Three $1 \times 18$ -in. steel plates are bolted to four $\mathrm{L} 6 \times 6 \times 1$ angles to form a beam with the cross section shown. The bolts have a $\frac{7}{8}-$ in. diameter and are spaced longitudinally every 5 in. Knowing that the allowable average shearing stress in the bolts is 12 ksi, determine the largest permissible vertical shear in the beam. (Given: $I_{x}=6123$ in $^{4}$.)