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A post tensioned beam of unsymmetrical I section is required to support a design ultimate moment of 1200 knm and determine the overall depth and thickness of the compression flange required if fck is 35n/mm2 and fp is 1500n/mm2?

A post tensioned beam of unsymmetrical I section is required to support a design ultimate moment of 1200 knm and determine the overall depth and thickness of the compression flange required if fck is 35n/mm2 and fp is 1500n/mm2? Correct Answer 1003mm2

For flanged sections, mud = 0.08fckbd2 assuming b = 0.5d and bw = 0.25b, d3 = ((1200×106)/0.10x35x0.5)), d = 1000mm, b = 500mm, thickness of top flange = hf = 0.2d = (0.2×10000) = 200mm, thickness of web = bw = 0.25b = (0.25×500) = 125mm Assuming the neutral axis depth, xu = hf = 200mm, mu = 0.87fpAp (d-0.42xu), Ap = (mu/0.87fp (d-0.42xu)) = (1200×106/0.87×1500(1000-0.42×200)) = 1003mm2.

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