Chapter 12 - Composite Beams

Abstract

A beam comprised solely of concrete has very little bending strength, because the concrete in the tension zone of the beam cracks at very low values of load. Concrete beams are therefore reinforced in their tension zones by steel bars embedded in the concrete. Generally, whether the beam is precast or forms part of a slab/beam structure, the bars are positioned in a mold into which the concrete is poured. On setting, the concrete shrinks and grips the steel bars; the adhesion or bond among the bars and the concrete transmits bending and shear loads from the concrete to the steel. The modern design of reinforced concrete structures relies on the ultimate load theory. The calculated moment of resistance of a beam section is therefore based on the failure strength of concrete in compression and the yield strength of the steel reinforcement in tension modified by suitable factors of safety. Typical values are 1.5 for concrete and 1.15 for steel. However, failure of the concrete in compression could occur suddenly in a reinforced concrete beam, whereas failure of the steel by yielding would be gradual. It is therefore preferable that failure occurs in the reinforcement rather than in the concrete.