SEISMIC RESPONSE ANALYSIS OF A CABLE-STAYED BRIDGE

Abstract

Behavior of a cable-stayed bridge under moving loads, seismic and wind forces is greatly dependent upon its structural features. Major structural components of cable-stayed bridge are deck, tower (pylons), cables and abutments/piers. The structure is inherently nonlinear and highly indeterminate. The development and application of computers opened up numerous possibilities for more accurate solutions of these systems for both static as well as dynamic analysis of their behaviour. This requires various elements of cable-stayed bridge to be modeled properly so as to represent the actual behavior as closely as possible. The lighter weight of the bridge, though a disadvantage under heavy wind, is an advantage during an earthquake. However, should uneven settling of the foundations occur during an earthquake or with time, the cable-stayed bridge can suffer damage. Fast development in India is taking place which has necessitated to construct fast erecting, aesthetically pleasing, safer and economical bridges even in seismically active areas. Cable-stayed bridges fulfill most of the above requirements. Many cablestayed bridges are being taken up for construction. In this thesis an investigation has been made using a finite element model to study the 3-D nonlinear seismic behaviour of a cable-stayed bridge with single inclined tower with unusual geometry and height with a special arrangement of cable stays like on one side of the pylon, semi harp shaped cable stays are provided on both the edges of the deck and on the other side of the pylon the cable-stays are anchored at the centre of the deck. Such a cable-stay arrangement is very unsymmetrical about the pylon and special pylon arrangement is necessary. A finite element model ofthe bridge is generated using finite element package SAP 2000. The static non-linear analysis under dead load is essential as a first step for the non-linear seismic analysis. Nonlinear static analysis under the action of static loads i.e. dead and superimposed dead loads is performed. The behavior of composite girder cable stayed bridges under static loads is highly nonlinear. The role of dynamic forces in cable stayed supported bridges is very important more than for any other type of bridge. Such forces can even determine the very feasibility of the project. The vibrations due to wind, traffic and seismic activity can result in inconvenience to users. These physiological effects are generally very subjective

experiences. If vibrations become large, damage can occur too. Analysis of all these dynamic phenomena, including seismic effects, calls for prior knowledge of the free vibration characteristics (frequencies and vibration modes) of the structure and accordinglya free vibrationanalysis has been conducted for the cable-stayed bridge. In general cable stayed bridges are regarded as structures on which vibrations due to earthquake have little effect as they rest on a limited number of point supports (abutments, piers, pylons) which absorb different displacements during seismic action. Differential movements of the supports are thus the causes of the most serious damage suffered by bridges, in particular if the seismic excitation acts along a longitudinal or transverse axis. Such damage, less marked in the case of vertical excitation, generally occurs at the junctions between the deck and the piers. For the present study the cable stayed bridge has a span of 251 m. The tower height is 164 m which is unusual for such spans. Because of the proximity of the bridge site to a major fault, the seismic study of this bridge gains more importance and thus seismic response analysis has been conducted for the cable-stayed bridge. The seismic demands in such unusual height cable-stayed bridges are of concern and the present study deals with the understanding of the seismic behaviour of such cable-stayed bridges along with the evaluation of various components. The study indicated that the seismic demands in the transverse directions are of greater concern. The nonlinearity due to cable sag and P-A effect has pronounced effect on the static and seismic response of cable-stayed bridges. Nonlinear static pushover analysis is used to check the seismic capacity of the pylon pier which is expected to take most ofthe seismic forces generated. The study indicated that the pier can easily withstand the considered earthquake. A parametric study is conducted to check the sensitivity under the transverse seismic excitation. The seismic response of the bridge under various parameters like the height of tower, cable areas, end span boundary conditions and the bearing boundary conditions are studied. The study indicated that the variation of cable areas, boundary condition of central girder and bearing conditions on piers has insignificant effect on the response of cable-stayed bridge.