Study Of Long Span Cable-Stayed Bridges Stochastic Seismic Response And Dynamic Reliability

The main span of cable-stayed bridges is developing from several hundred metres to nearly 1000 metres, and in the 21th century it is even beyond 1000 meters, such as Tatara Bridge(890 metres), Hong Kong Stonecutters Bridge(1018 metres), Su Tong Yangtze River Highway Bridge(1088 metres). With the rapid increasing of the length of main span, a large number of complex problems are produced. As the main load of superlong span structure,earthquake action is the control factor of structural design. It is becoming complex for earthquake dynamics as the main span becomes longer and longer without criterion to be followed. Consequently, seismic analysis is becoming more important than ever.The three elements of earthquake ground motion (duration, intensity,frequency spectrum) show uncertainty. Obviously, it is reasonable to apply the theory of stochastic vibration to the seismic analysis of cable-stayed bridges. The advantages of stochastic vibration analysis are as follows:(l) The results of stochastic vibration are resoponse value, it includes tatistical information about a large number of time-history curves. So its results are more representative and universal than one time-history curve. (2) It is simpler for the mean value of maxinum by stochastic vibration analysis than those by time-history analysis. (3) The input of stochastic vibration analysis is power spectrum, and it analyses problems in the point of energy. As a result, the method of stochastic vibration analysis will be used widely in earthequake engineering research field.It is the irregularity of the earthquake ground motion that decides to change it from deterministic model to stochastic processes model. Based on stochastic vibration theory and combined with SuTong Yangtze River Highway Bridge, the stochastic seismic response of superlong span cable-stayed bridge is developed, including the following several aspects:1. The first question for stochsatic seismic analysis is the input of earthquake ground motion. Some expressions about earthquake ground motion model are introduced in Chapter Two, and combined with some scholastic research to compute parameters of models. The above research provides references for future stochastic seismic analysis.2. In Chapter Three, computation formula of stochastic responses of structures under seismic excitations is presented based on stochastic vibration theory. The statistic of structural dynamic response of cable-stayed bridges under seismic ground motions is investigated based on the above computation formula.3. Based earthequake stochastic models which are used widely now, the input of earthquake ground motion is indicated by Duxiuli power spectral model improved on Kanai-Tajimi model. The structural dynamic responses under uniform excitations and seismic ground motions with spatial variations,including local site effect, incoherence effect and wave passage effect, are studied by stochastic vibration method.The results indicate that the vertical bending moment of girders ,especailly the girders of middle span,are influenced obviously by incoherence effect than those by local site effect. The axial force of girder is influenced greatly by wave passage effect and increases 70% than the response under uniform excitation. However, the vertical shear force is influnced lightly.The root mean square of vertical displacement for middle span girders is increased 74% by wave passage effect,60% by incoherence effect, 13% by local site effectc, compared with those responses under nuiform seismic excitation. For different responses of internal force and displacement, the impact and the rules are different for seismic ground motion with spatial varations. Therefore, in view of different cases ,the analyses would have diferent results.4. The stochastic vibration analysis of Sutong brudge duly takes into account the strucural elastic modulus , pile-soil-structure interaction,seismic ground motion models, the cable vibration and the number and nature of modes, which provide an important reference for seismic design and analysis of long span cable-stayed bridges.5. It is the trend to design bridges through probabilistic analysis method on the on the basis of reliability theory. In Chapter Six, based on stochastic vibration theory, seismic dynamical reliability of long span cable-stayed bridges is studied through probabilistic analysis method, which provide an important theoretical reference for improving seismic design method.Finally, the research work of this paper is summarized and some conclusions about the study are drawn, and the further research problems are indicated, too.