I usually treat them as pinned... conservative, but, there is little redistribution when a column fails...and not much financial benefit. Occasionally when designing a portal frame, I'll treat them as fixed, and design the base plate accordingly.
This has been asked numerous times. Usually steel column base is assumed pinned. But there are cases where same column base detail may be assumed as fixed say if its a csntilever column like a pole. Or if column is so short like only 3' high where moment is so low.
Some also do dual analysis. Pinned to check structure, fixed to check anchor bolt, base plate and foundation.
I have a couple thoughts on this (since its an area of structural design I regularly encounter)
1- I consider it fixed if the anchors are exterior to the column flanges and sized such that the anchors transfer a moment load thru the flanges to the anchors - then into the foundation.
or
If the anchors are designed to carry a specified moment load - (which is not necessarily the same capacity that the column flange is capable of resisting)
2- It wouldn't matter if fixed or pinned if the foundation is not capable of resisting moment loads (induced by a fixed base plate)
Fixed is the wrong word. Fixed means the base connection is capable of developing the full capacity of the member. Fully fixed column bases are rare, as they depend so much on whether the footing can rotate at all. "Rigid" is the correct terminology, as it includes partial fixity.
Most of us used the pinned assumption for building columns. The fixation on "fixed" undoubtedly results from a lot of computer programs, which only give F and R as choices.
It is best to analyse the steel columns as pinned, even if in reality would be semi-rigid, because BM in frame members from pinned base analysis is usually conservative so it is justifiable. Fixed Base = expensive foundation cost
