Time-Variant Reliability Assessment Of Prestressed Strand Anchor System Of Suspension Bridge

The suspension bridge’s safety relies on its Anchor system, whose working condition affect the whole bridge’s reliability level. In China, prestressed strand anchor system is widely used on suspension bridge,with cement mortar used for protection In the past. In recent years, a new type of prestressed strand anchor system has been invented, which is replaceable and uses oil to prevent corrosion. This is certainally a technological progress. However, the actual situation of corrosion of such anchor system and its reliability classes in design reference period has been rarely studied or reported. This thesis attempts to apply the theory of reliability with regard to prestressed anchor system under corrosion. A preliminary study has been maded with the main work as follows:By introducing the typical models used for degrading resistance and stochastic load and the ways to get their parameters, the resistance degradation model of strands under corrosion attack was established, based on the gathering and reinvestigating of experimental results improved by different researchers.Four computional methodologies of time-dependent reliability that ae used most often were studied in detail, namely time-integrated method, time-discretized method, time-discretized-integrated method. By applying them to two practical examples, their accuracy and shortcomings were compared carefully.The failure mode of suspension bridge’s prestressed strand anchor system was analyzed, and the degradated resistance of corroded strands as well as the live load were modeled. Thereafter the performance function of the prestressed strand anchor system was established. By calculating the time-dependent reliability index of the prestressed strand anchor system, its risk rate and the random variables’parametric sensibility were analyzed.The steel main cable’s reliability was also calculated to be cpmpared with its corresponding safety factor. Herein, the reliability level of the main cable designed with traditional safety factor method could be discussed.A lifetime optimization methodology for planning the inspection and repair of structures that deteriorate over time was introduced. The optimization was based on minimizing the expected total life-cycle cost while maintaining an allowable lifetime reliability for the structure. This method incorporates all repair possibilities based on an event tree and the time value of money. The methodology was illustrated using the prestressed strand anchor system. An optimum inspection/repair strategy was developed for the anchor system that are deteriorating due to corrosion in an aggressive environment.It is shown in this study that, the reliability of prestressed strand anchor system is sensitive to the corrosion rate. The reliability index will decrease fast in the design reference period if corrosion occurs. It is seen that resistance degradation contributes most to reliability deteriorating by analyzing the sensitivity of the random variables. The main cable designed by safe factor method is reliable despite of area loss due to corrosion in most case. The optimum inspection/repair strategy to the prestressed strand anchor system depends on loss due to failure and deterioration the strands, thus a health monitoring system should be established to inspect the corrosion states in time, so that an efficient assessment of the safety and serviceability of the prestressed strand anchor system, or an inspection/repair Strategy could be made.