Jinkyo F. Choo

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.

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.

Effect of Cable Loss in Cable Stayed Bridges – Focus on Dynamic Amplification

This paper investigates the dynamic response in case of cable loss with focus on dynamic amplification through case studies on an actual cable-stayed bridge. Cable-stayed bridges are designed to accommodate cable replacement as well as the abrupt loss of a stay cable and design recommendations are providing simplified method of the cable loss verification, which replaces the dynamic effect by factored static load. It is not unusual for cable loss to govern the design of the superstructure due to inadequate or overestimated dynamic amplification factors. This paper intends to derive proper load factor for cable-stayed bridges through adequate simulation of the instantaneous or progressive rupture of a stay cable as well as the use of appropriate model in the time history analysis. Accordingly, a process by which reasonable values of DAF can be derived for cable stayed bridges according to the stress of considered structural element or section is proposed. The proposed process applied to Seohae Bridge is seen to produce reasonable values of the DAFs according to the structural element with values below 1.5 for the critical sections.

Dynamic Characteristics of a Multi-Functional Bridge Bearing with Built-In Piezocomposite Element

A multi-functional bridge bearing with built-in piezocomposite electricity generating element (PCGE) is being developed by our research team to respond to the growing demand of self-powered sensing devices for the monitoring of bridges by harvesting the energy produced by the traffic-induced vibrations. For the intended application, a multilayered piezoelectric PCGE structure composed of layers of piezoceramic, glass/epoxy, and carbon/epoxy, has been developed to improve the durability, output voltage and power of the piezoceramic. The output voltage of this PCGE can be used for real-time traffic monitoring like in bridge-weigh-in-motion systems and can eventually be exploited to generate the electricity needed for the lighting and functioning of other embedded sensors. This paper presents the results of the dynamic loading tests conducted on a prototype of the proposed multi-functional bridge bearing to enhance its design details and verify the accuracy of the measurement. The results show that the bearing provides reliable measurement for traffic monitoring and enable to conceive details for the improvement of the output voltage of the PCGE. Since bridge bearings, as indispensable devices transferring the loads and movements from the deck to the substructure and foundations of the bridge, are continuously subjected to traffic loads, the proposed bridge bearing appears to be a natural and economic solution that can be applied to existing or newly built bridges without modification of the conventional design while providing additional and valuable functions for the maintenance of the structure.

Experimental Study on the Optimal Layering Position of the Piezocomposite Electricity Generating Element Built-in an Innovative Traffic Load-Measuring Bridge Bearing

The bearings of a bridge are indispensable structural members, which support the superstructure, transfer the loads of the superstructure to the substructure and, accommodate the horizontal and vertical motions of the superstructure caused by all types of loads like traffic. Such traffic loads induce the bridge to vibrate but this energy is wasted. Therefore, a piezocomposite electricity generating element (PCGE) has been introduced in the bearing to exploit effectively this vibrational energy. The bridge bearing with built-in PCGE was seen to be effective for measuring the traffic loads and, detecting eventual overloaded or overspeeding vehicles. In a will to optimize the amount of energy generated by this bearing, this study compares experimentally the amount of electricity produced by the PCGE with respect to its position in the bridge bearing through a series of dynamic loading tests to find out the position of the PCGE providing the most efficient electricity generation.

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.

Recent Major Bridges in Korea

During a relatively short period, bridge technology in Korea has recognized outstanding development. Owing to the governmental policy aiming to achieve balanced regional development, unprecedented bridge construction activities are conducted in the peninsula, especially in the southwestern coast to link some of the 3,000 islands with the mainland. Korean bridge engineers are now designing and constructing bridges using domestic technology, which has today reached a level enabling to realize the erection of numerous sea-crossing bridges counting among the longest in the world like Incheon cable-stayed bridge (main span 800m) and Gwangyang suspension bridge (main span 1545m). This paper reviews the evolution of modern bridge construction in Korea and presents major bridges recently erected or to be built. R&D programs dedicated to the advancement of bridge technology are also introduced.

Flexural and Shear Behaviors of Reinforced Alkali-Activated Slag Concrete Beams

The material properties of cement-zero concrete using alkali-activators have been studied extensively as the latest response to reduce the CO2 exhaust of the cement industry. However, it is also critical to evaluate the behavior of reinforced concrete beams made of alkali-activated slag (AAS) concrete in terms of flexure and shear to promote the applicability of AAS concrete as structural material. Accordingly, nine types of beam specimens with various ratios of tensile steel and stirrup were fabricated and subject to bending and shear tests. The results show that the flexural and shear behaviors of the reinforced AAS concrete members are practically similar to those made of normal concrete and indicate the applicability of the conventional design code given that the lower density of slag is considered. In addition, a framework using the elastic modulus and stress-strain relation from earlier research is adopted to carry out nonlinear finite element analysis reflecting the material properties of AAS concrete. The numerical results exhibit good agreement with the experimental results and demonstrate the validity of the analytical model.

Lifetime design of long-span bridges

Recently, lifetime has become a major design factor for long-span bridges due to their socio-economic importance. Current practices show that design lifetimes of 100 to 150 years seem to be achievable and have already been implemented in recent major bridges through appropriate maintenance planning and in terms of material durability. However, it is clear that the lifespan of materials alone cannot guarantee that of the structure itself. Accordingly, more rational and systematic design approach needs to be developed especially for long-span bridges. This paper reviews current practices for lifetime design and addresses issues to be considered to realise long-span bridges with extended lifetime since the design stage. Since the lifetime of a bridge is essentially influenced by issues related to material, design live load, fatigue and extreme or unexpected events, review is done for each of these factors. Possible solutions are suggested and additional requirements that will help designers in achieving extended lifetime of 200 years or more for long-span bridges are derived.

Dynamic Analysis of Cable-Stayed Bridge under Construction Considering Hoisting Deck Segment Motion

In this study, the authors perform dynamic response analysis for cable-stayed bridge system under construction including hoisting deck segment motion as well as pylon-cable-deck vibration. In order to consider the interaction between the bridge, derrick-crane and the hoisted deck segment, we formulate nonlinear equations of the motion for bridge-segment coupled system. Analysis results show that vortex induced resonant vibration of the bridge-segment coupled system can occur for a specific stage of the construction and hoisting motion, which results in excessive displacement response of the hoisted deck segment. As a feasibility study, we develop a vibration controller for the time-varying pendulum motion period of the hoisted deck, which consists of a combination of the multiple optimal controllers. The effectiveness of the proposed control method is presented by investigating the acceleration and displacement response of the system with respect to safety and workability criteria.