Hyun-Moo Koh

Active control of earthquake excited structures using fuzzy supervisory technique

A fuzzy supervisory technique for the active control of earthquake-excited building structures is studied in this paper. The method has a hierarchical structure, which consists of a supervisor at the higher level and several sub-controllers at the lower level. Each sub-controller is designed to reduce the story-drift of each floor by using an optimal control theory and a fuzzy logic is adopted to obtain a desirable supervisor. A fuzzy supervisor appropriately tunes the predesigned control gains at every moment by estimating the state of a structure through the fuzzy inference mechanism. The improved seismic control performance can be achieved by converting a simply designed static gain into a real-time variable dynamic gain through a fuzzy tuning process. Example designs and numerical simulations of an earthquake-excited three-story building are performed to prove the validity of the proposed control strategy.

System reliability analysis using dominant failure modes identified by selective searching technique

The failure of a redundant structural system is often described by innumerable system failure modes such as combinations or sequences of local failures. An efficient approach is proposed to identify dominant failure modes in the space of random variables, and then perform system reliability analysis to compute the system failure probability. To identify dominant failure modes in the decreasing order of their contributions to the system failure probability, a new simulation-based selective searching technique is developed using a genetic algorithm. The system failure probability is computed by a multi-scale matrix-based system reliability (MSR) method. Lower-scale MSR analyses evaluate the probabilities of the identified failure modes and their statistical dependence. A higher-scale MSR analysis evaluates the system failure probability based on the results of the lower-scale analyses. Three illustrative examples demonstrate the efficiency and accuracy of the approach through comparison with existing methods and Monte Carlo simulations. The results show that the proposed method skillfully identifies the dominant failure modes, including those neglected by existing approaches. The multi-scale MSR method accurately evaluates the system failure probability with statistical dependence fully considered. The decoupling between the failure mode identification and the system reliability evaluation allows for effective applications to larger structural systems.

Finite element formulation for the analysis of turbulent wind flow passing bluff structures using the RNG k−ε model

Abstract This paper presents a stable finite element formulation to predict behaviors of high-speed wind passing bluff structures using the Reynolds averaged Navier–Stokes equation and the k – e model. To incorporate the k – e model with the finite element framework, a stable and accurate solution strategy is proposed. The streamline-upwind/Petrov–Galerkin scheme is adopted to stabilize the Reynolds averaged Navier–Stokes equation as well as the k – e equations. The re-normalization group k – e model is employed to reduce the turbulence over-production around the stagnation points on the upwind side of structures. Detailed discussions on the flow behaviors around the bluff structures are made through examples of wind flows passing a square cylinder and actual bridge sections to validate the proposed formulation. It is shown that the periodicity and magnitude of unsteady forces acting on the square cylinder are well predicted. Aerodynamic forces acting on bridge girder sections with complex geometry are presented for high-speed wind with the Reynolds number over 10 7 , and compared with experimental results.

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.

Recent Development of Bridge Health Monitoring System in Korea

Since bridge structures constitute the very vulnerable part of civil transportation system that affect directly the public safety, focus has been done on the active development of bridge health monitoring in Korea since 1990’s. Gathering of field data for design verification and monitoring of long-term performance of bridges were performed in the scope of systematic and scientific inspection and management programs. Actually, modern and integrated monitoring systems are introduced in newly-built bridge structures since their design stage. This paper reviews recent development of bridge health monitoring systems in Korea in newly-built bridges with their objectives and major characteristics.

A Neural Network-Based Damage Detection Algorithm Using Dynamic Responses Measured in Civil Structures

A neural network (NN)-based technique making direct use of measured dynamic responses in civil structures is proposed to model the structure and detect eventual anomalies with their location and extent. Although numerous researches were conducted to apply NN for damage detection purposes, the problem constituted by the selection of an appropriate architecture for the networks still remains a major obstacle impeding their applicability. In order to avoid this shortcoming, the proposed algorithm performs the modeling of the structure stepwise by successive integration-like neural operations, which permits to reduce effectively the size of the networks and simplify effectively their architecture. The damage parameter is decided to be the restoring forces and corresponding stiffness of each major structural member. The trained network fed with data of the structure encountering diverse damage events under various loading episodes reconstructs the actual restoring force loops and the ones that should be obtained for the undamaged structure, of which comparison provides accurate estimation of damages. A shear building example verifies the efficiency and accuracy of the proposed method in detecting, locating and giving the extent of damages in real time.

Optimal Design of Linear Viscous Damping System for Vibration Control of Adjacent Building Structures

This paper proposes an optimal design method of linear viscous dampers for the seismic performance of two adjacent structures with different heights. Accordingly, connection method using diagonal bracing between two floors and connection method between two structures are considered, and the effectiveness of the latter method is confirmed through the comparison of the frequency response functions with respect to damping capacity. Moreover, optimal damping to minimize the response of the adjacent structures in the frequency domain is found. The sensitivity of natural frequency and modal damping according to the damper capacity at each floor is obtained for the optimally designed system. From the sensitivity analysis, the modal damping is evaluated to be very sensitive to the damper installed at higher floor. Therefore, sensitivity-based damping distribution method is proposed. Diagonal bracing connection method, uniform distribution method and sensitivity-based distribution method are compared to each other in terms of seismic performance. The comparative results demonstrate that the proposed method is an effective seismic design method for the adjacent structures.

Multi-Objective Integrated Optimal Design of Hybrid Structure-Damper System Satisfying Target Reliability

This paper presents an integrated optimal design technique of a hybrid structure-damper system for improving the seismic performance of the structure. The proposed technique corresponds to the optimal distribution of the stiffness and dampers. The multi-objective optimization technique is introduced to deal with the optimal design problem of the hybrid system, which is reformulated into the multi-objective optimization problem with a constraint of target reliability in an efficient manner. An illustrative example shows that the proposed technique can provide a set of Pareto optimal solutions embracing the solutions obtained by the conventional sequential design method and single-objective optimization method based on weighted summation scheme. Based on the stiffness and damping capacities, three representative designs are selected among the Pareto optimal solutions and their seismic performances are investigated through the parametric studies on the dynamic characteristics of the seismic events. The comparative results demonstrate that the proposed approach can be efficiently applied to the optimal design problem for improving the seismic performance of the structure.

Toward a Balanced Heritage Management Plan for Old Stone Bridges Considering the Embedded Cultural Significance

Old stone bridges in Korea are becoming short of safety while their cultural significance is becoming more important. Unlike existing modern bridges requiring strengthening mainly for structural safety, old stone bridges must be treated from both the points of view of historic significance and safety to advocate for the preservation of cultural and engineering landmarks–monuments for the people and communities. Therefore, engineers must balance preservation principles demanding authenticity of materials and visual characteristics with code requiring safety, strength, and stability and historic significance for the maintenance of historic bridges. This article proposes a framework for developing management plans for old stone bridges to consider cultural significance values as decision variables. To verify the concepts proposed in this article, a case study was simulated for the Supyo Bridge management plan, which is under consideration by the office of Seoul Metropolitan cultural heritage.

Reliability-based design optimisation combining performance measure approach and response surface method

The necessity of assessing uncertainties to guarantee safety in structural design arises the potential of reliability-based design optimisation (RBDO), which is a methodology based on reliability analysis and design optimisation through probabilistic models. RBDO differs from conventional design optimisation by the presence of probabilistic constraints, which are evaluated using reliability index approach (RIA) or performance measure approach (PMA). It is generally known that PMA is more stable and efficient than RIA. Despite the advantages brought by PMA, RBDO still requires excessive computational cost for large scale structures involving complex finite element analysis. Accordingly, this paper presents a new scheme for RBDO achieving improved stability and efficiency by combining response surface method with PMA. The moving least squares (MLS) method is used to approximate the limit state function. Applications to a mathematical example, the 10-bar truss problem and the vehicle side impact problem verify that the proposed method shows better convergence and efficiency than other approaches.