We seen in last blog different types of prestress losses as per IS 1343:2012, clause, 19.5.2, lets see remaining losses,
Loss due to Elastic Shortening of Concrete: (19.5.2.4)
This type of loss occurs when the prestressing tendons upon release from tensioning devices cause the concrete to be compressed. This loss is proportional to the modular ratio and initial stress in the concrete and shall be calculated as below,
For Pretensioning, the loss of prestress in the tendons at transfer shall be calculated on a modular ratio basis using the stress in the adjacent concrete. ( cl 19.5.2.4)
Loss = m. Fc
For post-tensioned, tendons which are not stressed simultaneously, there is a progressive loss of prestress during transfer due to the gradual application of the prestressing forces. This loss of prestress should be calculated on the basis of half the product of the stress in the concrete adjacent to the tendons averaged along their lengths and the modular ratio. Alternatively, the loss of prestress may be exactly computed based on the sequence of tensioning. ( cl 19.5.2.4)
Case - I, when all wires are tensioned simultaneously (i.e. all wires are tensioned at the same time) then total loss is 0, because the applied prestressing is recorded after the elastic shortening of concrete or wires are anchored only after elastic shortening of the concrete.
Case - II, when all the wires are tensioned successively or sequentially (i.e. one wire at a time tensioned) then stressing in one bar will cause loss in other bar or there will be loss previously stressed wires.
Where,
n = Number of wires or tendons
m = Modular ratio
Fc = Stress in concrete
Loss due to Slip of Anchorage: (19.5.2.5)
In post tension member, when the prestress is transfer to the concrete the wedges slips through a certain distance before they get properly seated in the conical space, therefore anchorage block moves before it settles on the concrete. Therefore there is loss of prestress due to the reduction in the length of tendon.
Any loss of prestress which may occur due to slip of wires during anchoring or due to the strain of anchorage shall be allowed for in the design. ( cl 19.5.2.5)
Loss due to Friction : (19.5.2.6)
The friction generated at the interface of concrete and steel during stretching of curved tendons, leads to drop in the prestress along the member from the stretching.
The design shall take into consideration all losses in prestress that may occur during tensioning due to friction between the prestressing tendons and the surrounding concrete or any fixture attached to the steel or concrete. (cl19.5.2.6)
For straight or moderately curved structures, with curved or straight cables, the value of prestressing force Px at a distance x metres from tensioning end and acting in the direction of the tangent to the curve of the cable, shall be calculated as below:
Where,
P0 = Prestressing force in the prestressed steel at the tensioning end acting in the direction of tangent to the force of the cable,
α = Cumulative angle in radians through which the tangent to the cable profile has turned between any two points under consideration,
μ = Coefficient of friction in curve,
k = Coefficient for wave effect.
Values of μ and k is given in cl 19.5.2.6 table
To Calculate α,
Jacking from one end :
Jacking from both end :
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