What are modes of failure of beam????

We know, in a reinforced section all the compressive forces are borne by concrete and all the tensile forces are borne by the steel.

If ratio of steel to concrete in a beam is such that both the materials fails simultaneously i.e. maximum strain in both the materials reach simultaneously. Such a beam is called balanced reinforced beam. In this type of beams, a sudden failure would occur with less alarming deflection. A Reinforced concrete beam may have following two types of failure- Tension Failure:-

When amount of steel is kept less than that in the balanced section, then neutral axis moves upward to satisfy equilibrium condition that is the force of compression must be equal to force of tension .

In this process, centre of gravity of compressive force shifts upward. Under increasing bending moment, steel is strained beyond the yield point and maximum strain in concrete remains less than 0.35 percent. If beam is further loaded, then strain in the section increases. Once the steel has yielded, it does not take any additional stress for the additional strain and total force of tension remains constant. However, compressive stresses in concrete do increase with the additional strains. Thus, neutral axis and centre of gravity of compressive forces further shift upward to maintain equilibrium. This process of shift in neutral axis continues until maximum strain in concrete reaches its ultimate value, i.e. 0.35 percent, and concrete is crushed. Such a beam is called under-reinforced beam. The failure is called Tension Failure because yielding of steel was responsible for continued higher strains in concrete, resulting in its failure. In this type of failure steel yields first while stress in structure is still in permissible limit of concrete. Because of the yielding of steel the cracks are formed in structure and indicates future collapse of section, thus sudden collapse of structure is avoided. That’s why we should prefer under reinforced beam sections. 2. Compression Failure: When amount of steel is kept more than that in the balanced condition, then neutral axis tends to move downward and strain in steel remains in the elastic region.

If beam is further loaded, then stress and strain in steel keep on increasing and so is the force in tension. The additional increase in the concrete stress is much slower. Thus, to maintain equilibrium of tension and compression forces, area of concrete resisting compression has to increase. In this process, neutral axis shifts further downward until maximum strain in concrete reaches its ultimate value i.e., 0.35 percent and concrete is crushed. The steel is still well within the elastic limit. Such a beam is called over-reinforced beam and the failure, is called compression failure. A compression failure is a brittle failure and is to be avoided. In simple words, In this the failure of concrete occurs first while stress occured in structure is still in the permissible limit of steel, Thus because of the failure of concrete the mass of concrete directly collapse while steel remains in its position as the stress induced is in permissible limit of steel. Thus in over reinforced section, sudden failure of structure occurs without any warning. That’s we should avoid over reinforced beam sections.