Xinqun Zhu

Damage Detection in Simply Supported Concrete Bridge Structure Under Moving Vehicular Loads

A method for damage detection of a simply supported concrete bridge structure in time domain is presented using the interaction forces from the moving vehicles as excitation. The vehicular loads are modeled as a group of vehicle-bridge interaction forces moving at a prescribed velocity, and the bridge is modeled as an Euler-Bernoulli beam simply supported at both ends. A damage function is used to simulate the crack damage in the reinforced concrete beam. The vehicle-bridge interaction forces and the structural damage in the bridge deck are identified from the measured responses in sequence of iteration without prior knowledge of the moving loads. The effects of parameters of the vehicle-bridge system and measurement noise on the damage detection result are studied. Simulation results show that the method is effective and noise insensitive to damage detection in the concrete bridge structure under moving vehicular loads.

Identification of Moving Loads on an Orthotropic Plate

A method is presented to identify indirectly loads moving on an orthotropic plate. The loads are in a group of two forces or four forces. The dynamic behavior of the plate under the action of these moving loads is analyzed. A method to identify these moving forces from the dynamic responses of the plate is developed basing on the modal superposition principle, and Tikhonov regularization procedure is applied to provide bounds to the solution in the time domain. Prior knowledge on the modal properties of the plate and the velocity of loads is required. The errors in the identified individual loads are discussed. The effect of different combinations of measuring locations on the identification is studied. Numerical results show that acceleration responses would give better and acceptable results than strain measurements.

Structural Health Monitoring Based on Vehicle-Bridge Interaction: Accomplishments and Challenges

The use of the vehicle-bridge interaction (VBI) data for structural health monitoring has received considerable interest in the last decade. Compared with the traditional bridge health monitoring, the VBI based approach allows the target bridges to be monitored or assessed under operating conditions. The VBI system has time-variant features and the vehicle can serve as a moving exciter and a mobile sensor in the system. Many bridge damage identification techniques based on VBI have been developed, and they could be divided into three categories, namely, technique based on the bridge responses, technique based on the vehicle responses and technique based on both the vehicle and bridge responses. This paper presents a review on the structural health monitoring based on VBI and the challenges for its general implementation in practice.

DYNAMIC ASSESSMENT OF UNDERWATER PIPELINE SYSTEMS USING STATISTICAL MODEL UPDATING

This paper presents a statistical model updating technique for damage detection of underwater pipeline systems via vibration measurements. To verify the reliability of the method, laboratory tests of a scaled pipeline model were carried out in a towing tank. The model includes a plastic pipe and some removable springs which are designed and fabricated to link the pipe and the steel base to simulate the bedding conditions. Different damage scenarios, in terms of location and severity of scouring under the pipe, were simulated by removing one or several springs. The natural frequencies, damping ratios and mode shapes of the pipeline system were extracted from the measured vibrations using a stochastic subspace identification technique. Both the numerical and the experimental results show that the method is effective and reliable in identifying the underwater pipeline bedding conditions and the damage in the pipe structure.

DAMAGE DETECTION OF RC SLABS USING NONLINEAR VIBRATION FEATURES

Studied herein are the signatures of nonlinear vibration characteristics of damaged reinforced concrete structures using the wavelet transform (WT). A two-span RC slab built in 2003 was tested to failure in the laboratory. Vibration measurements were carried out at various stages of structural damage. The vibration frequencies, mode shapes, and damping ratios at each loading stage were extracted and analyzed. It is found that the vibration frequencies are not sensitive to small damages, but are good indicators when damage is severe. The dynamic responses are also analyzed in the time–frequency domain by WT and the skeleton curve is constructed to describe the nonlinear characteristics in the reinforced concrete structures. The results show that the skeleton curves are good indicators of damage in the reinforced concrete structures because they are more sensitive to small damages than vibration frequencies.

Spectral Element Modelling of Wave Propagation with Boundary and Structural Discontinuity Reflections

Spectral element method is very efficient in modelling high-frequency stress wave propagation because it works in the frequency domain. It does not need to use very fine meshes in order to capture high frequency wave energy as the time domain methods do, such as finite element method. However, the conventional spectral element method requires a throw-off element to be added to the structural boundaries to act as a conduit for energy to transmit out of the system. This makes the method difficult to model wave reflection at boundaries. To overcome this limitation, imaginary spectral elements are proposed in this study, which are combined with the real structural elements to model wave reflections at structural boundaries. The efficiency and accuracy of this proposed approach is verified by comparing the numerical simulation results with measured results of one dimensional stress wave propagation in a steel bar. The method is also applied to model wave propagation in a steel bar with not only boundary reflection, but also reflections from single and multiple cracks. The reflection and transmission coefficients, which are obtained from the discrete spring model, are adopted to quantify the discontinuities. Experimental tests of wave propagation in a steel bar with one crack of different depths are also carried out. Numerical simulations and experimental results show that the proposed method is effective and reliable in modelling wave propagation in one-dimensional waveguides with reflections from boundary and structural discontinuities. The proposed method can be applied to effectively model stress wave propagation for structural damage detection.

Spectral Element Model Updating for Damage Identification Using Clonal Selection Algorithm

A spectral element model updating procedure is presented to identify damage in a structure using Guided wave propagation results. Two damage spectral elements (DSE1 and DSE2) are developed to model the local (cracks in reinforcement bar) and global (debonding between reinforcement bar and concrete) damage in one-dimensional homogeneous and composite waveguide, respectively. Transfer matrix method is adopted to assemble the stiffness matrix of multiple spectral elements. In order to solve the inverse problem, clonal selection algorithm is used for the optimization calculations. Two displacement-based functions and two frequency-based functions are used as objective functions in this study. Numerical simulations of wave propagation in a bare steel bar and in a reinforcement bar without and with various assumed damage scenarios are carried out. Numerically simulated data are then used to identify local and global damage of the steel rebar and the concrete-steel interface using the proposed method. Results show that local damage is easy to be identified by using any considered objective function with the proposed method while only using the wavelet energy-based objective function gives reliable identification of global damage. The method is then extended to identify multiple damages in a structure. To further verify the proposed method, experiments of wave propagation in a rectangular steel bar before and after damage are conducted. The proposed method is used to update the structural model for damage identification. The results demonstrate the capability of the proposed method in identifying cracks in steel bars based on measured wave propagation data.

Dynamic axle and wheel loads identification: laboratory studies

Two methods have been reported by Zhu and Law to identify moving loads on the top of a bridge deck. One is based on the exact solution (ESM) and the other is based on the finite element formulation (FEM). Simulation studies on the effect of different influencing factors have been reported previously. This paper comparatively studies the performances of these two methods with experimental measurements obtained from a bridge/vehicle system in the laboratory. The strains of the bridge deck are measured when a model car moves across the bridge deck along different paths. The moving loads on the bridge deck are identified from the measured strains using these two methods, and the responses are reconstructed from the identified loads for comparison with the measured responses to verify the performances of these methods. Studies on the identification accuracy due to the effect of the number of vibration mode used, the number of measuring points and eccentricities of travelling paths are performed. Results show that the ESM could identify the moving loads individually or as axle loads when they are travelling at an eccentricity with the sensors located close to the travelling path of the forces. And the accuracy of the FEM is dependent on the amount of measured information used in the identification.

Dynamic assessment of shear connection conditions in slab-girder bridges by Kullback-Leibler distance

Shear connectors are widely used in composite bridges that provide composite action. Their damage will reduce the load-carrying capacity of the structure. In this study, a novel method based on Kullback-Leibler distance (KLD) was developed to assess the integrity of the shear connectors. A bridge model was constructed in the laboratory and some removable anchors were specially designed and fabricated to link the beams and slab that were cast separately. Each anchor consists of a threaded bar that penetrates through the soffit of the beam and ties up into an embedded nut cap to simulate a shear connector in the real bridges. Different damage scenarios were introduced by pulling out some connectors. Vibration tests were carried out in each damage scenario. Various damage detection methods have been applied and results show that the method was able to detect all the assumed damage scenarios successfully and consistently.

Measurement System With Accelerometer Integrated RFID Tag for Infrastructure Health Monitoring

This paper presents a measurement system for measuring dynamic acceleration of infrastructure remotely using semipassive radio-frequency identification (RFID) tag. This measurement is critical to the vibration-based method for infrastructure health monitoring. Design considerations of accelerometer integrated ultrahigh-frequency RFID tag and dynamic acceleration measurements through an RFID wireless link are discussed. Measurement results of the system for a structural specimen have shown that it is capable of acquiring data which provides the information of natural frequency of the structural specimen. Moreover, the system can distinctively identify the state changes of the structural specimen by natural frequency shifts. These results are benchmarked against the results obtained with two commercial systems. It is shown that the standard deviation of the measurement of the natural frequency is ±0.01 Hz which is very close to the standard deviation of the commercial measurement systems.