Prestressing is the intentional application of compressive force to a structure before external loads are applied in order to improve its strength and behavior. It works by counteracting the internal tensile stresses that would otherwise develop under external loads. Historically, prestressing techniques were used in structures like wooden barrels, bicycle wheels, and guyed ropes, where internal compression resisted tension from loads. Prestressed concrete applies this concept by compressing steel within a concrete structure to counteract the tensile stresses from loads, improving the structure's load capacity compared to reinforced concrete alone.

1. welcome

3.  Prestressing, itself means, an intentional application
of a pre-determined force on a system, for resisting
the internal stresses that may be developed in the
system, due to external loads.
 It is meant for improving the behaviour and capacity
of the system.
 If the system is expected to have tensile stresses due
to external loads pre-compression can be given to the
system to a desired level even before the application
of the external loads.
 Then the tension developed due to the external loads,
will get largely nullified by the initially applied
compression.

4.  This principle was used long back in our
civilization. It is reflected in the design of
barrels to contain liquid or wheel of a bicycle
or guyed ropes of mast or inflation of tyres of
a vehicle
 Metal strip wound around wooden boards of
barrel are tightened to create pre-
compression on the barrels. When liquid is
stored in the barrels, tension is generated on
the wooden boards and the already provided
pre-compression nullifies this tension.

5. WOODEN BARREL

6.  Similar is the effect of spokes of the bicycle wheel,
which has an initial tension to resist subsequent
compression under loading. These are the cases of
uniform prestressing of the material, in an element, to
counteract the uniform stresses due to external loads.
It will be of the same nature (say tension or
compression) throughout the material.
 This philosophy of prestressing is extended later to
materials, which are subjected to different types of
stresses, namely tension or compression within the
element itself due to external loads

7. BICYCLE SPOKES

8.  In ordinary reinforced cement concrete, compressive
stresses are taken up by concrete and tensile stresses
by steel alone.
 The concrete below the neutral axis is ignored since it
is weak in tension.
 Although steel takes up the tensile stresses, the
concrete in the tensile zone develops minute cracks.
the load carrying capacity of such concrete sections
can be increased if steel and concrete both are
stressed before the application of external loads. This
is the concept of prestressed concrete.

9.  In prestressed concrete the weakness of concrete in
tension is compensated by the tensile strength of
steel - in this case, steel wires, strands, or bars
 The compressive strength of the concrete is used to
advantage by applying an external compressive force
to it which either keeps it permanently in
compression even when loads are applied to it
during its service life (fully-prestressed) or limits the
value of any tensile stress which arises under load
(partial prestressing)

10.  The pre-compressing or prestressing of concrete can be
likened to picking up a row of books by pressing the books
together. The greater the number of books (the longer the
span) the greater the force that has to be applied at either
end of the row to prevent the row (the beam) collapsing under
its own weight.
 The row of books has zero tensile capacity. Hence the beam
made of books cannot even carry its own self-weight.
 However, if the books are externally compressed (prestressed),
the beam can resist the self-weight and even some additional
loads kept at the top till the tensile stress developed by
bending overcomes the compressive stress induced by
prestress.
 The same resisting mechanism exists in a prestressed concrete
member

12. Thank you