Introduction

Civil engineering structures, during their service life, may experience damage caused by various sources such as harsh environmental conditions, overloading, ageing materials or inadequate maintenance. In order to prevent catastrophic failure and prolong the service life of the structures, early and reliable damage identification is necessary. In engineering practices, damage detection methods are categorized into two major groups as destructive and non-destructive methods. In general, the destructive methods due to their disadvantages are not appropriate; therefore, the use of non-destructive methods has attracted much attention. The non-destructive damage detection methods, which are restricted to local observations in a limited area, when applied to large structures, are very time consuming and expensive. The stress wave, ultrasonic, X-ray, acoustics and radiography are the examples of these methods. In contrast, vibration-based damage detection methods are global non-destructive ones based on the principle that the damage changes the physical properties leading to altering dynamic properties of a structure. Therefore, by utilizing the dynamic characteristics from the structural vibration, damage can be predicted.

In general, there are three types of measured dynamic features including modal parameters, frequency response functions (FRFs) and time history responses. Traditionally, modal parameters such as natural frequencies, mode shapes and damping ratios are the most common dynamic features for the damage detection. Especially, the use of resonant frequencies as the damage index was popular in the early years of vibration-based damage detection since they are easy to obtain [1,2]. Maity and Tripathy used the genetic algorithm to detect the structural damage from changes in natural frequencies [3]. Pawar and Ganguli used the change in frequencies and a genetic fuzzy system for determining the crack density and location in a thin-walled hollow circular cantilever beam [4]. Unfortunately, in many damage cases, the resonant frequencies were found to be insensitive to the structural damage especially in multiple damage cases in large structures [5,6]. For field application, another drawback is that the natural frequencies are heavily sensitive to environmental changes such as temperature or humidity fluctuations [7].