Column slab base: sketch and design steps

Slab base

A slab base is a rectangular steel plate welded (or connected by cleats/angles) to the foot of the column. The column end is machined to bear on the plate, so most of the load passes by direct contact and the weld is nominal. The plate spreads the load over the concrete and is held down by anchor bolts.

elevation                               plan
                                        |<------- B ------->|
   | |  column  | |                     +-------------------+ -
   | |          | |                     |  o             o  | ^
   | |          | |  weld               |    +---------+    | |
+==+=+==========+=+==+ base plate (t_s) | a  | column  |  a | L
|  concrete pedestal |                  |    +---------+    | |
|   |            |   |                  |  o      b      o  | v
|   anchor bolts     |                  +-------------------+ -

o = anchor bolt;  a, b = projections of the plate beyond the column

Design procedure (IS 800:2007 cl. 7.4)

1. Factored load $P$ and concrete grade; bearing strength of concrete $=0.45f_{ck}$ (cl. 7.4.1).
2. Required area $A_{req}=\dfrac{P}{0.45f_{ck}}$.
3. Plate size: choose $B\times L$ so that $B\cdot L\ge A_{req}$, with projections of about 50 mm or more beyond the column; keep the projections about equal so that the plate thickness is economical.
4. Actual bearing pressure: $w=\dfrac{P}{B\,L}\le0.45f_{ck}$.
5. Projections: $a$ = larger projection and $b$ = smaller projection of the plate beyond the column (beyond the flange tips and the flange faces).
6. Thickness (cl. 7.4.3.1):

$$t_s=\sqrt{\frac{2.5\,w\,(a^2-0.3b^2)\,\gamma_{m0}}{f_y}}\ \ge t_f$$

where $t_f$ is the column flange thickness and $\gamma_{m0}=1.10$. 7. Connection: connect the column to the plate by a nominal fillet weld all round (the load is carried by contact if the end is milled); provide at least 2 anchor bolts (usually 4) of 20 mm for fixing (design them for any uplift or moment, if present). 8. Check: the plate is thick enough ($t_s\ge$ calculated, rounded up to 12, 16, 20, 25 mm...), the concrete pressure is not exceeded, and the foundation (pedestal or footing) is checked separately for bearing and size.