HOW TO DETERMINE WHERE TENSILE AND COMPRESSIVE STRESSES OCCUR IN STRUCTURAL MEMBERS OF A BEAM
It is convenient to consider the behaviour of an elastic material.
For instance, the diagram above illustrates Indian Rubber under stress. A bar of rubber laid across and not fixed to two supports bends under load and the top surface shortens and becomes compressed under stress and the bottom surface stretched under tensile stress.
A member that is supported so that the support do not restrain bending under load is said to be simply supported as illustrated above. The maximum stretching due to tension occurs at the outwardly curved underside of the rubber bar. In that position the steel will be exposed to surrounding air and it would rust and gradually lose it strength.
Furthermore, if fire occur in a building near the beam, the steel might loss so much strength as to impaired it reinforcing effect and the beam would collapse.
In practise, is necessary to cast the steel reinforcement into the concrete so that there is at least 20mm of concrete cover between the reinforcement and the surface of the concrete.