Question $4$: The truss structure is loaded at joint C with a vertical lond N and horizontal load of $0\mathrm{.}4$ N $.$ In addition, the truss members are made of steel with an allowable yield strength of $250$ MPa and elastic maslulus of $E=200$GPa. Determine the maximum value of N for the loading conditions given. Use a factor of safety against buekling of $2\mathrm{.}5$ and consider buckling in the plane of the stricture only.

$($Remember that $I_x=I_y=\frac{r^4}{4}$ for a circular area$)$