Seismic Strengthening of Intze Type RC Elevated Water Tank ...

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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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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



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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.



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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.



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