Question

A singly reinforced rectangular concrete beam of width 300 mm and effective depth 400 mm is to be designed using M25 grade concrete and Fe500 grade reinforcing steel. For the beam to be under-reinforced, the maximum number of 16 mm diameter reinforcing bars that can be provided is

• A. 3
• B. 4
• C. 5
• D. 6

Answer 1

The correct option is C 5

$x_{u,lim}$ for Fe 500 $=0.46\times 400=184$ mm
For section to be under reinforced $A_{st}\le A_{st,lim}$
$A_{st,lim}=\frac{0.36f_{ck}b{x}_{ulim}}{0.87f_y}=\frac{0.36\times 25\times 300\times 184}{0.87\times 500}$
$=1142.06m{m}^2$
No.of bars for limited area of steel
$=\frac{1142.06}{\frac{\pi }{4}\times {16}^2}=5.68$

So, number of bars will be 5.