Numerically Extracting Bridge Modal Properties from Dynamic Responses of Moving Vehicles

The dynamic response of moving vehicles on a bridge carries the vibration information of the bridge, which a few researchers have studied to extract the bridge modal properties such as natural frequencies and mode shapes. However, the vehicle response contains not only the frequency characteristics of the bridge and vehicle, but also other information such as the driving-related frequencies and road roughness effects, which makes the extraction difficult. To this end, the present study proposes a method using a specialized test vehicle consisting of a tractor and two following trailers, which can eliminate those effects and more efficiently extract the bridge modal properties. The two trailers towed by the tractor moving along the bridge were used as dynamic response receivers. In order to eliminate the effects previously discussed, the responses of one trailer were subtracted by the responses of the other with a time shift to obtain the residual responses. The residual responses were then processed with the fast Fourier transformation (FFT) and short-time Fourier transformation (STFT) to extract the bridge modal properties. Firstly, a theoretical study was conducted to derive the residual responses of the two trailers in a simplified vehicle-bridge coupled (VBC) system and extract the bridge mode shape squares (MSS) by applying the STFT. Then, numerical studies were conducted to demonstrate and verify the proposed method that is very effective and robust in extracting bridge modal properties, especially under ongoing traffic flows and different road surfaces. Finally, vehicle parameters, such as the trailers' mass and stiffness, spacing, and traveling speed, were studied to design a proper test vehicle for field applications. (C) 2016 American Society of Civil Engineers.