AISC ASD Approach. The lowest columns of a structure usually are supported on a concrete foundation. To prevent crushing of the concrete, base plates are inserted between the steel and concrete to distribute the load. For very heavy loads, a grillage, often encased in concrete, may be required. It consists of one or more layers of steel beams with pipe separators between them and tie rods through the pipe to prevent separation.
The area (in^2) of base plate required may be computed from
Eccentricity of loading or presence of bending moment at the column base increases the pressure on some parts of the base plate and decreases it on other parts. To compute these effects, the base plate may be assumed completely rigid so that the pressure variation on the concrete is linear.
Plate thickness may be determined by treating projections m and n of the base plate beyond the column as cantilevers. The cantilever dimensions m and n are usually defined as shown in Fig. 5.35. (If the base plate is small, the area of the base plate inside the column profile should be treated as a beam.) Yield-line analysis shows that an equivalent cantilever
For columns subjected only to direct load, the welds of column to base plate, as shown
in Fig. 5.35, are required principally for withstanding erection stresses. For columns subjected to uplift, the welds must be proportioned to resist the forces.
Base plates are tied to the concrete foundation with hooked anchor bolts embedded in
the concrete. When there is no uplift, anchor bolts commonly used are 3â„4 in in diameter, about 1 ft 6 in long plus a 3-in hook.
Instead of welds, anchor bolts may be used to tie the column and base plate to the concrete foundation. With anchor bolts up to about 11â„4 in in diameter, heavy clip angles may be fastened to the columns to transfer overturning or uplift forces from the column to the anchor bolts. For large uplift forces, stiffeners may be required with the anchor bolts (Fig. 5.36).
The load is transferred from column to bolts through the stiffener-plate welds. The cap plate should be designed for bending.
Pressure under Part of Base Plate. The equivalent eccentricity of the moment acting at
the base of a column equals the moment divided by the concentric load: e = M/P. When
the eccentricity exceeds one-sixth the length of base plate (P lies outside the middle third
of the plate), part of the plate no longer exerts pressure against the concrete. The pressure diagram thus extends only part of the length of the base plate. If the pressure variation is assumed linear, the pressure diagram is triangular. For moderate eccentricities (one anchor bolt required on each side of the column), the design procedure is similar to that for full pressure over the base plate. For large eccentricities (several anchor bolts required on each side of the column), design may be treated like that of a reinforced-concrete member subjected to bending and axial load.
Planing of Base Plates. Base plates, except thin rolled-steel bearing plates and surfaces embedded in grout, should be planed on all bearing surfaces. Also, all bearing surfaces of rolled-steel bearing plates over 4 in thick should be planed. Rolled-steel bearing plates over 2 in but not more than 4 in thick may be planed or straightened by pressing. Rolled-steel bearing plates 2 in or less in thickness, when used for column bases, and the bottom surfaces of grouted plates need not be planed. Design thickness of a plate is that after milling. So finishing must be taken into account in ordering the plate.