The status of bridges which play an important role in the transportation lines is self-evident.The function of bridges not only depends on the design and soundness of the superstructure,but also it relates to the normal work of the substructure.In general,defect of the superstructure are often relevant to those of the substructure.As is well-known,bridges gradually deteriorate and even damage with time under action of the natural environment (atmospheric corrosion,temperature,and humidity changes) and service environment (load increase,material fatigue and damage) (Shu 2007).If design,and construction of bridges was inappropriate,operation or maintenance was not in time,the defects of bridge structures could be aggravated.This impacted on the durability and serviceability of bridges or even may led to collapse of bridges in extreme cases.According to incomplete statistics by the end of2011,China had 658,000bridges,among which there were more than 100,000suffering from defects or working with defects.And in the past 10 years,more than 110 bridges in China collapsed due to different causes.