A new damage index, called strain change based on flexibility index (SCBFI), is introduced to locate damaged elements of truss systems. The principle of SCBFI is based on considering strain changes in structural elements, between undamaged and damaged states. The strain of an element is evaluated using the columnar coefficients of the flexibility matrix estimated via modal analysis information. Two illustrative test examples are considered to assess the performance of the proposed method. Numerical results indicate that the method can provide a reliable tool to accurately identify the multiple-structural damage for truss structures. 1. Introduction Structural damage detection has a great importance in civil engineering. Neglecting the local damage may cause the reduction of the functional age of a structural system or even an overall failure of the structure. Therefore, damage detection is an important issue in structural engineering. The basis of many damage identification procedures is observing the changes in structural responses. Damage reduces structure’s stiffness and mass, which leads to a change in the static and dynamic responses of the structure. Therefore, the damage detection techniques are generally classified into two main categories. They include the dynamic and static identification methods requiring the dynamic and static test data, respectively. Because of the global nature of the dynamic responses of a structure, techniques for detecting damage based on vibration characteristics of structures have been gaining importance. Presence of a crack or localized damage in a structure reduces its stiffness leading to the decrease of the natural frequencies and the change of vibration modes of the structure [1–3]. Many researchers have used one or more of these characteristics to detect and locate the structural damage. Cawley and Adams  used the changes in the natural frequencies together with a finite element model to locate the damage site. Although it is fairly easy to detect the presence of damage in a structure from changes in the natural frequencies, it is difficult to determine the location of damage. This is because damage at two different locations associated with a certain amount of damage may produce the same amount of frequency change. Furthermore, in the case of symmetric structures, the changes in the natural frequencies due to damage at two symmetric locations are exactly the same. There is thus a need for a more comprehensive method of damage assessment in structures. To overcome this drawback, mode shapes have been used for
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