Column Bases — Slab Base, Gusseted Base & Grillage

Purpose

A steel column carries a large load on a small area; the concrete foundation or pedestal below cannot take such a high stress. The column base spreads the load over enough concrete area so the bearing stress stays within the permissible value, and anchors the column against uplift, shear and moment.

Bearing strength of concrete

Required base plate area: $A=\frac{P_u}{0.6f_{ck}}$.

Slab base

The base plate projects beyond the column; each projection bends as a cantilever under the uniform upward bearing pressure $w$.

Details:

• Column end machined (milled) for full bearing on the base plate, with welds or cleats sized for erection and for the load not transmitted by bearing; otherwise welds must carry the full load.
• Base plate levelled on grout (non-shrink cement grout, about 25–50 mm).
• Anchor bolts (holding-down bolts) — at least 2 (usually 4) — hold the column in position during erection and resist uplift and shear.

Gusseted base

For heavy columns, a thick slab base becomes uneconomical. Gusset plates on the flanges and cleat angles transfer part of the load from the column to the base plate over a larger area, reducing the cantilever projection and the plate thickness. The base plate is designed for bending between gussets; gussets and connecting welds/bolts are designed for the portion of the load they carry. In gusseted bases the column end, gussets and angles are usually machined so that load is shared by bearing and fasteners.

Bases with moment

With axial load $P$ and moment $M$, the eccentricity $e=M/P$:

• If $e\le L/6$ ($L$ = plate length), the whole plate is in compression: $\sigma =\frac{P}{BL}\left(1\pm \frac{6e}{L}\right)$.
• If $e>L/6$, part of the plate lifts; anchor bolts on the tension side resist uplift. Equilibrium of the bolt tension and the triangular (or rectangular block) concrete pressure gives the bolt force and the bearing length.