UC or UB Steel Column Base Plate Fixing Details

A steel column base plate is the connection between the bottom of the column and the foundation beneath. It distributes the concentrated column load over a larger bearing area and provides the fixing detail that locks the column in position.

Two Foundation Types

Type 1: Padstone on Existing Underbuild

Where the column sits on an existing masonry underbuild (e.g. inside a building where the column bears onto the existing masonry substructure), a C35 in-situ concrete padstone is typically cast at the intended bearing level.

The column base plate bears directly onto the padstone surface. Holding-down bolts are cast into the padstone or fixed with chemical resin anchors after the padstone has cured.

Use case: Internal columns where an existing ground floor slab or masonry foundation provides an adequate bearing surface.

Type 2: Concrete Pad Foundation

Where no existing foundation is available — or where a new pad must be formed because the column load exceeds the existing foundation capacity — a new reinforced concrete pad is cast.

The pad is sized by PorthouseDean to distribute the column load into the bearing stratum at an acceptable bearing pressure. Holding-down bolts are cast into the pad during concrete pours, projecting above the top face to receive the base plate.

Use case: New columns where no existing foundation is present, or where the load requires a larger bearing area.

Temporary Bracing

The column must be temporarily braced during construction. Before the base plate is fully bolted down and the masonry or encasement around the column is complete, the column is freestanding and unstable under lateral loads (wind, accidental impact). Temporary props or guys are required until the column is fully secured.

Base Plate Specification

| Item | Specification |
|—|—|
| Plate material | S275 structural steel |
| Plate thickness | Minimum 20 mm (heavier for high-load applications — see drawings) |
| Welds (column to plate) | 6 mm fillet, continuous around full column perimeter |
| Holding-down bolts | Grade 8.8 black, M20 minimum (see drawings for specific size) |
| Bolt resin (if post-fixed) | Structural epoxy, minimum embedment per manufacturer |

All material grades are as specified in the structural calculations — the above are minimum values.

Grout Under the Base Plate

After the column has been plumbed and the holding-down bolts tightened, the gap between the base plate and the foundation surface should be filled with non-shrink cementitious grout (e.g. Sika Grout or similar proprietary product). This ensures full bearing contact between the plate and foundation cushion — if the column sits on only the bolt heads or on a thin sliver of contact, stress concentrations can develop under the plate edge.

Final Inspection

Before closing in the column base (with concrete encasement, floor screed, or finishes), confirm with Building Control that the base plate installation has been inspected. Once covered, the fixing cannot be verified without opening up.

Contact us if you have questions about the base plate detail for your column.