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A prestressed concrete beam of section 200mm wide by 300mm deep of imposed load 4kn/m at a span of 6m, density of concrete is 24kn/m3. Find the concentric prestressing force necessary for zero fiber stress at the soffit?

A prestressed concrete beam of section 200mm wide by 300mm deep of imposed load 4kn/m at a span of 6m, density of concrete is 24kn/m3. Find the concentric prestressing force necessary for zero fiber stress at the soffit? Correct Answer 490

b = 200mm, d = 300mm, A = (200×300) = 6×104mm2, g = (0.2×0.3×24), Mg = (0.125×1.44×62) = 6.48knm, Mq = (0.125×4×62) = 18knm, Zb=Zt = (200×3002/6) = 3×106mm3 P/A = 8.16, P = (8.16×6×104) = 489.6kn.

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