Seismic Performance Assessment of an Existing Road Bridge using Standard Pushover Analysis

Roseenid Teresa. A1,a , Premavathi. N2,b , Umarani Gunasekaran3,c 1Ph.D. Research Scholar, Anna University, Chennai and Associate Professor, St. Joseph’s College

of Engineering, Chennai-119, India.

2 M.E. Student, Anna University, Chennai, India

3Associate Professor, Anna University, Chennai, India

aenidarose@gmail.com, bpremavathi.n@gmail.com, cumarani@annauniv.edu

Keywords: Road Bridge, Pushover analysis, Seismic response

Abstract. The inelastic seismic response of an existing multi-span concrete bridge is investigated

by performing nonlinear static pushover analysis. The bridge is subjected to lateral forces

distributed proportionally over the span of the bridge in accordance with the product of mass and

mode shape. The bridge is pushed up to the target displacement and the hinge formations of the

bridge in different steps of the pushover procedure in the transverse direction are obtained. The

expected capacity of the bridge is evaluated and compared with the displacement demand.

Introduction

The bridges and viaducts are the most critical elements of the road networks and they are lifeline

facilities that must remain functional even after major earthquakes. Since 1971 San Fernando

Earthquake and especially during 1994 M6.7 Northridge earthquake-USA,1995 M7.1 Loma Prieta-

California, few of which recorded in ATC-40 [1] and the most severe,1989 M7.2 Hanshin–Awaji

Kobe earthquake, many reinforced concrete bridge piers suffered severe damage, that was

considered unacceptable by the community as a whole. In-order to provide a measure of post-

earthquake serviceability, there has been a shift towards Performance-based seismic design (PBSD).

Thus, seismic design of any structure is expected to be based on multiple performance objectives.

To achieve such an objective, the inelastic behavior of the structure must be well understood over a

wide range of structural performance levels, rather than only at first yield or near Collapse. The

performance based Earthquake engineering concepts in recent guideline documents such as ATC-40

[1] and FEMA-356 [2] has led to increased utilization of nonlinear static methods to estimate the

seismic demands.

The Nonlinear static procedure which is often called as “Pushover Analysis”, constitute an

inelastic analysis which helps the engineers to gain a more realistic picture of the potential seismic

performance characteristics of the structure. Pushover analysis procedures are described in a

number of references, (Kim and D’Amore [3], Cosmin and Chiorean [4] and Kappos et al.[5]). In

this paper, the seismic performance assessment of an existing road bridge by Capacity spectrum

method using standard pushover analysis procedure is discussed.

Description of the study bridge

The performance of Koyembedu Bridge which is an existing road bridge located in Chennai is

studied by Standard Pushover Analysis (SPA) technique using nonlinear analysis software package

SAP 2000.The model which is taken for study which runs across Coovam River connects Guindy

and Thirumangalam. This bridge was widened in the year 2000 to meet the traffic demands by

providing expansion gap between older and the new one. The seismic performance of the widened

portion of the bridge is investigated and discussed. It is a simply supported RCC Slab cum T-Beam

Bridge of 129.7m span length with 8 equal simply supported spans of 16.21m. The bridge supports

Applied Mechanics and Materials Vol. 147 (2012) pp 278-282Online available since 2011/Dec/22 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.147.278

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one way traffic with a carriage way width of 7.1m.The superstructure consists of four longitudinal

T-Beam girders and five cross beams. It is supported on multi-column bents over Neoprene bearing

pads. Each multi-column bent has four columns which are transversely connected by the bent cap.

The well foundation of old bridge is extended for the widened portion. The cross sectional details of

the components of the bridge are presented in (Table 1).

Table 1 Cross sectional details of components of the bridge

Sl.No. Description Specification (m)

1.

Longitudinal girder

Top flange 2.5 x 0.22

Bottom flange 0.5 x 0.3

Web 0.25 x 1.4

2. Cross girder 0.2 x1.4

3. Bent cap 8.8 x 1.4 x 0.6

4. Bent column 0.8 diameter

5. Bearing pad 0.5 x0.32 x0.0335

The longitudinal view and the cross sectional elevation of the bridge is shown in (Fig.1, Fig.2)

Fig.1 Longitudinal view of the bridge Fig. 2 Cross sectional elevation

(All dimensions are in ‘m’)

Modeling of bridge

A three dimensional finite-element model of the bridge was created using SAP 2000. A spine model

was employed in modeling the superstructure. Due to the large in-plane rigidity, the superstructure

is assumed as a rigid body for lateral loadings. The framing action and coupling between columns

in the multi-column bent can contribute to the seismic resistance in terms of stiffness, resistance

capacity and axial load levels in the various frame members. In the analytical model, all of these

effects are incorporated in a planar frame model along the bent axis. Effective or cracked stiffness

properties are assigned to the moment of inertia of the entire cross section about the transverse axis

(Iy). The derivation of effective moment of inertia is based on the cracked section and it is sufficient

to use Ieff = 0.5Ig which results in an effective flexural stiffness for the beam (Priestley et al., [6]).

The cross-section of the bent cap is modeled as beam element and the columns are modeled as

inelastic column element using Section Designer in SAP 2000.Every bent is modeled as a plane

frame. Effective moment of inertia of RC column is taken as 0.7 Ig (Priestley et al., [6]). The

interface between the column face and centroid of bent cap is modeled using rigid element to

capture the actual behavior at junction. The default hinge properties of SAP 2000 are assigned to

each column at appropriate locations. PMM hinge is assigned to each column at both ends.

The bearing pads are modeled as linear spring elements. The initial stiffness, k (Mohamed et

al., [7]) is calculated using the relation,

Applied Mechanics and Materials Vol. 147 279

(1)

Where G is the shear modulus, A is the bearing cross-sectional area, and h is the bearing pad height.

The expansion joint between the deck slabs are represented by the Gap element in SAP 2000.

The effective stiffness, keff (Muthukumar [8]) is calculated using the relation,

(2)

The guide lines given in Caltrans [9] design aid are used to determine the stiffness values of the

abutment.

Results and discussion

The model is analysed using gravitational forces, considering the P-∆ effects. Following this, the

pushover analysis is done on the model in the transversal direction alone considering the

fundamental mode and P-∆ effects. From the dynamic characteristics of the bridge it is found that

the response of the bridge is governed by two modes in transverse direction to reach the total mass

participation above 90%.The fundamental mode in the transverse direction (mode 1) has a period of

0.3833 seconds and excited 80.81% of the system mass in the transverse direction and 0% in the

longitudinal direction. The mode 4 has a modal period of 0.2535 seconds contributed to the

response with the mass participation of 9.81%.As the fundamental mode is dominant, the response

of the bridge for the fundamental mode (mode 1) shown in (Fig. 3) alone is discussed. The pier top

displacements at various bent locations are shown in (Fig. 4).

Fig. 3 First mode shape in transverse direction Fig. 4 Pier top displacement at bent location

Performance Evaluation

Pushover analysis has been performed for the fundamental mode in the transverse direction. The

bridge is subjected to lateral forces distributed proportionally over the span of the bridge in

accordance with the product of mass and mode shape and the base shear-displacement curve of the

structure is obtained and shown in (Fig. 5). The performance point of the structure has happened at

a displacement of 50 mm and at an effective damping of 24.1% in the transverse direction is

observed from the capacity spectrum shown in (Fig. 6). Also, PMM hinge dominated at bottom of

the pier and the rotation of the structure at the performance point is 0.038 radians.

⋅= hGAk /

⋅= mheff kk δ

280 Computational Mechanics, Materials and Engineering Applications

Fig. 5: Base shear- Displacement curve Fig. 6 Capacity Spectrum curve

Performance condition of the bridge: The hinge at the bottom of the first column in the IV bent

was the first yielded hinge. At step 9, all of the plastic hinges yielded, and the structure continues to

push further until the last step when hinge at the bottom of the same column in IV bent has reached

the collapsed state. (Table 2) shows the hinge statuses at yielding at ultimate step in which

A,B,C,D,E are points defining the moment-rotation relation and the Immediate Occupancy(IO),

Life safety(LS), and Collapse prevention (CP) are performance levels.

Table 2 Hinge statuses of the Bridge at different steps in Transverse Pushover procedure

Steps Displacement (m) A-B B-IO IO-LS CP-C-D D-E > E Total

Initial 0 56 - - - - - 56

Yield 0.0351 0 14 36 6 - - 56

Ultimate 0.1038 0 04 31 20 1 - 56

At the initial step the bridge displaced under its self-weight and there was nil displacement.

Behaviour of the hinges under self-weight was still in linear elastic range. The yield point of the

structure was defined as the point when the first yield occurred at one of the plastic hinges, which is

indicated in the table as the status of “B-IO”. Table 3 shows the structural ductility of Koyembedu

bridge.

Table 3 Structural ductility of Koyembedu Bridge in Transverse direction

Yield

displacement

(mm)

Ultimate

displacement

(mm)

Performance

displacement

(mm)

Displacement

ductility

Performance

ductility

35.1 103.8 50 2.96 2.076

In the transverse direction, the performance displacement is 48.17% of its Ultimate capacity.

Conclusion

From the study, the seismic performance of the Bridge for the fundamental mode in the transverse

direction is found to be acceptable and the acceptance criterion is satisfied. Further research will be

carried out to assess the inelastic response of the bridge in both the directions. To account the

contribution of higher modes, Modal Pushover analysis procedure will be employed.

Applied Mechanics and Materials Vol. 147 281

Acknowledgment

The authors wish to express their sincere thanks to UGC, India for providing financial assistance

under UGC-CAS scheme.

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(1996)

[2] Federal Emergency Management Agency (FEMA), NEHRP Guidelines for the seismic

rehabilitation of buildings, FEMA 273 and 356, Washington, D.C, (1997).

[3] S.Kim, and E. D’Amore, Push-over Analysis Procedure in Earthquake Engineering,

Earthquake Spectra, 15(3), (1999), 417-434.

[4] Dr.Cosmin and G. Chiorean., Application of Pushover analysis on Reinforced concrete

bridge Model,Technical Report,POCTI NO.36019/99,Portugal, (2003).

[5] Kappos et al., “Modal pushover analysis as a means for the seismic Assessment of bridge

structures”, Proc. of the 4th European Workshop on the Seismic Behavior of Irregular and

Complex Structures, Greece, (2005).

[6] M.J.Priestly,F.Seible, and G.M. Calvi. Seismic design and retrofit of bridges, John wiley &

sons, Inc., New York. (1996).

[7] Mohamed ElGawady, William F.Cofer, and Reza Shafiei-Tahrany, Seismic assessment of

WSDOT bridge with prestressed hollow core piles, Technical research report,Washington State

Transportation Centre (TRAC) ,Pullman, Washinigton,(2009).

[8] Muthukumar, S. "A Contact Element Approach with Hysteresis Damping for the Analysis

and Design of Pounding in Bridge". Georgia Institute of Technology, (2003).

[9] Caltrans, Seismic design of highway bridge foundations: Training course manual, California

Department of Transportation, Sacramento, Calif., (1995).

282 Computational Mechanics, Materials and Engineering Applications

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