This set of Prestressed Concrete Structures Multiple Choice Questions & Answers (MCQs) focuses on “Design of Post Tensioned Beams”.
1.Calculate ultimate moment and shear of effective span is 30m, live load is 9kn/m, dead load excluding self weight is 2kn/m, load factors for dead load is 1.4 for live load is 1.6 cube strength of concrete fcu is 50n/mm2 cube strength at transfer is fci is 35n/mm2, tensile strength of concrete Ec is 34kn/mm2 loss ratio ɳ is 0.85 and 8mm diameter high tensile strength fpu is 1500n/mm2 are available for use and the modulus of elasticity of high tensile wires is 200kn/mm2?
a) 340 and 450kn
b) 240 and 340kn
c) 140 and 240kn
d) 100 and 200kn
Explanation: Wmin/Wud = (50x2400x9.81×0.125x25x30/50×106x0.852) = 0.31
Ultimate load excluding the factored selfweight = (1.4×2)+(1.6×9) = 17.2kn/m, Wud = 17.2/1-1.4×0.31) = 30KN/M, Wmin = (0.31×30) = 9.3kn/m, Ultimate moment , Mu = (0.125x30x302) = 3400knm, Ultimate shear, Vu = (0.5x30x30) = 450kn.
2. Find cross-sectional dimensions thickness of web if hf/d ratio is 0.23 and bw/b ratio is 0.25 and b is 0.5d?
Explanation: hf/d =0.23 and bw/b = 0.25 and b = 0.5d,
Mu = 0.10fcubd2 d = (3400×106/0.10x50x0.5)1/3 = 1130mm, h = (1130/0.85) = 1300, b = 600mm, hf = (0.2×1130) = 250mm, adopt an effective depth, d = 1150mm, thickness of web, bw = (0.6vu/fth) = (0.6x450x103/1.7×1300) = 120mm.
3. Calculate working moment if design working load is 19.8kn/m covered over a span of 30m( actual self weight of girder is 8.8kn/m)?
Explanation: Actual self weight of the beam and the girder = 8.8kn/m, span = 30m
Minimum moment Mmin = 990knm, Design working load = 19.8kn/m,
Working moment Md = (0.125×19.8×302) = 2230knm.
4. Find the Permissible stresses and range of stresses for class 1 structure fcu = 50n/mm2,fck = 35n/mm2 according to BS: 8110 recommendations for fcu = 50n/mm2 and fci = 35n/mm2,fct = 0.5fci= 17.5n/mm2?
Explanation: fcu = 50n/mm2,fck = 35n/mm2 according to BS: 8110 recommendations for fcu = 50n/mm2 and fci = 35n/mm2, fct = 0.5fci= 17.5n/mm2For class 1 structure fu = htw = 0, fbr = (ɳfct-ftw) = (0.85×17.5) = 15n/mm2, fcw = 0.33fcu = (0.33×50) = 16.5n/mm2, fcu = (fcw-ɳfu) = 16.5n/mm2.
5. Find prestressing force if area is 36.75mm2 of eccentricity 580given finf is 26.5kn/m and zb is 99×106?
Explanation: Area = 36.75mm2, e = 580, finf = 26.5kn/m, zb = 99×106
p =(AfinfZb/Zb+Ae) =(367500×26.5x99x106/(99×106)+(367500×580)) = 308x104kn/m2.
6. Find force in cable using Freyssinet cables 12-8mm diameter and stressed to 1100n/mm2 of eccentricity 50 and the prestressing force is given as 1000n/mm2?
Explanation: 12 diameter, stress = 1100n/mm2, e = 50, prestressing force =1000n/mm2
Force in each cable = ( (12x50x1100)/1000)) = 660kn.
7. Find ratio for ultimate flexural strength at the centre – span section given that Aps = 3000mm2, d= 1150mm, fcu = 50n/mm2, bw = 150mm, fpu = 1500n/mm2, b = 600mm, ht = 250mm, design ultimate moment mud = 3400knm?
Explanation: Aps = 3000mm2, d= 1150mm, fcu = 50n/mm2, bw = 150mm, fpu = 1500n/mm2, b = 600mm, ht = 250mm, design ultimate moment mud = 3400knm, according to BS: 8110-1985, Aps = (Apw+Apf) =
Apf = 0.45×50(600-150)(250/1500) = 0.45xfcu(b-bw)(hf/fpu) = 1680mm2, Apw = (1300-1680) = 1320mm2, ratio(fpuApw/fcubwd) = (1500×1320/50x150x1150) = 0.23.
8. Calculate the slope of cable section at support uncracked in flexure given that eccentricity is 410, length is 30m and stress induced is 1000?
Explanation: e = 410, length = 30m, stress induced = 1000
Slope of cable θ = (4e/l) = ((4×410)/(30×1000)) = 0.0547.
9. Calculate the span section cracked in flexure (M=M0) Fcp = 23.4n/mm2, zb is 99×106 and stress induced is 1000?
Explanation: Fcp = 23.4n/mm2, zb is 99×106, stress is 1000
m0 = (0.8fcpZb) = (0.8 x 23.4 x (99×106/1000)) = 1850knm.
10. Find resultant maximum long term deflection if ϕ is 2.6, αy is 38.5mm, αg is 46mm, αp is 74.7mm?
Explanation: Ece = (Ec/1+ϕ) = (Ec/2.6), ϕ = 2.6, αy = 38.5mm, αg = 46mm, αp = 74.7mm, resultant maximum long term deflection = (2.6×46)+38.5-(0.85×74.7) = 95mm which is less than the code limit (span/250) = 120mm, ɳ = 0.85.
Sanfoundry Global Education & Learning Series – Prestressed Concrete Structures.
To practice all areas of Prestressed Concrete Structures, here is complete set of 1000+ Multiple Choice Questions and Answers.