Pr$.1.$ The frame shown below is subjected to a load of $6kN$ acting on joint $D.$ The ultimate $($failure$)$ shear stress is ${}_{\text{f}\text{a}\text{i}\text{l}}=120$MPa for the pin connections at $D$ and $C,$ and the ultimate $($failure$)$ normal stress is ${}_{\text{f}\text{a}\text{i}\text{l}}=300$MPa in link $BC.$ Knowing that a factor of safety of F$.$S$.=2$ is desired, determine the required diameter of the pins at $D$ and $C,$ and side length of the section for the link BC $($which has a square section$).$

Note:

Support at $A$ is as a pin support, $E$ is a roller support.