fixed base for a column

huangyhg

fixed base for a column?
in digging into the nomograph for a previous thread i posted about fmc frames i happened upon another question that i am hoping i can get some opinions and insight on.
when using the nomograph to calc k (for either sway or non-sway frames), you can assume a g of 1 for a column rigidly attached to a properly design footing.  my question is what exactly is a properly designed footing?
initially, i thought it would be any footing designed to take the greatest load effects from the combinations assuming a fixed base.  when i started thinking about this more, i don't necessarily agree with this anymore.
now i think that the footing needs to be designed to take the full moment capacity of the column.  my reasoning is this:  as this column is loaded and reaches the point of buckling the column needs to be fixed at the base, not just be able to transfer some value of moment.  it is likely that the moment it will see when buckling could exceed the moment it sees under combined gravity and lateral loads.  i guess you could say that it won't see anything higher than gravity and lateral loads, but.. that is for strength, not stability.  the lower moment resistance will likely allow more rotation, true?
without checking an example, i would think that the axial load large enough to cause the column to buckle would contribute greatly to the p/a component of the footing, and it would not be as likely to rotate.
technically, i think you are correct.  but i am comfortable designing for the maximum load combination, and not worrying about any larger axial forces or moments that could cause buckling.  i also agree with pmr06.
daveatkins
i do not believe that the footing must be able to develop the column strength to use g=1.  why would we believe that the column base moment would be that large at the onset of bifurcation?  re  
agree with the above posters. i design for the maximum load combination from the footing. not the capacity of the column.
well, taken to the extreme case, if you have a gravity only column with no moment connection anywhere above the base, the moment at the base is going to be zero, even if you assume it fixed at the base.  
now the footing is designed for axial load only.  would you still use a g of 1 in this case, and if so why?  i would look at this case and say that it is not capable of resisting any appreciable amount of moment and therefore can't be considered fixed.  
there is a formula to approximate the fixity of the footing relative to the column based on the dimensions and the subgrade reaction modulus of the soil.
you can also derive the moment required for restraint at the base from the curvature of the buckled shape.  
haynewp-
how do you go about deriving the moment required for restraint at the base from the curvature of the buckled shape?
i hope this isn't a second grade question.
i think you have to go back to euler's equations where the buckling load is derived. these are interesting questions you have but i don't think i know anyone that has actually included this restraining moment in the design of a footing or in the connections between the column and the girders.
aci 318, 10.13.7-
"in sway frames, flexural members shall be designed for the total magnified end moments of the compression members at the joint."
i would happy if anyone could direct me to an example in any steel or concrete journal or text where the moment restraint required to resist column buckling is included in the design of a footing or the top connection of the column.