SEISMIC ANALYSIS OF FATIH SULTAN MEHMET SUSPENSION BRIDGE
Nurdan Apaydın Memişoğlu
(Thesis Supervisor: Mustafa Erdik)
Devastating earthquakes takes place in Turkey and other parts of the world. Suspension bridges represent critical nodes of major transportation systems. Especially in important metropolis, bridges have a strategic importance. Bridge failure poses not only the greatest threat of fatalities fiom a single devastating earthquake but causes a substantive interruption of emergency efforts. Istanbul has two suspension bridges and these bridges link two parts of Turkey and join together the European and Asian parts of Istanbul. One of them, Fatih Sultan Mehmet bridge, is a modern type suspension bridge and it is the 11 of the world's longest suspension bridge. The daily trffic of the bridge is approximately 180.000 vehicles. The seismicity of Istanbul and the location of the bridge increase the importance of the evaluation of the dynamic performance of the bridge. In this study three-dimensional finite element model of the bridge structural component is developed and the results of the free vibration analysis is presented. Linear and nonlinear behaviour of suspension bridges can be determined by means of the special algorithms. The three-dimensional finite element model of the bridge is developed using SAP2000 Structural Analysis Program. Natural frequencies and the corresponding mode shapes of the bridge models are determined. Physical and analytical understanding of the dynamic response of a suspension bridge is very dependent on the knowledge of its natural frequencies, darnping and normal mode shapes of vibration. Performing actual tests on hll-scale structures as the only sure way of assessing the dynamic parameters. Dynamic parameters of the Fatih Sultan Mehmet suspension bridge are determined by means of Ambient Vibration Test and Global Positioning System Techniques. Temporal and spectral analyses of recorded motions are also utilized and presented. Experimentally measured data is used for determining the parameters in the equations of motion of a system commonly are called system identification. System dentification integrates experimental, analytical and computational techniques in structural dynamics. In the third chapter dynamic characteristics of the Fatih Sultan Mehmet Bridge are explained and a comparative study is presented fiom analytical investigations, the ambient vibration survey and GPS records. Reasonable agreement is observed to exist between the calculated dynamic characteristics and those identified fiom the ambient vibration survey as well as those fiom the GPS records. The response of this type of bridge to earthquake excitations, especially to incoherent support motions, has special feature due to the complicated interaction of the threedimensional input motions with the whole structure. Furthermore, for these long-period structures, an understanding of the multi-modal contribution to the final response is of extreme importance to provide representative values of the seismic-response quantities. Thus, the problem is the seismic performance evaluation of the bridge subjected to multiple-support excitations. In the fourth chapter of the study, seismic performance of the bridge subjected to earthquake excitations is explained. The seismic performance of the bridge, in which maximum displacements and force levels that the structure and its components can tolerate from relatively stronger earthquake, is assessed. The bridge is analyzed for 200 mode shapes for each of the vertical, longitudinal and lateral directions, using the array 06 and array 07 records 1979 Imperial Valley Earthquake. According to the location of the bridge relative to the fault, the strong ground motion that will affect the Fatih Sultan Mehmet bridge in the most destructive way is considered. In order to achieve this, the strong ground motion record Array 06 that has higher acceleration amplitude values than the strong ground motion record Array 07 is chosen to be implemented as the input motion on the Asian side. In addition the higher amplitude value of the three orthogonal components of Array 06 and Array 07 are applied to the supporting points on the Asian and European side relatively in the lateral directions as the earthquake propagates in the north-south direction. It should be noted that the higher strong ground motion components of the records are used at the lateral direction as the input motion to be on the conservative side of the calculations. The seismic response of the bridge to traveling earthquake excitation is also investigated in the study. An artificial strong motion data is used for excitations. The strong ground motion input at the Asian supports is assumed to propagate with different velocities,. such that it hits the other supporting points (European supports) of the structure with no change of shape. A comparative study is presented for specified interval times. Finally a new vibration monitoring system encompassing the latest technology is designed and installed for Fatih Sultan Mehrnet Bridge. In order to detect and locate damage in structural components and to provide this information quickly, 12 acceleration transducers with x, y, z channels are instrumented to the bridge at the beginning of November 2001 by a team constituted from both Earthquake Engineering Department and 17. Division of the General Directorate of Turkish Highways. The system consists of the sensor (accelerometer) subsystem, the data acquisition 1 recording / monitoring subsystem. Temporal and spectral analyses of recorded motions fiom instrumentation are also presented. A comparative study is presented between analytical investigations instrumentation records. Reasonable agreement is observed to exist between the calculated dynamic characteristics and those identified from instrumentation records.