A beam having the cross section shown is subjected to a couple $M_0$ acting in a vertical plane. Determine the largest permissible value

of the moment $M_0$ of the couple if the maximum stress is not to exceed $104$MPa.

Given:

$l_y=I_z=\frac{b^4}{36}$

$l_{yz}=\frac{b^4}{72}$

The largest permissible value of the moment $M_0$ is