Stability design of crane columns in mill buildings

This paper deals with buckling aspects of the design of stepped columns in mill buildings. These columns are generally loaded axially at the top and at the section where the cross section changes. These loads usually act eccentrically and thus produce column moments. Additional bending from lateral loads also has to be accounted for. Starting from these considerations, a geometrically nonlinear model for buckling behaviour analysis is proposed and formulated using the differential equations of equilibrium. The concept of equivalent (or effective) lengths of the two column segments is successfully applied. Tables to determine effective length factors are provided for a range of the following parameters: end fixities, ratio of the end axial load to the intermediate axial load, the ratio of the length and moment of inertia of the upper segment to the lower segment and the splice mechanical properties. This is followed by a study of the imperfection sensitivity to the linearly evaluated critical load. A discussion on the variation of the load carrying capacity with the level of eccentricity and lateral load acting at the step on a practical column is also presented. The findings suggest that upper and lower segments are treated as separate beam-columns. Some implications for design, in the form of interaction equations, are deduced from the presented analysis.