A column base plate is a flat steel plate welded to the bottom of a steel column or post that spreads the column's load over the concrete below and anchors it to the foundation. The column itself is far too small, and the steel stress far too high, to bear directly on concrete. The plate enlarges the bearing area, takes the anchor rods that hold the column down, and gives the erector a way to set the column level and at the right height.
This guide explains how base plates work in a steel structure, the parts of a base plate connection, the common types, and how plates are sized and installed. (We're talking about structural steel here, not the other trades that use the term.)
What a base plate does
A base plate connection has four jobs:
- Spread the load. Concrete can take only a fraction of the stress steel can. The plate distributes the column load over enough concrete area to keep bearing pressure within what the footing or pier can carry.
- Anchor the column. Anchor rods through the plate hold the column against uplift (from wind on a light roof, for example) and transfer horizontal shear into the foundation.
- Keep the column stable during erection. Before beams and bracing connect it to the rest of the frame, a column stands on its anchor rods alone. CSA S16 requires columns to have at least four anchor rods for that reason, with special precautions where four rods arranged out of line aren't feasible.
- Set position and elevation. Levelling nuts or shims under the plate let the erector set the top of the column at the right height and plumb it before grouting.
Where the design needs it, a base plate also resists bending at the base. That's a fixed or moment base, and it's a much bigger connection than the typical pinned base.
Parts of a base plate connection
| Part | What it does |
|---|---|
| Base plate | The steel plate itself, usually square or rectangular |
| Column-to-plate weld | Fillet or groove welds around the column profile |
| Anchor rod holes | Usually oversized to allow for rod placement tolerance |
| Anchor rods | Threaded rods or headed bolts cast into the concrete |
| Plate washers | Thick washers that cover the oversized holes |
| Levelling nuts or shims | Support the plate at the right elevation before grouting |
| Grout | Non-shrink grout packed under the plate so it bears evenly |
| Stiffeners | Extra plates on heavy or moment bases to stiffen the plate |
On small posts the arrangement is simpler: a plate with two or four holes, set on the slab or a pier with expansion or adhesive anchors. Our post on base plate anchor bolts covers rod types, sizes and hole sizes.
Types of column base plates
Pinned base plates
Most building columns sit on pinned bases. The plate and anchor rods carry vertical load, shear and any uplift, but the connection isn't designed to resist significant bending. Pinned bases are smaller and simpler, and the frame relies on bracing or other elements for lateral stability.
Fixed (moment) base plates
Where the column has to resist bending at its base (moment frames, cantilevered columns, some canopy and sign structures), the plate is larger and thicker, with more anchor rods placed farther from the column and often with stiffener plates. The anchor rods on one side are in tension, and the concrete under the other side takes high bearing pressure.
Post base plates in houses
Steel posts and lally columns in basements have plates at each end. In houses designed under Part 9 of the 2024 Ontario Building Code, Article 9.17.3.2 requires steel columns to have plates at least 100 mm x 100 mm x 6.35 mm thick at each end, and the top plate under a wood beam must span the full width of the beam. That's a code minimum; engineered posts often need more.
Other variations
- Embedded bases, where the column foot is cast into a concrete pier or slab.
- Plates on piers or pedestals, with the pier sized to suit the plate and anchor rods.
- Round flange plates for light poles and bollards.
- Plates with shear lugs, a stub of plate or bar welded under the base plate and grouted into a pocket, used where shear is high.
Base plate vs other plates
Base plates are easy to confuse with other plates on a steel frame:
- Cap plate. The plate welded to the top of a column or post, which a beam sits on.
- End plate. A plate welded to the end of a beam, bolted to a column or another beam.
- Gusset plate. A plate that joins bracing members to the frame.
- Sole plate or sill plate. In wood framing, the bottom plate of a wall. In steel, "sole plate" sometimes means a plate under a beam bearing.
How a base plate is sized
A base plate for an engineered column is designed, not picked from a catalogue. In Canada, the steel is designed to CSA S16 and the concrete bearing to CSA A23.3. In outline, the designer:
- Works out the factored column load and any shear, uplift or moment.
- Sizes the plate area so the bearing pressure on the concrete is within its factored bearing resistance, which depends on concrete strength and how much larger the supporting concrete is than the plate.
- Sets the plate thickness so the parts of the plate that project past the column can resist the bending from that bearing pressure.
- Chooses anchor rods, edge distances, welds and grout thickness.
Our column base plate design post works a full example, and the base plate size and thickness chart shows common plate sizes and standard thicknesses.
How base plates are set on site
The usual sequence for a building column:
- Anchor rods are cast in using a template so their pattern matches the plate holes and they stand at the right projection.
- Elevation is set with levelling nuts on the rods or with steel shim packs on the concrete.
- The column is lowered over the rods, plumbed, and held with nuts and plate washers.
- The gap under the plate is grouted with non-shrink grout, packed so there are no voids. Grouting is covered in detail in our post on grouting base plates.
Getting the anchor rods in the right place is the step that most often causes trouble. A rod cast out of position means slotting holes, re-drilling or a design fix, all of which need the engineer's approval.
Getting column base plates made
Base plates are welded to columns and posts in the shop, so the column arrives ready to set. The fabricator needs the plate size and thickness, hole size and pattern, weld details, column length to the underside of the plate, and the finish. We cut, drill and weld base plates and cap plates to your drawings in our CWB certified shop, and we can prime or galvanize to suit.
Send your drawings or column schedule for a quote, or see our steel products, including posts with base and cap plates welded on.
Frequently asked questions
What is a base plate in construction?
In steel construction, a base plate is a flat steel plate welded to the bottom of a column or post. It spreads the column's load over the concrete footing, pier or slab below and is bolted down with anchor rods so the column stays put during erection and in service.
Why do steel columns need base plates?
The stress in a steel column is far higher than concrete can safely take in bearing, so the plate spreads the load over a larger area. It also gives the anchor rods something to bolt through, lets the column be levelled and set to the right elevation, and transfers shear and uplift into the foundation.
How do base plates work with anchor bolts?
Anchor rods are cast into the concrete in a set pattern. The base plate has matching holes, usually oversized to allow for small placement errors, and is set on levelling nuts or shims, then held down with washers and nuts. The gap under the plate is then filled with non-shrink grout so the plate bears evenly.
What is the minimum base plate size for a house column in Ontario?
For steel columns in houses designed under Part 9 of the 2024 Ontario Building Code, Article 9.17.3.2 requires steel plates at least 100 mm by 100 mm by 6.35 mm thick at each end of the column. That is a minimum; engineered columns get plates sized by calculation.
