Prestressed Concrete Structures Multiple Choice Questions on “Transmission Length”.
1. The transmission length required to attain uniform stress distribution in prestress member is acquired through __________
a) Zero bond stress
b) Middle bond stress
c) Safe bond stress
d) Edge bond stress
Answer: a
Clarification: The transmission length required to attain uniform stress distribution in prestress member with zero bond stress and the length required at the ends of a pretensioned member for the build up to of stress in concrete is of great importance, particularly in short pretensioned units, since it controls the working bending moment and the shear force allowable on the section.
2. The transmission length mainly depends up on how many factors?
a) 8
b) 4
c) 2
d) 12
Answer: b
Clarification: The transmission length mainly depends on 4 factors, diameter of wire, surface characteristics of wire, co-efficient of friction between steel and concrete, elastic properties of steel and concrete.
3. The expression for transmission length based on wedge action is given as?
a) Lt = ϕ/2μ (1+μc) (αc/μs-fpi/Ec) (fpe/2fpi-fpe)
b) Lt = ϕ/2μ (1+μc) (αc/μs-fpi/Ec)
c) Lt = ϕ/2μ (1+μc)
d) Lt = ϕ/2μ
Answer: a
Clarification: Hoyer has developed an expression for computing the transmission length, based on wedge action: Lt = ϕ/2μ (1+μc) (αc/μs-fpi/Ec) (fpe/2fpi-fpe), Lt = transmission length, ϕ = wire diameter, μ = coefficient of friction between steel and concrete, μc = Poisson’s ratio for concrete, μs = Possion’s ratio of steel, αc = modular ratio (Es/Ec), Ec = modulus of elasticity of concrete of concrete, fpi = initial stress in steel, fpe = effective stress in steel.
4. The transmission length is predicted considering the equation parameters varying from __________
a) 90 to 150ϕ
b) 100 to 200ϕ
c) 80 to 160ϕ
d) 20 to 60ϕ
Answer: c
Clarification: Under the normal ranges of values of transmission parameters, the transmission length is likely to vary from 80 to 160ϕ, several tests have been carried out by many investigators to determine the transmission length and these methods can be classified into different categories depending upon the principles under which the solutions are obtained.
5. Which one of the empirical formula is used for predicting transmission length?
a) Lt = ((fcu)1/2 x 103/β)1/2
b) Lt = ((fcu)1/2 x 103/β)
c) Lt = (fcu)1/2 x 103
d) Lt = (fcu)1/2
Answer: a
Clarification: The transmission length is predicted generally using the following empirical formula,
Lt = ((fcu)1/2 x 103/β)1/2, Lt = transmission length in mm, fcu = cube strength of concrete at transfer in n/mm2, β = constant depends on the strand and wire, the transmission length changes with time due to the effect of creep and shrinkage of the concrete.
6. The transmission length prevailing at the time of transfer does not remain __________
a) Zero
b) Constant
c) Without reaction
d) With reaction
Answer: b
Clarification: The transmission length prevailing at the time of transfer does not remain constant but increases at a decreasing rate with time due to the effect of creep and shrinkage of concrete, using radiographic strain measure techniques Evans and Robinson have conclusively shown that the influence of passage of time is to increase the transmission length and to move it bodily away from the end of the member.
7. The surface of drawn round wires is roughened by __________
a) Pickling
b) Bending
c) Bonding
d) Tensioning
Answer: a
Clarification: The surface of drawn round wires is roughened by pickling to produce fine irregularities which interlock with the concrete this is the method of obtaining shear bond, twisting of round wires into strands.
8. The Shear bonding is obtained by cold rolling of __________
a) Square indentations
b) Rectangle indentations
c) Elliptical indentations
d) Circular indentations
Answer: c
Clarification: Cold rolling of elliptical or rhombic shallow indentations on the surface of wires and hot rolling of oblique transverse ribs on wires which are subsequently heat treated methods of obtaining shear bond.
9. According to Ros, the transmission length of smooth round wires of 1.5-5mm diameter varies approximately from __________
a) 1500-600
b) 100-300
c) 200-100
d) 1100-800
Answer: b
Clarification: According to Ros, the transmission length of smooth, round wires of 1.5-5mm diameter varies approximately from 100-300 diameter respectively, after allowing for the time dependent in elastic effects like creep with crimping or indentations, these values could be considerably reduced.
10. Estimate the transmission length at the ends of a pretensioned beam prestressed by 7mm diameter wires. Assume the cube strength of concrete at transfer as 42n/mm2 (adopt empirical method)?
a) 550mm
b) 610mm
c) 420mm
d) 210mm
Answer: b
Clarification: Lt = ((fcu)1/2 x 103/β)1/2 = Transmission length,
7mm diameter smooth wires, β = 0.0174, fcu = 42n/mm2, Lt = ((42)1/2 x 103/ 0.0174) = 610mm = 87ϕ.