توسعه روش‌هاي COMAC و نشانه آسيب(DI) براي تشخيص آسيب در كنار تكيه‌گاه پل‌ها

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

روش‌ هاي COMAC و نشانه آسيب (DI) كه بر پايه تغيير در شكل‌هاي مودي است، در شناسايي مقطع طولي آسيب ديده در وسط دهانه عرشه پل ها قادر به تشخيص مقطع طولي آسيب ديده نيست و در برخي موارد مقطع عرضي آسيب ديده را نيز به درستي تشخيص نمي دهند. در اين پژوهش ابتدا ضعف روش هاي گفته شده، نشان داده مي شود و سپس روشي جديد براي تشخيص آسيب در عرشه پل از طريق توسعه روش‌هاي مذكور ارايه مي شود. براي اين منظور مدل عددي يك پل دو دهانه هفتاد و سه متري و مدل يك پل پنج‌دهانه بيست و پنج متري مورد استفاده قرار گرفته است. آسيب ايجاد شده روي عرشه پل، از طريق كاهش مدول الاستيسيته يك المان در كنار تكيه‌گاه پل ها تعريف مي‌شود، نشان داده شده است كه با در نظرگرفتن تنها شكل‌مودهاي يك مقطع طولي،اين روش‌ها قادر به شناسايي مكان آسيب‌ديده نمي‌باشند و بايد از شكل‌هاي مودي چندين مقطع طولي استفاده شود.

Abstract 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 new method base on developing the Co-Ordinated Modal Assurance Criteria and Damage Index (DI) is present. Mode shapes and natural frequencies extract from health bridge model and damage bridge model. A two-span bridge and a five-span bridge are modeled. To verifying the models, five natural frequencies that modeled in software, compared to natural frequencies of original models. The damage defined by reducing 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 inability of COMAC and DI methods to detecting the damage near the abutment is shown. Then a new method base on use of mode shapes that obtained from several longitudinal section instead of one longitudinal section is present. Results confirmed that if mode shapes are just extract from one longitudinal section like before, methods may detecting the damaged cross section or damaged longitudinal section wrongly. But if mode shapes obtain from several longitudinal sections, these methods will be able to assessment the damaged cross section and damaged longitudinal section. Although these methods usually detecting the elements that are near abutment as damage location wrongly. So it is necessarily to eliminate the result of this elements and then decided for the damage location. Besides it is concluded that to detecting the damage near the abutment, COMAC method has better result than DI method.