a. An unsymmetrical I section prestressed concrete beam of topflange (350 X 40 mm) in size, bottom flange of (200 X 40 mm), 30mmthickness of web and overall depth of 400 mm is prestressed with 19numbers straight steel wires of 7 mm diameter located at a distanceof 15 mm from the bottom of the soffit and 9 numbers of straightsteel wires of 6 mm diameter located at a distance of 15 mm fromthe top of the beam. The wires were initially tensioned on theprestressing bed with an initial prestress of 1.2 + 0.01 (54) GPa.The length of the beam is 10 m. Calculate the total percentage lossof stress in the wires at top and bottom. Assume the beam is posttensioned beam and all the wires stressed simultaneously by usingthe following data: Relaxation of steel stress= 4.5% of initialstress -6 Shrinkage strain in concrete for post tensioning = 200 x10 Creep Coefficient ɸ=1.6 Friction coefficient for waveeffect=0.0015 + 0.001 (54) per meter Slip at anchorage= 1.5 mmModulus of elasticity for steel = 210 GPa Modulus of elasticity forconcrete= 35 GPa
b. What are the measures that are suggested to be taken toreduce the losses in prestressed concrete members for the aboveproblem?
