Determine the largest load $\mathbf{P}$ that can be applied to the frame without causing either the average normal stress or the average shear stress at section $a-a$ to exceed $\sigma=150 \mathrm{MPa}$ and $\tau=60 \mathrm{MPa},$ respectively. Member $C B$ has a square cross section of $25 \mathrm{mm}$ on each side.