Behaviour Of Hollow Rc Beam Under Pure Cyclic Torsion

Abstract

Design of the Structures is based on the internal forces that structures experiences during its service life. Under the e ect of external load main emphasis is made on the safety and serviceability of the structure during expected service life. Beam is one of the basic building element of structure subjected to bending moments and shear forces under e ect of external loads. However in some cases the torsional moments are likely to be generated in beams. Various theories are proposed by di erent re- searchers for prediction of the torsional strength of reinforced concrete beam. In real the overall behavior of the reinforced concrete beam under torsion is very complex and no generalized theory is available for the reinforced concrete beam under torsion. It is observed from the literature that reinforcement of the beam, compressive strength of concrete, cross sectional dimensions a ect the behavior of solid reinforced concrete beam under static torsion. But now a days hollow reinforced concrete beams are also used especially in bridge structures. Due to the torsion, the stresses are generated which may crush concrete and sudden decrement of torsional sti ness occurs which results into sudden collapse of the structure. The present study focuses on behavior of Hollow Reinforced Concrete Beam under pure cyclic torsion. In this study e ect of longitudinal and transverse reinforcement on angle of twist, maximum torsional moment, energy dissipation, number of loading cycle are studied. To achieve this objective the scope of work was casting of 12 RC hollow beams having cross section dimension of 150mm 150mm 1500mm with hole of 50mm diameter at the centre of the beam. Six di erent types of beams were cast with varying amount of longitudinal and transverse reinforcement. For each type of RC beam two specimens were cast and tested under pure cyclic torsion. From this study it is observed that pre cracking behavior of RC hollow beam is mainly governed by the compressive strength of concrete and post cracking behavior is governed by the longitudinal and transverse reinforcement. The cracking torque for di erent beams were at par to each other. Diagonal crack were seen at the inclination between 42 to 44 degree with horizontal which is nearly 45 degree having maximum principle stress. After the cracking and failure of the beam not much distortion in stirrups was observed which shows that plane section remain plane after the applica- tion of torque. Ultimate torque was found to be same for same longitudinal reinforcement regardless of transverse reinforcement regardless of amount of transverse reinforcement. And it was observed decreasing by 36.10% as the amount of reinforcement increases. Total angle of twist in di erent beams was seen in the range of 0.1725 rad/meter to 0.492 rad/meter. While if angle of twist is considered for each cycle, for 8 mm longitudinal steel it was increasing between 2.91% to 5.71% , for 10 mm longitudinal steel it was increasing between 2.25% to 5.85%, and for 12 mm longitudinal steel it was increasing between 19.70% to 44.70%. Energy dissipation in di erent beams was seen in the range of 4.940 kJ to 13.597 kJ. If energy dissipation is considered for each cycle, specimen with four 8mm lon- gitudinal bars an increase between 12.44% to 16.52%, for specimen with four 10mm longitudinal bars an increase between 23.67% to 35.93%, and for specimen with four 12mm longitudinal bars an increase between 34.38% to 37.13% was observed. While increment in energy dissipation for same transverse reinforcement and di erent longi- tudinal reinforcement an increase in range of 21% to 571% was observed. Longitudinal reinforcement was found to be more e ective for energy dissipation than transverse reinforcement. So careful selection of amount of longitudinal and transverse reinforcement should be given more importance while designing of RC beam for torsion to achieve good behavior in terms of angle of twist and torsional stifness of RC beam.