Flexibility identification and deflection prediction of a three-span concrete box girder bridge using impacting test data

Abstract

Abstract Deflection is an important parameter for condition assessment of in-service bridges. However, accurately measuring bridge deflections especially that of long span bridges is still a challenging problem. In this article, field tests including multiple reference impact test (MRIT) and static test were performed on a three-span post-tensioned concrete box girder bridge to study structural deflections. Firstly, a new impacting device, which can generate high-magnitude and wide-frequency-band forces, has been developed and employed to execute the MRIT on the bridge. The output data from MRIT was utilized to estimate structural Frequency Response Function (FRF) and to identify deep-level structural parameters including modal scaling factor and structural flexibility, from which structural deflections under arbitrary loads were predicted. Field test results have shown that the developed impacting device can produce FRF with much better reciprocity than the FRF estimated from traditional sledge hammer, which demonstrates the superiority of the developed impacting device for bridge excitation and deflection prediction. Secondly, static test was also conducted on the bridge. The developed long gauge Fiber Bragg Grating (LG-FBG) strain sensors were mounted on the bottom of the box girder bridge. The measured long-gauge strains have been adopted to calculate structural deflections through a modified conjugate beam method which has high precision and there is no error accumulation from other span. Finally, data processing strategy was carried out according to the procedure of structural flexibility identification from MRIT data, and the identified flexibility matrix has been used to predict structural deflections by multiplying a static force vector corresponding to the tested truck load. The well agreement between the predicted deflections and the calculated deflections indicates that the proposed methods can be utilized to obtain structural deflections.