International Journal of Engineering Trends and Technology (IJETT) – Volume 21 Number 9 – March 2015
Behaviour of an elevated RC tank subjected to
various Earthquake responses
J. Yogeshwarana, C.Pavithrab
ab
Department of Civil Engineering,SRM university, Kattankulathur, India
Abstract— The main focus of this study is to evaluate the
performance of the elevated concrete tanks with frame staging
along with seismic behaviour of these construction types.
Normally in the design of the elevated tanks , the response of the
liquid with that of the tank structures is neglected. In this paper,
the spring mass model approach and the effect of hydrodynamic
pressure on tank wall are considered. An elevated concrete tank
with 900 cubic meters capacity, exposed to a pair of earthquake
records have been studied and analysed in time history analysis
using STAADpro.
Keywords— hydrodynamic forces, elevated tank, sloshing, spring
mass, base shear.
I. INTRODUCTION
The need for water storage tanks are considered as
important all over the world. They play the vital role in water
supply system and fire fighting systems. These water tanks are
placed over a supporting tower to acquire the necessary power
through gravity instead of heavy pumping. In the past
earthquakes, the failure of these lifeline services post
earthquake has caused some serious damages by water
shortcomings. Many studies have been conducted analysing
and investigating the behaviour of elevated tanks, some are
summarised below. Soheil Soroushnia and Sh. Tavoushi
Tafreshi F. Omidinasab, N. Beheshtian, Sajad Soroushnia[1]
determined which failure modes of shear forces in beam and
also ,the failure mode of axial force dominant in the structure.
S. Bozorgmehrnia, M.M. Ranjbar and R. Madandoust [2]
carried out a finite-element analysis of structure when exposed
to various earthquake records. Mostafa Masoudi, Sassan
Eshghi, Mohsen. G. Ashtiany [3] studied the response of both
shaft and frame stagings of elevated tanks. Moslemi , M.R.
Kianoush and W. Pogorzelski [4] verified the validity of the
current practice in estimating the seismic response of liquid
filled elevated water tanks. S.C. Dutta, S.K.Jain and C.V.R.
Murty [6] studied the in elastic behavior of elevated concrete
tanks to observe the strength deterioration under cyclic
loading. S.C. Dutta, S.K.Jain and C.V.R. Murty [7] proposed a
systematic approach for checking the torsional vulnerability of
tanks and choosing a suitable staging configuration. F.
Omidinasab and H. Shakib [8] showed that the difference in
tank’s sloshing period with that of structure’s main period
leads to displacement in joining place of frame to the tank.
staging. The height of the staging is 20m from the ground.
The inner dimensions of the tank are opted as 16 m in length,
12 m in breadth and 5.3 m including freeboard with a water
filling up to 4.7 m. The thickness of the tank wall is computed
as 0.3 m .
The roof slab is modelled to enclose the tank including the
wall with a thickness of 0.15 m. The base slab is having a
thickness of 0.35 m. The tank is supported on an RC staging
with beam dimensions 0.5 m in depth and 0.35 m in width.
The columns are square in shape with a dimension of 0.5 m.
Typical plan and elevation of tank is shown in Fig 1.
Fig. 1. Typical layout of the tank
III. PARAMETRIC STUDY
A detailed study on the behaviour of an elevated tank with
respect to seismic loading is analysed earthquake records of
bhuj and elcentro. These are conducted including the fluid
interaction with that of the structure when earthquake occurs.
A. Hydrodynamic pressure generation
The following depictions of hydrodynamic pressure
calculations are carried out as per IS 1893 (Part II): 2002
provisions. The elevated tank is modelled as a Spring mass
model. They are given as lumped masses namely impulsive
and convective. Impulsive pressure is the pressure generated
inside the tank due to the acceleration of the tank by
earthquake , where as in convective pressure the pressure
generated when water itself is excited into oscillations. This is
illustrated in the Fig2.
II. ELEVATED TANKS CHARACTERSTICS
A rectangular reinforced concrete elevated tank has been
considered of volume 900 m3 which is supported on a frame
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International Journal of Engineering Trends and Technology (IJETT) – Volume 21 Number 9 – March 2015
liquid and gravity acceleration. The results computed are
shown in Table1.
Table. 1. Impulsive and Convective pressure for the
respective tank geometry is given below.
Water
filling
level
Impulsive
pressure
kN/m2
Convective pressure kN/m2
At top
At bottom
4.7m
0.630387
0.7720
0.52764
2.7m
0.374945
0.57239
0.50005
Fig. 2(a). Impulsive Pressure
B. Modelling
STAAD Pro software is used to model the elevated tank
system. The columns and beams are modelled as frame
systems. A container walls and slabs are modelled as
quadrilateral plate elements. The cross section of the tank
model is given below in Fig 4.
Fig. 2(b). Convective Pressure
The frame staging which acts as the supporting tower for
the concrete tank is checked for its rigidity against a load of
10kN. The load is applied to the frame through a link which is
center of gravity of the tank,2.66m. the stiffness is calculated
from the STAADpro model of the RC frame staging as shown
below in Fig3.
Fig. 3. Stiffness calculation from STAADpro
Fig.4. STAAD Pro model of an elevated tank
Apart from the downward pressure of the water, added
pressure which is calculated as spring mass model is also
considered. The parameters of hydrodynamic pressure
generated are stated in table 2. An empty tank is also analysed,
which doesn’t have any effect due to hydrodynamic pressure.
An illustration of Hydrodynamic forces assigned on the
inside of the tank in STAAD Pro model is shown below in Fig
5.
Title The hydrodynamic pressure calculation is based on
the dimension of the tank, density of the liquid, quantity of the
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Page 441
Displacement
(mm)
International Journal of Engineering Trends and Technology (IJETT) – Volume 21 Number 9 – March 2015
Time (sec)
Fig.6(a).Time history of floor displacement under El centro earthquake in
empty filled condition
Displacement (mm)
Fig.5(a). Impulsive forces assignment on walls and base slab
Time (sec)
Fig.6(b).Time history of floor displacement under Bhuj earthquake in empty
Fig.5(b). Convective forces assignment on walls and base slab
A. Base shear
The base shear forces for the corresponding filling
parameters show that the maximum base shear occurs under
full filled condition. The difference in base shear for different
filling parameter is not the same for both the earthquakes. The
difference in base shear is shown in table 2.
Table.2. Comparison of base shear from analysis
Filling Parameter
El-Centro(kN)
Bhuj(kN)
4.7 m
6255.607
3141.17
2.7 m
4540.31
2140.162
0m
3993.45
1708.59
Time (sec)
Fig.6(c).Time history of floor displacement under El centro earthquake in half
filled condition
Displacement (mm)
IV.
RESULTS OF ANALYSIS
The responses of the tank for different filling parameters of
the elevated tanks subjected to a pair of earthquake records is
studied. The results obtained time history analysis for each
parameter is presented below.
Displacement (mm)
filled condition
Time (sec)
Fig.6(d).Time history of floor displacement under Bhuj earthquake in half
B. Tank displacements
A visual comparison of the roof and floor displacement of
the tank for various filling conditions of the tank is shown
below in Fig 6.
Displacement (mm)
filled condition
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Time (sec)
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International Journal of Engineering Trends and Technology (IJETT) – Volume 21 Number 9 – March 2015
Fig.6(e).Time history of floor displacement under El centro earthquake in Full
250
filled condition.
210
Time (sec)
Fig.6(f).Time history of floor displacement under Bhuj earthquake in Full
filled condition.
The variation in floor displacements for different filling
parameters among both the earthquakes is shown in Fig 7.
Similarly the time history tank roof displacement is shown in
Fig 8.
Floor displacement (mm)
Displacement (mm)
200
149
150
132
105
100
69.8
54.9
El centro
50
Bhuj
0
0m
250
2.7m
4.7m
Filling parameter
Floor displacement (mm)
207
200
Fig.8. Roof displacements from analysis.
147
150
C. Tank wall stresses
130
104
100
68.9
54.2
El
centro
Bhuj
50
The stresses developed on the tank walls due to
hydrodynamic forces accelerated from liquid is shown below
in Fig 9. The variation in tank wall stresses with respect to
different filling cases is also contrasted in it.
0
0m
2.7m
4.7m
Filling parameter
Fig.7. Floor displacements from analysis.
Fig.9(a).Wall Stress including hydrodynamic forces for liquid filled
upto 4.7m-El centro
Fig.9(b).Wall Stress including hydrodynamic forces for liquid filled
upto 4.7m-Bhuj
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International Journal of Engineering Trends and Technology (IJETT) – Volume 21 Number 9 – March 2015
D. The stresses developed on the tank walls are dependent
on the intensity of the earthquake, liquid fill at the time of
earthquake.
Elevated tank responses for the various earthquake gives
the seismic behavior and the areas where critical failure may
occur.
REFERENCES
[1]
[2]
Fig.9(c).Wall Stress including hydrodynamic forces for liquid filled
upto 2.7m-El centro
[3]
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[8]
Fig.9(d).Wall Stress including hydrodynamic forces for liquid filled upto
2.7m-Bhuj
V. CONCLUSIONS
The results reveal the performance of the elevated tank for
various earthquake responses. The analysis includes the effect
of the sloshing liquid inside the tank and pressure generated
by it. The following conclusions are made:
A. The hydrodynamic pressure generated from the liquid
excitation has significant impact on the walls and base
slab of the structure of the tank.
B. The maximum base shear varies along with percentage of
filling. Under fully filled condition, the free board
provided gives the enough space for the water to oscillate.
C. The variation in roof and floor displacement shows the
need for the wall to be designed as earthquake resistant to
ensure failure under these circumstances.
ISSN: 2231-5381
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