$10\mathrm{.}6.$ Select the lightest $W$ section for the two$-$span continuous beam of the accompanying figure. Use $F_y=50$ksi. The concentrated load is $20$ kips dead load and $30$ kips live load; the uniform load is $0\mathrm{.}8ki\frac{p}{f}t$ dead load and $2\mathrm{.}7$kip$\frac{s}{f}t$ live load. Specify the uniform laterally unbraced lengths $L_b$ to give the optimum design.