Optimization of Condensed Stiffness Matrices for Structural Health Monitoring

Optimization of Condensed Stiffness Matrices for Structural Health Monitoring

Kong Fah Tee (University of Greenwich, UK)
Copyright: © 2019 |Pages: 36
DOI: 10.4018/978-1-5225-7059-2.ch006

Abstract

This chapter aims to develop a system identification methodology for determining structural parameters of linear dynamic systems, taking into consideration practical constraints such as insufficient sensors. Based on numerical analysis of measured responses (output) due to known excitations (input), structural parameters such as stiffness values are identified. If the values at the damaged state are compared with the identified values at the undamaged state, damage detection and quantification can be carried out. To retrieve second-order parameters from the identified state space model, various methodologies developed thus far impose different restrictions on the number of sensors and actuators employed. The restrictions are relaxed in this study by a proposed method called the condensed model identification and recovery (CMIR) method. To estimate individual stiffness coefficient from the condensed stiffness matrices, the genetic algorithms approach is presented to accomplish the required optimization problem.
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Introduction

Civil engineering structures such as buildings, bridges and offshore platform continuously accumulate damage during their service life due to natural and man-made actions. Damage in a structure is often translated into change in physical properties of its structural components. When an element of the structure contains damage, the stiffness of the damage element as well as the load-carrying capacity of that element will change (Tee et al, 2013). If not monitored and rectified, damage would increase maintenance cost and render structures unserviceable. In extreme event, damage may even cause structures to collapse catastrophically, resulting in loss of lives and assets. The only way to safeguard the safety of human life and to reduce loss of wealth is to carry out regular monitoring for early detection of structural damage. It is therefore essential to detect the existence, location and extent of damage in the structure early and to carry out remedial work if necessary.

The science of monitoring (continuous or periodic) of the condition of a structure using built-in or autonomous sensory systems is now called Structural Health Monitoring (SHM). Some of the noteworthy efforts in SHM are reported in special issues in Journal of Engineering Mechanics, ASCE in July 2000 (Ghanem and Sture, 2000) and January 2004 (Bernal and Beck, 2004) and in Computer-Aided Civil and Infrastructure Engineering in January 2001 (Adeli, 2001). For civil engineering structures, the current methods used by practicing engineers are mainly visual inspection (Moore, 2001) and localized on-site methods such as acoustic or ultrasonic methods, magnetic field methods, radiography, eddy-current methods and thermal field methods (Doherty, 1987). All these latter on-site methods require that the vicinity of the damage is known a priori and that the portion of the structure being inspected is readily accessible. These experimental methods can usually be used to detect damage on or near the surface of the structure and are thus limited in application.

The need for quantitative global damage detection methods that can be applied to complex structures has led to research into SHM methods that examine changes in the vibration characteristics of the structure. Vibration-based inspection is currently an active area of research in SHM, on the basis of examining changes in the characteristics of a structure before and after damage occurrence based on analysis of input and output signals due to dynamic excitation (Tee et al, 2009; Koh et al, 2002). The general idea is that changes in the physical properties (i.e., stiffness, mass, and or damping) of the structure will, in turn, alter the dynamic characteristics (i.e., natural frequencies, modal damping and mode shapes) of the structure. A monitoring system can provide invaluable insight into the accuracy of these structural models and not only can assist engineers in refining them but also can verify design assumptions and parameters for future construction.

For the purpose of SHM, the use of vibration-based inspection or system identification provides a non-destructive means to quantify structural parameters based on measured structural response due to dynamic excitation. Using a monitoring system to measure structural responses, a damage detection strategy is then employed to monitor the structural health and to provide information for facilitating the planning of inspection and maintenance activities. Any health monitoring and damage detection methodology often involves some kind of system identification algorithm. Therefore, structural system identification will be briefly reviewed, and its correlation to structural damage identification will be highlighted in the following sections.

Key Terms in this Chapter

System Identification: The process of constructing models from experimental data.

Actuator: A device that converts energy into motion. It also can be used to apply a force.

Structural Health Monitoring: Process of implementing a damage detection and characterization strategy for engineering structures.

Sensor: A device that is used to record acceleration, velocity, or displacement of the structures.

Damage: Changes to the materials and/or geometric properties of structures, such as stiffness reduction.

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