A beam having a X-section in the form of a channel as shown in fig. is subjected to B.M about X-X axis. Calculate the thickness 't' of the channel in order that the bending stress at the top and bottom of the beam will be in the ratio of 7:3.

$\frac{p_t}{y_t}=\frac{p_b}{y_b}=\frac{M}{I}$

$\frac{p_t}{p_b}=\frac{y_t}{y_b}$

$\frac{p_t}{p_b}=\frac{7}{3}=\frac{250-\bar y}{\bar y}$

$7 \bar y=750- 3\bar y$

$\bar y=75mm$

$\bar y=\frac{2*(250*t)*125+(600-2t)*t*0.5t}{2(250*t)+(600-2t)*t}=75$

t=50mm