Hollow Section:
For a hollow section, with external diameter D & internal diameter d,
I = (π/64) (D4 - d 4 ); and A = (π/4) (D2 - d 2 )
∴ k 2 = I/A = D4 - d 4/16 (D2 - d 2 ) 16=( D 2 + d2 )/16
∴ e ≤ 2 (( D2 + d 2 )/16) = D2 + d 2/ 8D ------------- (11)
The core for a hollow circular section is thus, a concentric circle of diameter
(D2 + d 2 )/4D