Corbels and deep beams

Ordinary beam theory assumes plane sections remain plane and that span is large compared with depth. Corbels and deep beams break that assumption. A corbel is a short cantilever bracket projecting from a column or wall to support a beam or crane girder, with the load close to the face. A deep beam has a span comparable to its depth, such as a transfer girder, a pile cap or a wall acting as a beam. In both, load travels in a direct compression strut from the load to the support.

The strut-and-tie model is the natural tool. The concrete is idealised as inclined compression struts, the main reinforcement as horizontal ties, and the points where they meet as nodes. Equilibrium of the truss gives the tie force, the struts are checked for crushing and the nodes for bearing. For a corbel, the tie force is roughly the load times its distance from the column face divided by the lever arm, plus any horizontal force from shrinkage or restraint.

IS 456 treats corbels in clause 28 and deep beams in clause 29. For deep beams it gives the lever arm directly as a function of span and depth, because the linear strain assumption no longer holds. It defines a simply supported beam as deep when its effective span is less than twice its overall depth, and a continuous one when the ratio is below 2.5.

Detailing decides success. Corbel ties must be anchored at the outer face, often by welding to a cross bar or bending into a loop, because the load sits close to the end. Horizontal stirrups confine the strut. In deep beams, the main tension steel is spread over a band near the soffit, and a web mesh controls cracks. Bearing plates and edge distances must keep the node within the reinforced zone.