RCC IS A MONOLITHIC CONSTRUCTION OF STRUCTURES, SO IT IS NECESSARY TO DESIGN FOR THE ULTIMATE STRENGTH OF THE STRUCTURAL MEMBERS.

Concrete is strong in compression and very weak in tension. Its tensile strength, ie, modulus of rupture is only a fraction of its compressive strength. If a plane concrete beam is loaded, it will fail when the stress on tension side reaches the tensile strength of concrete. The stress on the compression side will be much less than the strength of concrete in compression. To make maximum use of strength of concrete, steel reinforcement is used to take up tensile stresses in reinforced concrete beams, as the Reinforced concrete is a monolithic construction. At small loads stresses are proportional to strains and hence the stress diagram will be a straight line. For very small loads tension is taken by concrete and steel on tension side and whole of compression is taken by concrete. As the load is increased further stress on tension side exceeds the modulus of rupture of concrete and concrete cracks. These cracks move upwards close to the neutral plane. The neutral plane gradually shifts with further cracking. In this stage all the tension is taken by steel . The strains are proportional to distance from Neutral axis and when the load is further increased, stresses are no longer proportional to strains and the failure takes place in two ways depending upon the amount of reinforcement used. For moderate amount of reinforcement, steel reaches its yield strength at some load. At this stage steel yields and extends by large amount with the results that the tension cracks widen and move upwards with considerable deflection of the beam. At this stage strains in compression zone of the concrete increase to such an extent that they cause crushing of concrete. Such a failure is known as secondary compression failure and it takes place at a load slightly greater than that which causes yielding of steel. Thus the load which causes yielding of steel determines the capacity of the beam. Obviously it is clear that it is desirable to design a beam in such a way that failure takes place by yielding of steel and subsequent crushing of concrete rather than crushing of concrete alone. Working stress method and another method of designing reinforced concrete structure is based on the ultimate strength of the structural memo, known as limit state method.