5350

Design a 25m span simply supported post tensioned bridge girder, the cross section of which is shown below: The girder supports a permanent load (G) of 27.5 kN/m and an imposed (Q) load of 25kN/m. It can be designed as a fully or partially prestressed member. Allocated mark for: (1) prestress force calculation: 20%, (2) stress checks: 10% and (3) ultimate capacity 15%. The design properties of the girder can be assumed as follows: ? Characteristic compressive strength of concrete at 28 days (fc’) = 50MPa ? Characteristic compressive strength of concrete at transfer fcp = 40MPa ? Breaking strength of prestressing steel (fpb) = 1870MPa ? Initial losses at mid-span = 12% ? Long term losses at mid-span = 20% ? Clear cover to duct = 65mm ? Duct diameter = 70mm

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