In absence of statistical data, characteristic strengths of concrete and steel may be taken as works cube strength and specified proof or yield strength respectively as provided for in codes.

In design of prestressed concrete member, minimum required resistance is predetermined for certain limit states such as collapse, deflection and cracking for each of se, design values of resistance must equal or exceed sum of design loads obtained by multiplying specified characteristic loads by appropriate partial safety factors.

Bond and anchorage failure of reinforcement, failure of connections between precast and cast insitu elements and failure due to elastic instability of members are factors which are considered in limit states due to ir characteristics of sustain.

W_cr = 3a_cr?_m / 1+2(a_cr ? l_min/h-x) is recommended by British code BS: 8110 ? 1985 for estimation of surface crack width based on work of beedy, a_cr = distance from point considered to surface of nearest longitudinal area, ?_m = average strain at level where cracking is being considered, C_min = minimum cover to tension steel, h = overall depth of member, x = neutral axis.

In general, a satisfactory design must ensure achievement of an acceptable probability that specified life of a structure is not curtailed prematurely due to attainment of unsatisfactory conditions or limit state, which covers various forms of failure.

It is important to note that limit state design proposals have been evolved from provisions of various earlier codes and resulting changes in design, being minimal, provides designer a greater freedom of choice.

In case of member subjected to axial tension, concrete section and minimum prestress are mainly controlled by Permissible stress at service loads and in some cases (walls of tanks) by practical considerations.

In case of flanged member on basis of collapse width and depth of compression flange is, while thickness of web is based on ultimate shear or a practical consideration of having curved cables with minimum cover requirements.

The bearing at support, anchorages or under concentrated imposed load, bond and various types of failure are also considered as bearing failure at supports, basically limit state design is a method of designing structures based on a statistical concept of safety and associated statistical probability of failure.

Serviceability limit state corresponds to excessive deflection and cracking and it is customary in most codes to safeguard against excessive deflection under serviceability limit state, eir indirectly by prescribing minimum span/depth ratio for member or directly specifying a maximum permissible deflection expressed as a fraction of span.