Soobong Shin

Design of accelerometer layout for structural monitoring and damage detection

The paper presents an algorithm for designing a layout of accelerometers for structural monitoring and damage detection. The maximum likelihood approach has been applied as a mathematical basis for the algorithm. Fisher information matrix is formulated in terms of mode shape sensitivity with respect to structural parameters. A scheme of an effective independence distribution vector has been applied to determine optimal locations of accelerometers. Singular value decomposition scheme has been applied to overcome the rank deficiency problem in the computed sensitivity matrix. The adequacy of the proposed algorithm has been examined by estimating structural parameters through a frequency-domain system identification. The identification results using the response data measured at the locations selected by the proposed algorithm are compared with those at arbitrary locations. In addition to the design of accelerometer layout for a structural monitoring of general purpose, the paper proposes another algorithm of layout design for damage detection with the assumption that some members critical for the structural safety are pre-determined. Damage possibility of each member computed from the static strain energy has been implemented as a weighting factor in the algorithm. Simulation studies have been carried out on a two-span multigirder bridge to examine the proposed algorithms.

A numerical study on detecting defects in a plane-stressed body by system identification

A parametric system identification algorithm is applied for detecting holes or cracks in an elastic plane-stressed body using measured static response at the boundaries. A linearly constrained nonlinear optimization problem is solved for optimal constitutive parameters by minimizing the error between the measured and computed displacements. Each finite element in the model is parameterized by decomposing its stiffness matrix into constitutive parameters and kernel matrices. Because locations and sizes of actual holes or cracks in a body are not the a priori knowledge, the finite element model for detecting such defects is simply set up for the defect-free state with the assumption of a linear elastic behavior. Defects in a plane-stressed body are predicted by the reduction in the constitutive parameters of each element from their baseline values without modifying the geometry and topology of the defined finite element model. The proposed defect-detection algorithm allows sparse measured data with respect to the number of degrees of freedom of the model and also provides statistical defect indices when considering noise in measurements. An adaptive parameter grouping scheme is applied to localize defects when limited measurements are provided. The proposed method is investigated through numerically simulated examples.

Bridge Health Monitoring with Consideration of Environmental Effects

Reliable response measurements are extremely important for proper bridge health monitoring but incomplete and unreliable data may be acquired due to sensor problems and environmental effects. In the case of a sensor malfunction, parts of the measured data can be missing so that the structural health condition cannot be monitored reliably. This means that the dynamic characteristics of natural frequencies can change as if the structure is damaged due to environmental effects, such as temperature variations. To overcome these problems, this paper proposes a systematic procedure of data analysis to recover missing data and eliminate the environmental effects from the measured data. It also proposes a health index calculated statistically using revised data to evaluate the health condition of a bridge. The proposed method was examined using numerically simulated data with a truss structure and then applied to a set of field data measured from a cable-stayed bridge.

Seismic Fragility Analysis of a FCM Bridge Considering Soil Properties

,, , , ABSTRACT >> This study investigates the influence of various soil properties on the seismic performance of a three-span FCM bridge. Piers that are vulnerable to seismic vibration are identified through numerical study of plastic hinges possibly occurring at the top and bottom of the piers. The fragility curve is obtained as a lognormal distribution function with respect to peak ground acceleration(PGA). The median and logarithmic standard deviation, which are two parameters of a lognormal distribution function, are estimated using the maximum likelihood method. In order to consider the different soil properties of each support, an equivalent spring based on the Korean Standard Specifications for Highway Bridges(KSSHB) is adopted in this study. For seismic fragility analysis, the rotational ductility demands of bridge piers are used as a damage index of the structure.

Damage assessment of complex structures from transient dynamic response

Damage detection and assessment algorithm using transient dynamic response is developed for complex structures. A parametric system identification method is implemented in the algorithm as the main tool to identify a structural system. A nonlinear constrained optimization problem is solved for estimating optimal structural parameters. To localize damaged members, the adaptive parameter grouping scheme is applied. To assess the severity of damage statistically, the time windowing technique is applied. Main considerations in developing the algorithm are noise and sparsity in the measurements. A simulation study is carried out with a truss structure excited by a harmonic force.

Determination of Optimal Accelerometer Locations using Mode-Shape Sensitivity

This paper proposes a new algorithm of MS-EIDV (modal sensitivity-effective independence distribution vector) for determining optimal accelerometer locations (OAL) by using the Fisher Information Matrix (FIM) derived from mode-shape sensitivities. Also, the paper provides a reasonable guideline for selecting OAL which can reflect dynamic responses of a structure effectively. Since OAL should be determined with known values of structural parameters but since the parameters can be estimated by applying an inverse method such as SI (system identification) using measured response, the paper proposes a statistical method to overcome the paradox by considering the error bound of the structural parameters. To examine the proposed methods, a frequency-domain SI method has been applied. By using the identified results, the minimum necessary number of accelerometers could be selected depending on the number of target measurable modes. Through simulation studies, the results by applying EIDV method directly using the information of mode shapes were compared with those by applying the proposed MS-EIDV.

Analytical Method to Determine the Dynamic Amplification Factor due to Hanger Cable Rupture of Suspension Bridges

A suspension bridge is a type of bridge in which the beam is suspended by load-bearing cables. There are two classifications: the self-anchored suspension bridge has the main cable anchored to the bridge girders, and the earth-anchored suspension bridge has the main cable anchored to a large anchorage. Although a suspension bridge is structurally safe, it is prone to be damaged by various actions such as hurricanes, tsunamis and terrorist incidents because its cables are exposed. If damage to a cable eventually leads to the cable rupture, the bridge may collapse. To avoid these accidents, studies on the dynamic behavior of cable bridges due to the cable rupture have been carried out. Design codes specify that the calculated DAF (dynamic amplification factor) should not exceed a certain value. However, it has been difficult to determine DAFs effectively from dynamic analysis, and thus no systematic approach has been suggested. The current study provides a guideline to determine DAFs reliably from the dynamic analysis results and summarizes the results by applying the method to an earth-anchored suspension bridge. In the study, DAFs were calculated at the location of four structural parts, girders, pylons, main cable and hangers, with variations in the rupture time.