Yeon-Sun Ryu

Damage identification in beam-type structures: frequency-based method vs mode-shape-based method

This paper presents a methodology to nondestructively locate and estimate the size of damage in structures for which a few natural frequencies or a few mode shapes are available. First, a frequency-based damage detection (FBDD) method is outlined. A damage-localization algorithm to locate damage from changes in natural frequencies and a damage-sizing algorithm to estimate crack-size from natural frequency perturbation are formulated. Next, a mode-shape-based damage detection (MBDD) method is outlined. A damage index algorithm to localize and estimate the severity of damage from monitoring changes in modal strain energy is formulated. The FBDD method and the MBDD method are evaluated for several damage scenarios by locating and sizing damage in numerically simulated prestressed concrete beams for which two natural frequencies and mode shapes are generated from finite element models. The result of the analyses indicates that the FBDD method and the MBDD method correctly localize the damage and accurately estimate the sizes of the cracks simulated in the test beam.

Vibration-based method to detect prestress loss in beam-type bridges

The loss of the prestress force is an uncertain parameter that may jeopardize the safety of PSC bridges. The prestress force is used to control crack formation in concrete, to reduce deflections, and to add strength to prestressed members; therefore, a substantial prestress-loss can lead to severe problems in the serviceability and safety of the PSC structures. A vibration-based method to detect prestress-loss in beam-type PSC bridges by monitoring changes in a few natural frequencies is presented. A SID (system identification) model is formulated to estimate changes in natural frequencies of the PSC bridges under various prestress forces. Also, an inverse-solution algorithm is proposed to identify the prestress-loss in the PSC bridges by using the changes in natural frequencies. The feasibility of the approach is evaluated using PSC beams for which a few natural frequencies are available for a set of prestress-loss cases.

Monitoring Cracks and Prestress-Loss in PSC Girder Bridges Using Vibration-based Damage Detection Techniques

In this study, a vibration-based method to simultaneously predict prestress-loss and flexural crack in PSC girder bridges is presented. Prestress-loss and flexural crack are two typical, but quite different in nature, types of damage which can be occurred in PSC girder bridges. The following approaches are implemented to achieve the objective. Firstly, two vibration-based damage detection techniques which can predict prestress-loss and flexural crack are described. The techniques are prestress-loss prediction model and mode-shape-based crack detection method. In order to verify the feasibility and practicality of the techniques, two different lab tests are performed. A free-free beam with external unbonded tendons is used to verify the feasibility of the prestress-loss prediction model. In additional, a PSC girder with an internal unbonded tendon is used to evaluate the practicality of the prestress-loss prediction model and the mode-shape- based crack detection method.

LRFD-based design optimization of steel box girder sections using genetic algorithms

A design optimization problem for steel box girder sections is formulated and a numerical solution procedure is presented. The formulation is based on the load and resistance factor design (LRFD) procedure. The application of genetic algorithms (GAs) for a class of numerical examples is investigated. The steel box girder may have longitudinal stiffeners on flanges and webs and transverse intermediate stiffeners. Thus, the design variables may be continuous or discrete, which naturally makes the use of GAs attractive. Design constraints represent the LRFD versions of the Standard Specifications of Korean Highway Bridges. The dimensions of concrete slab and other appurtenance are assumed to be pre-determined and minimization of the unit weight of the steel box girder is taken as a design objective. As the numerical optimizers, the Internet version of simple GA (SGA) and micro-GA (μGA) are used and their performance and results are compared. In addition, the sequential quadratic programming algorithm is also applied for purposes of comparison. It is found that μGA with the population size of 5 is a suitable and reliable tool for the design optimization of steel box girder sections. The resulting design can be effectively used as an initial design for practical proportioning

Smart baseline model for nondestructive evaluation of highway bridges

A methodology to identify a baseline modal model of a complicated 3D structure using limited structural and modal information is experimentally examined. In the first pat, a systems identification theory for the methodology to identify baseline modal responses of the structure is outlined. Next, an algorithm is designed to build a generic finite element model of the baseline structure and to calibrate the model by using only a set of post-damage modal parameters. In the second part, the feasibility of the methodology is examined experimentally using a field-tested truss bridge for which only post-damaged modal response were measured or a few vibration modes. For the complex 3D bridge with many members, we analyzed to identify unknown stiffness parameters of the structure by using modal parameters of the initial two modes of vibration.

Collision Behavior Evaluation of Flexible Concrete Mattress Depending on Material Models

The purpose of this study was to provide fundamental data for an anchor collision simulation of an FCM (flexible concrete mattress). Numerical material models (elastic-perfectly plastic model, Drucker-Prager model, and RHT concrete model) were compared. ANSYS Explicit Dynamics was used for collision analyses. An FE model was used for the anchor, FCM, andreinforcement bars. The results showed that the behavior of the FCM was verydifferent that those ofthe material models. In particular, the effect of the pressure dependen t strength was most noticeable among the properties of concrete.

Damage Detection Algorithm-Embedded Smart Sensor Node System for Bridge Structural Health Monitoring

In this study, a system using autonomous smart sensor nodes is developed for bridge structural health monitoring (SHM). In order to achieve the research goal, the following tasks are implemented. Firstly, acceleration-based and impedancebased smart sensor nodes are designed. Secondly, an autonomous operation system using smart sensor nodes is designed for hybrid health monitoring using global and local health monitoring methods. Finally, the feasibility and applicability of the proposed system are experimentally evaluated in a lab-scaled prestressed concrete (PSC) girder for which a set of damage scenarios are experimentally monitored by wireless sensor nodes and embedded software.

Cylindrical Guided Waves for Inspection of Clay-Steel Pile Interface

The feasibility of detecting damage at the steel pile-clay interface using cylindrical guided waves is investigated in this paper. It is shown that cylindrical guided waves (or cylindrical Lamb waves) can easily detect such damage. A special coupler between the steel pile and ultrasonic transducer has been used to launch non-axisymmetric (or flexural) cylindrical guided waves in the steel pile. This investigation shows that the cylindrical guided waves can propagate along the steel pile embedded in clay and are sensitive to the interface damage between pile and clay.

Damage Evaluation of Flexible Concrete Mattress Considering Steel Reinforcement Modeling and Collision Angle of Anchor

A flexible concrete mattress (FCM) is a structural system for protecting submarine power or communication cables under various load types. To evaluate its of protection performance, a numerical analysis of an FCM under an anchor collision was performed. The explicit dynamics of the finite element analysis program ANSYS were used for the collision analysis. The influences of the steel reinforcement modeling and collision angle of the anchor on the collision behavior of the FCM were estimated. The FCM damage was evaluated based on the results of the numerical analysis considering the numerical modeling and collision environment.

Vibration-Based Damage Monitoring Algorithms for Prestress-Loss in PSC Girder Bridges

Among many damage types, prestress-loss in tendon is the major one that should be monitored in its early stage in order to secure the safety of PSC girder bridges. This damage-type obviously change vibration characteristics, but with apparent difference depending on sensing mechanism as well as information analysis. Recently, there have been research efforts to develop wireless smart sensor nodes embedded damage monitoring algorithms for various sensing mechanisms. In this study, vibration-based damage monitoring algorithms which are appropriate for the smart sensor nodes to alarm the occurrence of prestress-loss in PSC girder bridges are presented. Firstly, two sensing mechanisms are considered for vibration characteristics: one is acceleration and the other is electro-mechanical impedance. Also, four acceleration-based algorithms and three impedance-based algorithms are selected to extract features from those signals. Secondly, the performances of those selected methods are evaluated using a large-scaled PSC girder for which a set of acceleration-impedance tests were measured for several prestress-loss scenarios by using both commercial instruments and a wireless smart sensor node.