50.1K

Verified Solution

Link Copied!

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 (35) 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 initial stress

Shrinkage strain in concrete for post tensioning = 200 x10-6

Creep Coefficient ɸ=1.6

Friction coefficient for wave effect=0.0015 + 0.001 (35) permetre 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 stressessuch that they do not exceed 25 N/mm2 at mid spansection.

b.What are the measures that are suggested to be taken to reduce thelosses in prestressed concrete members for the aboveproblem?