Given:

P = 10.8 kips
e = 0.8 in
diameter = 2 in
L = the effective length, since it is pinned at both ends.
Aluminum alloy 6061-T6

Equation 10.60 must be used:

Referring to page 640, for Aluminum alloy 6061-T6 and assuming that L/r > 66,

So the assumption was correct. Therefore, L = (84.0)(0.5 in) = 42.0 in.