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A concrete beam of rectangular cross-section of size 120 mm (width) and 200 mm (depth) is prestressed by a straight tendon to an effective force of 150 kN at an eccentricity of 20 mm (below the centroidal axis in the depth direction). The stresses at the top and bottom fibres of the section are
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- 2.5 N/mm2 (compression), 10 N/mm2 (compression)
- 10 N/mm2 (tension), 2.5 N/mm2 (compression)
- 3.75 N/mm2 (tension), 3.75 N/mm2 (compression)
- 2.75 N/mm2 (compression), 3.75 N/mm2 (compression)
Correct Option: A
Due to direct stress,
| stress = | = | = 6.25 N/mm2 | ||
| A | 120 × 200 |
Due to moment by eccentric load
| stress = | .y | I |
| = | × 100 | ||
| × 120 × (200)2 | |||
| 12 | |||
= 3.75
stress at top = 6.25 – 3.75 = 2.5 N/mm2
stress at bottom = 6.25 + 3.75 = 10 N/mm2
(compression)
