Prestressed Concrete Structures MCQ Set 1
1. The method of pre stressing by heated tendons, achieved by passing an electric current is known as:
a) Chemical pre stressing
b) Thermo electric pre stressing
c) Dynamic pre stressing
d) Thermo blasting pre stressing
Answer: b [Reason:] In this method the tendons are heated through the high tensile wires and by allowing electric current to flow through it, this process is referred as thermo electric pre stressing, in the erstwhile U.S.S.R, the electro thermal method has been widely used since 1958 for pretensioning bar reinforcements of precast units.
2. In thermo electric pre stressing, heating the bars is done to a temperature of:
Answer: a [Reason:] The temperature in the bars is raised from 300 to 400degrees for time duration of 3to 5 minutes, with this the bar elongates up to 0.4 to 0.5%, empirical relations for the estimation of the current, voltage and power requirements of the transformer are reported by Graduck.
3. The period of cooling in thermo electric pre stressing is observed as:
a) 20-30 minutes
b) 12-15 minutes
c) 5-10 minutes
d) 15-20 minutes
Answer: b [Reason:] After the process of cooling, the bars get shortened but which can be known with the fixed arrangement at both the ends and the period of cooling is observed as 12 to 15minutes, a steel metal sheath of enlarged diameter and sufficient length is generally used to cover the splice, screw threaded connectors are not recommended for splicing heat treated prestressing steels, which are highly suspectable for stress corrosion.
4. The thermo electric pre stressing, initial stress in tendons ranges between:
Answer: a [Reason:] Thermo electric pre stressing helps to cause on initial stress in tendons of magnitude from 500 to 600n/mm2 and concrete is placed in moulds only after the temperature of the wires falls below 90degrees.
5. In which place thermo electric pre stressing found to be more economical:
Answer: b [Reason:] In the erstwhile USSR, the Thermo electric pre stressing method has been widely used since 1958 for pre tensioning bar reinforcements of pre cast units and this method has been found to be more economical than conventional mechanical devices.
6. The thermo electric pre stressing is also used in:
a) Pre-block units
b) Pre-cast units
c) Pre-beam units
d) Pre-anchorage units
Answer: b [Reason:] Thermo electric pre stressing is adopted in many foreign countries as it is most economical when compared to the other conventional mechanical devices; it is also used in precast units, precast are only used within ranges of exterior and interior walls compressed in concrete and stone, creating a solid but maneuverable wall or face, precast concrete production performed on ground level, which helps with safety throughout a project.
7. The tensioning of oval section ribbed wires with an ultimate tensile strength was adopted in:
Answer: a [Reason:] Thermo electric pre stressing has also been adopted in Germany for the tensioning of oval section ribbed wires with an ultimate tensile strength of 1600n/mm2, the wrapping wire of 1mm diameter is generally used for splicing wires up to 6mm diameter and the length of splice may vary from 20-30cm and the splice formed in this manner has a strength almost equal to that of the normal wire and this type of splice is generally used for the wires of circular concrete tanks and anchorage loops.
8. The estimation of the current, voltage and power requirements of the transformer are for:
a) Theoretical equations
b) Empirical relations
c) Global relations
d) Quadratic equations
Answer: b [Reason:] A temperature of about 4600c was necessary to induce an initial stress of 55 percent of the ultimate tensile strength the heating time being 40-90s at 30v and 300-1100 A and Empirical relations for the estimation of the current, voltage and power requirements of the transformer are reported by Graduck.
9. In thermo electric pre stressing, the elongation is observed in case of:
a) Bar after cooling
b) Bar after heating
c) Bar after anchoring
d) Simple bar
Answer: b [Reason:] In case of heating the bar gets elongated and shortens after heating and after anchorages are applied on both sides which stretch the bar, clamp splices are equipped with bolts and nuts since there will be a considerable reduction in the tensile strength of up to 50 percent, these type of splice can be used only in locations where the prestressing force can be sufficiently reduced by the curvature of tendon due to friction.
10. How many wedges are considered in case of torpedo splices?
Answer: a [Reason:] Tendons are spliced in case of continuous pre stressed concrete members to gain continuity and the various types of splices used as tendons are screw connector, torpedo splices, clamp splice and wrapped splice and torpedo splices consists of triple wedges for securing the wires and the entire unit is covered and protected by a sleeve and this type of splice is largely used for splicing cold drawn wires.
Prestressed Concrete Structures MCQ Set 2
1. In case of structural concrete members subjected to torsion, shear stress develops depending upon the:
a) Type of bending
b) Type of tendon
c) Type of anchorage
d) Type of cross section
Answer: d [Reason:] In the case of structural concrete members subjected to torsion, shear stresses develop depending upon the type of cross section and magnitude of torque, the shear stresses in association with the flexural stresses may give rise to principal tensile stresses, the value of which when it exceeds tensile strength of the concrete results in the development of cracks on the surface of the member.
2. The distribution of torsion shear stress is uniform in:
a) Parallel sections
b) Rectangular sections
c) Trapezoidal sections
d) Circular sections
Answer: c [Reason:] The distribution of torsion shear stress is uniform in circular sections where the magnitude of the shear stress is proportional to the distance from the centre and in case of non circular sections involving warping of the cross section, approximate formula have been proposed based on elastic analysis.
3. The maximum shear stress of circular section is given as:
Answer: a [Reason:] An analysis of principal stresses in prestressed concrete members should include the combined effect of shear stresses due to transverse loads and torsion, together with direct stresses due to flexure and prestress and the maximum stress of circular section is given as:
16T/πD3, D = diameter, T = torsion.
4. The failure of prestressed concrete member without additional un tensioned reinforcement, to that of plain concrete is:
Answer: b [Reason:] The failure of a prestressed concrete member without additional un tensioned reinforcement, under pure torsion, is more or less similar to that of plain concrete where sudden failure is imminent almost simultaneously with the formation of first crack.
5. Which type of prestressing is advantageous for the members subjected to pure tension?
a) Concentric prestressing
b) Tangential prestressing
c) Circular prestressing
d) Overloaded prestressing
Answer: a [Reason:] The research by Humphery and Zia has shown that by suitably adjusting the value of the prestressing force the torsional resistance can be increased by as much as 2.5 times that for the corresponding plain concrete member and for members subjected to pure torsion; concentric prestress is more advantageous than eccentric prestress.
6. When both longitudinal steel and spirals are provided in prestressed members, the ultimate torsional resistance is:
Answer: b [Reason:] The use of longitudinal steel or spirals independent of each other does not increase the ultimate torsional resistance but when both longitudinal steel and spirals are provided in prestressed members, the ultimate torsional resistance is enhanced and according can be expressed as Ttp + Tts, where Ttp is the torsional resistance moment of the prestressed concrete section and Tts is the additional torsional resistance moment of the non-prestressed reinforcement, which must consist of spirals and longitudinal steel.
7. The behavior of a prestressed concrete member is affected by the relative magnitude of:
a) Internal actions
b) External actions
d) Constant actions
Answer: a [Reason:] The behavior of a prestressed concrete member is affected by the relative magnitude of the internal actions, such as torque, bending moment and shear force, in circular sections and if torsion is small, it has little effect on the overall behavior and the failure are controlled by either flexure or shear.
8. The members subjected to torque, bending and shear are generally reinforced with:
a) Parallel and perpendicular reinforcements
b) Longitudinal and transverse reinforcements
c) Rectangular and trapezoidal reinforcements
d) Circular and square reinforcements
Answer: b [Reason:] Members subjected to torque, bending and shear are generally reinforced with longitudinal and transverse reinforcements in order to study the contribution of the longitudinal and transverse reinforcement in resisting flexure, torsion and shear forces, it becomes necessary to analyze the system of forces acting on the warped cross sections of the structural element at the limit state of failure.
9. The skew bending theory is based on the:
a) Plane Elasticity
b) Plane Deformation
c) Plane Torque
d) Plane Strain
Answer: b [Reason:] The skew bending theory is based on the plane deformation approach to plane sections subjected to bending and torsion, the skew bending theory was initially proposed by Lessing with subsequent contributions from Collins, Hsu, Zia, GEsund, Mattock and Elfgreen, of the several researches in this field, Hsu has made the most significant contribution baasedd on the experimental investegations, his work forms the basis of the American, Australian(AS 1481) and Indian code (IS: 1343) provisions.
10. The space truss, which is composed of longitudinal bars and diagonal concrete truss subjected to twist is known as:
a) Skew bending theory
b) Space truss analogy theory
c) Space truss theory
d) Compression failed theory
Answer: b [Reason:] The space truss analogy theory, which is a modification of the planar truss analogy for shear and according to this theory the space truss, which is composed of longitudinal bars and diagonal concrete struts is subjected to twist in which the stirrups and longitudinal bars are considered the tension members and the diagonal concrete struts at an angle θ between the cracks and considered the compression members θ is idealized to 45 degrees.
Prestressed Concrete Structures MCQ Set 3
1. In a pretensioned system, when a wire is released from its temporary anchorage the end of the wire swells as a result of:
Answer: a [Reason:] In a pretensioned system, when a wire is released from its temporary anchorage on the prestressing bed, the end of the wire swells as a result of the recovery of the lateral contraction and develops a wedge effect and this is generally referred to as the Hoyer effect and this is to enable the prestressing force to become at the end of the wire.
2. The swelling of the wire produces which action on concrete:
a) Linear pressure
b) Axial pressure
c) Radial pressure
d) Upcoming pressure
Answer: c [Reason:] The swelling of the wire is only a few thousandth of a millimeter, but it nevertheless produces considerable radial pressures on the concrete, giving rise to large frictional forces, the general provisions in the Indian code for the transmission length are expressed in terms of the diameter of the wire, bar or strand, taking into considerations the surface characteristics of the tendons.
3. How many factors are considered while prestressing force in steel is transferred to concrete through bond?
Answer: c [Reason:] The prestressing force in steel is transferred to concrete considering three factors through the bond having adhesion, friction, shearing resistance; the limits were made for wire tendons of 100 to 140 diameter and for 7 wires strands 45 to 90 diameter of tendons.
4. The bond stress in the prestress members is mainly due to:
a) Friction and shrinkage
b) Compression and bondage
c) Tension and torsion
d) Anchorages and tendons
Answer: a [Reason:] The bond stress in the prestress members is mainly due to friction, shearing resistance and for perfect bond; friction is considered generally, the FIP recommendations regarding the anchorage lengths to ensure the transmission of the prestress to the concrete.
5. The bond stress at intermediate points is resisted by:
Answer: c [Reason:] Bond stress at intermediate points is resisted by adhesion and in transfer zone the adhesion is destroyed by invariable slip and sink of tendons into concrete, transversely ribbed steel type of wire has a diameter of 20 to 40 mm2 area of bond length 500mm.
6. Which zone attains maximum bond stress?
a) Transverse tension
b) Transverse anchorage
c) Transverse tendon
d) Transverse compression
Answer: d [Reason:] The zone of transverse compression attains maximum bond stress and when the bond stress becomes zero various changes occur, 2 or 3 strand wire of diameter 2 to 3 has a bond length of 700mm, 7 wire strand of diameter 2 to 4 has a bond length of 1000mm.
7. The steel and concrete reach their maximum values of stress when bond stress is:
Answer: a [Reason:] When the bond stress in zero, the steel and concrete reach their maximum values of stresses with uniform stress distribution in this section, the bond lengths recommended in German specifications are compiled in drawn steel of diameter 3 to 8 and bond length 600mm.
8. The term transmission refers to attainment of uniform:
a) Strain distribution
b) Stress distribution
c) Level distribution
d) Cross section distribution
Answer: b [Reason:] The length needed for achieving uniform stress distribution in a member is termed as transmission length, the transmission length for plain smooth wires is considerably greater than that for deformed bars or strands due to absence of a mechanical interlock.
9. At intermediate points along the length of a beam, the bond stress is resisted by:
Answer: b [Reason:] At intermediate points along the length of a beam, the bond stress is resisted by adhesion, while in the transfer zone the tendons invariably slip and sink into the concrete destroying most of the adhesion.
10. The shearing resistance is also termed as:
Answer: b [Reason:] Shearing resistance is also known as dilatancy, it is considered in case of transmission of prestressing force from steel to concrete through bond, the ACI code recommendations are based on the investigations of Kaar and Hatson it includes the both length required to develop the effective prestress as also the additional length over which the strand must be bonded to the concrete so that the tensile stresses develop in the strand at the limit state of collapse of the member.
Prestressed Concrete Structures MCQ Set 4
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 [Reason:] 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:
Answer: b [Reason:] 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 [Reason:] 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 [Reason:] 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 [Reason:] 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:
c) Without reaction
d) With reaction
Answer: b [Reason:] 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:
Answer: a [Reason:] 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 [Reason:] 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:
Answer: b [Reason:] 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)?
Answer: b [Reason:] 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.
Prestressed Concrete Structures MCQ Set 5
1. The transverse tensile stress in transmission zone is developed due to concentration of:
Answer: b [Reason:] Transverse tensile stresses of considerable magnitude develop in the transfer zone due to the concentration of tendons at the ends and these stresses are influenced by jacking and the method of releasing the tendons from the prestressing beds.
2. The transverse tensile stresses are found to be maximum near:
a) Centroidal section
b) Mid span section
c) End zone section
d) Shaft cracking
Answer: b [Reason:] The area at or near centroidal sections of the end faces of beams have maximum tensile stress and if the tensile stresses exceed the tensile strength of concrete, horizontal cracking occur, mid span of the section effects the deflection parameters of the structural member.
3. The method of distributing tendons at the ends has a greater influence on:
a) Mid zone cracking
b) End zone cracking
c) Equivalent cracking
d) Shaft cracking
Answer: b [Reason:] A number of cases of cracks developed in precast pretensioned members are reported by foundation and it has been found that the method of distributing tendons at the ends have a greater influence on the end zone cracking.
4. The problem of end zone cracking in pretensioned I beams has been experimentally investigated by:
a) Warren Hastings
Answer: c [Reason:] The problem of end zone cracking in pretensioned I beams has been experimentally investigated by Marshall, Mattock, Arther and also Ganguli at the centroid of cast in situ slab the compressive force applied is equivalent to direct compressive force acting at composite section together bending moment.
5. The tensile stress is determined by the empirical equation of the type:
a) fv = KM/bwd2
Answer: a [Reason:] The transverse tensile stress is determined by the empirical formula fv = KM/bwd2 fv– Transverse tensile stress at centroid of the end face, M – Resulting bending moment between the prestress force and internal prestress developed in the concrete on the centroidal axis, bw = thickness of web, d = overall depth of beam, K = constant depending upon the slope and distribution of tendons at the ends.
6. The transverse tensile stress distribution in transfer zones can be computed by an expression of the type:
a) 10M/bwhLt (1-x/Lt) e-3.5xLt
b) 10M/bwhLt (1-x/Lt)
Answer: b [Reason:] Investigations by Marshall and Krishna Murthy involving extensive tests on pretensioned I beams indicate that the transverse tensile stress distribution in transfer zone can be computed by an expression of the type: 10M/bwhLt (1-x/Lt) e-3.5xLt Lt = transmission length, x = distance from end face.
7. The magnitude of maximum tensile stresses found to occur at the centroid of end face when x is zero is:
Answer: a [Reason:] The magnitude of the maximum tensile stress which is found to occur at the centroid of each end face when x = 0 is given by:
fv(max) = 10M/bwhLt,
M = moment, Lt = transmission length.
8. The creep coefficient of the concrete varies from:
a) 1 to 6
b) 1 to 3
c) 1 to 9
d) 1 to 12
Answer: b [Reason:] The creep coefficient varies from 1 to 3 and it is denoted by ϕ, the composite section carries all the applied loads when the effect of creep is more and on the precast element the cast in situ slab is casted in which the slab has already undergone shrinkage and creep, but creeping starts only after thee in situ slab is casted and is larger due to wet concrete and both precast beams and cast insitu slabs contains differential creep and shrinkage which induced stresses from one element to the another element.
9. The term iw in a section of I beam is termed as:
a) Thickness of web
b) Depth of web
c) Self weight of I section
d) Total weight of I section
Answer: a [Reason:] The term iw is thickness of web and this is used in problems of end zone cracking in pretensioned I beams, on applying compressive forces of equal magnitude along the same line the tensile forces can be balanced.
10. A time dependent deformation of concrete is termed as:
c) Final stress
Answer: b [Reason:] A time dependant formation of concrete is termed as creep, this deformation is seen due to loadings and applied stress or strain failures and the main effects of creep are it decreases the effects due to shrinkage strain; there is a mutual transfer of loads from beam to the composite section.