Seismic Performance Evaluation of Existing Bridge

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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

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 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. DESCRIPTIONOFTHESTUDYBRIDGE

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.

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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

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0 2000 4000 6000 8000 10000 12000

Hard Soil Medium

Soil

Soft Soil

1572.981 2139.255 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

Hard Soil Medium

Soil

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

Hard Soil Medium

Soil Soft Soil 2516.77 3422.807 4203.006 10756.724 9805.114 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.

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0 5000 10000 15000

Hard Soil Medium

Soil

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 2139.2kN. 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.

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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. 

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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.

Figure

Fig 2. Effect of Seismic Isolation on Spectral Acceleration

Fig 2.

Effect of Seismic Isolation on Spectral Acceleration p.2
Fig 1. Determination of Performance Point.

Fig 1.

Determination of Performance Point. p.2
Table 2. Demand and Capacity of the bridge for different  types of soil in Zone – II

Table 2.

Demand and Capacity of the bridge for different types of soil in Zone – II p.3
Fig 5.1. Pushover curve in transverse direction for type – 2  soil

Fig 5.1.

Pushover curve in transverse direction for type – 2 soil p.3
Fig 5. Pushover curve in longitudinal direction for type – 2  soil

Fig 5.

Pushover curve in longitudinal direction for type – 2 soil p.3
Fig 6. Chart representing demand and capacity of the  structure for Zone – II in x – direction

Fig 6.

Chart representing demand and capacity of the structure for Zone – II in x – direction p.4
Fig 7. Pushover curve in longitudinal direction for type – 2  soil

Fig 7.

Pushover curve in longitudinal direction for type – 2 soil p.4
Fig 9. Plastic Hinge formation for gravity loads  B. Time History Analysis Results

Fig 9.

Plastic Hinge formation for gravity loads B. Time History Analysis Results p.5
Fig 11. Comparison of Base moment of a typical pier in                        x - direction

Fig 11.

Comparison of Base moment of a typical pier in x - direction p.5
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

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 p.5
Fig 12. Comparison of Base moment of a typical pier in                      y - direction

Fig 12.

Comparison of Base moment of a typical pier in y - direction p.6
Fig 14. Comparison of Base shear of a typical pier in                            y - direction

Fig 14.

Comparison of Base shear of a typical pier in y - direction p.6
Fig 13. Comparison of Base shear of a typical pier in                         x - direction

Fig 13.

Comparison of Base shear of a typical pier in x - direction p.6

References

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