the proportions of the conical and the spherical bottom

domes are selected so that the outward thrust from the bottom dome balances the inward thrust

due to the conical domed part of the tank floor.

Structural elements of Intze tank:

The various structural elements of an Intze type tank are as follows

1. The top spherical dome

2. The top ring beam


4. Bottom ring beam

5. Conical dome



8. Tower with columns and braces

9. Foundations

5.2 Description of the model

An Intze shape water container of 1000 m3 capacity is supported on RC staging of 8 columns

with horizontal bracings of 500 x 500 mm at three levels. Details of staging configuration are

shown in figure below. Grade of concrete and steel are M25 and Fe415, respectively. Tank is



located on medium soil in seismic Zone IV. Density of concrete is 25kN/m3. Analyze the tank

for seismic loads.

Table 5.2.1: Structural data for frame type

Capacity of water tank 1000m3

Unit weight of concrete 25 kN/m3

Thickness of top dome 0.15m

Rise of top dome 2.2m

Size of top ring beam 0.35m x 0.35m

Diameter of tank 13.6m

Height of cylindrical wall 6.8m

Size of middle ring beam 1.2m x 0.6m

Rise of conical dome 2.35m

Rise of bottom dome 1.6m

Thickness of bottom dome shell 0.2m

Number of columns 8

Size of bottom ring beam 1m x 1.2m

Distance between intermediate bracing 4m

Height of sagging above foundation 16m

Diameter of columns 0.75m

Size of bracing 0.5m x 0.5m

Thickness of cylindrical wall 0.33m

Number of bracing level 3

Thickness of conical shell 0.5m



Fig 5.1 Elevation View- RC Intze water Tank

Fig 5.2 Plan View- Octagonal cross bracing



Fig 5.3 Octagonal normal and radial bracing (Plan)

Fig 5.4 FEM Model in SAP-2000 (3D-view)



CHAPTER 6

RESULTS AND DISCUSSION

The obtained results such as mode period, base shear, maximum displacement and acceleration

are summarized and tabulated as shown below.

6.1a Mode Period- Octagonal and normal bracing

Table 6.1: Mode Period for different fluid level condition- Octagonal and normal bracing

Fluid level condition

Mode period (sec)

Octagonal and Normal Bracing

Mode-1

(X-dir)

Mode-2

(Y-dir)

Mode-3

(Torsion)

Empty 1.032 1.032 0.8961

Full 2.059 2.059 1.648

Fig 6.1: Mode Period for different fluid level condition- Octagonal and normal bracing


(X-dir) (Y-dir) (Torsion)Mode-1 Mode-2 Mode-3

0

0.5

1

1.5

2

2.5

Octagonal and normal bracing

EmptyFull

mod

e pe

riod

(sec

)


In the modal analysis carried out: the mode period of structure are found to be 1.032 sec (mode-1

and mode-2), both in X and Y-directions and 0.8961 sec, in torsion mode (i.e. mode-3) for empty

condition (water tank) with Octagonal and normal bracing.

In the modal analysis carried out: the mode periods of structure are found to be 2.059 sec (mode-

1 and mode-2), both in X and Y-directions and 1.648 sec, in torsion mode (mode-3) for tank full

condition (water tank) with Octagonal and normal bracing.

From the observation of figure 6.1, it is clear that the increase in mode period was nearly 0% and

14% i.e. mode-2 and mode-3 compared to mode-1. The mode period in full water tank condition

is more than mode period in empty condition and the mode period goes on decreases from mode-

l to mode-3 in empty and full conditions. In each case the maximum mode period is observed for

tank full condition of the structure. This shows the time period (mode period) and mass of

structure (structure and water) are directly proportional to each other.

Empty Full0

0.5

1

1.5

2

2.5


Mode-1 and 2 (X-dir and Y-dir)

mod

e pe

riod

(sec

)

Fig 6.2: Mode Period for different fluid level condition- Octagonal and normal bracing (X

and Y direction).

From the observation of figure 6.2, it is clear that the increase in mode period was nearly 50%

i.e. tank full condition compared to empty condition. The mode period in full water tank

condition is more than mode period in empty condition and the mode period goes on increase

from empty condition to tank full condition.



Mode Shapes

Fig 6.4: Octagonal and normal bracing for empty condition- Mode-2 (Y-Y direction)



Fig 6.5: Octagonal and normal bracing for empty condition- Mode-3 (Torsion)



Fig 6.6: Octagonal and normal bracing for full condition- Mode-1 (X-X direction)



Fig 6.7: Octagonal and normal bracing for full condition- Mode-2 (Y-Y direction)



Fig 6.8: Octagonal and normal bracing for half full condition- Mode-3 (Torsion)



6.1b Mode Period- Octagonal and Cross Bracing

Table 6.2- Mode Period for different fluid level condition- Octagonal and Cross Bracing

Fluid level

condition

Mode period (sec)

Octagonal and Cross Bracing

Mode-1 Mode-2 Mode-3

(X-dir) (Y-dir) (Torsion)

Empty 0.981 0.981 0.8881

Full 1.935 1.935 1.621


0

0.5

1

1.5

2

2.5

Octagonal and cross bracing

EmptyFull

Mod

e pe

riod

(sec

)

Fig 6.9- Mode Period for different fluid level condition- Octagonal and Cross Bracing

In the modal analysis carried out: the mode period of structure are found to be 0.981 sec (mode-1

and mode-2), both in X and Y-directions and 0.8881 sec, in torsion mode (i.e. mode-3) for empty

condition (water tank) with Octagonal and cross bracing.






From the observation of figure 6.9, it is clear that the increase in mode period was nearly 0% and

10% i.e. mode-2 and mode-3 compared to mode-1. The mode period in full water tank condition

is more than mode period in empty condition and the mode period goes on decreases from mode-

l to mode-3 in empty and full conditions. In each case the maximum mode period is observed for

tank full condition of the structure. This shows the time period (mode period) and mass of

structure (structure and water) are directly proportional to each other.

Empty Full0

0.5

1

1.5

2

2.5


Mode-1 and 2 (X-dir and Y- dir)

mod

e pe

rios(

sec)

Fig 6.10- Mode Period for different fluid level condition- Octagonal and Cross Bracing (X

and Y direction)







Fig 6.11: Octagonal and cross bracing for empty condition- Mode-1 (X-X direction)



Fig 6.12: Octagonal and cross bracing for empty condition- Mode-2 (Y-Y direction)



Fig 6.13: Octagonal and cross bracing for empty condition- Mode-3 (Torsion)



Fig 6.14: Octagonal and cross bracing for full condition- Mode-1 (X-X direction)



Fig 6.15: Octagonal and cross bracing for full condition- Mode-2 (Y-Y direction)



Fig 6.16: Octagonal and cross bracing for full condition- Mode-3 (Torsion)



6.1c Mode Period- Octagonal and Radial Bracing

Table 6.3- Mode Period for different fluid level condition- Octagonal and Radial Bracing

Fluid level

condition

Mode period (sec)

Octagonal and Radial Bracing

Mode-1 Mode-2 Mode-3

(X-dir) (Y-dir) (Torsion)

Empty 0.9833 0.9833 0.895

Full 1.937 1.937 1.632


0

0.5

1

1.5

2

2.5

Octagonal and Radial bracing

EmptyFull

mod

e pe

riod

(sec

)

Fig 6.17- Mode Period for different fluid level condition- Octagonal and Radial Bracing

In the modal analysis carried out: the mode period of structure are found to be 0.9833 sec (mode-

1 and mode-2), both in X and Y-directions and 0.8950 sec, in torsion mode (i.e. mode-3) for

empty condition (water tank) with Octagonal and cross bracing.







and 9% i.e. mode-2 and mode-3 compared to mode-1. The mode period in full water tank

condition is more than mode period in empty condition and the mode period goes on decreases

from mode-l to mode-3 in empty and full conditions. In each case the maximum mode period is

observed for tank full condition of the structure. This shows the time period (mode period) and

mass of structure (structure and water) are directly proportional to each other.

Empty Full0

0.5

1

1.5

2

2.5

Octagonal and Radial bracing

Mode-1 and 2 (X-dir and Y-dir)

Axis Title

Mod

e pe

riod

(sec

)

Fig 6.18- Mode Period for different fluid level condition- Octagonal and Radial Bracing

(X and Y direction)







Fig 6.19: Octagonal and radial bracing for empty condition- Mode-1 (X-X direction)



Fig 6.20: Octagonal and radial bracing for empty condition- Mode-2 (Y-Y direction)



Fig 6.21: Octagonal and radial bracing for empty condition- Mode-3 (Torsion)



Fig 6.22: Octagonal and radial bracing for full condition- Mode-1 (X-X direction)



Fig 6.23: Octagonal and radial bracing for full condition- Mode-2 (Y-Y direction)



Fig 6.24: Octagonal and radial bracing for full condition- Mode-3 (Torsion)

6.2 Base shear



Table 6.4- Base shear for different fluid level condition

Fluid Level

Condition

Base shear (kN)

Bracing types

Octagonal and normal

bracing

Octagonal and cross

bracing

Octagonal and radial

bracing

Empty 610.72 694.70 697.54

Full 1099.48 1196.41 1197.07

Empty Full0

200

400

600

800

1000

1200


Base

She

ar(k

N)

Fig 6.25: Base shear for different fluid level condition- Octagonal and normal bracing

From figure 6.25, it is observed that the increase in base shear in tank full condition was nearly

45% i.e. Octagonal and normal bracing compared to empty condition.



Empty Full0

200

400

600

800

1000

1200

1400


Base

She

ar(k

N)

Fig 6.26: Base shear for different fluid level condition- Octagonal and cross bracing


42% i.e. Octagonal and cross bracing compared to empty condition.

Empty Full0

200

400

600

800

1000

1200

1400

Octagonal and radial bracing

Base

She

ar(k

N)

Fig 6.27: Base shear for different fluid level condition- Octagonal and radial bracing


41% i.e. Octagonal and radial bracing compared to empty condition.






0

200

400

600

800

1000

1200

1400

EmptyFull

Base

She

ar(k

N)

Fig 6.28: Base shear for different fluid level condition



The increase in base shear in tank full condition was nearly 42% i.e. Octagonal and cross bracing

compared to empty condition.

The increase in base shear in tank full condition was nearly 41% i.e. Octagonal and radial

bracing compared to empty condition.

The base shear force is more in tank full condition in all the 3 cases (i.e. Octagonal and normal

bracing, Octagonal with cross bracing and Octagonal with radial bracing) compared to empty

condition. This shows mass of water participates more in tank full condition.



6.3 Maximum displacement

Table 6.5: Maximum displacement for different fluid level condition

Fluid Level

Condition

Maximum Displacement(mm)

Bracing types

Octagonal and

normal bracing

Octagonal and cross

bracing

Octagonal and

radial bracing

Empty 22.5 22.4 22.1

Full 39.7 38.1 38.0

Empty Full0

5

10

15

20

25

30

35

40

45


Max

imum

Dis

plac

emen

t(m

m)

Fig 6.29: Maximum displacement for different fluid level condition- Octagonal and normal

bracing

From figure 6.29, it is observed that the increase in maximum displacement in tank full condition

was nearly 44% i.e. Octagonal and normal bracing compared to empty condition.



Empty Full0

5

10

15

20

25

30

35

40

45


Max

imum

Dis

plac

emen

t(m

m)

Fig 6.30: Maximum displacement for different fluid level condition- Octagonal and cross

bracing


was nearly 41% i.e. Octagonal and cross bracing compared to empty condition.

Empty Full0

5

10

15

20

25

30

35

40


Max

imum

Dis

plac

emen

t(m

m)

Fig 6.31: Maximum displacement for different fluid level condition- Octagonal and radial

bracing


was nearly 42% i.e. Octagonal and radial bracing compared to empty condition.






0

5

10

15

20

25

30

35

40

45

EmptyFull

Max

imum

Dis

plac

emen

t(m

m)

Fig 6.32: Maximum displacement for different fluid level condition


was nearly 44% i.e. Octagonal and normal bracing compared to empty condition.

The increase in maximum displacement in tank full condition was nearly 41% i.e. Octagonal and

cross bracing compared to empty condition.

The increase in maximum displacement in tank full condition was nearly 42% i.e. Octagonal and

radial bracing compared to empty condition.

The maximum displacement is more in tank full condition in all the 3 cases (i.e. Octagonal and

normal bracing, Octagonal with cross bracing and Octagonal with radial bracing) compared to

empty condition. This shows mass of water participates more in tank full condition.

6.4 Acceleration



Table 6.6- Acceleration for different fluid level condition-Response spectrum method

Fluid Level

Condition

Acceleration(m/sec2)

Bracing types

Octagonal and

normal bracing

Octagonal and cross

bracing

Octagonal and radial

bracing

Empty 1.027 1.120 1.127

Full 1.00 1.035 1.0351

Empty Full0.985

0.99

0.995

1

1.005

1.01

1.015

1.02

1.025

1.03


Acc

eler

ation

(m/s

ec2)

Fig 6.33: Acceleration for different fluid level condition- Octagonal and normal bracing

From figure 6.33, it is observed that the decrease in acceleration in tank full condition was nearly




Empty Full0.98

1

1.02

1.04

1.06

1.08

1.1

1.12

1.14


Acc

eler

ation

(m/s

ec2)

Fig 6.34: Acceleration for different fluid level condition- Octagonal and cross bracing


8% i.e. Octagonal and cross bracing compared to empty condition.

Empty Full0.98

1

1.02

1.04

1.06

1.08

1.1

1.12

1.14


Acc

eler

ation

(m/s

ec2)

Fig 6.35: Acceleration for different fluid level condition- Octagonal and radial bracing

From figure 6.35, it is observed that the decrease in acceleration in tank full condition was nearly 9% i.e. Octagonal and radial bracing compared to empty condition.






0.9

0.95

1

1.05

1.1

1.15

EmptyFull

Acc

eler

ation

(m/s

ec2)

Fig 6.36: Acceleration for different fluid level condition



The decrease in acceleration in tank full condition was nearly 8% i.e. Octagonal and cross


The decrease in acceleration in tank full condition was nearly 9% i.e. Octagonal and radial


The acceleration is less in tank full condition in all the 3 cases (i.e. Octagonal and normal

bracing, Octagonal with cross bracing and Octagonal with radial bracing) compared to empty

condition. This shows mass participation factor more in tank full condition (i.e. mass is inversely

proportional to acceleration).



CHAPTER 7

CONCLUSION

The response quantity including mode period, base shear and acceleration are accessed

under the earthquake exestuation. The seismic responses of tank have been determined

using equivalent static force method and response spectrum analysis in two cases (i.e.

empty and full). Based on the obtained results and their analysis the following

conclusions are drawn.

1. The critical response quantities of filled elevated tanks is less than empty elevated

tanks.

2. The elevated intze tank showed that increase in mass of leads to increase in natural

period.

3. The mode period of octagonal tank with normal bracings has higher value compared to

octagonal with radial cross bracings because the stiffness participation factor is less in

normal bracing compare to all other cases (i.e. stiffness inversely proportional to time

period).

4. Base shear increase as bracing level increases irrespective of types of bracings. Base

shear is more for octagonal tank for tank full condition than empty tank condition

compared to tank with other types of bracings.

5. The octagonal with crossed type bracings gives less storey acceleration as compared to

other bracing types (octagonal with radial and normal bracing) this shows that increase in

stiffness leads increase in acceleration values noticed in full tank condition



CHAPTER 8

REFERENCES

1. Ayazhussain M. Jabar and H. S. Patel “Seismic behaviour of RC elevated water tank

under different staging pattern and earthquake characteristic” International Journal of

Advanced Engineering Research and Studies, Vol. I, April-June, 2012, pp.293-296

2. H. Shakib, F. Omidinasab and M.T. Ahmadi “Seismic Demand Evaluation of Elevated

Reinforced Concrete Water Tanks”, International Journal of Civil Engineering. Vol. 8,

No. 3, September 2010, pp 204 – 220.

3. Asari Falguni P and Prof. M.G. Vanza, “Structural control system for elevated water tank

, International Journal of Advanced Engineering Research and Studies, Vol. I, Issue III,

April-June, 2012,pp325-328

4. Syed saifuddin “Seismic analysis of liquid storage tanks” , International journals of

advanced trends in computer science and engineering, vol.2 ,No. 1, January 2013

Pages :357-362

5. S.Bozorgmehrnia ,M.M.Ranjbar and R.Madandoust, “Seismic behavior Assessment of

concrete elevated water tank, Journal of rehabilation in civil engineering 1-2 ,2013

6. M.V. Waghmare and S.N.Madhekar “Behaviour of elevated water tank under sloshing

effect” , International Journal of Advanced technology in civil Engineering

7. Gaikwad Madhurar V and Prof. Mangulkar Madhuri N “Comparison between static and

dynamic analysis of elevated water tank”,International journal of scientific and

engineering research ,vol. 4.issue 6, june 2013.

8. Draft IS: 1893 (Part-II, Liquid Retaining Tanks) “Criteria for Earthquake Resistant

Design of Structures”, Bureau of Indian standards, New Delhi, India.



9. IS 456:2000 – “Code of practice for plain and reinforced concrete, bureau of Indian standards, New Delhi”.


dynamic behavior

Documents

elevated water tanks

storage of water

water tank design

base of water tank

elevated tanks

supply of drinking water

drinking water supply

assured water supply