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Reinforced Cement Concrete

Slip Loss and Relaxation of Steel

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Question

The percentage loss of prestress due to anchorage slip of 3 mm in a concrete beam of length 30 m which is post-tensioned by a tendon with an initial stress of $1200 N / m m^{2}$ and modulus of elasticity equal to $2.1 \times 10^{5} N / m m^{2},$ is

A

0.0175

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B

0.175

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C

1.75

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D

17.5

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Solution

The correct option is C 1.75
Loss of prestress
$= \frac{Δ l}{l} \times E_{b} = \frac{3}{30 \times 10^{3}} \times 2.1 \times 10^{5} = 21 N / m m^{2}$
Initial stress $= 1200 N / m m^{2}$
$∴$ Loss of prestress $= \frac{21}{1200} \times 100 = 1.75 %$

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