Prestressed Concrete Structures MCQ Set 1
1. The most common type of composite construction consists of:
a) I beams
b) T beams
c) L beams
d) U beams
Answer: b [Reason:] The most common type of composite construction consists of a number of precast prestressed inverted T beams placed side by side and connected by a continuous top slab of insitu concrete and this type of construction is widely used in the construction of bridge decks.
2. The dead weight of the deck can be considerably reduced by using:
Answer: d [Reason:] The dead weight of the deck can be considerably reduced by using voids or light weight longitudinal cores in the space between the precast prestressed units and this light weight concrete for the cast in situ slab results in reduced dead loads, leading to economy in the overall costs.
3. The spans exceeding 30m for large span composite bridge decks the precast concrete unit consists of:
a) J sections
b) K sections
c) T sections
d) Rectangular sections
Answer: c [Reason:] For large span composite bridge decks of spans exceeding 30m, the commonly used precast prestressed concrete units consist of I section unsymmetrical T or box sections and the concrete cast insitu forms the deck slab interconnecting the precast units and typical cross section of bridge deck with different type of precast units are compiled.
4. The precast prestressed I and T beams have been standardized by:
a) Cement and concrete association
b) American standard association
c) British standard association
d) Indian standard association
Answer: a [Reason:] The precast prestressed I and T beams have been standardized by the cement and concrete association for use in the construction of bridge decks of span varying from 7 to 36m and standard I and T units are extensively used as highway bridge beams in U.S.A.
5. The use of prestressed concrete tie beams in a reinforced concrete truss reduces:
b) Cross section
c) Curing properties
Answer: b [Reason:] The use of prestressed tie beams in a reinforced concrete truss considerably reduces the cross sectional dimensions of the bottom chord member, which is subjected to high degree of tension in the case of large span trusses.
6. Reinforced and prestressed concrete truss generally used for spans ranging from:
a) 18 to 36m
b) 14 to 30m
c) 20 to 30m
d) 11 to 19m
Answer: a [Reason:] Reinforced and prestressed concrete trusses are generally used for spans ranging from 18 to 36m and this form of construction is ideally suited for industrial structures transfer prestressing is also used to develop monolithic action in the lateral direction.
7. The dead load stress developed in the precast prestressed units can be minimized by:
Answer: c [Reason:] The dead load stress developed in the precast prestressed units can be minimized by Propping them while casting the concrete in situ this method of construction is termed as propped construction and if the precast units are not propped while placing the insitu concrete, stresses are developed in the unit due to self weight of the member and dead weight of the insitu concrete and this method of construction is referred as unpropped construction.
8. The main advantage of composite member is:
a) Ease of work
b) Decreased workability
c) Increased depth of foundation
d) Decreased depth of foundation
Answer: a [Reason:] The main advantage of composite member is it facilitates the ease of work at the construction site and it is the fast approaching process by utilizing the self supporting precast stems the shuttering can be minimized.
9. The precast double tees are used in:
b) Parking structure
c) Balcony & Parking structure
d) None of the mentioned
Answer: b [Reason:] Precast double tees have been used in partially structures, bridge decks are cast in situ slab, it has long term durability and gives aesthetic flexibility, fire resistance and low maintenance and resists gravity seismic and using loads.
10. The depth variation for L shaped composite beams is:
a) 2 to 3 feet
b) 6 to 8 feet
c) 14 to 16 feet
d) 2 to 6 feet
Answer: a [Reason:] In this L section the depth varies from 2 to 3 feet deep, web thickness is usually taken as 16”(0.4m) for the steams of deck sections the ledge provides a bearing surface in which the compacted L beam is subjected to different eccentric loading.
Prestressed Concrete Structures MCQ Set 2
1. The failure due to fracture of steel in tension in the beam is because of:
a) Excess amount of steel reinforcement
b) Excess amount of prestressed concrete
c) Least amount of reinforcement
d) Least amount of prestressed concrete
Answer: c [Reason:] The percentage of steel provided, both tensioned and un tensioned taken together should be sufficient so that when the concrete in the pre compressed tensile zone cracks, the steel is in a position to bear the additional tensile stresses transferred to it by the cracking of the adjacent fibers of the concrete, thereby preventing a sudden failure of the beam due to fracture of steel in tension.
2. If the concrete in tension zone get cracks will lead to development of:
a) Principal stresses
c) Tensile stresses
Answer: c [Reason:] The sudden failure of a prestressed member without any warning is generally due to fracture of steel in the tension zone, this type of failure is imminent when the percentage of steel provided in the section is low that when the concrete in the concrete in the tension zone cracks, the steel is not in a position to bear up the additional tensile stress transferred to it by the cracked concrete.
3. The failure due to fracture steel in tension can be prevented by providing a steel reinforcement:
a) 0.15 – 0.2%
b) 0.20% – 0.35%
c) 0.2% – 0.6%
d) 0.6% – 0.10%
Answer: a [Reason:] In order to prevent this failure a minimum steel reinforcement is provided in the cross section of beam IS: 1343 recommended a minimum reinforcement of 0.15% – 0.2% of the cross sectional area in pretensioned units of small sections when a high yield strength deformed reinforcement is used, the minimum steel percentage is reduced to 0.15 per cent.
4. In under reinforced section failure the beam observes:
a) Excess elongation
b) Less elongation
c) Edge elongation
d) Mid span elongation
Answer: a [Reason:] In this mode of failure, the beam observes excess elongation of steel along with crushing of concrete this is because large amount of steel is provided in the compression zone (greater than minimum steel reinforcement in the tension zone).
5. Due to excess elongation of steel, the neutral axis near the compression face gets:
Answer: a [Reason:] If the cross section is provided with an amount of steel greater than the minimum, the failure is characterized by an excessive elongation of steel followed by the crushing of concrete and as the bending loads are increased excessive elongation of steel raises the neutral axis near the compression face at the critical section of beam.
6. Which type of deflections is observed in compression face during under reinforced section failure?
a) Small deflections
b) Large deflections
c) Bondage deflections
d) Shrinkage deflections
Answer: b [Reason:] Large deflections and wide cracks are observed in compression face and this is because the area of concrete provided is unable to resist the resultant internal compression forces and the member also fails in flexure due to crushing of concrete considerable warnings are observed before impending failure.
7. The over reinforced section fails due to sudden crushing of:
Answer: c [Reason:] Over reinforced section fails due to sudden crushing of concrete and this section observes small deflections and narrow cracks the effective reinforcement due to which the compressive strength of concrete and tensile strength of steel are increased up to certain range of values then the section is said to be over reinforced.
8. The amount of steel provided in this section should not be greater than:
a) Steel required
b) Concrete required
c) Water required
d) Tension required
Answer: a [Reason:] In this section large amount of steel is provided which resists the stresses developed at failure to reach the tensile strength of steel and the amount of steel provided in this section should not be greater than the steel required for balancing the section.
9. Which one of the following is other mode of failure?
a) Failure due to tension
b) Torsion failure
c) Reinforcement failure
d) Transverse shear failure
Answer: d [Reason:] Other modes of failure like transverse shear failure and web crippling are caused due to improper design of member in shear and by providing thin webs in the section, in case of pretensioned members, the failure of bond between the steel and the surrounding concrete is likely due to the inadequate transmission lengths at the ends of members, in post tensioned members anchorage failures may take place if the end block is not properly designed to resist the transverse tensile forces.
10. The failure due to other modes like transverse shear failure is mainly caused due to:
a) Principal stresses
b) Tensile stresses
c) Compressive stresses
Answer: a [Reason:] These failures are mainly caused due to excessive principle stresses developed in the members when they are subjected to transverse loading, prestressed concrete members subjected to transverse loads may fail in shear before their full flexural strength is attained, if they are not adequately design for shear, web shear cracks may develop if the principal stresses are excessive and if thin webs are used, the failure may occur due to web crushing.
Prestressed Concrete Structures MCQ Set 3
1. The reduction in the magnitude of prestressing force occurs due to:
a) Re block
d) Re tension
Answer: b [Reason:] According to Bennet the main effects due to axial deformation and tertiary moments are: Reduction in the magnitude of prestressing force in a particular member due to the restraint of the adjacent members, the development of tertiary moments due to bending deformation of the indeterminate structure due to axial contraction under the action of the prestressing force.
2. The ultimate load carrying capacity is controlled by:
a) Tensile strength
b) Compressive strength
c) Flexural strength
d) Axial strength
Answer: c [Reason:] The ultimate load carrying capacity of a statically determinate prestressed concrete structure is mainly controlled by the flexural strength of a critical section in the member and in contrast to this, the ultimate load carrying capacity of an indeterminate prestressed concrete structure depends upon the flexural strength of several critical sections and their rotation capacity.
3. A conservative estimate of the ultimate resistance is obtained by assuming:
a) Extra reinforcement
b) Tensile reinforcement
c) Factor of safety
Answer: d [Reason:] However a highly conservative estimate of the ultimate resistance of the structure is obtained by assuming the collapse to occur when the moment, calculated by the linear elastic theory first reaches a value equal to the flexural strength at any section.
4. Due to large deformations at the critical sections which concentration develops?
Answer: b [Reason:] Due to large deformations at the critical sections in a statically indeterminate structure a concentration of curvature builds up over a few clearly defined short length of the member, such as the interior supports and midspan points of a continuous beam and these plastic zones are idealized as hinges at which a rotation is considered to take place.
5. If the partial redistribution takes place by the limited hinge rotations, the ultimate strength of the structure lies between:
a) Upper limit
b) Lower limit
c) Upper limit above and lower limit
Answer: c [Reason:] If the partial redistribution takes place due to the constraints imposed by limited hinge rotations, the ultimate strength of the structure lies in between the upper limit above and the lower limit corresponding to the computations based on the linear elastic theory.
6. The equilibrium between the internal forces and the external loads is maintained for:
a) Stress combinations
b) Load combinations
c) Deformation combinations
d) Elastic combinations
Answer: b [Reason:] The redistribution of moments is permitted under the following conditions: equilibrium between the internal forces and the external loads is maintained for approximate load combinations, at sections where the moment capacity after redistribution is less than that from the elastic maximum moment diagram the following relationship should be satisfied (xu/d) +δm/100)) < 0.8, xu = depth of neutral axis, d = effective depth, δm = percentage reduction in moment.
7. The ultimate moment of resistance provided at any section of a member is not less than:
Answer: a [Reason:] The ultimate moment of resistance provided at any section of a member is not less than:
80% of the moment at that section obtained from an elastic maximum moment diagram covering all appropriate combination of loads, the elastic moment at any section in a member due to partial combination of loads shall not be reduced by more than 20 percent of the numerically largest moment covering all appropriate combination of loads.
8. In the design of continuous prestressed concrete members, it is often necessary to determine a cable profile lying within:
a) Distribution zone
b) Limiting zone
c) Edge zone
d) Span zone
Answer: b [Reason:] In the design of continuous prestressed concrete members, it is often necessary to determine a cable profile lying within the limiting zone of thrust and also satisfying the conditions of concordancy and a method based on the principle of virtual work is outlined for determining the concordant cable profile.
9. The difference of the maximum and minimum moments at a cross section in design of continuous prestressed beam is given as:
a) Mt = Mmax-Mmin
b) Mf = Mmax+Mmin
c) Mt = Mmax
d) Mt = Mmin
Answer: a [Reason:] The maximum positive and negative moments due to the live and dead load moments at various sections of the member are calculated and hence computing the range of moment which is obtained as the difference of the maximum and minimum moments at a cross section, Mt = Mmax-Mmin In the design of statically indeterminate prestressed concrete structures involves computation of maximum and minimum moments at various cross sections of the members.
10. The overall cross sectional dimensions are fixed using the:
a) Permissible tensile stresses
b) Permissible compressive stresses
c) Permissible principle stresses
d) Permissible strains
Answer: b [Reason:] The overall cross sectional dimensions are fixed using the permissible compressive stresses in concrete, fc by equation
Z = bh2/6 = My/fc assuming a suitable values of b = 0.4-0.5h, the overall depth this obtained so as obtain the range of moments which generally determines the cross sectional dimensions of the member.
11. In the design of unidirectional members like continuous beams, the axial contraction due to effect of prestressing is:
Answer: c [Reason:] In the design of unidirectional members like continuous beams, the axial contraction due to effect of prestressing is: Neglected as it has very little influence on the force and moments in the structure but in the case of two dimensional structures, such as portal frames and bents, the axial contraction of the members significantly influences the primary and secondary moments in the structure.
12. The data that should be available for purpose of design is:
a) Overall shape
b) Overall area
c) Overall height
d) Overall depth
Answer: a [Reason:] The data available for purpose of design is the overall shape of the structure and the loading conditions and the computations of maximum and minimum live load bending moments require a knowledge of the stiffness of the members of the frame and the moments and thrusts determined by assuming a suitable stiffness ratio determined by assuming a suitable stiffness ratio by the normal methods of structural analysis.
Prestressed Concrete Structures MCQ Set 4
1. How many types of modes of shear cracking are present in structural concrete beams?
Answer: b [Reason:] Research over the years have shown that there are two major modes of shear cracking in structural concrete beams, web shear cracks, flexure shear cracks and different modes of shear failure patterns are also considered like diagonal tension failure, shear compression failure, web compression failure.
2. The web shear cracks generally start from:
a) Interior point
b) Exterior point
d) Mid span
Answer: a [Reason:] Web shear cracks generally start from an interior point, when the local principal tensile stress exceeds the tensile strength of concrete, the British code (BS:8110:-1985) and the Indian code (IS:1343-1980) specify a modified version of this relation given by: Vcw = 0.67bwh(f12+0.8fcpft)1/2 .
3. The web shear cracks are developed when the beam is subjected to:
a) Point load
b) Concentrated loads
c) Shear loads
d) Uniformly distributed load
Answer: b [Reason:] Web shear cracks are likely to develop in highly prestressed beams with thin weds, particularly when the beam is subjected to large concentrated loads near a simple support, in which the value of 0.67 h is somewhat lower for flanges sections this together with reduced value of 0.8fcp results in conservation estimates of the shear resistance of flanges sections and if there are inclined cables, the sheaing force vcw is increased by an amount equal to the vertical component of the prestressing force, in the above expression for computing vcw the tensile strength of concrete may be assumed as ft = 0.24(fck).
4. In which direction the flexural cracks are first initiated:
Answer: c [Reason:] Flexure – shear cracks are first initiated by flexural cracks in the inclined direction and they are developed when the combined shear & flexural tensile stresses produce a principal tensile stress exceeding the tensile strength of concrete.
5. What is provided in order to protect a member from collapsing suddenly after the development of shear cracks?
a) Edge reinforcement
b) Principal reinforcement
c) Span reinforcement
d) Shear reinforcement
Answer: d [Reason:] In order to protect a member from collapsing suddenly after the development of shear crack, a minimum shear reinforcement is provided (Asv ) is provided in the form of stirrups which is obtained by satisfying the following condition:
Asv/bsv = 0.4/0.87fy In member without shear reinforcement, the inclined shear cracks extend to the compression face resulting in sudden explosive failure this is sometimes referred to as the diagonal tensile modes of failure.
6. The ultimate shear resistance of prestressed concrete with web shear cracking but without flexural crakes & mainly governed by:
a) Limiting value
b) Constant value
Answer: a [Reason:] The ultimate shear resistance of prestressed concrete sections with web shear cracking but without flexural cracks, is mainly governed by the limiting value of the principal tensile stress developed in concrete, the failure is assumed to take place when the principal tension exceeds the tensile strength of concrete.
7. The relation for computing the ultimate shear force is given as:
a) Vcw = bw dp((0.3λ )fcy1/2 – 0.3fcp)+ vp
b) Vcw = bw dp((0.3λ )fcy1/2+ 0.3fcp)+ vp
c) Vcw = bw dp(0.3λ + vp)
d) Vcw = bw dp
Answer: b [Reason:] The relation for computing the ultimate shear force, which includes a multiplying factor to multiplying factor to account for the type of concrete is given as Vcw = bw dp((0.3λ )fcy1/2 + 0.3fcp)+ vp Bw = breadth of web, dp = effective depth to tender or 0.8h, λ = 1.0 for nominal weight concrete and less for light weight concrete, fcy = cylinder compressive strength of concrete(n/mm2), fcp = compressive prestress at centroid of a section, vp = the vertical component of the effective prestressing force at section.
8. The ultimate shear resistance vcf of section cracked in flexure is expressed as:
a) vcf = (1-0.55fpc/fp)τ cbwd+(m0/m)v
b) vcf = (1-0.55fpc/fp)τ cbwd
c) vcf = (1-0.55fpc/fp)
d) vcf = (1-0.55fpc)
Answer: a [Reason:] vcf = (1-0.55fpc/fp)τ cbwd+(m0/m)v less than or equal to 0.1bwdfck1/2, fpc = effective prestress, fp = characteristic strength of prestressing steel, τ c = ultimate shear stress capacity of concrete, bw = breadth of the member, d = effective depth of tendons, m0 = moment necessary to produce zero, v and m = shear force and bending moment at section.
9. The flexure shear cracking load in a prestressed member is given by:
Answer: a [Reason:] The American code (ACI: 318-1989) recommendations for the flexure shear cracking load in a prestressed member are based on experimental observations that flexure shear cracking initiates at the flexural cracking load plus an additional shear which is a flexural cracking load plus an additional shear which is a function of the strength and density of concrete & the dimensions of the section.
10. According to Mast, a complete shear analysis is necessary only in beams of:
a) High span
b) Mid span
c) Low span
d) Edge span
Answer: c [Reason:] In general, the web shear and flexural shear resistance of the beam at important sections are compared with the ultimate shear requirements to identify zones where web reinforcement are required, According to Mast, a complete shear analysis is necessary only in beams of low span/depth ratio and in other cases the specified minimum proportions of web reinforcement is sufficient.
Prestressed Concrete Structures MCQ Set 5
1. The maximum permissible stresses are mainly governed by:
a) Tensile stresses
b) Compressive stresses
c) Ultimate stress
d) Principle stresses
Answer: b [Reason:] The maximum permissible stresses in the precast prestressed concrete and the insitu cast concrete are mainly governed by the compressive strength of concrete in the respective elements and in general the permissible stresses in a precast prestressed concrete are governed by the normal rules for prestressed concrete.
2. Certain expectations in serviceability limits of British code are made regarding:
a) High stresses
b) Compressive stresses
c) Limited stresses
d) Elongation stresses
Answer: a [Reason:] Certain expectations are made regarding high stresses developed at the interface of the precast and insitu cast elements and in British code BS: 8110 provides for a higher value of compressive for a higher value of compressive stress equal to 0.5fcu which is 50 percent higher than the normally allowable value in prestressed element.
3. The higher value of compressive stress is permissible only in:
a) I sections
b) Composite sections
c) L sections
d) Prestressed sections
Answer: b [Reason:] The higher value of compressive stress is permissible only in composite sections with the stipulation that the failure of the section is due to excessive elongation of steel and this requirement is to safe guard against the explosive compressive failure of the concrete at the limit state of collapse.
4. The permissible flexural tensile stress in the insitu concrete at the:
a) Contact surface
b) Longitudinal surface
c) Zig zag surface
d) Dump surface
Answer: a [Reason:] The permissible flexural tensile stress in the insitu concrete at the contact surface with the prestressed element, as prescribed in the British code BS: 8110 varies from 3.2 to 5.0n/mm2 corresponding to various grades of concrete.
5. The stresses may be increased up to 50 percent for prestressed unit by the use of:
a) Allowable tensile stresses
b) Permissible tensile stresses
c) Permitted tensile stresses
d) Constant tensile stresses
Answer: a [Reason:] Theses stresses may be increased by upto 50 percent, provided that the allowable tensile stresses for the prestressed unit is reduced by the same amount the higher values of flexural tensile stresses are permitted since it has been proved by experiments that the development of cracking which are visible is prevented by the uncracked prestressed concrete which is bonded to the insitu concrete.
6. The horizontal shear is generally governed by:
a) Shear limit state
b) Ultimate limit state
c) Principle limit state
d) Tensile limit state
Answer: b [Reason:] Horizontal shear is generally governed by the ultimate limit states the British code (BS:8110-1985) identifies three types of surfaces in the design provisions for the maximum permissible design ultimate horizontal shear stresses at the interface.
7. The horizontal shear force, at the interface of the precast and insitu components, are due to:
a) Design ultimate loads
b) Design shear loads
c) Design flexural loads
d) Design tensile loads
Answer: a [Reason:] The horizontal shear force at the interface of the precast and insitu components due to design ultimate loads is the total compression from that part of the compression zone above the interface calculated from the ultimate bending moment.
8. The average horizontal design shear stress is computed by dividing:
a) Horizontal principle force/Area
b) Precast force/Area
c) Horizontal shear force/Area
Answer: c [Reason:] The average horizontal design shear stress is computed by dividing the horizontal shear force by the area which is obtained as the product of the contact width of the bear and its length between the points of zero moment.
9. To obtain the horizontal shear stress the design shear stress are:
Answer: c [Reason:] The average design shear stress is then distributed in proportion to the vertical design shear force diagram to obtain the horizontal shear stress at any point along the length of the members and the design shear stresses should not exceed the values compiled for different types of surfaces.
10. When the links are provided, their cross sectional area should be at least:
Answer: b [Reason:] When links are provided their cross sectional area should be at least 0.15 percent of the contact area and the spacing of the links in T beam ribs with composite flanges should not exceed neither four times the minimum thickness of the insitu concrete nor 600mm.