This paper presents the results of an experimental investigation on the behaviour of steel column base connections. The purpose of the study was to determine the value of the rigidity ratio at the column base (GL). The connection is made of a steel base plate welded to the lower end of the column and attached to the footing by means of two or four anchor bolts. It is generally admitted that this type of connection behaves like a hinge and it is recommended to use GL = 10.0 to evaluate the effective length factor of the column.The test results indicate that for weak-axis buckling a conservative value of the rigidity ratio is GLY = 0.50 and for strong-axis buckling GLX = 1.50, if there is no relative displacement of column ends. The effective length of the column is therefore reduced. A design example shows that the column strength is increased by approximately 30% for slenderness ratios currently met in practice. Key words: steel column, column base connection, moment–rotation curve, buckling.