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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]
International Journal of Advanced Engineering Research and Studies E-ISSN2249 – 8974
IJAERS/Vol. I/ Issue I/October-December, 2011/78-87
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.
International Journal of Advanced Engineering Research and Studies E-ISSN2249 – 8974
IJAERS/Vol. I/ Issue I/October-December, 2011/78-87
Figure 5: Two mass idealization of elevated tank [IITK- GSDMA, 2005]
Figure 6: Seismic zone map of India [IS 1893 (Part 1), 2002]
International Journal of Advanced Engineering Research and Studies E-ISSN2249 – 8974
IJAERS/Vol. I/ Issue I/October-December, 2011/78-87
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
International Journal of Advanced Engineering Research and Studies E-ISSN2249 – 8974
IJAERS/Vol. I/ Issue I/October-December, 2011/78-87
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
International Journal of Advanced Engineering Research and Studies E-ISSN2249 – 8974
IJAERS/Vol. I/ Issue I/October-December, 2011/78-87
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
International Journal of Advanced Engineering Research and Studies E-ISSN2249 – 8974
IJAERS/Vol. I/ Issue I/October-December, 2011/78-87
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
International Journal of Advanced Engineering Research and Studies E-ISSN2249 – 8974
IJAERS/Vol. I/ Issue I/October-December, 2011/78-87
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
International Journal of Advanced Engineering Research and Studies E-ISSN2249 – 8974
IJAERS/Vol. I/ Issue I/October-December, 2011/78-87
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
International Journal of Advanced Engineering Research and Studies E-ISSN2249 – 8974
IJAERS/Vol. I/ Issue I/October-December, 2011/78-87
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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