# Prestressed Concrete Structures Questions and Answers – Estimation of Self Weight of Beams

This set of Prestressed Concrete Structures Questions and Answers for Campus interviews focuses on “Estimation of Self Weight of Beams”.

1. The computation of total ultimate moment required for the design of prestressed beams, knowledge of is necessary?
a) Spans
b) Self weights
d) Deflection

Explanation: Generally, the self weight may be assumed on the basis of previous experience and the use of design chart containing dimensions of beams for various spans and applied loads as recommended by magnel is very useful in the regard.

2. The estimation of self weight is expressed as ____________
a) wmin/wud = KDcgβ(L/h)L/fcu(d/h)2
b) wmin/wud = KDcgβ(L/h)L/fcu
c) wmin/wud = KDcgβ(L/h)
d) wmin/wud = KDc

Explanation: Bennelt has recently proposed a simple formula for estimating the self weight of the girder by considering several influencing parameters
wmin/wud = KDcgβ(L/h)L/fcu(d/h)2
wmin = Self weight or minimum load, L = effective span, K = numerical constant, Dc = density of the concrete member, g = acceleration, β = moment coefficient, h = overall depth of girder,

3. In the case of unsymmetrical I girders the range of values of hf/d for economical designs is generally?
a) 0.15 to 0.10
b) 0.15 to 0.25
c) 0.8 to 1.0
d) 3.4 to 6.0

Explanation: In the case of unsymmetrical I girders the range of values of ht/d and bw/b for economical designs is generally 0.15 to 0.25 and 0.2 to 0.3 respectively, however the thickness of web, bw is designed based on the dual criteria shear and housing the cables with adequate cover.

4. The breadth of the compression face may be assumed by considering the number of ____________
a) Anchorages
b) Splices
c) Girders
d) Ridges

Explanation: In dimensioning prestressed concrete flexural members the effective depth and breadth of the section at the compression face are determined solely on basis of the ultimate flexural strength requirements and The breadth of the compression face may be assumed by considering the number of covering a given width of bridge deck of a suitable ratio of b’d being in the range of 0.4 to 0.6.

5. The thickness of the web is generally determined on which of the basis?
a) Shear stress
b) Shear strength
c) Principle shear
d) Tensile shear

Explanation: The thickness of the web is generally determined on the basis of shear strength considerations discussed according to british code recommendations shear reinforcements are not required where V is less than 0.5vc and in members of minor imporatance when the shear for V exceeds (Vc to 0.4bwd), shear reinforcement are designed at spacing Sv = Asv0.8fy/0.4bw.
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6. The small span girders with straight tendons, bw is?
a) 0.85vu/fth
b) 0.60vu/fth
c) 0.70vu/fth
d) 0.69vu/fth

Explanation: In the case of small span prestressed members, thinner webs of about 40 to 60mm may be used however in the case of long span, heavily loaded girders when large, curved cables have to pass through the webs a minimum thickness of 120 to 150mm is mandatory to accommodate the cables with adequate cover.

7. The condition that the principal tensile stress is not to exceed the tensile strength of concrete yields a criterion of the type ____________
a) bw > (vu/ (I/s) ft(1-fcp/ft)1/2
b) bw > (vu/ (I/s) ft(1+fcp/ft)1/2
c) bw > (vu/ (I/s)
d) bw > (vu/ (I/s) ft

Explanation: The value of the shear moment arm I/S varies between 0.67 and 0.85h for I sections, the ratio fcp/ft generally varies between 2 and 3 for small span girders with straight tendons for long span girders with curved tendons, the ratio, fcp/ft can be taken between 3 and 4 and the effective shear as 0.8vu since the curved cables contribute to the ultimate shear resistance of the section.

8. The ultimate design load includes?
a) Partial factor of safety and live load

Explanation: The ultimate load includes the self weight enhanced by partial factor of safety
γf1q+γf2wmin, Wud = γf1q/1-γf2(Wmin/Wud).

9. The value of numerical constant K is between ____________
a) 4 to 5
b) 6 to 7.5
c) 4 to 8
d) 5 to 9

Explanation: The value of numerical constant K is between 6 to 7.5 for rectangular sections and I section girders of short spans, while it takes a value between 4 and 5 for the flanged T or I section girders of long spans for self weight equation.

a) 1.0
b) 1.5
c) 1.8
d) 2.0

Explanation: Load combinations: Dead and imposed (and earth and water pressure) – Dead beneficial is 1.0, Dead and wind (and earth and water pressure) – Dead beneficial is 1.0, Dead and wind and imposed (and earth and water pressure) – Dead beneficial is 1.2.

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