AbstractThe deflection of a bridge under a dominant load, called key deflection is an important parameter that reflects the load-bearing capacity of in-service bridges. The impact vibration test is a good candidate for obtaining structural flexibility as well as deflection. However, traditional impact tests require accelerometers to be installed on the full bridge, which makes the test work very complicated. To address this issue, mobile impact testing with optimized substructures was proposed in this paper for rapid estimation of bridge key deflection. Using the basis that the deterioration of an in-service bridge is suitable for structural perturbation theory, the key deflection of an in-service bridge can be approximately expressed in the form of the weighted superposition of several main eigenvectors, which are calculated from the flexibility matrix of the structural finite element model. An optimized substructural impact test and virtual loading method were proposed to determine the weighting coefficients corresponding to the main eigenvectors and to obtain bridge key deflection. A numerical simulation illustrates that the proposed method can estimate bridge key deflection under different structural damage conditions and dominant loads. To verify the effectiveness of the proposed method in practice, the impact testing data of a continuous girder bridge are analyzed. The same key deflection estimation quality can be obtained from both the full bridge impact test and the proposed test method with a limited substructural area, which indicates that the proposed optimized substructural testing method greatly improves the test efficiency while ensuring the estimation accuracy.