Wonsuk Park

Minmax optimum design of active control system for earthquake excited structures

A minmax optimization method for the vibration control system considering uncertain dynamics is presented in this paper. In the proposed method, structural modeling errors and various excitations, which may cause performance degradation, are treated as plant parameters. The minmax problem considered in this study is not only to find an optimal controller that minimize structural responses but also to search the worst-case plant parameters that maximize structural responses, whereas the conventional optimal design usually deals with a minimization problem for a certain performance index. In order to solve the proposed minmax problem, the bimatrix co-evolution algorithm is adopted. It approximates the minmax problem as the bimatrix game between two fitness measures and finds the optimal solution effectively through the co-evolutionary process, as a result, the optimal controller and the worst-case plant parameters are searched simultaneously. Example design of an earthquake-excited building structure with active tendon control system is performed and comparative results of numerical simulation are presented to validate the proposed method.

Preference-based maintenance planning for deteriorating bridges under multi-objective optimisation framework

A new preference-based method is presented to determine optimal schedule of maintenance actions for deteriorating bridges. The maintenance planning of deteriorating bridges is formulated as a multi-objective optimisation problem involving the maintenance cost as well as the condition grades of the bridge deck, girder and pier. The maintenance cost corresponds to life-cycle maintenance cost that is defined as the sum of the total expenses caused by the maintenance interventions during the life span of the bridge. The condition grades of the bridge components are characterised by time-dependent performance profiles that have been constructed by statistical regression with the actual condition grade data of bridges in Korea. To effectively address the multi-objective optimisation problem, a genetic algorithm is adopted as a numerical searching technique of multiple optimal solutions. As for the explored optimal solutions, a preference-based decision making process is then introduced to select reasonable maintenance scenario that most fits the designers preference while balancing the mutually conflicting design objectives. The effectiveness of the proposed method is verified numerically on a typical pre-stressed concrete girder bridge. The numerical results demonstrate that the optimal maintenance scenario determined by the proposed method can be a well-balanced maintenance strategy in terms of cost and bridge performances.

Robust multi-objective maintenance planning of deteriorating bridges against uncertainty in performance model

This study proposes a new robust multi-objective maintenance planning approach of the deteriorating bridges against uncertainty in performance degradation model. The main focus is to guarantee the performance requirements of the bridge by the scheduled maintenance interventions even in the presence of uncertainty in time-dependent performance degradation model. The uncertainties are modeled as the perturbation of the system parameters. These are simulated by a sampling method, and incorporated into the GA-based multi-objective optimization framework which produces a set of optimal preventive maintenance scenarios. In order to focus the searching on the most preferable region, the performance models of the bridge components are all integrated into single overall performance measure by using the preference-based objective-space reduction method. Numerical example of a typical prestressed concrete girder bridge is provided to demonstrate the new robust maintenance scheduling approach. For comparison purpose, non-robust multi-objective maintenance planning without considering uncertainty of the bridge performance is also provided. It is verified that the proposed approach can produce successfully-performing maintenance scenarios under the perturbation of bridge condition grades while maintaining well-balanced maintenance strategy both in terms of bridge performance and maintenance cost.

Candidate model construction of a cable-stayed bridge using parameterised sensitivity-based finite element model updating

An efficient method for constructing multiple candidate finite element (FE) models which are consistent with the measured dynamic properties of a civil structure is presented. A parameterised sensitivity-based FE model updating method was developed to permit a feasible FE model set for the target structure to be produced. In this method, an aggregated multi-objective function, which is defined by the weighted sum of the error functions, is parameterised by its relative weighting factors. By introducing multiple parameter sets for the weighting factors, a number of optimal updated FE models are produced that permit both the natural frequency errors and mode shape errors to be simultaneously minimised. The effectiveness of the proposed method is illustrated by an example using an existing cable-stayed bridge. The findings show that error-minimised, well-distributed FE models can be obtained in terms of modal frequency errors and mode shape errors. The high quality of the candidate model sets is also verified by observations showing that the distributions of the structural parameters are consistent through each updated FE models, and the characteristic features of the target structure such as non-symmetric mode shapes are relatively well captured.

Improvement of Seismic Performance of Long-span Bridges using Complex Dampers

This paper presents a new vibration control method for long-span bridges using complex damper system. The new system presents simple mechanical configuration with oil and elasto-plastic dampers which have velocity and displacement dependency in vibration energy absorbing. This system can produce various damping forces according to the applied external forces by the velocity and displacement-dependent characteristics of the dampers. The oil damper dissipates vibration energy for relatively frequent and small amplitude like in the case for small to moderate earthquakes, whereas the elasto-plastic damper system works for rare and large amplitude vibration such as high seismic excitation. Thus, the proposed system exhibits the advantage of low cost with high performance since the roles of the two different dampers are effectively separated. A numerical model is established for the complex damper system, and the response characteristics and effectiveness of the proposed system are presented through numerical simulations. Numerical results show that the proposed complex damper system can significantly improve the seismic performance of long-span bridge structures with much more effective damping mechanism than single conventional passive damper systems.

Risk assessment of dolphin protected bridge pier considering collision point analysis

The collision of ship embraces some uncertainties, such as a direction, velocity, and location of the aberrant vessel. These uncertainties may affect on the risk from the collision and the final design of bridge and protective structures against the collision of ship. The importance of accurate consideration for uncertainties is more obvious on the protected bridges by the dolphin, since it alters the motions during the collision. This paper considers the effect of collision point along the height of the pier that usually is ignored as using a simple twodimensional outline. In addition, it also analyzes the influence of the angle from the collision between vessel and pier. With the numerical example, it is possible to conclude that it is necessary to consider various vertical impact locations for safe design. Besides, it is sure to reduce the risk from the collision of ship, considering impact angle.

Preference Based Genetic Algorithm for the Optimum Design of Integrated Structural Control System

This paper proposes an optimal design of a hybrid system for the mitigation of wind-induced vibration in high-rise buildings. Even if effective control performance can be achieved by the hybrid system which applies simultaneously both active and passive control devices, its design is extremely complicated due to the increase of the design variables. This paper considers a hybrid system composed of hybrid mass damper and viscous dampers. The corresponding design variables are the mass, tuning ratio, damping ratio, damper locations and capacity of the dampers. Since the minimization of cost factors is also a matter of utmost importance, the required control efforts and dynamic responses of the target structure are selected as objective functions to be minimized. Therefore, the problem is formulated as a multi-objective optimization problem involving the design of two different types of systems. The validity of the proposed optimization technique is verified through a wind-excited 20-story building with hybrid control system and comparative results with non-hybrid design approach are presented.

A Study on Cost-Effectiveness Evaluation and Optimal Design of ant dampers for Cable-Stayed Bridges

A method is presented for evaluating the economic efficiency of a semi-active magneto-rheological (MR) damper system for cable-stayed bridges under earthquake loadings. An optimal MR damper capacity maximizing the cost-effectiveness is estimated for various seismic characteristics of ground motion. The economic efficiency of MR damper system is addressed by introducing the life-cycle cost concept. To evaluate the expected damage cost, the probability of failure is estimated. The cost-effectiveness index is defined as the ratio of the sums of the expected damage costs and each device cost between a bridge structure with the MR damper system and a bridge structure with elastic bearings. In the evaluation of cost-effectiveness, the scale of damage cost is adopted as parametric variables. The results of the evaluation show that the MR damper system can be a cost-effective design alternative. The optical capacity of MR damper is increased as the seismic hazard becomes severe.

Vibration Control and Cost-Effectiveness Evaluation of Cable-Stayed Bridges with Semi-Active Control System

This paper presents cost-effectiveness evaluation of semi-active control system for cable-stayed bridge under earthquake excitations with various magnitudes and frequency contents. Semi-active control system, which is operated by using Bi-stale control method on the basis of linear quadratic Gaussian (LQG) optimal controller, is designed for the benchmark control problem proposed by Dyke et at. The cost-effectiveness of the proposed control system is defined by the ratio of life-cycle costs between a bridge structure with shock transmission units and a bridge structure with the semi-active control devices. The simulated results show that the damper cost has little influence on the cost-effectiveness of the semi-active control system while the cost-effectiveness is quite sensitive to the damage cost induced by the bridge failure. It is also found that the semi-active control system guarantees relatively high cost-effectiveness for the cable-stayed bridge subject to the ground motions in the regions of moderate seismicity with soft soil condition and strong seismicity with stiff soil condition.

Reliability-based structural design framework against accidental loads – ship collision

Abstract As civil infrastructures become highly complex and interconnected, accidental events such as fire, explosion, dropped objects and collision tend to occur more frequently, and are causing critical structural damage and socio-economic costs. Therefore, accidental loads should be considered appropriately during the structural design. To achieve this goal, the paper presents a reliability-based structural design framework against accidental loads. The proposed framework employs a scenario-based simulation in which the occurrence of an accidental load event and the resulting structural loads are modelled probabilistically. The probabilistic distribution of accidental loads is then determined based on the occurrence probabilities of identified scenarios and the conditional probability of structural loads given each accident scenario. In order to estimate the structural responses under accidental loads, an appropriate type of structural analysis is carried out for the given limit states of the structural resistance at the critical sections. The proposed framework is illustrated by an example of a cable-stayed bridge whose piers are under ship collision risk. To consider the dynamic amplification of bridge response caused by ship–bridge collision, time history analysis is performed for the equivalent static ship collision loads. The paper also identifies research needs and challenges in applying the proposed framework to other accidental loads.