Design of Slabs Questions and Answers

Answer: D

The minimum reinforcement for Fe 250 in a slab is 0.15% of its gross sectional area.

IS 456 (clause 26.5.2.1) requires the reinforcement in either direction in slabs to be at least 0.15 percent of the gross cross-sectional area when mild steel (Fe 250) is used. The limit is reduced to 0.12 percent for high strength deformed bars, which are stronger.

0.12 percent is the HYSD value, the usual trap, and 0.10 and 0.14 percent are not code values.

Answer: C

The nominal shear stress is \(\tau_{v}\) = 0.189 MPa.

Effective span is the lesser of clear span plus d (4.165 m) and centre-to-centre of supports (4.23 m), so 4.165 m. Loads per square metre:

\[ \begin{aligned} w &= 0.2 \times 25 + 1 + 4 = 10\ \text{kN/m}^2 \\ w_u &= 1.5 \times 10 = 15\ \text{kN/m}^2 \end{aligned} \]

For a 1 m wide strip:

\[ \begin{aligned} V_u &= \frac{15 \times 4.165}{2} = 31.24\ \text{kN} \\ \tau_v &= \frac{31.24 \times 10^3}{1000 \times 165} = 0.189\ \text{MPa} \end{aligned} \]

Leaving out the factor 1.5 gives about 0.126 MPa, and other options come from wrong spans or loads.

Answer: C

The thickened part of the slab over a column is the drop panel.

In a flat slab the slab rests directly on the columns, so shear and negative moment are concentrated around each column. A drop is a local thickening of the slab itself over the column, which increases the depth there to resist punching shear and hogging moment.

A column head, also called a capital, is the flared top of the column, not part of the slab. 'Beam head and panel' is not a flat slab term; a flat slab has no beams at all.

Answer: C

The minimum reinforcement for Fe-250 is 0.15%.

IS 456:2000, clause 26.5.2.1, requires slab reinforcement in either direction of at least 0.15 percent of the gross cross-sectional area when mild steel (Fe 250) is used, reduced to 0.12 percent for high strength deformed bars.

0.12% is the near miss, the figure for HYSD bars. 0.08% and 0.04% are below any minimum in the code.