Category: Prestressed Concrete Structures Objective Questions

Prestressed Concrete Structures Multiple Choice Questions on “Strain Compatibility Method”.

1. The estimation based on compatibility of the strain of prestressed concrete involves __________
a) Flexural strength
b) Tensile strength
c) Compressive strength
d) Bulking strength
Answer: a
Clarification: The method by which the flexural strength of prestressed concrete is estimated based on the compatibility of strains and equilibrium of forces acting on the section at the stage of failure is known as strain compatibility method.

2. The distribution of concrete in the strain compatibility method is?
a) Aligned
b) Curved
c) Linear
d) Bent
Answer: c
Clarification: The distribution of concrete strain is linear (plane section normal to axis remains plane even after bending), the basic theory is applicable to all structural concrete sections whether reinforced or prestressed and some assumptions are made.

3. The resistance of concrete is neglected at __________
a) Tension
b) Compression
c) Shear
d) Breakage
Answer: a
Clarification: The resistance of concrete in tension is neglected in strain compatibility method at the end of the section, the maximum compressive strain in concrete at failure reaches a particular value are the assumptions made in strain compatibility method.

4. The stress distribution in the compression zone of concrete can be defined by means of __________
a) Specific gravity
b) Coefficient
c) Modulus of elasticity
d) Span moment
Answer: b
Clarification: The stress distribution in the compression zone of concrete can be defined by means of coefficients applied to the characteristic compressive strength and the average compressive stress and the position can be assumed.

5. The flexural compression stress in the compressive zone follows the __________
a) Block curve
b) Anchorage curve
c) Mid span
d) Stress strain curve
Answer: d
Clarification: The flexural compressive stress in the compressive zone closely follows the stress strain curve of concrete and the properties of the concrete stress block can be expressed in terms of the characteristic ratios of k₁ & k₂.

6. The stress-strain characteristics of steel used as prestressing tendons are necessary for __________
a) Principal computation
b) Stress computation
c) Flexural computation
d) Strain computation
Answer: c
Clarification: Knowledge of the stress-strain characteristics of steel that is used as prestressing tendons is necessary for flexural strength computations by the strain compatibility method and a typical short- term design stress-strain curve for concrete recommended in the British & Indian standard codes.

7. The number of steps to be followed in the strain compatibility method is?
a) 4
b) 7
c) 10
d) 6
Answer: d
Clarification: The major steps to be followed in the strain compatibility method are summarized below compute the effective strain, assume a trail value for the neutral axis, using the stress-strain curve for steel compute the total compression L_u & tension T_u, if the compressive and tensile forces are equal, evaluate the ultimate moment M_u.

8. Who suggested a graphical version compatibility method?
a) Cornd
b) Morsch
c) Lin
d) Musy
Answer: b
Clarification: Morsch has suggested a graphical version of the strain compatibility method in which the failure of the compressive zone is assumed when the extreme compressive fiber reaches a strain limit of 0.2%, many codes have recommended simplified procedures for calculating the flexural strength of concrete sections which are reinforced with high tensile steel in the tension zone.

9. How much percentage of tensile strain is assumed at the failure of under reinforced sections?
a) 0.7%
b) 0.5%
c) 0.4%
d) 0.2%
Answer: c
Clarification: For under reinforced sections, the failure of the prestressing steel is assumed to take place at a maximum tensile strain of 0.5% however the method can be considerably simplified under reinforced sections, in which the stress in tensile stress at the collapse stage is more or less equal to the characteristic tensile strength of tendons.

10. The strain compatibility method is generally applicable for __________
a) Under & over reinforcement sections
b) Partially prestressed sections
c) Mid span sections
d) Fully prestressed sections
Answer: a
Clarification: Many codes have recommended simplified procedures for calculating the flexural strength of concrete sections which are reinforced with high tensile steel in the tension zone; the use of the strain compatibility method, generally applicable for both under & over reinforced sections is illustrated.

Prestressed Concrete Structures Multiple Choice Questions on “Deflections of Composite Members”.

1. In the case of composite members, deflections are computed by taking into account the different stages of ___________
a) Tension
b) Loading
c) Ranging
d) Hogging
Answer: b
Clarification: In the case of composite members, deflections are computed by taking into account the different stages of loading as well as the differences in the modulus of elasticity of concrete in the precast prestressed unit and then insitu cast element.

2. The initial deflection is due to ___________
a) Self weight
b) Cross section
c) Beam weight
d) Foundation
Answer: a
Clarification: The initial deflection due to prestress of self weight of the beam and the weight of the insitu cast concrete if the beam is not propped is computed on the basis of the section and the modulus of elasticity of the precast unit.

3. The live load deflection is always estimated using ___________
a) Beam sections
b) Composite sections
c) Bent sections
d) Rolled sections
Answer: b
Clarification: The live load deflection is always estimated using composite properties if the precast beam is propped during construction and the deflections due to dead weight of in situ concrete is also computed on the basis of composite sections.

4. When the modulus of elasticity of the precast and insitu cast concrete are different the flexural rigidity is worked out by?
a) First moment of area
b) Second moment of area
c) Limit states
d) Composite beam
Answer: b
Clarification: When the modulus of elasticity of the precast and insitu cast concrete are different the flexural rigidity is worked out by computing the equivalent second moment of area of the composite sections using the modular ratio and the deflections computed under service loads should not exceed the limiting values prescribed in the codes.

5. One of the knowledge required for planning of any structural component is ___________
a) Seismic nature of terrain
b) Regional nature of terrain
c) Wind nature of terrain
d) Loads
Answer: a
Clarification: Planning of any structure like a building, bridge, marine structures or storage structures requires comprehensive knowledge of the various parameters like site conditions availability of skilled labour and materials, transportation facilities, seismic nature of the terrain, sub soil water conditions, choice of material like steel, reinforced or prestressed concrete, weather and durability considerations, clients requirements and the funds earmarked for the structures.

6. In general appropriate technology must necessarily involve the prevailing?
a) Structures
b) Infastructure
c) Codes
d) Books
Answer: b
Clarification: In general, appropriate technology must necessarily involve the prevailing local infrastructure like raw materials, man power, plant and machinery, power and financial resources and an excellent example of appropriate technology can be found in the low cost suspension bridges built by Girish bharadwaj in south India.

7. The factors which influence the cost of a bridge are ___________
a) Method of erection
b) Length of beam
c) Cost of water
d) Testing of materials
Answer: a
Clarification: In general the quantities of concrete and steel expressed per unit area of deck can be considered as indicative of economy although these figures are not the only ones which governs the overall cost of the bridge and the various factors which influence the cost of a bridge are: the length of individual spans, the type of cross section of deck, the number of longitudinal girders, the width of bridge deck, depth and type of foundation, excavations etc, the cost of form work, the cost of materials and labour, the type of construction, such as cast in situ or precast, the method of erection of precast elements.

8. For spans less than 35m, SSB beams are?
a) Costly
b) Cheapest
c) Zero
d) Constant
Answer: b
Clarification: For spans less than 35m, SSB are the cheapest type of construction and as the span increases, the cost of simply supported beams increases rapidly and it is prohibitively costly for spans exceeding 60m.

9. Which are more expensive than continuous beams of unequal spans?
a) Portals
b) Hinges
c) Layers
d) Arches
Answer: a
Clarification: Portals are slightly more expensive than continuous beams of unequal spans and however for very long spans especially in deep ravines, reinforced concrete arched bridges are more economical and concrete arches are seldom prestressed because prestressing adds very little to the natural advantage of thrust in arch design.

10. In early 1960, based on which analysis have reported the variation of the cost of bridge decks?
a) Rigorous computations analysis
b) Tension analysis
c) Strain analysis
d) Stress analysis
Answer: a
Clarification: As early as in 1960, Sarkar based on a rigorous computational analysis have reported the variations of the cost of bridge deck in relation to span and the number of longitudinal girders in a tee girder bridge and although the cost was more or less the spans up to 15m the cost increased with the increasing number of girders for spans in the range of 15 to 35m.

Prestressed Concrete Structures Multiple Choice Questions on “Prestressed Concrete Bridge Decks”.

1. The prestressed concrete bridge decks generally comprise __________
a) Precast pretensioned
b) Precast post tensioned
c) Partially pretensioned
d) Partially post tensioned
Answer: a
Clarification: Pretensioned prestressed concrete bridge decks generally comprise pretensioned units used in conjunction with cast in situ concrete, resulting in composite bridge decks which are ideally suited for small and medium spans in the range of 20 to 30m and in general pretensioned girders are provided with straight tendons and to use of seven wire strands has been found to be advantageous in comparison with plain or indented wires.

2. The precast prestressed I and inverted T beams have been standardized by __________
a) Pulverization association
b) Cement and concrete association
c) Brick association
d) Steel association
Answer: b
Clarification: The precast prestressed I and inverted T beams have been standardized by the cement and concrete association for use in the construction of bridge decks of spans varying from 7 to 36m and standard I and T units are widely employed in highway bridge beams in USA.

3. The post tensioning is ideally suited for prestressing of __________
a) Short span girders
b) Long span girders
c) Effective span
d) Limited span
Answers: b
Clarification: Post tensioning is ideally suited for prestressing long span girders at the site of construction, without the need for costly factory type installations like pretensioning beds and segmental construction is ideally suited for post tensioning work and in this method, a number of segments can be combined by prestressing resulting in an integrated structure.

4. In India, a large number of long span bridges have been constructed using __________
a) Simply supported beam
b) UDL
c) Point
d) Cantilever
Answer: d
Clarification: In India a large number of long span bridges have been constructed and some of the notable examples being the barak bridge at silchar built in 1960 with a main span of 130m and the lunha bridge in assam with a span of 130m between the bearings.

5. Long span continuous prestressed concrete bridges are built of which type of box girders?
a) Single celled
b) 3 celled
c) Multi celled
d) 4 celled
Answer: c
Clarification: Long span continuous prestressed concrete bridges are built of which type of box girders multi celled box girders segments of variable depth using the post tensioning system, and typical cross sections of post tensioned prestressed concrete bridge decks and salient features of the cantilever construction method using cast insitu segments and precast concrete elements.

6. Calculate Permissible stresses compressive stresses in concrete at transfer and working loads, as recommended given f_ct = 15n/mm², f_st = 0.8, f_cw is 12n/mm²?
a) 13.2
b) 13.4
c) 13.8
d) 13.0
Answer: a
Clarification: Permissible stresses compressive stresses in concrete at transfer and working loads, as recommended f_ct = 15n/mm² < 0.4f_ci = (0.45×35) = 15.75n/mm², f_st ratio, ɳ = 0.80, f_cw = 12n/mm² < 0.33f_ck = (0.33×40) = 13.2n/mm², f_u = f_tw = 0.

7. Calculate bending moment of dead load given total load = 14n/mm², Dead load bending moment dead weight of slab = 12kn/m², dead weight of w.c = 1.76kn/m², span length is 10.4m?
a) 120knm
b) 130knm
c) 190knm
d) 160knm
Answer: c
Clarification: Dead load bending moment dead weight of slab = 12kn/m², dead weight of w.c = 1.76kn/m², total load = 14n/mm²,
Dead load bending moment (m_g) = (14×10.42)/8 = 190knm.

8. Calculate effective length of Road on a highway given the overall thickness is 500mm and thickness of wearing coarse is 80mm (x is 3.6m)?
a) 4.76m
b) 34.8m
c) 2.34m
d) 1.45m
Answer: a
Clarification: Overall thickness = 500mm and thickness of wearing coarse = 80mm, x is 3.6m
Effective length of road = (3.6+2(0.5+0.08)) = 4.76m.

9. Calculate the mid support section given that A_p = 7980mm², f_ck = 60n/mm², b = 800mm, f_p = 1862n/mm², b_w = 200mm, M_u(required) = 16865knm, D_t = 400mm, d = 1750mm, failure by yielding of steel?
a) 1204
b) 23402
c) 1298
d) 45367
Answer: b
Clarification: A_p = 7980mm², f_ck = 60n/mm², b = 800mm, f_p = 1862n/mm², b_w = 200mm, M_u(required) = 16865knm, D_t = 400mm, d = 1750mm, M_u = 0.9dA_pf_p = (0.9x1750x7980x1862) = 2340x 10⁶ nmm = 23402 knm.

10. Calculate failure by crushing of concrete such that b_w is 200, d is 1750, f_ck is 60n/mm², b is800, D_f is 400?
a) 12253x106nmm
b) 15253x106nmm
c) 18253x106nmm
d) 16253x106nmm
Answer: c
Clarification: b_w is 200, d is 1750, f_ck is 60n/mm², b is800, D_f is 400,
M_u = 0.176bwd²f_ck+2/3 x 0.8(b-bw)(d-0.5D_f)D_ff_ck = ((0.176x200x1750²x60)+0.67×0.8(800-200)(1750-0.5×400) 400×60) = 18253×10⁶nmm.

11. Check for ultimate shear strength such that shear force, V_u = 2834 kn, According to IRC: 18-2000, the ultimate shear resistance of the support section uncracked in flexure loss ratio is 0.8, p is 12000×10³, area is 0.88 x 10⁶mm²?
a) 11.3n/mm²
b) 23.5n/mm²
c) 19.9n/mm²
d) 10.9n/mm²
Answer: d
Clarification: Flexure loss ratio is 0.8, P = 12000×10³, Area = 0.88 x 106mm²,
f_cp = (ɳp/A) = (0.8x12000x103/0.88×10⁶) = 10.9n/mm².

12. Calculate the slope angle such that eccentricity is 750, length is 40m and stress induced is 1000n/m²?
a) 0.89
b) 0.075
c) 0.054
d) 0.065
Answer: b
Clarification: Here, e = 750, l = 40m and stress induced is 1000n/m²,
θ = (4e/L) = (4×750/40×1000) = 0.075.

13. Calculate spacing of a prestressed concrete T slab using 12mm diameter of two legged stirrups such that f_c is 415, v_cw = 1900kn, balance shear is 934×10³?
a) 165kn
b) 345kn
c) 230kn
d) 450kn
Answer: a
Clarification: f_c is 415, v_cw = 1900kn, balance shear = 934kn, using 12mm diameter two legged stirrups spacing,
s_v = (0.87x415x2x113x1900/934×10³) = 165mm.