seismic performance evaluation of existing...

ISSN: 2278 – 7798

International Journal of Science, Engineering and Technology Research (IJSETR)

Volume 5, Issue 5, May 2016

1596 All Rights Reserved © 2016 IJSETR

Abstract — Earthquakes are the most devastating forces

that structures are likely to be subjected to. The observed

behaviour of bridges during the past earthquakes has

indicated several deficiencies in their design, in view of

the fact that many of them were not designed in

accordance with the recent seismic design procedures. As

a result many of the bridges constructed earlier fail to

meet requirement of current codes. Such bridges may

need seismic assessment and retrofitting. In order to

address this problem, the aim of the present project is to

carry out a seismic evaluation case study for an existing

RC bridge using push over analysis. An existing 11-span

integral reinforced concrete slab deck bridge is

considered for the present study and it is seismically

evaluated. Computer software Csi Bridge is used to

analyze the bridge. Push over analysis is adopted to

evaluate seismic performance of a bridge. Result

obtained from the analysis is taken as the demand for the

applied lateral load. This demand is compared with the

capacity of the structural elements of the bridge. In any

case capacity of the elements is less than the demand then

the capacity of those elements has to be increased with

the suitable retrofitting method. The best retrofitting

method can be achieved by either the use of dampers or

by the adoption of base isolation. Among these base

isolation is gaining significant popularity in the recent

years. When the bridge was subjected to an earthquake

similar to the Bhuj Earthquake in transverse and

longitudinal directions, from the time history analysis

base shear and base moment is compared with integral

bridge & isolation of bridge.

Index Terms— Demand, Capacity, Pushover Analysis,

Base shear, Time history analysis.

I. INTRODUCTION

Bridges are lifeline structures. They are an important link in

surface transportation networks, and their failure during a

seismic event will seriously hamper relief and rehabilitation

work. Due to their structural simplicity, bridges are

particularly vulnerable to damage and can even collapse

when subjected to earthquake motions. General earthquake

design philosophy is to design the structure to prevent

complete collapse in case of very strong ground motion.

There are many literatures available on the seismic

evaluation procedures of multi-storied buildings. There is no

much effort available in literature for seismic evaluation of

existing bridges although bridge is a very important structure

in any country. The attention for existing bridges is

comparatively less. However, bridges are very important

components of transportation network in any country. The

Manuscript received May, 2016.

1. Vinay Kumar M, PG Student, The Department of Civil Engineering, The

Oxford College of Engineering, Bangalore, India.

2. Shivanand C.G, Asst. Professor, The Department of Civil Engineering,

The Oxford College of Engineering, Bangalore, India.

bridge design codes in India have no seismic design

provision at present. A large number of bridges are designed

and constructed without considering seismic forces.

Therefore, it is very important to evaluate the capacity of

existing bridges against seismic force demand. Seismic isolation is a method that attempts to reduce the

seismic forces to or near the elastic capacity of the structural

member, thereby reducing the inelastic deformations. The

main concept in isolation is to reduce the fundamental

frequency of structural vibration to a value lower than the

predominant energy-containing frequencies of the

earthquake. The other purpose of an isolation system is to

provide a means of energy dissipation, which dissipates the

seismic energy transmitted to the system. Thus, the isolation

device, which replaces the conventional bridge bearings,

isolates the bridge deck which alone is responsible for the

majority of the pier base shear from the bridge substructure

during earthquakes, thereby significantly reducing the deck

acceleration and, consequently, the forces transmitted to the

piers. Refer to the fig.2.

A. Pushover Analysis

The use of the nonlinear static analysis (pushover analysis)

came in to practice in 1970‟s but the potential of the pushover

analysis has been recognized for last 10-15 years. This

procedure is mainly used to estimate the strength and drift

capacity of existing structure and the seismic demand for this

structure subjected to selected earthquake. This procedure

can be used for checking the adequacy of new structural

design as well. The effectiveness of pushover analysis and its

computational simplicity brought this procedure in to several

seismic guidelines (ATC 40 and FEMA 356) and design

codes (Euro code 8 and PCM 3274) in last few years.

Pushover analysis is defined as an analysis wherein a

mathematical model directly incorporating the nonlinear

load-deformation characteristics of individual components

and elements of the building shall be subjected to

monotonically increasing lateral loads representing inertia

forces in an earthquake until a „target displacement‟ is

exceeded. Target displacement is the maximum

displacement (elastic plus inelastic) of the building at roof

expected under selected earthquake ground motion.

Pushover analysis assesses the structural performance by

estimating the force and deformation capacity and seismic

demand using a nonlinear static analysis algorithm. The

seismic demand parameters are global displacements (at roof

or any other reference point), storey drifts, storey forces, and

component deformation and component forces. The analysis

accounts for geometrical nonlinearity, material inelasticity

and the redistribution of internal forces. Response

characteristics that can be obtained from the pushover

analysis are summarized as follows:

Estimates of force and displacement capacities of the

structure. Sequence of the member yielding and the progress

of the overall capacity curve.

Estimates of force (axial, shear and moment) demands on

potentially brittle elements and deformation demands on

ductile elements.

Vinay Kumar M1 , Shivanand C G

2

Seismic Performance Evaluation of Existing Bridge

ISSN: 2278 – 7798




Estimates of global displacement demand, corresponding

inter-storey drifts and damages on structural and

non-structural elements expected under the earthquake

ground motion considered.

Sequences of the failure of elements and the consequent

effect on the overall structural stability.

Identification of the critical regions, where the inelastic

deformations are expected to be high and identification of

strength irregularities (in plan or in elevation) of the

building.

Fig 1. Determination of Performance Point.

Fig 2. Effect of Seismic Isolation on Spectral Acceleration

II. DESCRIPTION OF THE STUDY BRIDGE

The bridge is situated Karnataka, India. It is multi-span

simply supported reinforced cement concrete integral bridge

having the total length of 264 m with 11 equal spans of 24 m

length. It is supported on single pier type bents, which are

transversely connected by the bent cap. The bridge piers and

abutments are supported on well foundations.

Fig 3. Deck Section Details

Fig 4. Elevation of Longitudinal Girders

Table 1. Cross sectional details of bridge

Bridge

Componen

t

Dimensions

(mm)

Deck Slab Width 12000

Depth 2430

Bent Cap Cross Section 2300 x 1800

Length 11150

Bent

Pier

Diameter 2000

Height 6400

Abutment Fixed Fixed

Materials: M40 concrete and Fe-415 steel

Loadings:

Dead Load – Self weight of the superstructure.

Moving Loads – IRC_AA_W (IRC 6 Code).

Earthquake Load – Response Spectra

Pushover Analysis in Csi Bridge –Target

Displacement 4% of bridge height.

Time Period – Program Calculated.

ISSN: 2278 – 7798




Lead Rubber Isolator Properties:

Effective Stiffness (kN/m) – 54894.92

Bearing Horizontal Stiffness (kN/m) – 405

Post yield stiffness ratio – 0.11

Yield Strength (kN) - 138.6

Damping – 0.05

B. MODELLING OF THE BRIDGE

A three dimensional (3D) finite element model (FEM) of the

bridge was created using Structural Analysis and Program

Software Csi Bridge. The Bridge modeller can be used to

bridge wizard generates a bridge model. The Bridge wizard

provides a step-by-step guide through the modelling process

using Csi Bridge Information Modeller. The deck edges in

each simply supported span were considered rigid. Due to the

large in-plane rigidity, the superstructure was assumed as a

rigid body for lateral loadings. The bridge consists of six

equal spans and five wall type bent was modelled as a frame.

The framing action and coupling between columns in the

column bent provides seismic resistance in terms of strength

and stiffness. The pier cap and the piers were modelled as

beam-column elements. Deck is modelled as shell thin

elements. The default hinge properties (PMM – P stands for

axial force, M stands for M2 moment, and M stands for M3

moment in Csi Bridge) were assigned to each end of the

columns. The base of the column was assumed as fixed. The

deck of the bridge is integrally connected to the pier cap. The

bridge is also analyzed using non-linear time history method.

The time history acceleration data of Bhuj earthquake (20

Jan 2011) is used as the time history function for analysis.

Assuming in case if it were to be constructed with lead rubber

bearings and this bearing was placed in between deck and

pier cap, study is carried out by nonlinear time history

analysis to investigate time period, base shear etc. Finally

results are compared with integral bridge and isolated bridge.

Below figure showing modelling of study bridge.

III. RESULTS AND DISCUSSION

A. Pushover Analysis Results

a) Pushover curve for Zone – II

Fig 5. Pushover curve in longitudinal direction for type – 2

soil

Fig 5.1. Pushover curve in transverse direction for type – 2

soil

Table 2. Demand and Capacity of the bridge for different

types of soil in Zone – II

SOIL

CO -

EFFICIENTS

(DBE)

DEMAND

OF THE

STRUCT

URE

(BASE

SHEAR)

(kN)

CAPACITY OF

THE

STRUCTURE

(BASE SHEAR)

(kN)

DISPLACEMENT

(MM)

Ca Cv X Y X Y

TYPE I

(HARD)

0.05 0.05 1572.9 10013 12236 0.0305 0.0049

TYPE II

(MEDIUM

)

0.05 0.07 2139.2 10084 12400 0.0311 0.0051

TYPE III

(SOFT)

0.05 0.08 2626.8 10013 12737 0.0285 0.0056

ISSN: 2278 – 7798




0

2000

4000

6000

8000

10000

12000

Hard Soil MediumSoil

Soft Soil

1572.9812139.255 2626.879

10013.496 10084.039 10013.023

Z O N E - 2

Base Shear Capacity of the Structure in X - direction

Fig 6. Chart representing demand and capacity of the

structure for Zone – II in x – direction.

0

2000

4000

6000

8000

10000

12000

14000


Soft Soil

1572.981 2139.255 2626.879

12236.73 12400.278 12737.587

Z O N E - 2

Base Shear Capacity of the Structure in Y - direction

Fig 6.1 Chart representing demand and capacity of the

structure for Zone – II in y – direction.

b) Pushover curve for Zone – III

Fig 7. Pushover curve in longitudinal direction for type – 2

soil

Fig 7.1. Pushover curve in transverse direction for type – 2

soil

Table 3. Demand and Capacity of the bridge for different

types of soil in Zone – III

SOIL

CO -

EFFICIENTS

(DBE)

DEMAND

OF THE

STRUCTU

RE (BASE

SHEAR)

(kN)

CAPACITY OF

THE STRUCTURE

(BASE SHEAR)

(kN)

DISPLACEMENT

(MM)

Ca Cv X Y X Y

TYPE I

(HARD)

0.08 0.08 2516.77 10756 12528 0.0420 0.0053

TYPE II

(MEDIUM)

0.08 0.11 3422.807 9805 12418 0.0268 0.0051

TYPE III

(SOFT)

0.08 0.13 4203.006 9910 12552 0.0268 0.0053

0

2000

4000

6000

8000

10000

12000


Soft Soil

2516.773422.807

4203.006

10756.7249805.114 9910.033

Z O N E - 3

Base Shear Capacity of the Structure in X - direction

Fig 8. Chart representing demand and capacity of the

structure for Zone – III in x – direction.

ISSN: 2278 – 7798




0

5000

10000

15000


Soft Soil

1572.981 2139.255 2626.879

12528.619 12418.485 12552.141

Z O N E - 3

Base Shear Capacity of the Structure in Y - direction

Fig 8.1. Chart representing demand and capacity of the

structure for Zone – III in x – direction.

Discussion on Pushover Results

The pushover analysis was conducted in both the

transverse and the longitudinal directions. It is assumed that

the shape of the global pushover curve reflects the global or

local mechanism involved when the structure approaches

dynamic instability. The capacity curve (pushover curve) is

the graphical plot of the total lateral force or base shear (Vb)

on a structure against the lateral deflection (δ) of the control

node of the bridge structure.

The pushover curve for longitudinal direction is shown in

Figure 6. The figure indicates that the performance point

occurred at a base shear of 10084 kN with the control node

displacement of 0.0311m for the soil type-2 in Zone-2. The

pushover curve for transverse direction is shown in Figure

6.1. The figure indicates that the performance point occurred

at a base shear of 12400 kN with the control node

displacement of 0.0051 m for the soil type-2 in Zone-2.

Demand of the bridge in both direction occurred at base shear

of 2139.2 kN. Similarly varying soil type, zones demand and

capacity of bridge for each soil and zones is found out.

Demand of the bridge is obtained from the linear static

earthquake analysis (using IS 1893-2002 part-1). Capacity of

the bridge is obtained from the nonlinear static pushover

analysis. In this case demand and capacity of the bridge is

found for both zone-2 & zone-3.In both zones demand of the

bridge is less than capacity of the bridge, so this bridge does

not required any retrofitting work.

Fig 9. Plastic Hinge formation for gravity loads

B. Time History Analysis Results

Preliminarily, it is intended to compare the seismic

behaviour of an integral bridge and an isolated bridge. The

modal time periods for different modes of the bridge with and

without isolation are shown in figure.

Fig 10. Comparison of Modal Time Periods for the bridge

It can be clearly seen that, isolated bridges shows much

higher time periods compared to integral bridges. Hence the

effect of isolation is to impart flexibility to the structure.

Fig 11. Comparison of Base moment of a typical pier in

x - direction.

ISSN: 2278 – 7798




Fig 12. Comparison of Base moment of a typical pier in

y - direction.

Fig 13. Comparison of Base shear of a typical pier in

x - direction.

Fig 14. Comparison of Base shear of a typical pier in

y - direction.

The base shear in global x and y directions are plotted as a

function of time for the time history function. The maximum

base shear experienced by an integral bridge is significantly

higher compared to an isolated bridge. Internal forces in one

of the pier are also plotted for the time history function as

shown in above figure. The members in the base isolated

structure developed much lesser bending moments and shear

forces compared to non-isolated structure. This is a clear

illustration of the energy absorbing capacity of the base

isolated system which reduces the earthquake loads that are

transferred to the superstructure from the foundation.

IV. CONCLUSION

Following conclusions are drawn from the present study

of non-linear static (pushover) analysis and nonlinear

dynamic time history analysis on integral bridge and isolated

bridge.

The bridge selected has been evaluated for the seismic

performance. Capacity and demand the structure is

obtained by pushover analysis. Demand of the structure is

less than the capacity. Therefore retrofitting is not

required.

The survivability of the bridge structure under Bhuj

earthquake was checked using capacity spectrum method.

It was found that the study bridge could survive Bhuj

Earthquake.

Base isolation is an effective and efficient method of

reducing the effect of seismic forces by lengthening the

time period of the structure and reducing the forces

transferred to the super structure.

Structures with base isolation shows higher modal time

periods compared with integral bridge indicating the

increased flexibility of the bridge.

The internal forces in the members are also considerably

reduced due to the incorporation of isolation system.

The flexibility imparted to the structure by the base

isolation system increases with decrease in the lateral

stiffness of the isolator.

ISSN: 2278 – 7798




ACKNOWLEDGMENT

I would like to thank my guide and advisor, Mr. Shivanand

CG, Assistant Professor of civil department at the Oxford

College of Engineering, Bangalore for his guidance. And my

special thanks to the Head of the Department, management,

faculty and friends of the Oxford College of Engineering,

Bangalore, Karnataka for their support.

REFERENCES

[1] Agarwal P. and Shrikhande M (2006) “Earthquake

Resistant Design of Structures”.

[2] R.E.T. Amaladosson and U. Gunasekaran ―Analysis

of T–beam Bridge for seismic characterisation” 2014

NZSEE conference.

[3] ATC-40, Applied Technology Council, (1996)

“Seismic Evaluation and Retrofit of Concrete

Buildings”, Vol. 1-2, Applied Technology Council,

Redwood City, California.

[4] IS: 1893 - 2002 (Part 1), Indian Standard Criteria for

Earthquake Resistant Design of Structures, fifth

revision, Bureau of Indian Standards, New Delhi.

[5] IS: 456-2000, Indian Standard Plain and Reinforced

Concrete-Code Of Practice (Fourth Revision), Bureau

of Indian Standards, New Delhi.

[6] IS: 875 - 1987 Reaffirme d 2003 (Part 2), Code of

Practice for Design Loads (other than earthquake) for

Buildings and Structures, Imposed Loads, Bureau of

Indian Standards, New Delhi.

[7] IS: 875 - 1987 Reaffirme d 2003 (Part 3), Code of

Practice for Design Loads (other than earthquake) for

Buildings and Structures, Wind Loads, Bureau of Indian

Standards, New Delhi.

[8] Kaliprasanna Sethy (2011) “Application of Pushover

Analysis to RC Bridges”, Report, Department of Civil

Engineering, National Institute of Technology,

Rourkela.

[9] Kappos A. J, Paraskeva T. S and Sextos A. G (2005)

“Modal Pushover Analysis as a Means for the Seismic

Assessment Of Bridge Structures”, No. 49, Proceedings

of the 4t hEuropean Workshop on the Seismic Behaviour

of Irregular and Complex Structures, Thessaloniki,

Greece.

[10] Ranjit S Abeysinghe, Evgenia Gavaise, Marco

Rosignoli and Theodoros Tzaveas (2002) “Pushover

Analysis of Inelastic Seismic Behavior of Greveniotikos

Bridge”, Journal of Bridge Engineering, Vol. 7, No. 2,

ASCE.

BIOGRAPHY

1. Vinay Kumar M D.C.E., B.E., A.M.I.E., (M.Tech)

PG Student,

Civil Engineering Department,

The Oxford College of Engineering, Bangalore, Karnataka,

India.

2. Shivananad C G B.E., M.Tech (Ph.D)

Assistant Professor,

Civil Engineering Department,

The Oxford College of Engineering, Bangalore, Karnataka,

India.

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