The eccentric axial force $\mathbf{P}$ acts at point $D,$ which must be located $25 \mathrm{mm}$ below the top surface of the steel bar shown. For $P=60 \mathrm{kN}$ (a) determine the depth $d$ of the bar for which the tensile stress at point $A$ is maximum, $(b)$ the corresponding stress at $A$