SEISMIC PERFORMANCE OF A CONTINUOUS BRIDGE: ROLE OF BEARINGS AND SUBSTRUCTURE

Abstract

Bridges play a crucial role in the lifeline system of a country. Failure ofbridges can severely hamper the post-earthquake relief and rescue work. Further, these are costly assets and their failure can have severe impact on the economy of the country. Great developments have taken place in the art and science ofbridge design and construction and seismic safety of bridges has been at the centerstage ofthe bridge engineering in the last few decades. Still, several cases of failure ofbridges have been observed, World over, in the past earthquakes. This highlights the need for further research in understanding the seismic behaviour and earthquake resistant design of different types of bridges. During the past earthquakes, it has been observed that generally, the failure of bridges takes place due to failure of substructure and bearings. Therefore, there is a need to improve the design ofbearings and substructure. In the present study, an effort has been made to understand the role of various substructure components, including bearings and the soil beneath the foundation, during an earthquake. Some inferences have also been drawn, which may be useful in design and retrofitting of bridge. In the present study, a three span continuous bridge, existing in the Northern high seismic zone of India, has been considered. The superstructure of the bridge has been kept unchanged throughout the study, but different alternatives for bearings, piers and foundation have been considered. Site specific design response spectrum for site of the bridge is available. An ensemble of five ground motion histories recorded at different source-site conditions have been used, with and without scaling to make them

compatible with the design spectra. The behaviour of various components has been studied at different (DBE and MCE) levels of earthquake excitation. A 3D nonlinear model of the bridge has been developed in the software SAP2000 Nonlinear, and Nonlinear Static and Nonlinear Dynamic Analysis procedures have used to determine the response of the bridge in different cases. Seismic behaviour of the bridge with different types of Traditional and Isolation Bearings, and Bearing Protection Devices has been studied. It has been observed that the Isolation Bearings have better performance than the Traditional Bearings, under seismic, as well as, under thermal loading. Among the various isolation systems, considered in the study, the Lead Rubber Bearing, High Damping Rubber Bearing, and Friction Pendulum System have resulted in lower displacements and pier forces, but these are deficient in terms of restoring effect at longer periods and higher damping. Conventional Elastomeric Bearings with supplemental damping have good restoring effect but these result in comparatively higher pier and abutment forces. The coupled plasticity behaviour in the bearings has been modeled and the effect of bi-directional ground shaking on the performance of the isolated bridge has been investigated. Performance of a number of types of bearing protection devices, viz. Rigid Stopper, Yielding Stopper, steel restrainer, and SMA Restrainer has been studied. Models of the different devices have been developed using the combination of various elements available in SAP2000. A sensitivity analysis has been performed to select the optimal values of various design parameters. It has been shown that these devices can be used in the existing, as well as, in the new bridges, alongwith the Conventional Elastomeric Bearings to restrict the deformations in the bearings, within the safe limits. IV Effect offoundation-soil flexibility and soil amplification, on the seismic performance of the bridge, has been studied, considering the bridge to have well foundations. A comparative study of the various soil spring models available in literature has been made with a 3D finite element model of soil-foundation system. To study the effect of soil flexibility and amplification, bridge has been considered with integral connections between the superstructure and piers, and also, with Isolation Bearings. It has been found that the combined effect of soil flexibility and amplification can be significant enough to cause collapse of the integral bridge, which has 10 level of seismic performance when founded on rock outcrop. Similarly in case of isolated bridge, the effect of soil flexibility and amplification has resulted in an increase ofmore than 60% in the peak displacement. Finally, a comparative study between the conventional Force-Based Design Method and the upcoming Performance Based Design Method has been performed for the considered bridge. Acomparison of the FEMA-356 performance levels recommended for building frame components has been made with the limit states suggested by Kowalsky for the bridge columns. Relative accuracy of the two Pushover Analysis procedures originally formulated in ATC-40 and FEMA-273 and further developed in FEMA-440 has been studied by comparing the results with Nonlinear Time History Analysis. Correlation between the Response Reduction Factor (or Behaviour Factor) used in the conventional design practice, and the performance levels ofthe Performance Based Design has also been investigated.