Updated: Jun 12
BASIC PRINCIPLES OF PRESTRESSING
A beam resting on supports at each end trends to bend under its own weight and under applied loading. This causes compression along the top part of the beam and tension along the bottom part. In other words, there is a tendency for the bottom of the beam to stretch.
Concrete is strong in compression, but weak in tension, and for this reason, a plain concrete beam has little strength. The tensile weakness of concrete is overcome by casting steel bars into the sections where tension is likely to occur. When a load is applied on beam, cracks still occur in the concrete, but the tension is carried by the steel reinforcement.
The principle of prestressing is to compress the beam before it is loaded in such a way that stresses are induced in the section which is opposite in action to those arising under loading. Thus the bottom of the beam is compressed by the prestressing so that tension arising when it is loaded will be entirely neutralized.
Furthermore, the compression in the concrete is also of great importance in resisting shear. If one imagines a prestressed beam as a row of blocks pressed together, it is easy to see that if they are pressed together sufficiently tightly they will not fall out when a load is applied. This condition aided by the device of sweeping cables upwards at the end of the beam will usually eliminate the need for steel reinforcement to resist shear stresses.
ADVANTAGES OF PRESTRESSED CONCRETE
The rapid increase in the use of prestressed concrete is due to the fact that it is technically and economically superior to other methods of construction.
Since the size of prestressed members is less than that of the conventional reinforced concrete members, the dead load of the structure is often reduced sufficiently, hence savings of mate