中文摘要：

由于长时间的使用、超载、环境侵蚀以及性能劣化等人为或自然因素的作用,既有桥梁在服役期间会出现各种结构损伤。如何对其承载能力及安全性能进行评估已成为世界各国桥梁工程界普遍关心的热点问题之一。该文简要介绍了用于既有结构安全评估的结构可靠性理论,并在该理论框架下分析了既有桥梁评估的特点和与新建桥梁设计的不同之处。在此基础上,讨论了结构可靠性理论在既有桥梁评估规范当中具体应用,并具体比较了中美两国桥梁评估规范间的异同,指出：1）依据实测的材料强度、几何尺寸、整体效应,并考虑构件损伤造成的不确定性增加对既有桥梁承载力进行评价;2）按照多层次的车辆荷载等级,考虑实测车流评估既有桥梁的安全状况,这两点是美国规范先进于我国规范的地方,也是我国规范发展的方向。既有桥梁的承载历史是对其承载能力的证明,依据承载历史对既有桥梁的承载力进行修正能有效提高既有桥梁的安全等级,减少不必要的加固和限行。该文最后针对我国车辆荷载的特点,通过可靠性分析提出了依据实测日通行车辆数ADTT修正活荷载分项系数的方法,并对依据既有桥梁承载历史修正其承载力提出了建议。

英文摘要：

Bridge structures are at risk from aging,leading to structural deterioration from service demands,from increasing traffic and heavier loads,from aggressive environmental attack and other physical mechanisms.The loading capacity and safety level of existing bridges are of world-wide concern.This paper introduced briefly the structural reliability theory,which is currently a powerful approach for condition assessment of existing bridges,and presented the difference between new bridge design and existing bridge assessment in the framework of reliability theory.By comparing Chinese code with American Code,the paper pointed out： 1） the load capacity of existing bridges is evaluated based on the in-site measured strength dimension,system effect and the increased uncertainty due to the development of deterioration;2） the safety condition of existing bridges is assessed multi-levelly according to the measured Average Daily Truck Traffic（ADTT）;are two points in which American Code is more advanced than Chinese Code.The loading history of existing bridges is a kind of proof test,which may increase effectively the evaluated load capacity of bridges,and avoid the unnecessary posting or strengthening.At the end,the paper proposed the modification of a live-load factor according to ADTT and the modification of load capacity of existing bridges according to the loading history.