Category: Prestressed Concrete Structures Objective Questions

Prestressed Concrete Structures Multiple Choice Questions on “Thermo Electric Prestressing”.

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
Clarification: 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 ___________
a) 300-400degrees
b) 500-600degrees
c) 200-300degrees
d) 150-450degrees
Answer: a
Clarification: The temperature in the bars is raised from 300 to 400degrees for a 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
Clarification: 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 ___________
a) 500-600n/mm²
b) 200-300n/mm²
c) 100-150n/mm²
d) 600-800n/mm²
Answer: a
Clarification: 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?
a) Canada
b) USSR
c) France
d) Switzerland
Answer: b
Clarification: 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
Clarification: 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 ___________
a) Germany
b) France
c) Switzerland
d) Britain
Answer: a
Clarification: 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
Clarification: 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
Clarification: 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 the case of torpedo splices?
a) 3
b) 4
c) 5
d) 2
Answer: a
Clarification: 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 Multiple Choice Questions on “Long Term Deflections”.

1. The deformation of prestressed members change with time as a result of __________
a) Creep and shrinkage
b) Friction and torsion
c) Deformation and flexibility
d) Cracking moment
Answer: a
Clarification: The deformation of prestressed members change with time as a result of creep and shrinkage of concrete and relaxation of stress in steel, practically the change in stress obtained is relatively very small and hence it may be assumed that at constant stress the formation of creep occurs in concrete.

2. The deflection of prestressed members can be computed relative to given __________
a) Bending moment
b) Strain diagram
c) Curvature
d) Datum
Answer: d
Clarification: The deflection of prestressed members can be computed relative to a given datum if the magnitude and longitudinal distribution of curvatures for the beam span are known based on load history including prestressing forces and live loads.

3. The prestressed concrete member develops deformation under the influence of __________
a) Flexural moments
b) Stress strain diagram
c) Prestress and transverse loads
d) Self weight
Answer: c
Clarification: The prestressed concrete members develop deformation under the influence of two usually opposing effects, which are the prestress effects and transverse loads and the deflections caused are to be changed first because the loss incurred due to prestress which in turn decreases the deflection and effects of creep which increases the deflection are suggested in the method for long term deflections.

4. The net curvature ϕ_tat a section at any given stage is given as __________
a) ϕ_t = ϕ_w + ϕ_e
b) ϕ_t = ϕ_m + ϕ_n
c) ϕ_t = ϕ_p + ϕ_s
d) ϕ_t = ϕ_mt + ϕ_pt
Answer: d
Clarification: The net curvature ϕ_t at a section at any given stage is obtained
ϕ_t = ϕ_mt + ϕ_pt, ϕ_mt = change of curvature caused by transverse loads,
ϕ_pt = change of curvature caused by prestress, as the time changes the compressive stress distribution in the concrete also changes under sustained transverse loads.

5. The section of sustained transverse loads under compressive stress distribution in the concrete changes with __________
a) Time
b) Intensity
c) Bending moment
d) Curing
Answer: a
Clarification: Under the section of sustained transverse loads, the compressive stress distribution in the concrete changes with time, axial force is a measurement of the forces required to pull something such as rope, wire or structural beam to the point where it breaks compression force is the application of power, pressure and erection against an object.

6. The creep strain due to the transverse loads is directly computed as a function of __________
a) Strain coefficient
b) Creep coefficient
c) Stress coefficient
d) Bending
Answer: c
Clarification: The creep strain due to transverse loads is directly computed as a function of the creep coefficient so that the change of curvature can be estimated by the expression,
Φ_mt = (1+ϕ)ϕ_i,
ϕ = creep coefficient, ϕ_i = initial curvature immediately after the application of transverse loads.

7. Which of the following person made attributions to evaluate the curvature under simplified assumptions?
a) Mchenry
b) Rose
c) Daglon
d) Carnwallis
Answer: a
Clarification: Several methods have been proposed to evaluate the curvature under simplified assumptions and important ones are attributed by Buseman, Mchenry, Douglas, Corley, Sozen and Siessand and the numerical solutions developed ignore the influence of the tensile concrete zone on the strain distribution in the section, which considerably effect deflection the equation for long term deflection of cracked members.

8. The creep curvature due to prestress is obtained on the simplified assumption that creep is induced by the average prestress acting over the given time is according to __________
a) Busemann
b) Douglas
c) Lorley
d) Neville
Answer: d
Clarification: According to Neville and the ACI committee report, the creep curvature is obtained due to prestress based on a very simplified assumption that the creep is induced into the concrete by the average prestress acting over with respect to the given time.

9. A simplified but an approximate procedure for computing long time deflections is given by __________
a) Siess
b) Corley
c) Lin
d) Doglas
Answer: c
Clarification: “Lin” suggested a procedure which is not exactly accurate but it helps in calculating long term deflections in a very simplified manner, this procedure helped in calculating the long term deflections.

10. The long time deflections are expressed as __________
a) a_f = (a_il-a_ipxp_t/p_i) (1+ϕ)
b) a_f = (a_il-a_ipxp_t/p_i)
c) a_f = (a_il-a_ipxp_t/p_i)
d) a_f = (a_il-a_ipxp_t/p_i)
Answer: a
Clarification: The principle of reduced modulus involving the creep coefficient is used to amplify the initial deflections and according to this method, the final long time deflections are expressed as:
a_f = (a_il – a_ipxp_t / p_i) (1+ϕ).

Prestressed Concrete Structures Multiple Choice Questions on “Investigations”.

1. In the anchorage zone or the end block of a post tensioned prestressed concrete element, the state of stress distribution is considered as ____________
a) Zero
b) Complex
c) Easy
d) Unity
Answer: b
Clarification: In the anchorage zone or the end block of a post tensioned prestressed concrete element, the state of stress distribution is complex and three dimensional in nature, in most post tensioned members the prestressing wires are introduced in cable holes or ducts, preformed in the members, and then stressed and anchored at the end faces.

2. The stress distribution at a distance far away from the loaded face is computed from ____________
a) Simple bending theory
b) Complex bending theory
c) Axial bending theory
d) Parallel bending theory
Answer: a
Clarification: According to st.venant’s principle, the stress distribution at a distance far away from the loaded face (normally at a distance equal to or greater than the depth of the beam) can be computed from the simple bending theory.

3. The term anchorage zone or end block is termed as ____________
a) Zone between end and section
b) Zone between middle and section
c) Zone between edge and section
d) Zone between tension and section
Answer: c
Clarification: The zone between the end of the beam and the section where only longitudinal stress exists is generally referred to as anchorage zone or end block, as a result of this, large forces, concentrated over relatively small areas are applied on the end blocks, the highly discontinuous which are applied at the end while changing progressively to continuous linear distribution, develop transverse and shear stresses.

4. The transverse stresses developed in the anchorage zone are ____________
a) Tensile
b) Compressive
c) Shafted
d) Jacked
Answer: a
Clarification: The transverse stresses developed in the anchorages zone are tensile in nature over a large length and since concrete is weak in tension, adequate reinforcement should be provided to resist this tension.

5. Which knowledge is essential at the anchorage zone according to view of designer?
a) Transverse stresses
b) Distribution of stresses
c) Zone stresses
d) Anchorage stresses
Answer: b
Clarification: From the point of view of the designer, it is essential to have a good knowledge of the distribution of stresses in the anchorage zone, so that he can provide an adequate amount of steel, properly distributed to sustain the transverse tensile stresses.

6. The investigations studied the stress distribution in the anchorage zone stresses using ____________
a) Theoretical solutions
b) Linear equations
c) Pie diagrams
d) Charts and blocks
Answer: a
Clarification: A number of investigations have studied the stress distribution in the anchorage zone using empirical equations or theoretical equations or theoretical solutions based on two or three dimensional elasticity or experimental techniques.

7. Who among the one has done investigations on anchorage zone stress?
a) Kern
b) Dalton
c) Guyon
d) Lacey
Answer: c
Clarification: The important investigations were those done by Magnel, Guyon, Iyengar, Zielinski, Rowe, Yettram, Robbins and Chandra sekhara, since the lines of force follow the same pattern with half the radius of curvature the length of the anchorage zone is halved, the transverse tension developed is also proportionately reduced, in similar way the greater the number of points of application of the prestressing force on the end block, the more uniform is the stress distribution.

8. The main aim of stress analysis in the anchorage zone is to obtain ____________
a) Zone tensile stress
b) Transverse tensile stress
c) Longitudinal tensile stress
d) Jack tensile stress
Answer: b
Clarification: The main aim of stress analysis in the anchorage zone is to obtain the transverse tensile stress distribution in the end block from which the total transverse bursting tension could be computed, the effect of transverse tensile stresses is to develop a zone of bursting tension in a direction perpendicular to the anchorage forces.

9. During anchorage of the end faces results in application of ____________
a) Small forces
b) Neutral forces
c) Large forces
d) Tensile forces
Answer: c
Clarification: In most post tensioned members, the prestressing wires are introduced in cable holes or ducts, performed in the members and then stressed and anchorage at end faces and as a result of this, large forces, concentrated over relatively small areas, are applied on end blocks.

10. The discontinuous forces applied at the end while changing progressively to develop ____________
a) Transverse and shear
b) Principle and shear
c) Bursting and shear
d) Longitudinal and shear
Answer: a
Clarification: The highly discontinuous forces which are applied at the end, while changing progressively to continuous linear distribution, develop transverse and shear stresses, the ratio of transverse tensile stresses to the average compressive stress gradually decreases with the increase in the ratio of the depth of anchor plate to that of the end block.

Prestressed Concrete Structures Questions and Answers for Entrance exams on “Design of Partially Prestressed Members”.

1. In partially prestressed members, to which extent tensile stresses are permitted?
a) Unlimited
b) Limited
c) Constant
d) Zero
Answer: b
Clarification: In partially prestressed members limited tensile stresses are permitted in concrete under service wads with controls on the maximum width of cracks and depending upon the type of prestressing and environmental conditions and the use of partial prestressing was first proposed by Emperger in 1939 and further progress in this field was mainly due to the sustained work of abeles, Goschy and others and the west German code DIN 42227 has provided for partial prestressing even before the C.E.B-FIP provisions were introduced in 1970.

2. Which type of reinforcement is required for the advantage of partial prestressing?
a) Un tensioned reinforcement
b) Tensioned reinforcement
c) Forced reinforcement
d) Embedded reinforcement
Answer: a
Clarification: The main point in favour of partial or limited prestressing is that required in the cross section of a prestressed member for various reactions such as to resist the differential shrinkage temperature effects and handling stresses.

3. The reinforcement can cater for which requirements?
a) Limited requirement
b) Serviceability requirements
c) Range requirements
d) Termed requirements
Answer: b
Clarification: The reinforcement can cater for the serviceability requirements such as control of cracking and partially for the ultimate limit state of collapse which can result in considerable reductions in the costlier high tensile steel and the saving in prestressing steel contributes to an overall saving in the cost of the structure.

4. The fully prestressed members are prone to excessive ___________
a) Downward deflections
b) Upward deflections
c) Side deflections
d) Transverse deflections
Answer: c
Clarification: Fully prestressed members are prone to excessive upward deflections especially in bridge structures where dead loads form a major portion of the total service loads and these deflections may increase with time due to the effect of creep.

5. The energy capacity is comparatively low in case of ___________
a) Fully
b) Partial
c) Post
d) Prestressed
Answer: a
Clarification: It is well established that fully prestressed members due to their higher rigidity have a lower energy absorption capacity in comparison with partially prestressed members, which exhibit a ductile behavior.

6. Which are used as untensioned reinforcement?
a) Yield steel and mild steel
b) High tensile steel and mild steel
c) Tensile steel and mid steel
d) Principle steel and mild steel
Answer: b
Clarification: High tensile steel and mild steel have been used as un tensioned reinforcement and the present practice is to use high yield strength deformed bars which are considerably cheaper than prestressing steel and at the same time have higher yield strength and better crack control characteristics by virtue of their surface configuration as compared to mild steel bars with plain surface.

7. The width of crack is influenced by ___________
a) Cover
b) Limit states
c) Steel
d) Factor of safety
Answer: a
Clarification: The method of calculating the crack width is of considerable importance in checking the limit state of cracking at service loads and it is well established that the width of crack primarily depending upon the stress in the reinforcement is also influenced by the cover and the type of reinforcement.

8. The calculation based on the stress in the reinforcement is obtained by ___________
a) Limit state theory
b) Convential theory
c) Retardation theory
d) Principle theory
Answer: b
Clarification: Several empirical relations have been developed to estimate the width of cracks but it is considered that a calculation based on the stress in the reinforcement obtained by the convential theory of cracked reinforcement concrete section is inherently more accurate than a computation based on the fictitious tensile stress in an uncracked section.

9. The tensile strength of the concrete below the neutral axis is neglected in ___________
a) Cracked section analysis
b) Uncracked section analysis
c) Beam section analysis
d) Strain analysis
Answer: a
Clarification: The cracked section analysis of a partially prestressed flange section with tensioned high tensile steel and un tensioned reinforcement is carried out under the following assumptions: The strain distribution across the section is linear, the tensile strength of the concrete below the neutral axis.

10. The stresses and strains developed and the forces acting on cracked prestressed concrete sections which are subjected to a moment M_cr is given as?
a) ε_pe = P/A_pE_p
b) ε_pe = P/A_eE_p
c) ε_pe = P/A_sE_p
d) ε_pe = P/A_lE_p
Answer: c
Clarification: In excess of the cracking moment M_cr just prior to the application of the moment the tensile strain in the prestressing steel is εpe and the compressive strain in the concrete at the tendon level is ε_ce and these strains can be evaluated from the prestressing force acting at this stage ε_pe = p/A_pE_p , ε_ce = P/E_c(e²/I_c+1/A_c), P = effective prestressing force, e = eccentricity of tendons, A_c = cross sectional area of the concrete section, I_c = second moment of area of the concrete section.

Prestressed Concrete Structures for Entrance exams,