TO TAKE MAXIMUM USE OF STRENGTH OF CONCRETE, STEEL REINFORCEMENT IS USED TO TAKE UP TENSILE STRESSES IN REINFORCED CONCRETE BEAMS.

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 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. For moderate amounts of reinforcement, steel reaches its yield strength at some load. At this stage steel yields and extends by large amount with the result 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. This type of failure is a gradual one and is preceded by the widening of cracks and significant increase in deflection. If the amount of reinforcement used is large, the steel does not reach its yielding stress but before that concrete has reached its compressive strength. It fails by crushing when the strains in concrete are large enough to cause disruption of concrete. The failure takes place when the concrete has reached strain of 0.3 to 0.4 percent. The failure of this type is sudden and of explosive nature and occurs without visible signs. It is obvious from the above discussion held in the structure 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. The design of Reinforcement concrete structures based on the working stress method and another method of designing reinforced concrete structure is based on the ultimate strength of the structural member and also known as the limit state method.