Showing papers by "Yong Xia published in 2015"

L1 regularization approach to structural damage detection using frequency data

Abstract: Structural damage often occurs only at several locations that exhibit stiffness reduction at sparse elements out of the large total number of elements in the entire structure. The conventional vibr...

Hysteretic behaviour of tubular T-joints reinforced with doubler plates after fire exposure

Abstract: The hysteretic behaviour of tubular T-joints reinforced with doubler plates was experimentally and numerically investigated in this study. Two specimens were tested to failure under cyclic axial load at the brace end, one under ambient temperature and the other after fire exposure. Specimens with identical material and geometric properties were compared. The failure modes of the specimens were cracking along the weld toe at the intersection of the plate and brace. The results indicated that the hysteretic behaviour and energy dissipation of the joint after fire exposure were smaller than those at ambient temperature. The finite element package ABAQUS was then used to simulate the joint specimens. In a comparison of the hysteretic curve, skeleton curve, energy dissipation and failure location, the simulation and experimental results were in good agreement. The finite element method was subsequently used to carry out a parametric study. Parameters τ and e had little influence on the post-fire hysteretic behaviour of the joint, but joints with a large γ or small α, β, or ξ values had a low capacity for hysteretic behaviour after fire exposure.

Measurement of rivulet movement on inclined cables during rain–wind induced vibration

Abstract: The large amplitude vibration of stay cables has been observed in several cable-stayed bridges under the simultaneous occurrence of rain and wind, which is called rain–wind induced vibration (RWIV). During RWIV, the upper rivulet oscillating circumferentially on the inclined cable surface is widely considered to have an important role in this phenomenon. However, the small size of rivulets and high sensitivity to wind flow make the measurement of the rivulet movement challenging. This study proposes a digital image processing method to measure the rivulet movement in wind tunnel tests. RWIV of a cable model was excited during the test and a digital video camera was used to record the video clips of the rivulets, from which the time history of the rivulet movement along the entire cable is identified through image processing. The oscillation amplitude, equilibrium position, and dominant frequency of the upper rivulet are investigated. Results demonstrated that the proposed non-contact, non-intrusive measurement method is cost-effective and has good resolution in measuring the rivulet vibration. Finally the rivulet vibration characteristics were also studied when the cable was fixed. Comparison demonstrates the relation between the upper rivulet and cable vibration.

Vibration of Timoshenko beam on hysteretically damped elastic foundation subjected to moving load

Abstract: The vibration of beams on foundations under moving loads has many applications in several fields, such as pavements in highways or rails in railways. However, most of the current studies only consider the energy dissipation mechanism of the foundation through viscous behavior; this assumption is unrealistic for soils. The shear rigidity and radius of gyration of the beam are also usually excluded. Therefore, this study investigates the vibration of an infinite Timoshenko beam resting on a hysteretically damped elastic foundation under a moving load with constant or harmonic amplitude. The governing differential equations of motion are formulated on the basis of the Hamilton principle and Timoshenko beam theory, and are then transformed into two algebraic equations through a double Fourier transform with respect to moving space and time. Beam deflection is obtained by inverse fast Fourier transform. The solution is verified through comparison with the closed-form solution of an Euler-Bernoulli beam on a Winkler foundation. Numerical examples are used to investigate: (a) the effect of the spatial distribution of the load, and (b) the effects of the beam properties on the deflected shape, maximum displacement, critical frequency, and critical velocity. These findings can serve as references for the performance and safety assessment of railway and highway structures.

Damage assessment of shear connectors with vibration measurements and power spectral density transmissibility

Abstract: Shear connectors are generally used to link the slab and girders together in slab-on-girder bridge structures. Damage of shear connectors in such structures will result in shear slippage between the slab and girders, which significantly reduces the load-carrying capacity of the bridge. Because shear connectors are buried inside the structure, routine visual inspection is not able to detect conditions of shear connectors. A few methods have been proposed in the literature to detect the condition of shear connectors based on vibration measurements. This paper proposes a different dynamic condition assessment approach to identify the damage of shear connectors in slab-on-girder bridge structures based on power spectral density transmissibility (PSDT). PSDT formulates the relationship between the auto-spectral densities of two responses in the frequency domain. It can be used to identify shear connector conditions with or without reference data of the undamaged structure (or the baseline). Measured impact force and acceleration responses from hammer tests are analyzed to obtain the frequency response functions at sensor locations by experimental modal analysis. PSDT from the slab response to the girder response is derived with the obtained frequency response functions. PSDT vectors in the undamaged and damaged states can be compared to identify the damage of shear connectors. When the baseline is not available, as in most practical cases, PSDT vectors from the measured response at a reference sensor to those of the slab and girder in the damaged state can be used to detect the damage of shear connectors. Numerical and experimental studies on a concrete slab supported by two steel girders are conducted to investigate the accuracy and efficiency of the proposed approach. Identification results demonstrate that damages of shear connectors are identified accurately and efficiently with and without the baseline. The proposed method is also used to evaluate the conditions of shear connectors in a real composite bridge with in-field testing data.

Integration of health monitoring and vibration control for smart building structures with time-varying structural parameters and unknown excitations

Abstract: When a building structure requires both health monitoring system and vibration control system, integrating the two systems together will be cost-effective and beneficial for creating a smart building structure with its own sensors (nervous system), processors (brain system), and actuators (muscular system). This paper presents a real-time integrated procedure to demonstrate how health monitoring and vibration control can be integrated in real time to accurately identify time-varying structural parameters and unknown excitations on one hand, and to optimally mitigate excessive vibration of the building structure on the other hand. The basic equations for the identification of time-varying structural parameters and unknown excitations of a semi-active damper-controlled building structure are first presented. The basic equations for semi-active vibration control of the building structure with time-varying structural parameters and unknown excitations are then put forward. The numerical algorithm is finally followed to show how the identification and the control can be performed simultaneously. The results from the numerical investigation of an example building demonstrate that the proposed method is feasible and accurate.

Structural damage and force identification under moving load

Abstract: Structural damage and moving load identification are the two aspects of structural system identification. However, they universally coexist in the damaged structures subject to unknown moving load. This paper proposed a dynamic response sensitivity-based model updating method to simultaneously identify the structural damage and moving force. The moving force which is equivalent as the nodal force of the structure can be expressed as a series of orthogonal polynomial. Based on the system Markov parameters by the state space method, the dynamic response and the dynamic response derivatives with respect to the force parameters and elemental variations are analytically derived. Afterwards, the damage and force parameters are obtained by minimizing the difference between measured and analytical response in the sensitivity-based updating procedure. A numerical example for a simply supported beam under the moving load is employed to verify the accuracy of the proposed method.Structural damage and moving load identification are the two aspects of structural system identification. However, they universally coexist in the damaged structures subject to unknown moving load. This paper proposed a dynamic response sensitivity-based model updating method to simultaneously identify the structural damage and moving force. The moving force which is equivalent as the nodal force of the structure can be expressed as a series of orthogonal polynomial. Based on the system Markov parameters by the state space method, the dynamic response and the dynamic response derivatives with respect to the force parameters and elemental variations are analytically derived. Afterwards, the damage and force parameters are obtained by minimizing the difference between measured and analytical response in the sensitivity-based updating procedure. A numerical example for a simply supported beam under the moving load is employed to verify the accuracy of the proposed method.

Analysis of dynamic characteristics of the Canton Tower under different earthquakes

Abstract: This paper presents the experimental modal analysis of the Canton Tower from the ambient vibration measurements under different earthquake excitation conditions. Firstly, the time history and Power Spectral Density (PSD) of the acceleration response under different earthquake excitations have been compared. Secondly, two output-only modal identification techniques are applied to the field measurements under earthquake excitations to identify the dynamic properties of the tower, namely, the Frequency Domain Decomposition (FDD) and the Subspace Stochastic Identification (SSI). At last, the Hilbert-Huang transform (HHT) method is applied to obtain the instantaneous frequencies and energy distribution under different loadings in the time-frequency domain and the results are compared with those from wavelet transform. The modal properties of the Canton Tower presented in this paper can be used as baseline for structural health monitoring and the analysis of dynamic characteristic of the high-rise structures un...