Experiments on the strength of reinforced and prestressed concrete beams and of concrete-encased steel joists in combined bending and torsion

Abstract

Summary A series of reinforced and prestressed concrete beams and concrete-encased steel joists was tested in combined bending and torsion, with ratios of bending moment to twisting moment varying from pure bending to pure torsion. In all, 33 full-size beams were tested. Measurements of defiexions and strains due to bending and to torsion were made at selected points. The agreement between theory and experiment was generally good. The torsional strength of concrete can be augmented by suitable shear reinforcement. The resistance of the reinforced concrete beams to torsion, however, still depends on the tensile strength of the concrete; it is not therefore possible to gain an increase in strength comparable with that resulting from the addition of tension reinforcement to a plain concrete beam subject to bending. On the other hand the insertion of even a small quamity of shear reinforcement turns the abrupt torsion failure of plain concrete into a gradualfailure accompanied by considerable cracking and deformation. The torsional strength of a concrete-encased steel beam is approximately the same as that of a similar plain concrete beam. The “1” shape is fundamentally a weak one in torsion and the concrete encasement therefore results in a considerable increase in the torsional strength of the steel joist, provided that the joist is surrounded with a nominal amount of stirrup reinforcement or expanded metal to ensure that the steel and concrete act together. Very large increases in torsional strength can be obtained by prestressing, since the initial prestress in compression must be overcome before the concrete can be stressed in tension. The prestressing force, however, increases the rate at which cracks propagate during failure, which therefore tends to assume a violent character. Two distinct types of failure can be distinguished in beams subject to combined bending and torsion. A primary torsion failure occurs with a cleavage fracture normal to the direction of the principal tension. It is governed by the criterion of a constant maximum tensile stress. In bending, tension failure is prevented by the steel, and a primary bending failure therefore results in the crushing of the concrete on the compression face. In reinforced concrete and concrete-encased steel beams the addition of bending increases the resistance to torsion. The paper concludes with detailed recommendations for the design of reinforced and prestressed concrete beams and concrete-encased rolled steel joists in torsion, and in combined bending and torsion.