Jan Ming Ko

Dynamic response of damper-connected adjacent buildings under earthquake excitation

A formulation of the multi-degree of freedom equations of motion for fluid damper-connected adjacent multi-story buildings under earthquake excitation is presented. The ground acceleration due to earthquake is regarded as a stochastic process, and a pseudo-excitation algorithm in the frequency domain is implemented in a computer program to handle non-classical damping properties of the system. The 1940 El Centro earthquake time history is also used for dynamic analysis of the system in the time domain. The effectiveness of fluid joint dampers is then investigated in terms of the reduction of displacement, acceleration and shear force responses of adjacent buildings. Finally, an extensive parametric study is carried out to find optimum damper properties for adjacent buildings of different stiffness ratios and different heights. Results show that using fluid dampers to connect the adjacent buildings of different fundamental frequencies can effectively reduce earthquake-induced responses of either building if damper properties are appropriately selected.

Fatigue analysis and life prediction of bridges with structural health monitoring data — Part I: methodology and strategy

Abstract This paper is aimed at developing a methodology and strategy for fatigue damage assessment and life prediction of bridge-deck sections of existing bridges with online structural health monitoring data. A fatigue damage model based on the continuum damage mechanics (CDM) is developed for evaluating accumulative fatigue damage of existing bridges. A structural model for the fatigue stress analysis of bridge-deck structures is proposed, in which structures are modeled by elastic members and welded connections with possible accumulative damage. Based on the proposed model, an analytical approach for evaluating the fatigue damage and service life of bridge-deck sections based on strain history data from an online structural health monitoring system and the CDM fatigue model are suggested. The updating of the representative block of cycles of the local stress by online monitoring data in the future is included in the computational approach. In order to compare results of fatigue damage and service life prediction evaluated by the CDM fatigue damage model, a modified Palmgren–Miner rule is developed for the same fatigue problem.

Neuro-control of cable vibration using semi-active magneto-rheological dampers

A neuro-control method is proposed for semi-active vibration control of stay cables using magneto-rheological (MR) dampers. A finite element model with multiple degrees-of-freedom is formulated for the hybrid system of a sagged stay cable incorporated with MR dampers. Two neural network control strategies are developed by using the full-order system model directly and introducing a reduced-order modal model, respectively. In the first strategy, which eschews a model reduction process, the neural network controller is designed and trained to have the dual functions of controller and observer, so that the resulting neuro-controller is able to perform the clipped linear quadratic Gaussian (LQG) control with incomplete state observation. In the second strategy, with the aid of a reduced-order modal model, the neural network controller is designed to consist of a control network and a state estimator network. The resulting neuro-controller performs the clipped LQG control using only a few observed states. The control effectiveness of the proposed neural network control strategies is verified numerically by application to a 12 m long scale-model cable prototype transversely connected with an MR damper near the lower anchorage. The response mitigation ability of the proposed control strategies using incomplete output observation is compared with that of the clipped LQG control with full state observation. The adaptive capability of the configured neuro-controllers under dynamic excitation distinct from training loading is examined. The analysis results show that the proposed control strategies can effectively implement semi-active vibration control of stay cables with the use of MR dampers.

Fatigue analysis and life prediction of bridges with structural health monitoring data — Part II: application

Abstract This paper is a continuation of the paper titled “FATIGUE ANALYSIS AND LIFE PREDICTION OF BRIDGES WITH STRUCTURAL HEALTH MONITORING DATA — PART I: METHODOLOGY AND STRATEGY” with the emphasis on application of the developed method to the fatigue damage assessment of the Tsing Ma Bridge. Based on the methodology and strategy of the fatigue analysis presented in Part I, fatigue damage analysis and service life prediction of the bridge–deck section of the Tsing Ma Bridge (TMB) are carried out by using the strain-time history data measured by the structural health monitoring system of the bridge. The stress spectrum of the representative block of cycles at the location of strain gauges in a typical longitudinal truss is obtained by rainflow counting of cycles of stress history and statistical analysis on daily samples of daily stress spectrum. The effect of low stress cycles on the fatigue and service life is considered by modifying the stress range when it is less than a limit value of stress range. Results of fatigue damage and service life are calculated respectively by the model developed in Part I and Miners rule are compared. The influence of updating on the calculation of fatigue damage and predicted service life is numerically investigated. The result shows that the magnitude of the stress range of the bridge–deck section considered on TBM is in the region of greatest concern of bridge fatigue, and values of fatigue damage and predicted service life, as an estimation of the state of fatigue for the bridge, are found to be reasonable.

Tuned liquid column damper for suppressing pitching motion of structures

Abstract Tuned liquid column damper (TLCD) was developed mainly for the purpose of suppressing horizontal motion of structures. No relevant research has been found on the suppression of structural pitching vibration by using TLCD. This paper thus aims to investigate the possibility and effectiveness of applying TLCD to suppress pitching motion of structures. Both theoretical and experimental investigations are carried out. A mathematical model of tuned liquid column damper for suppressing structural pitching vibration is developed. The TLCD-structure interactive equations are derived and solved in both time domain and frequency domain. A series of free and forced vibration experiments with different TLCD configuration and parameters are performed. The influences of variable TLCD parameters on control effectiveness are determined. Numerical simulations corresponding to the experimental cases are carried out and compared with the experiments. A close agreement is obtained between the experimental results and theoretical simulation. This also verifies the developed theoretical model. Both theoretical and experimental studies show that TLCD can efficiently reduce structural pitching motion.

Investigation concerning structural health monitoring of an instrumented cable-stayed bridge

In Hong Kong, a sophisticated long-term structural health monitoring system has been devised by the Highways Department of HKSAR Government to monitor the structural performance and health conditions of three cable-supported bridges. On-structure instrumentation systems for two new long-span bridges are also being implemented. The implementation of these monitoring systems highlights the necessity for developing a monitoring-based structural health evaluation paradigm for long-span bridges. This paper describes the research directed towards this that has been conducted in the Hong Kong Polytechnic University. Taking the instrumented cable-stayed Ting Kau Bridge as a paradigm, the research covers the development of an index system and a database system for monitoring data management, the modelling of the environmental variability of measured modal properties with the intention of eliminating environmental effects in vibration-based damage detection, and the feasibility of using measured modal properties from the deployed vibration sensors for structural damage identification.

Fatigue Damage Model for Bridge under Traffic Loading: Application Made to Tsing Ma Bridge

A fatigue damage model is developed to account for the damage accumulation process in bridges subjected to in-field traffic loading. Continuous damage mechanics (CDM) is applied to formulate damage kinetic constitutive equations. On-line strain gauge measurements are then made on the orthotropic steel deck structure of the Tsing Ma Bridge, an essential portion of the transport network for the Hong Kong airport. Fatigue life prediction analyses are then made. The results agree well with those obtained by the tests.

Determination of effective stress range and its application on fatigue stress assessment of existing bridges

This paper presents a unified approach on determination of the effective stress range based on equivalent law of strain energy and fatigue damage model, so as to provide an efficient approach for accurately assessing effective fatigue stress of existing bridge under traffic loading. A new theoretical framework to relate variable- and constant-amplitude fatigue is proposed in this paper. Different formulation for calculating effective stress range can be derived by the proposed theory, which include the effective stress range by the root mean square, by Miners law and a new effective stress range based on the nonlinear fatigue damage model. Comparison of the theoretical results of fatigue damage under the effective stress range of the variable-amplitude stress spectrum and experimental data of fatigue damage under realistic traffic loading has confirmed the validity of the proposed theory. As a way to relate variable-amplitude fatigue data with constant-amplitude data, the effective stress range provides the most convenient way for evaluating fatigue damage under variable-amplitude loading. The proposed theory is then applied to provide an efficient approach for accurately assessing fatigue damage of existing bridges under traffic loading, in which online strain history data measured from bridge structural health monitoring system is available. The proposed approach is applied to evaluate the effective stress range for the purpose of the fatigue analysis of a deck section of a long-span steel bridge-the Tsing Ma Bridge in Hong Kong.

Modal analysis of tower-cable system of Tsing Ma long suspension bridge

A three-dimensional dynamic finite element model is established for the tower-cable system of the Tsing Ma long suspension bridge which is currently under construction. The two bridge towers, made up of reinforced concrete columns and deep prestressed concrete beams, are modelled by three-dimensional Timoshenko beam elements with rigid arms at the connections between columns and beams. The main span and side span cables are modelled by three-node cable elements accounting for geometric nonlinearity and large elastic deflection. The modal analysis is then performed to determine the dynamic characteristics and dynamic interaction between the towers and cables. The results show that at lower natural frequencies, the modes of vibration of the system can be reasonably separated into in-plane modes and out-of-plane modes. Dynamic interactions between the towers and cables are significant at global natural frequencies in either in-plane or out-of-plane vibration. There are many local natural frequencies at which the cables vibrate but the towers remain stationary or have relatively small modal motion only. The dynamic interactions between the main span and side span cables are also observed at some local natural frequencies. The finite element model and the analytical results presented in this paper have been verified by measuring the dynamic properties of the system.

Modal sensitivity analysis of Tsing Ma bridge for structural damage detection

This study addresses model-based structural damage simulation and identification of the Tsing Ma Suspension Bridge through modal sensitivity analysis. For this purpose, a precise three- dimensional finite element model has been developed with the attributes: (1) the spatial configuration of the original structure remains in the model; (2) the geometric stiffness of cables and hangers has been accurately accounted for in the model; (3) the mass and stiffness contribution of individual structural members is independently described in the model, so damage to any structural member can be directly and precisely simulated. The model was validated using the measured modal data obtained at different erection stages and after the bridge completion. It is then used as a baseline for structural damage simulation and modal sensitivity analysis. Due to the intensive distribution of natural frequencies of the bridge, modal assurance criterion (MAC) is first utilized to check the correlation of mode pairs between the damaged and intact structure. Ten damage cases are simulated and the sensitivities of various modal parameters including natural frequency, mode shape and modal flexibility to different types of damage are evaluated. The goal of this study is to analytically determine which modal parameter is most sensitive to damage for a large-scale suspension bridge. The analysis results show that, in most cases, the frequency sensitivity to damage is low, while the modal flexibility method clearly indicates the damage locations only using a few lowest frequency modes.