Optimization of Condensed Stiffness Matrices for Structural Health Monitoring

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.