$11\mathrm{.}4-15$ A rigid L$-$shaped frame is supported by a steel pipe column $AB($see figure$)$ and is subjected to a horizontal load $F=140$kips. If the pipe has an outside diameter $d=4in.$ and a factor of safety of $2\mathrm{.}5$ is required with respect to Euler buckling, what is the minimum acceptable thickness $t$ of the pipe? Assume that $E=30,000$ksi for $AB.$

Pin connection

PROBLEM $11\mathrm{.}4-15$