A 16-ft-long column is made of aluminum alloy 2014 - T6. If it is fixed at its top and bottom, and a concentric load $\mathbf{P}$ and a moment $M=P(4.5 \text { in. })$ are applied at point $A$ determine the maximum allowable magnitude of $\mathbf{P}$ using the equations of Sec. 13.6 and the interaction formula with $\left(\sigma_{b}\right)_{\text {allow }}=20 \mathrm{ksi}$