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Seismic Strengthening of Intze Type RC Elevated Water Tank by Jacketing: A
Pushover Approach Hariram Rimal
Department of Earthquake Enginering
IOE Thapathali Campus
Kathmandu, Nepal [email protected]
Abstract— Earthquake is one of the natural
calamities that produces the vibration to the ground
which cause to produce most destructive forces on
earth, and have potential to cause damage to lives and
lifeline facilities. Elevated storage reservoir need to be
functional even after the major earthquake event.
During past earthquakes elevated storage tank
experience damage or collapse all over the world.
This paper presents nonlinear static analysis
(Pushover Analysis) to evaluate seismic demand for
600 m3 capacity of intze type elevated water tank at
zone V, soil type III in reservoir full, reservoir 60%
full and reservoir empty condition. This gives the
plastic hinge formation and plots the total base shear
verses top displacement curve, which is known as
‘capacity curve’ of the structure. The analysis is
performed using CSI SAP2000 V17 software package
to evaluate base shear demand and performance point.
The initial structure consisting of inadequate frame
staging system and study is focused on column
jacketing and braces replacement technique of retrofit
and then base shears, fundamental time periods and
performance points are compared. Final structure
after retrofit is safe in major seismic event as it have
performance point base shear greater than design base
shear and all the column and braces hinges are at
Immediate Occupancy (IO) performance level at
Design Base Earthquake (DBE).
Keywords— Column Jacketing, Capacity Curve,
Performance Point, Pushover analysis, DBE, Intze
type Elevated water tank.
1. INTRODUCTION
Earthquake is known to produce one of the most
destructive forces on earth. It cause damage to man-
made structures, like Buildings, Chimneys, Towers
and Public Infrastructures like, Bridge, Roads, Dams
and Irrigation structures, Water supply and
Sewerage systems, Telecommunications systems,
Power Plants Industries, Life line systems etc. The
earthquakes are also known to cause landslides,
liquefaction, slope-instability and damage to earth
and rack structures. The earthquake causes loss of
life and property and shakes the moral of people.
Elevated storage reservoir is the very important
component of water distribution system for any
country therefore it is necessary to remain function
even after the major earthquake event.
Elevated reservoirs are constructed in order to
distribute water in the gravity flow in the
distribution network. Typically Department of
Water Supply and Sewerage (DWSS) and other
water distribution agencies construct the Intze type
RC reservoir as the RC elevated water reservoir in
Nepal. Intze type RC water tank are quite
susceptible to seismic forces due to their basic
configuration consisting of large mass concentrated
at the top with relatively slender supporting system.
The poor seismic performance of many elevated
water tanks was observed during past major
earthquakes due to the inappropriate structural
design of framing system and joints, low overall
strength and ductility.
Present study is primarily focused on understanding
seismic behavior and performance characteristic of
elevated water tank using a non-linear static analysis
approach: a pushover approach. Study assumes all
the components above the staging like top ring
beam, tank wall, bottom ring beam, conical wall, top
and bottom spherical domes are intact and the
vulnerable part is the staging. And hence the focus
of study is towards the seismic strengthening of the
staging i.e. columns and braces using RC jacketing.
Three cases; reservoir full case, half full case and
empty case are modeled in SAP 2000 v17 and
analyzed.
2. METHODOLOGY
In the present study, two cases were separately
analyzed one is the structure before retrofitted and
another is the structure after retrofitted with column
and braces jacketing.
For each cases; three conditions i.e. reservoir full,
partially full and empty were analyzed in SAP 2000
v17 software. Hydrodynamic forces exerted by
liquid on tank wall shall be considered in the
analysis in addition to hydrostatic forces. These
hydrodynamic forces are evaluated with the help of
spring mass model of tanks.
2.1 Spring Mass Model for Seismic Analysis:
When a tank containing liquid vibrates, the liquid
exerts impulsive and convective hydrodynamic
pressure on the tank wall and the tank base in
addition to the hydrostatic pressure. In order to
include the effect of hydrodynamic pressure in the
analysis, tank can be idealized by an equivalent
spring mass model, which includes the effect of tank
KEC Conference 2021, April 18, 2021“3rd International Conference On Engineering And Technology”
Kantipur Engineering College, Dhapakhel, Lalitpur, Nepal
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wall – liquid interaction. The impulsive mass of
liquid, mi is rigidly attached to tank wall at height
hi. Similarly, convective mass, mc is attached to the
tank wall at height hc by a spring of stiffness Kc as
shown in the figure 1.
Fig.1: Spring mass model for elevated tank
2.2 Plastic Hinge Properties:
Basically a hinge represents localized force-
displacement relation of a structural element
through its elastic and inelastic simulation under
seismic loading.
Hinges are of various types namely
• Hinges for Flexural
• Hinges for Shear
• Hinges for Axial
Nonlinear behaviour of structural member is the
nonlinearity of the material which does not allow
only the plastic behaviour of member thus it is
necessary to generate the moment-rotation curve
which characterizes the yield criteria of nonlinear
frame. For each
and every
degree of
freedom define
a moment-
rotation relation
curve that gives
the plastic
deformation,
yield value and
the following
yield. This is
done in terms of
an idealized
curve with
values at five points AB-C-D-E as following figure.
The following points should be noted:
• Point A is always will be the origin.
• Point B represents start of yielding. Deformation
does not occur in the hinge up to point B. Only the
plastic deformation beyond point B will be shown
by the hinge.
• Point C represents the ultimate capacity of structure
by pushover analysis.
• Point D represents a residual strength or after
damage of structure.
• Point E shows total failure of structure. Beyond E
point the hinge will drop shear down to point F, which
is not visible in figure, directly below point E on the
horizontal axis. If user does not want fail hinge this
way, user need to be sure to give a large value for the
deformation at point E.
2.3 Model Descriptions:
600 cu.m. intze type elevated RC tank with following
dimensions is selected for the study:
Diameter of the cylindrical portion (D)= 11.0 m
Rise of top dome (h1)= 2.2 m
Diameter of lower ring beam (D0)= 7.0 m
Rise of bottom dome (h2) = 1.65 m
Height of conical dome (h0) = 2.2 m
Height of cylindrical portion (h) = 5.2 m
Height of staging from top of foundation to circular
ring beam = 15.0 m
Thickness of cylindrical portion = 250 mm
Thickness of conical dome = 400 mm
Thickness of top spherical dome = 100 mm
Thickness of bottom spherical dome = 250 mm
Size of top ring beam = 300X300 mm
Size of bottom ring beam = 750X400 mm
Size of circular ring beam = 450X750 mm
The structure above the circular ring beam is
assumed to be intact. The staging consist of 300
mm diameter column with 6-16 mm diameter
HYSD 500 re-bars and 6 mm diameter helical ties
at 300 mm c/c. Braces consists of 300X300 mm
section and 3-16mm diameter HYSD 500 re-bars on
the top and 3-16 diameter re-bars on the bottom and
6 mm diameter stirrups at 300 mm c/c spacing. All
the column and braces sections are assigned in the
model by section designer. The parameters for the
spring mass model are calculated as per the
Guidelines of IS 1893: Part 2 (Draft) and values are
given in following table 1:
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Kantipur Engineering College, Dhapakhel, Lalitpur, Nepal
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2.4 Analysis Results Before Retrofit:
The elevated water tank with staging height of 15m
is located in seismic zone V (Location Kathmandu).
For the analysis of the structure, the basic computer
model was created using SAP 2000 v17. The
fig.4.shows the 3-D model of the structure. For the
pushover analysis of the structure, the properties of
the various plastic hinges such as flexural, shear,
torsional and joint hinges are defined. After the
hinges are assigned, the maximum roof
displacement of 400mm is applied and the hinges
formation belonging to different performance level
was studied. The section in which the performance
level exceeded the targeted performance level was
retrofitted using Jacketing.
Fig. 4. 3-D model for reservoir full, 60% full and empty condition
ATC 40 provides information about seismic
coefficient to construct elastic response spectra and
these are dependent on zone factor and soil profile
type. Kathmandu lies in higher seismic zone and the
soil type of Kathmandu is very soft soil, hence, Ca
(Effective peak acceleration of the ground) and Cv
(5 percent-damped response of a 1 second system)
value is adopted as 0.18 and 0.30 for Design
Earthquake (IS 1893:2000).
The analysis is performed for three conditions that is
reservoir full condition, reservoir 60% full condition
and reservoir empty condition. The results of each
conditions with pushover are stipulated in table 1.
As the elevated water tank is very essential and
lifeline facilities the structure should be in IO level
at DBE and also the performance point base shear
should be higher than design base shear. To meet
this requirement; the structure should be retrofitted.
From the result, the fundamental time period of the
Table - 2
S
.
N
o
.
Parameter
s
Conditions
Reservoir Full
condition
Reservoir 60%
full condition
Reservoir
empty
Condition
1
Column
Hinge
( figure 5)
At top of bottom
storey, one
column hinge at D
level, two column
hinges at C level
At bottom of first
storey and top of
second storey,
two-two columns
hinges are at LS,
some of column
hinges are at IO
and most of
column hinges are
at B level
Some
column
hinges are at
IO and most
of the
column
hinges are at
B
2
Braces
Hinge
(figure 5)
All the braces
hinges are at B
(just start of
plastic yielding)
Some braces hinge
are at IO and most
of hinges are at B
(i.e. just start of
plastic yielding)
All the
braces
hinges are at
B (just start
of plastic
yielding)
3
Performan
ce Point at
DBE N.A. N.A.
230.667
KN, 142
mm
4
Displacem
ent of
joint 48 at
above
hinge
result and
correspon
ding base
force
99 mm, 151.923
KN
197.4 mm,
180.899 KN
144.89 mm,
230.374 KN
5
Base
Shear
(Auto
Seismic)-
Design
Base
Shear 386.645 KN 350.40 KN 288.095 KN
6
Time
period of
fundament
al mode 3.03746 Sec 2.743244 Sec
2.357798
Sec
7 Result
Capacity of
structure is
Inadequate at
DBE
Capacity of
structure is
Inadequate at
DBE
Capacity of
structure is
inadequate
as the
performance
point base
shear is less
than design
base shear
KEC Conference 2021, April 18, 2021“3rd International Conference On Engineering And Technology”
Kantipur Engineering College, Dhapakhel, Lalitpur, Nepal
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KEC Conference
structure is quite higher and at reservoir empty
condition, performance point base shear is lower
than design base shear which means the section is
less stiff. Even if braces sections are within
Immediate Occupancy (IO) level for all conditions,
base shear criteria indicate the section also needs
retrofitting or replacement. At roof displacement of
only 99 mm (corresponding shear force = 151.923
KN), some column hinges are at the level of failure
which means ductility of the structure is less in the
reservoir full condition and hence this is most
critical condition among empty and partially full
conditions. There is no performance point for
reservoir full and 60% full condition but seismic
demand and capacity of structure meets at base
shear force of 230.667 KN and corresponding
displacement of 142 mm for reservoir empty
condition at the design based earthquake (DBE) but
seismic event can produce base shear of 288.095
KN which is beyond the performance point base
shear of 230.667 KN.
As the elevated water tank is very essential and
lifeline facilities the structure should be in IO level
at DBE and the stiffness of the structure also needs
to increase and period of structure have to reduce.
To meet these requirements, the column and braces
sections should be strengthened.
.
Fig. 5. Column and braces hinge result before retrofit for
reservoir full, 60% full and empty condition
2.5 Column jacketing technique:
Jacketing is a technique used to increase the strength
of existing structural members (e.g. Columns,
Beams etc.) by providing a “Jacket” of additional
material around the existing member. This
additional material can be of several types e.g.
concrete, steel or FRP etc. Concrete Jacketing is
pivotal for strengthening to add or restore ultimate
load capacity of reinforced concrete columns. It is
used for seismic retrofitting, supporting additional
live load or dead load that is not included in the
original design, to relieve stresses generated by
design or construction errors, or to restore original
load capacity to damaged structural elements.
The case of this study is that, the structure is
inadequate to withstand the seismic loading demand.
Figure 6(a) illustrates the column section of
previously constructed and retrofitted structure. The
retrofitted section (outer core) consists of #20 mm -
10 numbers of bars as longitudinal bars and 10 mm
dia circular ties @ 100 mm C/C and concrete
material is M20 and having total of 480 mm
diameter.
Figure 6(b) illustrates the braces section of
previously constructed and renovated structure. The
replaced section consists of #20 mm -10 numbers of
bars as longitudinal bars and 10 mm dia stirrups @
100 mm C/C and concrete material is M20 and
having size of 450 x 450 mm.
KEC Conference 2021, April 18, 2021“3rd International Conference On Engineering And Technology”
Kantipur Engineering College, Dhapakhel, Lalitpur, Nepal
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Fig 6(a): column section before and after retrofit
Fig 6(b): braces section before and after retrofit
The section is finalized after the series of trial
sections. From table 1, only few column sections are
deficient and retrofitting done to only those deficient
sections does not make sense. That means local
retrofitting technique does not work and hence all
the column sections are replaced with jacketed
section (i.e. global retrofitting) and braces sections
are replaced with another heavy section (450 x
450mm) then pushover analysis is run again for
three conditions. The result should be positive for all
three conditions otherwise section should be revised
and analysis should run for next trial section.
2.6 Analysis Results After Retrofit:
The pushover analysis result after the concrete
jacketing of column (no braces jacketing) with all
three conditions are illustrated in the table 3 and
corresponding hinge results are shown in figure 7.
Table - 3
S.
No. Parameters
Conditions
Reservoir
Full
condition
Reservoir
60% full
condition
Reservoir empty
Condition
1
Column Hinge
(Figure 7)
All bottom
storey column
hinges are at
B and rest of are at elastic
deformation
Few of the
column
hinges are at B and rest of
are at elastic
deformation
Some bottom storey column
hinges are at B
and rest of are at elastic
deformation
2 Braces Hinge
(Figure 7)
All the
braces hinges are at B (just
start of
plastic yielding)
All the
braces hinges are at B (just
start of
plastic yielding)
some braces hinges are at B
(just start of
plastic yielding) and few braces
hinges are at
elastic deformation
3 Performance Point at DBE
1059.542KN, 87.00 mm
1051.923
KN, 80.00 mm
1035.169 KN, 68.00 mm
4
Displacement
of joint 48 at
above hinge result and
corresponding
base force
91.95 mm,
1073.023 KN
82.069 mm,
1062.53 KN
72.726 mm,
1063.314 KN
5
Base Shear
(Auto
Seismic)- Design Base
Shear 952.619 KN 874.87 KN 744.227 KN
6
Time period
of first 3 modes 1.34069 Sec 1.21226 Sec 0.992275 sec
7 Result
Capacity of structure is
adequate at
DBE
Capacity of structure is
adequate at
DBE
Capacity of
structure is
adequate at DBE
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Fig 7: Column and braces hinge result after retrofit for
reservoir full and 60% full and empty condition
3. RESULTS AND DISCUSSION
To evaluate the seismic performance of the structure
models, the pushover curve (Base shear versus
Roof displacement) and response spectrum curve
(Acceleration versus Time period) should be
overlaid in acceleration displacement response
spectrum (ADRS) format after converting their
domain to spectral acceleration and spectral
displacement. The intersection of both curves gives
the performance point, which is analyzed for the
evaluation process. The storey drift and the roof
displacement of the structure also give the overall
performance of that structure.
After retrofitting, the column and braces sections are
increased, due to this moment of inertia also
increased and ultimately the stiffness of the whole
staging frame increases. The design base shear is
increased from 386.645 KN to 952.619 KN for
reservoir full condition, 350.4 KN to 874.87 KN for
reservoir 60% full condition and 288.095 KN to
744.225 KN for reservoir empty condition after
retrofit. That means the seismic demand of the
structure also increased and the capacity of the
structure is increased from 230.667 KN to 1035.169
KN at reservoir empty condition. The time period of
the fundamental mode is decreased from (3.03746 to
1.34069 Sec), (2.743244 to 1.21226 Sec) and
(2.357798 to 0.992275 sec) for reservoir full,
reservoir 60% full and reservoir empty condition
respectively. Figure 8(a) shows the variation of base
shear demand of the structure and figure 8(b) shows
the variation of fundamental time period of structure
before and after the retrofit.
Fig 8(a): Variation of base shear demand before and after
retrofit
Fig 8(b): Variation of fundamental time period before and
after retrofit
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For all three conditions the performance point base
shear is higher than the design base shear. The
pushover curve is shown in figure 9, which shows
increases in base shear and corresponding
displacement ultimately the capacity of the structure
is increased.
Fig 9(a): Pushover curve before and after retrofit (Reservoir
Full Condition)
Fig 9(b): Pushover curve before and after retrofit (Reservoir
60% Full Condition)
Fig 9(c): Pushover curve before and after retrofit (Reservoir
Empty Condition)
Figure 10 shows response spectrum curve
(Acceleration versus Time period) overlaid in
acceleration displacement response spectrum
(ADRS) format after converting their domain to
spectral acceleration and spectral displacement
which shows performance point. Before retrofit,
there is no performance point in reservoir full and
60% full condition. That means capacity of the
structure is inadequate to the seismic demand of the
structure. But there is performance point in reservoir
empty condition in parent structure.
After the retrofit, the seismic demand of the
structure is fulfilled by the capacity for all three
conditions.
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Fig 10(a): capacity vs. demand in ADRS format before and
after retrofit (Reservoir Full Condition)
Fig 10(b): capacity vs. demand in ADRS format before and
after retrofit (Reservoir 60% Full Condition)
Fig 10(b): capacity vs. demand in ADRS format before and
after retrofit (Reservoir empty Condition)
4. CONCLUSIONS:
The performance of reinforced concrete frame
staging was investigated using the pushover
analysis. As a result of the work that was completed
in this study, the following conclusions were made:
1. After the structure retrofitted with column
Jacketing and braces replacement, the hinge
formation on the structure is within the
Immediate Occupancy Level.
2. The performance point base shear is greater
than design base shear, the elevated water tank
frame staging is seismically safe.
3. Fundamental time period of the structure
decreases as the water level in the tank
decreases.
4. Seismic demand of the structure decreases as
the water level in the tank decreases, reservoir
full case is found to be the most critical.
5. LIMITATIONS:
The study performed here is based on following
assumptions:
1. The upper structure above staging (i.e. tank
portion) is assumed to be intact and vulnerable
portion is staging only.
2. The elevated water tanks usually consists of RC
spiral staircase which may produce
eccentricities in analysis but the effect of this is
not considered in the analysis.
3. The study is carried out in seismic zone V and
soil type III (soft soil), the result obtained here
used in other zone and other soil type but for
same reservoir type may be uneconomical.
4. The study only focuses on seismic safety.
Economical retrofitted section is not analyzed
which can be found after doing pushover
analysis of different varying retrofitted sections
(with different concrete thickness and steel
rebar quantity).
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REFERENCES
[1] Jignesh A Amin and D P Soni. Assessment Response Reduction Factor of Elevated Tanks with Alternate RC Frame Staging Configurations. 4, 12 , 8.
[2] Rupal Gondalia and Asst Dhananjay Patel. NON-LINEAR STATIC (PUSHOVER) ANALYSIS ON ELEVATED STORAGE RESERVOIR (ESR). International Journal of Advance Engineering and Research Development 4, 4 , 11.
[3] Rupal Gondalia and Asst Dhananjay Patel. NON-LINEAR
STATIC (PUSHOVER) ANALYSIS ON ELEVATED STORAGE RESERVOIR (ESR). International Journal of Advance Engineering and Research Development 4, 4 , 11.
[4] O R Jaiswal and Sudhir K Jain. 2005. Modified proposed provisions for aseismic design of liquid storage tanks: Part II – commentary and examples. 32, (2005), 14.
[5] Afshin Mellati. 2018. Predicting Dynamic Capacity Curve
of Elevated Water Tanks: A Pushover Procedure. Civ Eng J 4, 11 (November 2018), 2513. DOI:https://doi.org/10.28991/cej-03091177
[6] Suyash Nerkar and Chittaranjan Nayak. SEISMIC
BEHAVIOUR OF ELEVATED STORAGE RESERVOIR BY FINITE ELEMENT METHOD. 11.
[7] Dakshes J Pambhar. PERFORMANCE BASED
PUSHOVER ANALYSIS OF R.C.C. FRAMES. 4.
[8] 350.3-06 Seismic Design of Liquid-Containing Concrete
Structures and Commentary. 67.
[9] Draft of IS 1893 (Part-2).pdf.
[10] FEMA356-2000.pdf.
[11] IRJET-V2I7104.pdf.
KEC Conference 2021, April 18, 2021“3rd International Conference On Engineering And Technology”
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