Abstract: Long-term impacts due to traffic loads may result in the fatigue damage of bridge expansion joints that is difficult to repair and replace.However,there are few researches on the fatigue lifetime of modular expansion joints,especially for the middle-steel beam-welded points.In order to solve this problem,the damage and replacement process of the SD-160 modular expansion joints of a bridge is taken as an example and a survey of the bridge traffic load is performed,based on which five different types of simplified vehicle load frequency spectrum are established.Then，the ANSYS finite element software and the Miner fatigue cumulative damage theory are used to estimate the fatigue lifetime of the SD-160 modular expansion joints in the anchorage concrete,the welding points between anchored and pre-embedded bars,as well as the middle welded steel beam.The results show that ( 1) the lifetimes of the anchorage concrete，the middle steel beam only considering the vehicle load and the welding points of embedded bars are respectively 7. 7,29. 3 and 10. 7 years; ( 2) with the increase of the space between two supporting beams,the lifetime of the middle steel beam decreases; ( 3) greater strength of the concrete in the anchorage zone helps to prolong the lifetime of the concrete itself and the welding points between anchored and pre-embedded bars; ( 4) the lifetime of elastic resin concrete is about 2 times that of C50 concrete; and ( 5) according to the replacement program of the SD-160expansion joints,the simulated lifetime of the welded points in the middle steel beam is 6. 2 years，and the service lifetimes of programs Ⅰ and Ⅱ are respectively 7. 7 and 20. 8 years.This research provides a reference for the design and replacement of the expansion joints similar to the investigated one.

Key words: bridge, expansion joint, fatigue damage, fatigue lifetime