Question

a. An unsymmetrical I section prestressed concrete beam of top flange (350 X 40 mm) in size, bottom flange of (200 X 40 mm), 30mm thickness of web and overall depth of 400 mm is prestressed with 19 numbers straight steel wires of 7 mm diameter located at a distance of 15 mm from the bottom of the soffit and 9 numbers of straight steel wires of 6 mm diameter located at a distance of 15 mm from the top of the beam. The wires were initially tensioned on the prestressing bed with an initial prestress of 1.2 + 0.01 (35) GPa. The length of the beam is 10 m. Calculate the total percentage loss of stress in the wires at top and bottom. Assume the beam is post tensioned beam and all the wires stressed simultaneously by using the following data:

Relaxation of steel stress= 4.5% of initial stress

Shrinkage strain in concrete for post tensioning = 200 x 10-6

Creep Coefficient ɸ=1.6

Friction coefficient for wave effect=0.0015 + 0.001 (35) per metre Slip at anchorage= 1.5 mm

Modulus of elasticity for steel = 210 GPa

Modulus of elasticity for concrete= 35 GPa

b. In the above problem, how to control the resultant stresses such that they do not exceed 25 N/mm2 at mid span section.

b. What are the measures that are suggested to be taken to reduce the losses in prestressed concrete members for the above problem?

Answer 1