DYNAMIC ANALYSIS OF HIGHWAY BRIDGES UNDER MULTIPLE RANDOM EXCITATIONS

Abstract

Many long-span bridges throughout the world are subject to very complicated loading, especially those that are located in wind-prone regions or high seismic zones and that carry both trains and road vehicles. Taking into consideration the multiple types of loading that are concerned and the complexity of the loading combinations, the response analysis of long-span bridges under vehicle, wind loading and seismic forces stand as a great scientific challenge. Moreover, mitigation of the studied vibrations generated by those dynamic loads is of more concern because of their several consequences on the construction components safety. This thesis focuses on the bridge vibrations and their mitigation under multiple dynamic loads to be: the vehicles, wind and earthquake. The work was divided into two parts; the first was an analytical study of separated coupled systems which are: the bridge-vehicle, the wind-bridge-vehicle and the bridge-seismic with a numerical simulation for each one. In the second part a comparison study was developed between a bridge structure provided by vibrations mitigation mechanism and the same structure without vibrations absorbers under a strong earthquake. The bridge-vehicle system was modelled as two separated three dimensional subsystems, the equations of motion were established for each model and the interacting forces were determined by the method of moments. A numerical simulation was given to illustrate the analysis using SAP2000 V 16 structural software applied on a cable stayed bridge. The effect of aerodynamic loads applied on the bridge-vehicle coupled system was studied as a second step, the equations of motion of the three dimensional models were extracted and wind forces acting on the bridge and the vehicle were simulated using analytical approaches. To investigate the dynamic response of the vehicle-bridge system under wind actions an incremental-iterative approach was used where the nonlinear aerodynamic curves were simulated by a sequence of piecewise connected linearly. Seismic forces present the most critical loads applied on bridge structure, for that reason, the comparative study on isolated bridges was against earthquake excitation. The study put most emphasis on the time variation of base shear and bearing displacement in order to understand the behaviour of seismically isolated bridges with a comparison between isolated and non isolated bridges. For this purpose seismic forces were applied as a spectrum obtained from recorded strong earthquakes by specialized stations on the three dimensional bridge model using structural software. Three kinds of bearings were studied including N-Z, LRB and FPS since they are considered as the most popular bearings in the field of bridges vibration control.