# Design of RC Structures Questions and Answers – Slabs

This set of Design of RC Structures Multiple Choice Questions & Answers (MCQs) focuses on “Slabs”.

1. For freely supported slab, the effective span is taken equal to the distance between centre to centre of supports.
a) True
b) False

Explanation: In case of freely supported slab, the effective span is taken equal to the distance between centre to centre of supports. This can also be taken as the clear distance between the supports plus the effective depth of the slab whichever is less.

2. Which of the following is the basic value of effective depth ratio for control of deflection for cantilever slabs (for span up to 10)?
a) 7
b) 20
c) 26
d) 33

Explanation: The deflection of a structure or part there of shall not adversely affect the appearance or efficiency of the structure or finishes or partitions. For control of deflection the basic values of span of effective depth ratios, for span up to 10 m are as follows:
Cantilever = 7
Simply Supported = 20
Continuous = 26

3. What is the value of minimum mild steel reinforcement provided in either direction while designing slabs?
a) Not less than 0.15% of total cross sectional area
b) Not less than 0.12% of total cross sectional area
c) Not more than 0.15% of total cross sectional area
d) Not more than 0.12% of total cross sectional area

Explanation: The mild steel reinforcement in either direction in slabs shall not be less than 0.15% of the total cross-sectional area. This value can be reduced to 0.12% when high strength deformed bars shall deformed bars or welded wire fabrics are used.

4. Which of the following is correct spacing of bars between parallel main reinforcement bars in slab design?
a) 500 mm
b) 450 mm
c) 400 mm
d) 300 mm

Explanation: The horizontal distance between parallel main reinforcement bars shall not be more than three times the effective depth of a solid slab or 300 mm, whichever is smaller. The horizontal distance between parallel reinforcement bars provided against shrinkage and temperature shall not be more than five times the effective depth of the slab or 450 mm whichever is small.

5. For curtailment of tension reinforcement in flexural members, reinforcement shall extend beyond the point at which it is no longer required to resist flexure.
a) True
b) False

Explanation: For curtailment of tension reinforcement in flexural members, reinforcement shall extend beyond the point at which it is no longer required to resist flexure. This is for a distance equal to the effective depth of the member or 12 times the bar diameter, whichever is greater except at simple support or end of cantilever. The point at which reinforcement is no longer required to resist flexure is where the resistance moment of section, considering only the continuing bars, is equal the design moment.

6. Which of the following equation is used to calculate the effective depth (d) in the design of slabs?
a) 0.42 × (xumax / d) (1 – (0.36xumax / d)) bd2fck
b) 0.36 × (xumax / d) (1 – (0.42xumax / d)) bdfck
c) 0.42 × (xumax / d) (1 – (0.36xumax / d)) bdfck
d) 0.36 × (xumax / d) (1 – (0.42xumax / d)) bd2fck

Explanation: According to IS: 456 – 2000 the equation used for calculating effective depth (d) of the in the slab design is given by:
Mu,lim = 0.36 × (xumax / d) (1 – (0.42xumax / d)) bd2fck
Here, (xu) is depth of neutral axis,
(d) is effective depth,
(fy) is characteristic strength of reinforcement,
(Ast) is area of tension reinforcement,
(fck) is characteristic compressive strength of concrete,
(b) is width of compression face,
(Mu,lim) is limiting moment of resistance of a section without compression reinforcement, and
(xumax) is limiting value of xu.

7. Which of the following equation is used to calculate the area of steel or reinforcement in the design of slabs?
a) 0.80 × fy Ast d (1 – (fy Ast / bdfck))
b) 0.90 × fy Ast d (1 – (fy Ast / bdfck))
c) 0.87 × fy Ast d (1 – (fy Ast / bdfck))
d) 0.86 × fy Ast d (1 – (fy Ast / bdfck))

Explanation: The equation suggested by IS: 456 – 2000 for calculating the area of steel or reinforcement in the slab design is given by:
Mu = 0.87 × fy Ast d (1 – (fy Ast / bdfck))
Here, (d) is effective depth,
(fy) is characteristic strength of reinforcement,
(Ast) is area of tension reinforcement,
(fck) is characteristic compressive strength of concrete, and
(b) is width of compression face.

8. Which of the following equation is used to determine the nominal shear stress (τv) in the design of slabs?
a) (Vu / 2bd)
b) (Vu / 4bd)
c) (2Vu / bd)
d) (Vu / bd)

Explanation: The nominal shear stress (τv) is computed as:
v) = (Vu / bd)
Here, (Vu) is maximum shear force at the critical section,
(b) = width of the section, and
(d) = effective depth of the section.
If τ ≤ ($$\frac {1}{2}$$)τc or Vu = ($$\frac {1}{2}$$) Vuc where Vuc = (τcbd),
No shear reinforcement is really necessary.
If ($$\frac {1}{2}$$) Vuc ≤ Vu ≤ Vuc. Provide a nominal shear reinforcement given by:
(Asv / bsv) ≥ (0.4 / 0.87fy)
Choosing bar diameter Asv is known and hence sv can computed from the above equation. However maximum spacing is restricted to 0.75d or 300 mm, whichever is less.

9. Which of the following is used to determine (L0) in the equation of check for development length at the ends?
a) (Ls / 3) – x’
b) (Ls / 2) – x’
c) (Ls / 4) – x’
d) (Ls) – x’

Explanation: Check for development length at the end: for simple supports check if
Ld ≤ (M1 / V) + L0
Ld = development length = (0.87 fy / 4τbd) Φ,
M1 = moment of resistance of the section at the support,
V = shear force at the section = wuL / 2,
L0 = sum of anchorages beyond the centre of support and the equivalent anchorage value at any hook or mechanical anchorage at the simple support = (Ls / 2) – x’.

10. The IS: 456 – 2000 recommends value of (M1 / V) in the check for development length can be increased by 30%.
a) True
b) False

Explanation: The code IS: 456 – 2000 recommends that the value of (M1 / V) in the expression for check for development length can be increased by 30% when the ends of the reinforcement are confined by a compressive reaction. This condition of confinement of reinforcing bar may not be available at all the types of simple supports.

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