اثر مقاومت بتن، مقدار ميلگرد، و خسارت اوليه بر نتايج آزمايش ديناميكي مودال در تيرهاي خمشي

EFFECT OF CONCRETE STRENGTH, FLEXURAL REINFORCEMENT AND INITIAL DAMAGE ON THE DYNAMIC PROPERTIES OF REINFORCED CONCRETE BEAMS

در اين نوشتار، اثر مقاومت بتن، مقدار ميلگرد، و خسارت اوليه بر نتايج آزمايش مودال در تيرهاي خمشي بررسي مي‌شود. بدين منظور، 8 عدد تير بتن مسلح با ابعاد 200×150×2200 ميلي‌متر ساخته و آزمايش شد. نمونه‌ها از نظر مقاومت بتن، مقدار ميلگرد، و خسارت اوليه با يكديگر متفاوت بودند. آزمايش بر روي نمونه‌ها شامل مراحل استاتيكي و ديناميكي بود. فرآيند آزمايش‌ها در مراحل مختلف به‌صورت هم‌گام با يكديگر تا تخريب نمونه‌ها ادامه يافت و پارامترهاي ديناميكي ثبت شد. براي كشف خسارت از دو روش شكل مودهاي اول و دوم و جذر مجموع مربعات شكل مودهاي اول و دوم استفاده شد. نتايج نشان داد كه مي‌توان با كمك شكل مود دوم و نيز جذر مجموع مربعات شكل مودها، به مكان خسارت در نمونه‌ها پي برد. همچنين با تغيير در مقدار ميلگرد و مقاومت بتن، تغييرات محسوسي در نتايج آزمايش مودال حاصل شد.

This paper investigates the effect of concrete strength, the amount of flexural reinforcement and initial damage of reinforced concrete beams on the results of modal tests. A total of eight reinforced concrete beam specimens with the dimensions of 150×200×2200 mm were manufactured and tested. The beams were reinforced using steel bars with 6, 8, 12 and 18 mm diameter (a 6 mm bar diameter was used for transverse reinforcement, while other diameters were used as longitudinal reinforcement). In order to investigate the effect of concrete strength on dynamic parameters, two different concrete mixtures were used to produce concrete with compressive strengths of 21MPa and 40MPa (standard cube specimens). Modal tests, involving static and dynamic stages, were carried out in sequence. In the static stage, under the application of static load, damage was induced to the specimen in the form of shear and flexural cracks. A four-point loading system was employed in the static stage by a hydraulic jack. The mid-span deflection of the beam and the applied load were recorded by a displacement transducer (LVDT) and a load-cell, respectively. The load-cell was in direct contact with the hydraulic jack and measured the magnitude of the applied load at every moment. The data from the LVDT and the load-cell were recorded by a data-logger acquisition system. After the end of each static stage, the specimen was unloaded and then dynamic tests were carried out. For the dynamic tests, 23 degrees of freedom (DOF) were defined for the beam. Three specimens were damaged before the tests to study the effect of initial damage on the dynamic properties of beams. In the dynamic tests, the frequency changes and damping ratios of beams were measured at different ages of the specimens.Test results showed that the frequencies of beams decreased with increasing the damage level of the specimens. The increase of tensile reinforcement and concrete strength increased the frequencies of the beams. The intensity of the damage level in each specimen affected its mode shape. The location of damage was determined by the results of mode shape curvatures.