Base Plate for Steel Column

The base of a column which is supported by a concrete footing must have a bearing plate large enough to distribute the load over an area sufficient to preclude excessive bearing stresses. The base plate may be shop-connected or shipped loose. Except where end moments are involved, the connection need only hold the parts in line if the ends of the column are finished to a plane surface, since the load is transmitted by bearing at the contact surfaces.

It may be cheaper to omit planning the ends of lightly loaded columns and to design the connection between base and shaft for the total load. The distribution of the pressure of the plate on the footing depends upon the relative stiffness of the two. Even if the distribution were known, the resulting stresses in the plate could not be determined easily since bending in two directions is involved. The usual analysis is based on two assumptions:

1. The pressure of the footing on the plate is uniformly distributed

2. Those portions of the plate which project from the column shaft act as cantilever beams. Since the sections of zero share, and consequently of maximum moment, in the bearing plates are inside the area of contact between the column shaft and plate, the length of the cantilevered portions are usually assumed to be longer than the actual projection o the plate. 


The AISC recommended analysis for H-shaped columns assumes that the maximum moments occur at sections which are 0.95d apart in one direction and 0.8b apart in the other direction, where d and b are, respectively, the depth and flange width of the shape.

Bases of columns in industrial buildings, and those in tier-building construction designed to resist wind forces, may need to be proportioned to resist end moment resulting from lateral forces.