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Losses in Prestress Concrete

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hossam emam

on 28 April 2015

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Transcript of Losses in Prestress Concrete

Relaxation
Prestressed Concrete
Losses
Losses
The force which is used to stretch the wire to the required length must be available all the time as prestressing force if the steel is to be prevented from contracting. Contraction of steel wire occurs due to several causes, effecting reduction in the prestress. This reduction in the prestressing force is called loss in prestress. In a prestressed concrete beam
Creep is the time dependent deformation due to permanent force. In prestressed concrete, prestress is the permanent force in the member, causing compressive stress at the level of steel. Hence there is creep strain in the member.
Creep strain = Ce x Elastic strain
There is contraction due to drying of concrete and shrinkage strain occurs in concrete. Shrinkage strain causes the steel to lose its stretch, resulting in the loss of prestress.
Loss of stretch = shrinkage strain
Creep & Shrinkage
Prestressed concrete is a method for overcoming concrete's natural weakness in tension. It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete. Prestressing tendons are used to provide a clamping load which produces a compressive stress that balances the tensile stress that the concrete compression member would otherwise experience due to a bending load.
Elastic shortening
Creep & Shrinkage
Friction
Anchorage

Post Tension Only
Post tension Only
Relaxation
When the stresses in steel is more than half of its yield stress there is creep in steel also. Force of prestress falls as a result of creep in steel. Then there is a loss of prestress. Percentage creep varies from 1 to 5%. Creep in steel is also termed as relaxation of steel.
Friction

Post Tension Only
Friction
Post Tension Only
Frictional loss occurs only in post tensioned beams. When the cable is stressed, friction between the sides of the duct and the cable does not permit full tension to be transmitted. Therefore at a point away from the jacking end prestress is less.

Frictional loss is due to

a) Length effect, and

b) Curvature effects.
Anchorage

Post tension Only
Anchorage set losses are caused by the movement of the tendon prior to seating of the wedges or the anchorage gripping device. A common value of these losses is about 10mm
Elastic
shortening
When the prestress is transmitted to the concrete member, there is contraction due to prestress. This contraction causes a loss of stretch in the wire. When some of the stretch is lost, prestress gets reduced. Let Fc be the compressive stress at the level of steel.
Solving
خ
s
this Chart Show that lossesat (ACI Code) is the smallest value by (29%)
and the (Indian code) has the biggest value (36%)
types of losses
According to time
Immediate
Time dependent
Elastic
Creep
Friction
Anchored
shrinkage
Relaxation
According to material
Steel
Concrete
Relaxation
friction
Anchorage
Creep
Elastic
Shrinkage
Full transcript