1. The wobble effect due to loss of stress is also known as ____________
a) Wave effect
b) Ray effect
c) Bubble effect
d) Bulb effect
Explanation: Loss of stress due to wobble effect, which depends upon the local deviations in the alignment of the cable and the wobble effect is also known as wave effect, the friction coefficient values for wave effect k are 0.15 per 100m for normal conditions, 1.5 per 100m for thin walled ducts where heavy vibrations are encountered and in other adverse conditions.
2. The wobble effect is the result of ____________
a) Misalignment
b) Extreme alignment
c) Tensile alignment
d) Anchorage alignment
Explanation: The wobble or wave effect is the result of accidental or unavoidable misalignment since ducts cannot be perfectly located to follow a predetermined profile throughout the length of the beam, the coefficient due to wobble effect may be reduced to zero where the clearance between the duct and cable is sufficiently large to eliminate wave effect so as the sheath is made up of heavy gauge steel tube with water tight joints, where a deformation of duct profile is prevented during the vibration of concrete.
3. The loss of stress due to friction is given as ____________
a) 1-(μα+Kx)
b) 1-(μα+Kx)
c) 1-(μα+Kx)
d) 1-(μα+Kx)
Explanation: The prestressing force at section is given by:
Px = p˳e-(μα+kx), Loss of stress (Δf)f = 1-(μα+Kx),
Px = Prestressing force at section x, P˳ = initial prestressing force, μ = coefficient of friction between the cable and concrete, k = wobble effect, d = cumulative angle.
4. The value of ‘μ’ in loss of stress equation depends upon ____________
a) Type of curing
b) Type of concrete
c) Type of steel
d) Type of aggregates
Explanation: The values of ‘μ’ (coefficient of curvature effect) depend upon the type of steel and concrete used in construction and are given in Indian standard codes of practice, coefficient of friction can be considerably reduced by variety of lubricants, particularly greases, oil, graphite mixtures, paraffin, the use of paraffin wax gives by far the coefficient of friction especially with high contact pressure.
5. A post tensioned concrete beam 200mm wide and 450mm deep, of span 10m, initial stress of 840n/mm2 is available in the un jacked end immediately after the anchoring. Find the angle between tangents to the cable at supports?
a) 0.13
b) 0.12
c) 0.10
d) 0.23
Explanation: b = 200mm, d = 450mm, l = 10m, r = 84m, d = 5m,
Angle between the horizontal tangent drawn to the cable at support sinα = (5/84) = 0.06radians,
Cumulative angle between tangents to the cable at supports = (2×0.06) = 0.12radians.
6. A concrete tank if has a minimum stress in wires 600n/mm2 immediately after tensioning and the coefficient of friction is 0.5. Calculate the maximum stress to be applied to the wires at the jack?
a) 900n/mm2
b) 960n/mm2
c) 850n/mm2
d) 800n/mm2
Explanation: Px = 600n/mm2, e = 2.7183, μ = 0.5
Px = P˳e-μα,
600 = P˳e-(0.5×π/2),
P˳ = (600) × (2.71830.79) = 1320n/mm2, Average stress in wires = (1320+600/2) = 960n/mm2.
7. A cylindrical concrete tank, 40m external diameter is to be prestressed circumferentially by means of a high strength steel wire (Es = 210kn/mm2) jacked at 4 points and 90 degrees apart. Find the expected extension at the jack?
a) 150mm
b) 130mm
c) 144mm
d) 133mm
Explanation: d = 40m, Es = 210kn/mm2, n = 4points, θ = 90˚,
Length of wires = (π×40×1000/4) = 104πmm,
Extension at the jack = (960/210×103×104π) = 144mm.
8. The structural concrete members if not designed to have adequate stiffness, it affects ____________
a) Serviceability
b) Local conditions
c) Aggregates
d) Cement
Explanation: In general, according to various national codes, the structural concrete members should be designed to have adequate stiffness to limit deflections, which may adversely affect the strength or serviceability of the structure at working loads
9. The large deflections under dynamic effects may cause ____________
a) Likely appearance to users
b) Discomfort to users
c) High bending
d) Requires more construction materials
Explanation: Large deflections under dynamic effects and under the influence of variable loads may cause discomfort to users, the members of structural concrete are designed in such a way that it possess adequate stiffness to control the deflections or it may leads to a very great impact on serviceability conditions and strength of a structure at working loads.
10. The excessive deflections are likely to cause damage to ____________
a) Partitions
b) Footing
c) Column
d) Beam
Explanation: Excessive deflections are likely to cause damage to finishes, partitions and associated structures, the principal structural members which resulted in excessive sagging condition makes the floor area not suitable for further use.