Limit State

in civil engineering, the point at which a structure or a foundation no longer meets the service requirements. The concept of limit state is employed in the design method that was developed in the USSR and introduced into the Construction Code in 1955. Compared to the methods previously used, with respect to working stresses and breaking loads, the limit state design is more accurate. It offers a complete evaluation of the bearing capacity and reliability of structures through consideration of the probability characteristics of the loads acting on the structures and the resistances to these loads. It also evaluates the specific performance of various types of structures and the plastic properties of the materials used.

Instead of the single safety factor used heretofore, several independent factors are used in the limit state design, each of which plays a particular role to ensure the reliability of the structure and the guarantee against the occurrence of a limit state. These factors include the safety factor for a material and the soil, which takes into account the statistical variability of the strength properties for materials and the soil. It also takes into account certain other factors for which statistical evaluation is either ruled out or is very difficult, such as the difference between the strength of a material in a structure and that determined by sample tests.

The load factor takes into account the possible deviation of the actual loads from the design (standard) values owing to the variability of loading and departures from normal service conditions. The factor for the operating conditions accounts for those aspects of the actual performance of the structural elements, the foundations, and the buildings and structures as a whole that are not directly built into the calculations. There is also a reliability factor that takes acount of the endurance limit of a structure and the comparative importance of the limit state of various parts of the structure.

A distinction is made between the limit state at which a structure becomes unfit for normal service and that at which it completely loses its bearing capacity. For normal service, a structure usually requires adequate rigidity, limited settling, and a certain degree of crack resistance. The loss of bearing capability may become evident in a change in the structural configuration or in the form of brittle, ductile, and fatigue failure of the material. The basic objective of the limit state design is to prevent such failure throughout the serviceable life of a building or structure.

Limit state design has been used extensively in the USSR and in member countries of the Council for Mutual Economic Assistance, the European Organization on Standardization, and the European Committee for Concrete. In the USSR, the limit state is calculated in the design of such mechanical-engineering structures as the metal framework of bridge cranes, overhead cranes, and tower cranes.