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
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
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
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
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
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 fct = 15n/mm2, fst = 0.8, fcw is 12n/mm2?
Clarification: Permissible stresses compressive stresses in concrete at transfer and working loads, as recommended fct = 15n/mm2 < 0.4fci = (0.45×35) = 15.75n/mm2, fst ratio, ɳ = 0.80, fcw = 12n/mm2 < 0.33fck = (0.33×40) = 13.2n/mm2, fu = ftw = 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?
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 (mg) = (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)?
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 Ap = 7980mm2, fck = 60n/mm2, b = 800mm, fp = 1862n/mm2, bw = 200mm, Mu(required) = 16865knm, Dt = 400mm, d = 1750mm, failure by yielding of steel?
Clarification: Ap = 7980mm2, fck = 60n/mm2, b = 800mm, fp = 1862n/mm2, bw = 200mm, Mu(required) = 16865knm, Dt = 400mm, d = 1750mm, Mu = 0.9dApfp = (0.9x1750x7980x1862) = 2340x 106 nmm = 23402 knm.
10. Calculate failure by crushing of concrete such that bw is 200, d is 1750, fck is 60n/mm2, b is800, Df is 400?
Clarification: bw is 200, d is 1750, fck is 60n/mm2, b is800, Df is 400,
Mu = 0.176bwd2fck+2/3 x 0.8(b-bw)(d-0.5Df)Dffck = ((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, Vu = 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?
Clarification: Flexure loss ratio is 0.8, P = 12000×103, Area = 0.88 x 106mm2,
fcp = (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?
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 fc is 415, vcw = 1900kn, balance shear is 934×103?
Clarification: fc is 415, vcw = 1900kn, balance shear = 934kn, using 12mm diameter two legged stirrups spacing,
sv = (0.87x415x2x113x1900/934×103) = 165mm.