Question

Four identical pin-pin ended solid concrete cylindrical columns that are L=10m tall need to be able to withstand 6.8*10^8 N of force before Euler buckling. What should be the diameter, d, in meters, of each column if the force is uniformly distributed? The Young’s modulus, E, for concrete is 17GPa, and 𝐼=(πr^4)/4 . Assume n=1.

I used: P_critical = (n^2*π^2*E*I)/(L^2*(πr^2)) to solve for r then multiplied by two for diameter but this isn't correct. Any advice for correcting my formula? Thank you.