Development of COMAC and Damage Index Methods for Damage Detection in the Near of Abutments of Bridges

Bridges are exposed to damage during their service life which can severely affect their safety. Thus, it is important to monitor bridges for existence of damage. A damage in a structure alters its dynamic characteristics. Changes in properties such as the flexibility or stiffness matrices derived from measured modal properties and changes in mode shape curvature have shown promise for locating structural damage. Since damage alters the dynamic characteristics of a structure, namely its eigenproperties (natural frequencies and modes of vibration), several techniques based on experimental modal analysis have been developed in recent years. Therefore vibration characteristics of a structure can be used as the basis for vibration based damage detection (VBDD) techniques. These techniques have been recently subjected to a considerable amount of attention for damage detection due to their relative simplicity and the moderate cost of dynamic measurements. Damage detection methods based on the dynamic measurements of structures are one of the most important techniques for damage evaluation in bridges. VBDD methods use damage-induced changes to the dynamic properties of a structure to detect, locate, and sometimes quantify the extent of damage.VBDD methods are able to detect damage with information from the dynamic response of the bridge only. The performance of these methods for damage detection in bridges has not been fully proven so far and more research needs to be done in this direction. In this article a new method base on developing the Co-Ordinated Modal Assurance Criteria and Damage Inedx (DI) is present. For applying these methods, mode shapes and natural frequencies that came from health bridges model and damage bridges model are used. The bridges that used are a two-span bridge and a five-span bridge that modeled and verified. To verifying the models, five natural frequencies of the models that created with software, copmared to natural frequencies of the original models. In this article just one element defined as damage location. The damage created by redusing stiffness in one element near the abutments. The four level damage that considered are 15%, 30%, 70% and 90% reduce in module of elasticity. At first the unability of COMAC and DI methods to detecting the damage near the abutment is shown. Then the new method base on these method is presented. This new method is use of mode shapes that obtained from several longitudinal section instead of one longitudinal section. Results confirmed that if mode shapes are just extract from one longitudinal section like before, methods can not always detecting the damaged cross section or damaged longitudinal section. But if mode shapes obtain from several longitudinal sections, these methods will be able to assessment the damaged cross section plus damaged longitudinal section. Although in the most of the times these methods detecting the elements at the abutment as damage location wrongly. So it is necessarily to eliminate the result of the element at the abutment and then decided for the damage location. Besides it is concluded that for detecting the damage near the abutment, COMAC method has better