STRENGTH AND STIFFNESS OF CONTINUOUS REINFORCED CONCRETE BEAMS WITH OPENINGS

Abstract

This thesis deals with the strength and deformation characteristics of reinforced concrete beams that contain a large rectangular opening and are subjected to combined bending and shear. Such openings are often required in beams for the passage of utility ducts and pipes. The problem is treated both analytically and experimentally. In the analytical part, two methods are presented to predict the piecewise linear load-deflection relationship for such beams. The first method (Equivalent Stiffness Method) adopts equivalent flexural stiffness for the segment traversed by the opening. Deflections are then computed by successive elastic analysis using the conjugate segment analogy. The second method (Alternative Method) assumes the flexural stiffness of the segment traversed by opening as equal to the stiffness of the solid beam minus that of the void, but deflections are calculated by linear superposition of the deflection component due to simple bending to that caused by Vierendeel effect that the opening section. The ultimate strengths of the beams are computed by following the collapse load approach in both the methods. A total of eight beams, consisting of three two-span continuous beams and five here-span continuous beams, was tested to failure under a point load. Test results indicate that cracking as well as the collapse loads decrease while the deflection increases with increasing opening size, either in length or depth. The location of opening virtually has no influence on the cracking load, but the collapse load was found to increase as the absolute magnitude of bending moment at the centre of opening is decreased. Higher deflection was obtained for beams with the opening located at a relatively high moment region. The test results of these beams as well as those available in the literature are compared with the analytical prediction. Both methods give correct predictions of the mode of failure. The predicted collapse loads and deflections at calculated service load also agree reasonably well with experimental data. However, the Alternative Method appears to prove relatively better predictions than the Equivalent Stiffness Method.