Hyun-Man Cho

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.

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

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.

Hybrid Vibration-Impedance Approaches for Damage Detection in Plate-Girder Bridges

In this paper, a hybrid vibration-impedance approaches is newly proposed to detect the occurrence of damage, the location of damage, and extent of damage in steel plate-girder bridges. Firstly, theoretical backgrounds of the hybrid structural health monitoring are described. The hybrid scheme mainly consists of three sequential phases: 1) to alarm the occurrence of damage in global manner, 2) to classify the alarmed damage into subsystems of the structure, and 3) to estimate the classified damage in detail using methods suitable for the subsystems. Damage types of interest include flexural stiffness-loss in girder and perturbation in supports. In the first phase, the global occurrence of damage is alarmed by monitoring changes in acceleration features. In the second phase, the alarmed damage is classified into subsystems by recognizing patterns of impedance features. In the final phase, the location and the extent of damage are estimated by using modal strain energy-based damage index methods. The feasibility of the proposed system is evaluated on a laboratory-scaled steel plate-girder bridge model for which hybrid vibration-impedance signatures were measured for several damage scenarios.

Fundamental guided wave attenuations and sensor location estimation in fluid-filled steel pipes

The sound intensities required to sustain a certain interval between two ultrasonic sensors that are installed outside a steep pipe for structural health monitoring are numerically investigated. Two fundamental guided wave modes, longitudinal and flexural, are considered for the calculation of their attenuations via the concept of parametric density and a sink inside steel pipes, and these attenuations are used for the sound-intensity calculations. The results, attenuations, and sound intensities are useful as a guideline when guided-ultrasonic-wave-based inspection is planned for pipeline maintenance and safety, especially for estimation of the axial location between ultrasonic sensors.

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.