Question

In a pre-stress concrete beam section, net loss 10% and final pre-stressing force applied at X is 750kN. The initial fibre stresses (N/mm${}^2$) at top and bottom are

• A. 4.166 and 20.833
• B. - 4.166 and - 20.833
• C. 4.166 and 20.833
• D. - 4.166 and 20.833

Answer 1

The correct option is D - 4.166 and 20.833

Given, net loss = 10%
After loss, final prestressig force = 750 kN
Hence, initial prestressing force

$=\frac{750}{0.9}=833.33kN$

Initial stress at top and bottom

$=\frac{P}{A}\pm \frac{M}{Z}$

$d=\frac{833.33\times {10}^3}{250\times 400}\pm \frac{833.33\times {10}^3\times 100\times 6}{250\times {400}^2}$

$=8.33\pm 12.50$

$Attop=8.33-12.50$

= - 4.166 (tensile)

$Atbottom=8.33+12.50$
= 20.833 (compressive)