The primary object of structural design is to obtain a structural solution which can result in result greater overall economy by providing maximum assistance in satisfying all or requirements of structure.

Nawy based on extensive research work, have developed empirical relations to predict maximum width of cracks and ir mean spacing in pre tensioned and post tensioned beams and according to investigations stabilized mean crack spacing, a_cs can be expressed as C (A₁/?0)
A_t = effective concrete area in tension, ?0 = sum of circumferences of reinforcing elements, C = empirical constant.

The minimum prestressing force required and corresponding eccentricity are controlled by serviceability limit states at which stresses are not to exceed Permissible values are se values are permitted or limited to a certain extent.

Failure of one or more critical sections in flexure, shear, and torsion or due to ir combinations is one of criteria concerning prestressed concrete for ultimate limit state, ultimate load analysis of slabs by yield line ory also suffers from similar deficiencies in that deformation characteristics of slab at service loads cannot be predicted by ultimate loads.

Tensile stresses of magnitude exceeding both modulus of rupture of concrete and visible cracking of limited width are permitted under service loads as compiled and members are referred to as partially prestressed class-3 type constitute most economical of prestressed concrete structures due to limited prestressing forces and high tensile steel requirements.

The structure may be rendered unfit for its intended purpose due to various serviceability limit states being reached, re are six factors which effect limit states and different failures of members are also considered in this factors in case of limit states.

The trial and error method of assuming a certain section and checking its resistance using appropriate design formula is often time consuming since a section satisfying requirements at serviceability limits may be deficient at limit state of collapse.

DL is dead load, LL is live load and WL is wind load while considering earthquake effects substitute EL for WL and se loads are considered while calculation of loads and deflections of beam.

The inadequacy of working load design in predicting ultimate loads of a structure was recognized after first world war factor of safety applied to consistent materials does not present a realistic picture of degree of safety against collapse of composite materials such as reinforced concrete used in structural component.

The design loads for various limit states are obtained as products of characteristic loads and partial safety factor and are expressed as:
F_d = ?_f F_k, where F_d = appropriate design load, ?_f = partial safety factor for loads, F_k = characteristic load.