Failure modes for a structural member are categorized as follows:
• Flexural buckling. Bending about the axis of the least resistance.
• Torsional buckling. Twisting about the longitudinal (x) axis. It may occur if the torsional rigidity of the section is low, as for a member with a thin-walled open cross section.
• Lateral-torsional buckling. Synchronized bending and twisting. A member which is bent about its major axis may buckle laterally.
• Local buckling. Buckling of a plate or shell element that is a local part of a member
General speaking, the nonlinear buckling analysis means two types of nonlinearity which are material and geometrical nonlinearities.
Also the initial imperfection may be critical issue for almost all the buckling problems.
Even if your analysis is beam type, the allowable for compression may well be taken into account of Euler type buckling effect.
RECOMMENDED PRACTICE DNV-RP-C201
https://exchange.dnv.com/publishing/codes/download.asp?url=2010-10/rp-c201.pdf
For initial imperfection of a pole, you can find it as a tolerance in "Design of Steel Transmission Pole Structures (Manual 72)" or "Guide Specifications for Steel Single Pole and H-Frame Structures"
http://www.rurdev.usda.gov/SupportDocuments/UEP_Bulletin_1724E-204.pdf