مطالعه آزمايشگاهي رفتار تيرهاي عميق بتن مسلح پيش-تنيده و مقاوم شده با CFRP

Experimental Study of prestressed RC Deep Beams retrofitted by CFRP

- در سال‌هاي اخير براي افزايش ظرفيت برشي تيرهاي عميق از روش پيش تنيدگي و يا تقويت با CFRP استفاده شده است و اثر هم‌زمان پيش-تنيدگي و مقاوم سازي تير عميق مورد بررسي قرار نگرفته است. در اين مقاله به مطالعه آزمايشگاهي رفتار تيرهاي عميق دو سر ساده در حالات مختلف تيرهاي عميق معمولي، تيرهاي عميق پيش تنيده و تيرهاي عميق مقاوم شده با CFRP پرداخته شده و رفتار آن‌ها با هم مقايسه شده است. در اين راستا 10 تير عميق با نسبت دهانه به ارتفاع 2 ساخته شده و تا نقطه خرابي تحت بار تك نقطه اي قرار گرفته اند. نتايج به دست آمده از تحليل داده هاي آزمايشگاهي نشان مي دهد كه اگر تيرهاي عميق معمولي و پيش تنيده با ظرفيت برشي يكسان، با آرايش مناسبي از CFRP تقويت شوند، تير عميق پيش تنيده تقويت شده شكل‌پذيري بيشتري داشته و مقاومت نهايي آن حدود 7% بيشتر از تير عميق معمولي تقويت شده خواهد بود. همچنين رابطه اي براي تعيين بار نهايي برشي تيرهاي عميق معمولي و پيش تنيده پيشنهاد شده است كه در مقايسه با ساير روش‌هاي موجود از هم‌خواني مناسبي با نتايج آزمايشگاهي برخوردار است.

Deep beams are the members that their behavior is different from conventional beams due to their special geometry and loading condition. Due to the low thickness compared with the height of the beams, the flexural reinforcement’s ratio is usually high and need to be placed in several layers. One of the most effective ways to reduce the ratio of the flexural reinforcement is to use of the prestressed reinforcement instead of conventional reinforcement which more conventional reinforcement can be replaced by a prestressed reinforcement. If that happens, there will be discussion of prestressed deep beams. In recent decades, along with the serious discussion of prestressed deep beams, reinforced concrete members retrofitted with FRP are also considered and in the last years the similar studies have also done on deep beams. The girders are usually prestressed deep beams in the structures such as reinforced concrete bridges, and if the retrofitting of them with FRP is considered, it will encounter with FRP strenghened prestressed deep beams and it is necessary to have knowledge of the behavior of such members. However, the simultaneous effect of prestressing together with retrofitting has not been studied. For this, the experimental study was carried out in this paper for a better understanding of their behavior and comparing of their behavior with other deep beams. This paper study the behavior of simply supported deep beams experimentally by different conditions and has been examined their behavior compared to conventional deep beams, prestressed deep beams, and deep beams strengthened with CFRP. For this purpose, 10 deep beams with span to depth ratio of 2 are constructed and subjected to concentrated load. Considering of this span to depth ratio is due to more compatibility with existing codes. The concrete cylindrical strength is considered greater than 400 kg/cm2 because of prestressed specimens. The test indicates that the idea of replacing of the prestressing cable instead of conventional reinforcements is appropriate and can increase the shear strength and initial stiffness of deep beams in addition to their bending strength. The analysis of experimental results shows that the effects of prestressing and strengthening are not the sum of prestressing and strengthening individually. Moreover, if two conventional and prestressed deep beams with equal shear capacity strengthen with the appropriate arrangements of CFRP, ultimate strength of prestressed deep beam will be %7 higher than conventional deep beam. The energy absorption and ductility of prestressed deep beams strengthened with CFRP are higher than strengthened conventional deep beams. Furthermore, the new model is peoposed to precdict the ultimate shear load of conventional and prestressed deep beams. Comparison of the experimental results with analytical results obtained from proposed model shows that the proposed model are capable to predict the behavior of deep beams.