Design of reinforced concrete structures started in the beginning of this century
following purely empirical approach. Thereafter came the so called rigorous elastic
theory where it is assumed that concrete is elastic and reinforcing steel bars and
concrete act together elastically. The load-deflection relation is linear and both
concrete and steel obey Hooke’s law.
The method is designated as working stress method as the loads for the design of
structures are the service loads or the working loads. The failure of the structure will
occur at a much higher load. The ratio of the failure loads to the working loads is the
factor of safety. Accordingly, the stresses of concrete and steel in a structure designed
by the working stress method are not allowed to exceed some specified values of
stresses known as permissible stresses.