A Study On The Working Conditions Analysis For The Bridge Indirect Measurement Method

The damage level of a bridge structure can be assessed by comparing the changes in the modal parameters,i.e.frequencies and mode shapes,of the bridge.The conventional approach for measuring the modal parameters of a bridge is called the direct approach,since the vibration sensors need to be directly mounted on the bridge.The indirect approach was first put forward by Prof.Yang and co-workers in 2004,which extracts the frequencies and mode shapes of the bridge indirectly from the dynamic response of a test vehicle during its passage over the bridge.This new approach has advantages of being convenient and rapid in operation and economical in expense.The assumption that the test vehicle is modeled as an undamped single-degree-of-freedom lumped mass moving at a constant speed was adopted in the derivation of the analytical solution of the bridge and vehicle’s interaction responses.The finite element simulation programs for the dynamic response of the vehicle-bridge system are implemented using the vehicle-bridge interaction(VBI)element in the Matlab environment.In this study,three working conditions are considered,i.e.,damped vehicle,non-uniform speed and two-degree-of-freedom vehicle model.The purpose is to analyze their effects on the dynamic response of the vehicle-bridge system and on the measuring results of the indirect approach.The main conclusion and innovative points of this paper are as follows:(1)Using Hann’s window and band-pass filter to distill the component responses associated with the bridge frequencies.Hann’s window can be used to remove the obscured interference from spectral leakage to the high-order bridge frequencies,which is helpful for obtaining better high-order mode shapes of the bridge even at high vehicle speeds.Thus,the constraint on the low vehicle speed mentioned in the previous research was basically removed.(2)Using the vehicle location x on the bridge,rather than time t,as the variable in the equation of the instantaneous amplitude history of the bridge component response.Theoretically,the instantaneous amplitude history is representative of the mode shape of the bridge whether the vehicle is moving at a uniform speed or non-uniform speed.This gives another viewpoint of the equation of instantaneous amplitude history,while removing the constraint on the constant speed requirement in the previous research.(3)Three working conditions,i.e.damped vehicle,non-uniform speed and two-degree-of-freedom vehicle model,are considered to study their effects on the indirect measurement.The phenomenon and concluding remarks drawn in this paper provide a useful reference for future field tests and researches of the indirect measurement.