The value of Leff depends upon the column length and the termination or boundary conditions for the column. You may wish to review the reading for sub-subunit 3.3.4.
Attribution:
Stephanie Redfern, Ranjeet (Ron) Agarwala, and Dr. Steve Gibbs. Mechanics I. The Saylor Academy 2014, http://www.saylor.org/courses/me102/