research article seismic performance of elevated … i/ijaers vol i issue i... · this paper...

International Journal of Advanced Engineering Research and Studies E-ISSN2249 – 8974

IJAERS/Vol. I/ Issue I/October-December, 2011/78-87

Research Article

SEISMIC PERFORMANCE OF ELEVATED WATER TANKS Dr. Suchita Hirde

1, Ms. Asmita Bajare

2, Dr. Manoj Hedaoo

3

Address for Correspondence 1Professor in Applied Mechanics Dept., Govt. College of Engineering, Karad, 415124, Dist. Satara (M.S).

2Assistant Engineer Grade I, Bridge Design Unit, Nagpur, Maharashtra, India

3Associate Professor in Civil Engineering, Govt. College of Engineering, Karad, 415124, Dist. Satara (M.S)

ABSTRACT Elevated water tanks are one of the most important lifeline structures in earthquake prone regions. In major cities and also in

rural areas elevated water tanks forms an integral part of water supply scheme. These structures has large mass concentrated

at the top of slender supporting structure hence these structures are especially vulnerable to horizontal forces due to

earthquake. Elevated water tanks that are inadequately analyzed and designed have suffered extensive damage during past

earthquakes. The elevated water tanks must remain functional even after the earthquakes as water tanks are required to

provide water for drinking and fire fighting purpose. Hence it is important to check the severity of these forces for particular

region. This paper presents the study of seismic performance of the elevated water tanks for various seismic zones of India

for various heights and capacity of elevated water tanks for different soil conditions. The effect of height of water tank,

earthquake zones and soil conditions on earthquake forces have been presented in this paper with the help of analysis of 240

models for various parameters.

KEY WORDS: Earthquake effects, elevated water tank, seismic analysis.

INTRODUCTION

Indian sub-continent is highly vulnerable to natural

disasters like earthquakes, draughts, floods, cyclones

etc. Majority of states or union territories are prone to

one or multiple disasters. These natural calamities are

causing many casualties and innumerable property

loss every year. Earthquakes occupy first place in

vulnerability. Hence, it is necessary to learn to live

with these events. According to seismic code IS:

1893(Part I):2002, more than 60% of India is prone

to earthquakes. After an earthquake, property loss can

be recovered to some extent however, the life loss

cannot. The main reason for life loss is collapse of

structures. It is said that earthquake itself never kills

people; it is badly constructed structures that kill.

Hence it is important to analyze the structure

properly for earthquake effects.

WATER TANK

Water supply is a life line facility that must remain

functional following disaster. Most municipalities in

India have water supply system which depends on

elevated tanks for storage. Elevated water tank is a

large elevated water storage container constructed for

the purpose of holding a water supply at a height

sufficient to pressurize a water distribution system. In

major cities the main supply scheme is augmented by

individual supply systems of institutions and

industrial estates for which elevated tanks are an

integral part. These structures have a configuration

that is especially vulnerable to horizontal forces like

earthquake due to the large total mass concentrated at

the top of slender supporting structure. So it is

important to check the severity of these forces for

particular region.

DAMAGES TO ELEVATED WATER TANK

DURING PAST EARTHQUAKES

Water supply is essential for controlling fires that

may occur during earthquakes, which cause a great

deal of damage and loss of lives. Therefore, elevated

tanks should remain functional in the post-earthquake

period to ensure water supply is available in

earthquake-affected regions. Nevertheless, several

elevated tanks were damaged or collapsed during past

earthquakes. Figure 1 shows the collapsed slender

and weak framed staging of water tank in Manfera

village.

Figure 1: Collapsed water tank in Manfera village

[Rai, 2003]

Figure 2 shows the water tank in Bhachau pulled

down due to severe damage in staging. Brace and

column members of tanks in Manfera and Bhachau

do not meet the ductility and toughness requirements

for earthquake resistance.

Figure 2: Collapsed water tank in Bhachau

[Rai, 2003]



Figure 3 shows the tank of capacity 1 lakh British

gallons on staging height of 18.3 m which was

damaged in Bihar Nepal earthquake (magnitude 6.7)

of August 1988. The tank was located at Khagaria in

Bihar which is at about 100 km from the epicenter.

Figure 3: Water tank damaged at Khagaria

[Sajjad and Jain, 1993]

Figure 4 shows the frame-supported elevated water

tank of Kautha collapsed straight down into its

crumpled supports in Killari earthquake, 1993.

Figure 4: Frame-supported elevated water tank of

Kautha [Ramancharla, 2003]

The study of damage histories revealed

damage/failure of reinforced concrete elevated water

tanks of low to high capacity. Elevated water tank is

a very important lifeline facility and damage of the

same often results in significant hardships even after

the occurrence of the disaster, claiming human

casualties and economic loss to built environment.

Investigating the effects of earthquakes has been

recognized as a necessary step to understand the

natural hazards and its risk to the society in the long

run. Most water supply systems in developing

countries, such as India, depend on reinforced cement

concrete elevated water tanks. The strength of these

tanks against lateral forces, such as those caused by

earthquakes, needs special attention. It is very

important to analyze reinforced cement concrete

elevated water tank properly. Therefore in this paper

an attempt has been made to study the performance

of elevated water tank for earthquake forces for

different height and capacity of the elevated water

tank. The study of effect of height and capacity of

elevated water tanks on earthquake forces for

different zones and different soil conditions has been

presented in this paper with the help of analysis of

240 models of water tank. SEPL Esr-Gsr software

has been used to analyze the elevated water tank.

MODELING AND ANALYSIS OF ELEVATED

WATER TANK FOR EARTHQUAKE

Two mass model for elevated tank was proposed by

Housner [Housner, 1963] which is more appropriate

and is being commonly used in most of the

international codes. The pressure generated within

the fluid due to the dynamic motion of the tank can

be separated into impulsive and convective parts.

Impulsive liquid mass-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.

Convective liquid mass- 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. Thus, total liquid mass of elevated water tank

shown in Figure 5 (a) gets divided into two parts, i.e.,

impulsive mass and convective mass. In spring mass

model of tank-liquid system, these two liquid masses

are to be suitably represented as shown in Figure 5

(b).

Structural mass ms, includes mass of container and

one-third mass of staging. Mass of container

comprises of mass of roof slab, container wall,

gallery, floor slab, and floor beams. Staging acts like

a lateral spring and one-third mass of staging is

considered based on classical result on effect of

spring mass on natural frequency of single degree of

freedom system. Most elevated tanks are never

completely filled with liquid. Hence a two-mass

idealization of the tank shown in Figure 5 (c) is more

appropriate as compared to a one mass idealization,

which was used in IS 1893: 1984.

The response of the two-degree of freedom system

can be obtained by elementary structural dynamics.

However, for most elevated tanks it is observed that

the two periods are well separated. Hence, the system

may be considered as two uncoupled single degree of

freedom systems as shown in Figure 5 (d). This

method will be satisfactory for design purpose, if the

ratio of the period of the two uncoupled systems

exceeds 2.5. If impulsive and convective time periods

are not well separated, then coupled 2-DOF system

will have to be solved using elementary structural

dynamics. There are two cases for seismic analysis

namely tank empty condition and tank full condition.

For tank empty condition, tank will be considered as

single degree of freedom system and empty tank will

not have convective mode of vibration whereas tank

full condition is considered as two degree of freedom

system.



Figure 5: Two mass idealization of elevated tank [IITK- GSDMA, 2005]

Figure 6: Seismic zone map of India [IS 1893 (Part 1), 2002]



The important factors that affect the magnitude of

earthquake forces are-

(a) Seismic zone factor, Z

India has been divided into four seismic zones as per

IS 1893 (Part 1): 2002 for the Maximum Considered

Earthquake (MCE) and service life of the structure in

a zone. Different zone have different zone factor.

Figure 6 shows seismic zone map of India. India is

divided into four seismic zones. There are three types

of soil considered by IS 1893 (Part 1): 2002 i.e. soft

medium and hard soil.

(b) Importance factor, I

Importance factor depends upon the functional use of

the structures, characterized by .hazardous

consequences of its failure, post-earthquake

functional needs, historical value, or economic

importance. Elevated water tanks are used for storing

potable water and intended for emergency services

such as fire fighting services and are of post

earthquake importance. So importance factor is 1.5

for elevated water tank.

(c) Response reduction factor, R

Response reduction factor depends on the perceived

seismic damage performance of the structure,

characterized by ductile or brittle deformations. R

values of tanks are less than building since tanks are

generally less ductile and have low redundancy as

compared to building. For frame confirming to

ductile detailing i.e. special moment resisting frame

(SMRF), R value is 2.5.

(d) Structural response factor, (Sa/g)

It is a factor denoting acceleration response spectrum

of the structure subjected to earthquake ground

vibrations, and depends on natural period of vibration

and damping of the structure.

STUDY PARAMETERS

In this paper, the study is carried out on reinforced

cement concrete circular elevated water tanks which

are commonly used in practice. Grade of concrete

and steel used are M25 and Fe415. In the analysis

special moment resisting frame (SMRF) are

considered. Elevated water tanks having 50,000 liter

and 1,00,000 liter capacity with staging heights of 12

m,16 m, 20 m, 24 m and 28 m considering 4 m height

of each panel are considered for study. For 50,000

liter capacity reinforced cement concrete elevated

water tank for tank full condition 12 m, 16 m, 20 m,

24 m, 28 m staging heights are considered as shown

in Figure 7. For each staging height three soil

conditions are considered i.e. soft, medium and hard

soil condition and for each soil condition, four zones

are considered. Total sixty models are studied for

50,000 liter capacity reinforced cement concrete

elevated water tank for tank full condition and sixty

models are studied for 50,000 liter capacity

reinforced cement concrete elevated water tank for

tank empty condition, i.e. total 120 models are

prepared for 50,000 liter capacity reinforced cement

concrete elevated water tank. Same 120 models are

studied for 1,00,000 liter capacity reinforced cement

concrete elevated water tank. So in all study of 240

models have been presented in this paper. Other

relevant data is tabulated in Table 1

Figure 7: Models for earthquake analysis

Table 1: Data of elevated water tanks for analysis Capacity 50,000 liter 1,00,000 liter

Dia. Of container 4.65 m 5.89 m

Depth of water in container 3.0 m 4.0 m

Free board 0.3 m 0.3 m

Roof slab 120 mm 140 mm

Bottom slab 200 mm 270 mm

Bottom beam 250 x 600 mm 300 x 700 mm

Wall 200 mm 200 mm

Bracing 300 x 450 mm 250 x 350 mm

column 4 nos.- 450 mm dia. 4 nos.- 500 mm dia

Depth of footing below ground level 2.0 m 3.0 m

c/c distance between column 3.43 m 4.31 m



RESULTS AND DISCUSSIONS

In this paper an attempt is made to study the seismic

performance of the elevated water tanks. For all the

above mentioned 240 water tanks, analysis has been

carried out by using Esr-Gsr software. Earthquake

analysis is carried out for different soil conditions

and different earthquake zones. Tank empty and tank

full conditions are considered for earthquake

analysis. The main objective of this paper was to

study the effect on seismic forces on reinforced

cement concrete elevated water tank in seismic zones

II, III, IV and V for soft, medium and hard soil

conditions for different capacities and heights of the

elevated water tank.

Effect of earthquake zone on earthquake forces:

The results obtained from analysis are analyzed and

shown in graphical form. To study the effect of

earthquake zones on earthquake forces, graphs are

plotted taking staging height as abscissa and the

earthquake forces as ordinate for 50,000 liter and

1,00,000 liter capacity elevated water tank.

Earthquake forces for different staging height for

soft, medium and hard soil conditions for tank empty

and tank full condition are shown in Figure 8 to

Figure 13. From these graphs, it is observed that,

earthquake forces decreases with increase in staging

height and increases with increase in zone factor for

soft, medium and hard soil conditions for tank empty

and tank full condition. Earthquake forces for zone II

is about 37-38% less than zone III, about 58-59% less

than zone IV and about 72-73% less than zone V.

Earthquake forces for zone III is about 33-34% less

than zone IV and about 55-56% less than zone V.

Earthquake forces for zone IV is about 33-34% less

than zone V. Earthquake forces increases from zone I

to zone V for soft, medium and hard soil conditions

for tank empty and tank full condition. Since zone

factor value increases from zone I to zone V,

earthquake forces increases in that order. Earthquake

forces for tank full condition are about 21-28%

greater than that of tank empty condition. Hence tank

full condition is more severe as compared to tank

empty condition.

Effect of staging height on earthquake forces

Earthquake forces decreases with increase in staging

height because as staging height increases the

structure becomes more flexible. Therefore time

period increases due to which structural response

factor decreases from lower to higher staging height.

This affect the earthquake forces.

Effect of type of soil on earthquake forces

Graphs are plotted taking staging height as abscissa

and the forces as ordinate for reinforced cement

concrete elevated tanks of 50,000 liter and 1,00,000

capacity to study the effect of soil type on earthquake

forces. The effect of soil condition on earthquake

forces is shown in figure 14 to figure 21. From these

graphs it is observed that, earthquake forces

decreases with increase in staging height. Earthquake

forces for soft soil is about 18-19% greater than that

of medium soil, Earthquake forces for medium soil is

about 26-27% greater than that of hard soil,

Earthquake forces for soft soil is about 40-41%

greater than that of hard soil for all earthquake zones

and tank full and tank empty condition. The

responses for soft soil are more because of structural

response factor (Sa/g). Since this value is more for

soft soil as compared to medium and hard soil; soft

soil condition is more severe than medium soil

condition and hard soil condition and medium soil

condition is more severe than hard soil condition.

Table 2 gives time period for elevated water tanks of

50,000 liter and 1,00,000 liter capacity having

different staging heights for tank empty and tank full

condition.

Figure 8: Earthquake forces for tank empty condition for soft soil



Figure 9: Earthquake forces for tank full condition for soft soil

Figure 10: Earthquake forces for tank empty condition for medium soil

Figure 11: Earthquake forces for tank full condition for medium soil



Figure 12: Earthquake forces for tank empty condition for hard soil

Figure 13: Earthquake forces for tank full condition for hard soil

Figure 14: Earthquake forces for tank empty condition for zone II



Figure 15: Earthquake forces for tank full condition for zone II

Figure 16: Earthquake forces for tank empty condition for zone III

Figure 17: Earthquake forces for tank full condition for zone III



Figure 18: Earthquake forces for tank empty condition for zone IV

Figure 19: Earthquake forces for tank full condition for zone IV

Figure 20: Earthquake forces for tank empty condition for zone V



Figure 21: Earthquake forces for tank full condition for zone V

Table 2: Time period for elevated water tanks of 50,000 liter and 1,00,000 liter capacity having different

staging heights for tank empty and tank full condition.

Time Period for tank

empty condition

(sec)

Impulsive Time Period for

tank full condition

(sec)

Convective Time Period

for tank full condition

(sec)

Staging

height

50 liter 100 liter 50 liter 100 liter 50 liter 100 liter

12m 0.708 1.163 0.875 1.52 2.274 2.55

16m 0.874 1.437 1.071 1.866 2.274 2.55

20m 1.043 1.691 1.267 2.182 2.274 2.55

24m 1.217 1.936 1.467 2.48 2.274 2.55

28m 1.4 2.175 1.674 2.77 2.274 2.55

Present study will be useful to Civil Engineers to

understand the behaviour of elevated water tank for

various staging height and also to get the feel of

effect of earthquake zones of India and soil

conditions on earthquake forces.

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research article seismic performance of elevated … i/ijaers vol i issue i... · this paper...

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