Wei-Xin Ren

State-of-the-art review on seismic induced pounding response of bridge structures

Seismic induced pounding damage to bridge structures was repeatedly observed in many previous major earthquakes. To avoid this adverse effect, extensive research efforts have been made by many researchers. This paper presents a state-of-the-art review in this field. It includes a brief review of the numerical modeling of bridge structures and impact models, numerical simulation of pounding responses between different components of bridge structures, experimental investigations, and pounding mitigation methods.

Strain modes based dynamic displacement estimation of beam structures with strain sensors

This paper presents a new technique to estimate the dynamic displacement based on strain mode shapes of beam structures with strain sensors. Strain mode shapes are first estimated from the cross-correlation function of the measured dynamic strain data. Then, the displacement mode shapes can be estimated from the strain mode shapes based on the displacement-strain relation. For an oscillating beam structure under service conditions, the dynamic response can be expressed as the superposition of their corresponding mode shapes weighed by the corresponding modal coordinates. Thus, by knowing the strain mode shapes and strain data, the corresponding modal coordinates can be estimated. The dynamic displacement can then be estimated by the obtained displacement mode shapes and modal coordinates. This method is verified by numerical simulations of a simply supported beam subjected to impulsive excitation and earthquake excitation. Experimental tests of a simply supported beam under various hammering excitations are also conducted to verify the effectiveness of the proposed method. Both the numerical and test results show that the proposed method can estimate the dynamic displacement of beam structures with high accuracy.

Time-Varying Linear and Nonlinear Structural Identification with Analytical Mode Decomposition and Hilbert Transform

AbstractAnalytical mode decomposition (AMD) of a time series concerning any preselected bisecting frequency with Hilbert transform has been developed for closely spaced multicomponent signal decomposition. For this class of structures, it is often challenging, if not impossible, to apply empirical mode decomposition. In this study, the instantaneous structural frequencies are directly derived from the decomposed modal responses for systems with single and multiple degrees of freedom with both free and force vibrations, based on AMD combined with Hilbert transform analysis. The results show that the slow varying component of the instantaneous frequency of the signal is approximately equal to the instantaneous frequency of the systems for slowly time varying linear or weakly nonlinear structures. Both numerical simulations and experimental tests show that the proposed method is capable of tracking the frequency variations with high accuracy for time varying linear structures or weakly nonlinear structures.

Deflection Estimation of Bending Beam Structures Using Fiber Bragg Grating Strain Sensors

This paper uses strains measured by fiber Bragg grating (FBG) sensors to estimate the static or dynamic deflection curve of bending beam structures. The deflection estimation method is only based upon the geometric equations of a beam structure without knowing the material information. At each cross section of a beam structure, two FBG strain sensors are installed, and the curvature at the cross section can be estimated by using the two measured strains from the geometric equation. Then the curvature function is assumed as a polynomial function and the coefficients can be estimated using least squares method. Finally, the deflection curve is estimated by integrating the curvature function twice. For dynamic deflection estimation, since only the geometric equations are used, and at each time step, the geometric equations can be also used for a dynamic system. Therefore, at each time step, the deflection can be estimated using the proposed method and the dynamic deflection can be finally obtained. The method is verified by the simulations of a continuous beam under static loads and experimental tests of a simply supported beam under various static loads. A simply support beam under moving loads is also simulated to verify the method for dynamic deformation estimation. All the numerical and test results show that the method can estimate the static and dynamic deflection curve of beam structures with a high accuracy.

Seismic response of a concrete filled steel tubular arch bridge to spatially varying ground motions including local site effect

The construction of concrete filled steel tubular (CFST) arch bridge has become widespread all over the world and especially in China since 1990. This paper studies the nonlinear seismic response of a CFST arch bridge on a canyon site subjected to multi-component spatially varying ground motions. The three-dimensional (3D) finite element (FE) model of the CFST arch bridge is developed with consideration of the material and geometric nonlinearities of the arch ribs. The spatially varying ground motions with consideration of wave passage effect, coherency loss effect and local site effect are stochastically simulated based on the combined one-dimensional (1D) wave propagation theory and spectral representation method. The effects of multi-component earthquake excitations, spatial variations of ground motions and varying site conditions on the seismic response of the CFST arch bridge are analysed. Numerical results show that for a reliable seismic analysis of a CFST arch bridge, multi-component earthquake excitations with consideration of ground motion spatial variations and local soil conditions should be considered.

Hilbert low-pass filter of non-stationary time sequence using analytical mode decomposition

In this paper, the recently developed analytical mode decomposition with a constant or time-varying cutoff frequency is extended into the decomposition of a non-stationary discrete time sequence. The discretization of the signal and the selection of the cutoff frequency may cause the failure of low frequency component extraction. In this study, to eliminate the effects of the signal discretization, the one-step, two-step, and four-step low-pass filters with cutoff frequencies are proposed. Based on the theoretical derivation, the previous one-step low-pass filter is effective only when the cutoff frequency is not greater than a quarter of the sampling frequency and the maximum frequency of the signal not greater than a half of the sampling frequency. In this study, if the cutoff frequency is less than or equal to a quarter of the sampling frequency, a two-step low-pass filter is proposed to extract the low frequency component. If the cutoff frequency is greater than a quarter of the sampling frequency, a four-step low-pass filter with frequency shifting process is proposed. When the time-varying cutoff frequency is not always larger than or less than a quarter of the sampling frequency, a sufficient condition, which is the sampling frequency is greater than four times of the maximum frequency of the signal component, is provided in this study. Two numerical examples are used to validate the effectiveness of the proposed low-pass filters. Both the theoretic derivation and numerical simulations show that the proposed filters can analytical extract the discrete low frequency component with an appropriate cutoff frequency.

Time-varying system identification of high voltage switches of a power substation with slide-window least-squares parameter estimations

This paper is aimed at identifying the time-varying parameters and ultimate behavior of high voltage switch structures based on a series of full-scale shake table tests with harmonic excitations. Each structure involves a mechanical device for switch-on and switch-off, a friction-based switch, and three porcelain pillars. To identify the structural properties over time, a novel slide-window least-squares estimation method is developed. Each time-varying parameter is firstly approximately expressed by a simple polynomial or exponential function with time in a short slide-window. The time-invariant coefficients of the polynomial or exponential function are then estimated using a least-squares method. Finally, the time-varying parameters can be simply calculated from the estimated polynomial or exponential function. The proposed method is validated by simulated one- and two-story buildings with three kinds of time-varying parameters (stiffness varying abruptly, gradually, and periodically) under earthquake excitations. The application of the proposed method to the tested switch structures demonstrated that the time-varying fundamental frequency of the structures decreased from 7.5 to 6.5 Hz near resonance, which is consistent with the shake table test observations under an excitation of 1.27 and 2.54 mm in stroke. During the shake table tests, all switch structures failed at the bottom of the mechanical device under cyclic loading.

Experimental and three-dimensional finite element method studies on pounding responses of bridge structures subjected to spatially varying ground motions

Pounding and unseating damages to bridge superstructures have been commonly observed in many previous major earthquakes. These damages can essentially attribute to the large closing or opening relative displacement between adjacent structures. This article carries out an experimental study on the pounding responses of adjacent bridge structures considering spatially varying ground motions using a shaking table array system. Two sets of large-scale (1:6) bridge models involving two bridge frames were constructed. The bridge models were subjected to the stochastically simulated ground motions in bi-direction based on the response spectra of Chinese Guideline for Seismic Design of Highway Bridge for three different site conditions, considering three coherency levels. Two types of boundary conditions, that is, the fixed foundation and rocking foundation, were applied to investigate the influence of the foundation type. In addition, a detailed three-dimensional finite element model was constructed to simulate an experimental case. The nonlinear material behavior including strain rate effects of concrete and steel reinforcement is included. The applicability and accuracy of the finite element model in simulating bridge pounding responses subjected to spatially varying ground motions are discussed. The experimental and numerical results demonstrate that non-uniform excitations and foundation rocking can affect the relative displacements and pounding responses significantly.

Effectiveness of Stoppers and Shock Absorber Devices in Mitigating Earthquake Damage to Curved Viaducts with Steel Bearing Supports

Viaducts with steel bearings are subject to damage during earthquakes. Damage to conventional steel bearings could cause viaduct decks to collapse. This study attempts to prevent large displacements by installing steel stoppers at both sides of roller bearings. Shock absorber devices (SADs) are also installed between roller bearings and stoppers to reduce pounding forces. The study then explores the effectiveness of different stopper gaps and SADs installed between roller bearings and stoppers in mitigating viaduct damage. The effectiveness of stoppers without and with SADs is also investigated. The pounding forces between roller bearings and stoppers, the displacements on the tops of piers, the displacements of superstructures, and the bending moment–curvature relationship at pier bases are evaluated accordingly. Numerical results reveal that earthquake damage to viaducts with small stopper gaps is not significant. The application of SADs is also found to significantly mitigate viaduct damage.

Structural Modal Parameter Identification from Forced Vibration with Analytical Mode Decomposition

In this paper, a recently developed analytical mode decomposition method is proposed for modal parameters identification of structures subjected to impulsive, harmonic, and ambient excitations. The decomposed modal response for a structure subjected to an impulsive load is free vibration, thus, the instantaneous amplitude and phase angle of each decomposed modal response can be directly used to identify the modal parameters by using the least-squares fit procedure. For a structure subjected to a harmonic load, the transient response is first extracted from the measured response. Then, the extracted transient response can be decomposed into modal responses and the modal parameters can be evaluated. The proposed method in combination with the conventional random decrement technique is proposed for modal parameter identification for structures under ambient vibration. The random decrement technique is used to extract the free vibration information from which modal parameters are evaluated. A 3 degree-of-freedom mechanical system with closely-spaced modes subjected to impulsive, harmonic, and ambient loads is simulated as a numerical example. The new method is then validated with shake table testing of a 3-story building frame subjected to white noise and earthquake excitations. Both experiments and simulations showed high accuracy and effectiveness of the new method for structural modal parameters identification.