Prestressed Structure

an engineering structural component in which stresses are optimally distributed among the elements during manufacturing, assembly, or erection. In modern construction, prestressing is most commonly used with rein-forced-concrete structural members and units that are intended for various purposes. It is also common in metal structures. Prestressed structures are very efficient owing to their use of high-strength materials and fuller exploitation of the physical and mechanical properties of these materials.

In prestressed concrete structures, compressive stresses in the concrete and tensile stresses in the reinforcement are usually induced. The use of high-strength steel permits significant savings (up to 70 percent) in the required amount of reinforced steel —cable, wire, or deformed bars. Prestressed structures are highly resistant to cracking. In comparison with conventional structures, they have much greater rigidity, and their ability to endure repeated loads is increased. Prestressed concrete structures are most efficient for buildings and engineering structures (for example, bridges) where the spans, loads, or working conditions are such that the use of structures with unstressed reinforcement involves significant technical difficulties or large outlays of concrete and steel. Prestressed concrete is also suitable for pressure pipes, tanks, silos, and other containers that must be impermeable.

Prestressed metal components are used in bridge spans, crane beams, masts, towers, power transmission line supports, and other engineering structures.

Prestressed structures are designed by the limiting state method, taking into account the actual physical and mechanical properties of the concrete and steel. It is assumed in the design that the induced stresses do not remain constant until service loads are applied. Losses of prestress may be caused by technological factors, such as heat treatment of products and structural members, or by such physical and mechanical properties of the concrete and steel as shrinkage and creep in the concrete and relaxation of stresses in the steel. Prestress may also be lost as a result of design characteristics of the prestressed structures and of the equipment used for tensioning the reinforcement. Such engineering factors may lead to loss of prestress caused by such factors as deformation of the anchors and friction between the tendons and the surface of the concrete in the channels and grooves.

The tendons of prestressed concrete structures may be tensioned before the concrete hardens, in which case the tendons are stretched in a mold or between restrainers in a casting bed. If the tensioning is done after the concrete sets, the tendons are tensioned within the hardened concrete. In this case, the reinforcement is placed in channels that pass through the structure or in exterior grooves. Concrete can also be prestressed in the process of hardening, using self-stressing (expensive) cement. The tendons may be tensioned mechanically, using special jacks and other devices. Electrical heating and other methods may also be used. Prestressing in metal structures is accomplished by elastic bending of the individual elements that are welded together into a single beam, by using tie beams of high-strength steel to compress individual truss members and whole truss systems, by deliberately setting up strains in the supports of continuous girders, arches, and frames, and by other methods.