In recent years,numbers of bridges have been constructed in China.Research on maintaining of these bridges becomes more and more important,especially for long span bridges.A structure usually wont be tested for some small deteriorations and damages until the problem becomes severe and obvious enough.When the necessity of such tests is finally noticed,damages have already severely exacerbated the structure reliability in many cases and some are even on the verge of collapse.Although routine visual inspection is mandatory for important structures in some countries,for instance,bridges in the United States,it is not guaranteed to find all the possible defects.A recent survey by US Federal Highway Administration revealed that at most 68% of the condition ratings were correct and in-depth inspections could not find interior deficiencies considering the fact that vision examination of inspectors has its limitation.