A steel column meets its foundation through a base plate bolted to anchor rods cast into concrete. The column can bear on a spread footing, on a concrete pedestal that rises from the footing to a set elevation, or, in a house, on a footing below the basement slab, but never on an ordinary slab alone. The engineer sizes the footing, pedestal, plate and anchor rods; the details below are what makes that design work on site, and what to do when the concrete and the steel don't line up.
Structural columns need a designer or engineer and usually a building permit. This article explains the details so you know what to expect, not how to design them.
Three ways a column meets the concrete
| Support | How it works | Typical use |
|---|---|---|
| Directly on a spread footing | Base plate bears on the footing; the slab is poured around the column later | Basement columns in houses, interior columns in some low buildings |
| On a pedestal (pier) | A short reinforced concrete block rises from the footing; the plate bears on top | Industrial and commercial columns, exterior columns, deep footings, columns above wet floors |
| On a slab-on-grade | Not done without a footing below; the slab is not a footing | Thickened slab or isolated footing designed for the column load |
On a footing. For house columns, Article 9.17.2.1 of the 2024 Ontario Building Code requires columns to be centrally located on a footing conforming to Section 9.15, and Article 9.17.3.2 requires steel columns to have end plates of at least 100 mm x 100 mm x 6.35 mm. The column usually stands on the footing and the basement slab is poured around its base.
On a pedestal. When the footing has to go deep (for frost, for bearing soil, or below a pit), a pedestal carries the load up to a convenient elevation. It also keeps the base plate above the floor or grade, where it stays drier, and gives the anchor rods a solid, reinforced block of concrete. In the 2004 edition of CSA A23.3 we checked, a pedestal is defined as an upright compression member whose unsupported height is less than three times its least lateral dimension. A taller one is designed as a concrete column.
On a slab. A slab-on-grade is designed to carry floor loads spread over the ground, not a column's concentrated load. A column landing on a slab needs a footing, either an isolated footing below the slab or a thickened slab section designed for it.
How a pedestal is reinforced
A pedestal looks like a block of concrete, but it is a reinforced element with a few specific details:
- Vertical bars run from the footing up through the pedestal, usually as dowels cast into the footing and lapped with the pedestal bars. Our guide to footing-to-wall dowels covers how dowels are sized and placed.
- Ties wrap the vertical bars, as in a column.
- Extra ties at the top around the anchor rods. In the 2004 edition of CSA A23.3 (Clause 7.6.5.8), anchor bolts in the tops of columns or pedestals must be enclosed by lateral reinforcement that also surrounds at least four vertical bars, distributed within 125 mm of the top and consisting of at least two 10M bars. Confirm the current requirement with your engineer; the 2024 OBC references the 2019 edition.
- Size to suit the base plate and rods. The pedestal is usually larger than the base plate so the anchor rods have enough concrete around them. Edge distance in concrete is a design check of its own, covered in anchor bolts for base plates.
- Cover. The bars still need the concrete cover required for the exposure under CSA A23.1.
The rods, ties and vertical bars all compete for space at the top of a pedestal. It pays to have the rebar detailer and the steel detailer working from the same anchor rod layout.
Getting the elevations right
Three elevations have to agree:
- Top of concrete (footing or pedestal).
- Underside of base plate, which sits above the concrete by the grout thickness shown on the drawings.
- Top of steel, where the beams land.
The column length is detailed from these numbers. If the concrete is poured high, there is no room for grout and levelling, and the fix is usually grinding concrete. If it is poured low, the grout gap gets too thick or the column is short. Either way, the problem shows up when the steel arrives.
That is why the top of concrete is set below the base plate by the grout gap, and why columns are set on levelling nuts, steel shims or a setting plate and then grouted. Our guide to grouting base plates explains grout types and thickness.
For a basement post sitting directly on a footing, the fabricator measures the actual distance from footing to beam on site, because the post has to fit the space exactly.
Anchor rods: setting and checking
Anchor rods are cast into the footing or pedestal before the steel arrives. A few points decide whether the column goes on smoothly:
- Use a template. A plywood or steel template drilled to match the base plate holds the rods in position and at the right projection while the concrete is placed. See base plate layout and anchor bolt templates.
- Four rods for building columns. CSA S16 requires columns to have at least four anchor rods for erection safety, with special precautions where that is not feasible, as the Canadian Institute of Steel Construction explains with reference to the 2014 edition.
- Projection. Rods must stick up far enough for the levelling nut, the plate, a washer and a full nut with thread to spare, and not so far that they foul anything.
- Survey before steel. Check rod positions, projection and top-of-concrete elevation against the drawings before the column is shipped. A problem found on paper costs far less than one found with a column on the hook.
When anchor rods are out of position
It happens on many jobs: a rod moved during the pour, a template was rotated, or a layout dimension was misread. The fix always comes from the engineer of record, because the anchor rods carry real forces. Typical options the engineer may approve:
- Enlarged or slotted holes in the base plate, with heavy plate washers over them, when the rod is only slightly off.
- A modified or larger base plate that reaches the rods where they are.
- Abandoning the rods and installing post-installed anchors (adhesive or mechanical) designed for the loads.
- Adjusting the rods themselves, only where the engineer allows it and specifies how.
What not to do: cut rods off, bend them with heat, or torch new holes in the plate on site without approval. Each of those can weaken the connection in ways that are hard to see later.
Setting the column
On a typical building column the sequence is: confirm rods and elevations, set levelling nuts or shims, land the column, install nuts, plumb it, brace it until the frame is stable, connect the beams, then grout. The full on-site process, including residential posts, is in installing steel columns and posts.
Columns in slabs and outdoors
Inside, where the slab meets the column. The slab and the column foundation move differently, so slabs are usually separated from columns with an isolation joint: a compressible material that lets the two move independently. The Ontario Cast-in-Place Concrete Development Council (OCCDC) reference guide describes isolation joints as typically 13 mm of compressible material. Without one, the slab can crack from the column outward.
Outside. Exterior column footings have to be deep enough to stay below frost. For Part 9 buildings, Table 9.12.2.2 of the OBC sets minimum foundation depths by soil type and drainage, and engineered buildings follow the geotechnical report. Bring the pedestal above finished grade so the plate isn't sitting in soil or standing water, and slope the top of the grout away from the column. Exterior steel columns susceptible to corrosion need at least one coat of rust-inhibitive paint (Article 9.17.3.3).
Columns and base plates fabricated to your drawings
We fabricate steel and HSS columns with base and cap plates welded on, detail them in-house from your foundation and framing drawings, and weld in a CWB certified shop. We also supply and install structural steel and rebar, so the pedestal reinforcement and the columns on top of it can come from one supplier. Delivery is across the GTA and Ontario.
Send your foundation and steel drawings for a quote, or see the services we offer.
Frequently asked questions
Can a steel column sit directly on a concrete slab?
Not on an ordinary slab-on-grade. A column needs a footing sized for its load, and in a house the Ontario Building Code requires columns to be centred on a footing. The slab is usually poured around the column base, separated from it by an isolation joint.
Do steel columns need a concrete pedestal?
Not always. Basement columns often bear directly on a footing below the slab. Pedestals are used when the footing is deep, to bring the base plate up to a set elevation above the floor or grade, to keep steel out of soil and water, or to give anchor rods enough concrete around them.
What happens if anchor bolts are in the wrong place?
The engineer of record decides the fix. Common options include slotting or enlarging the base plate holes with heavy plate washers, modifying the plate, or abandoning the rods and installing post-installed anchors. Rods should not be cut, bent or heated, or holes torched in the field, without the engineer's approval.
How much gap is left under a column base plate?
Building columns are usually set with a gap between the top of concrete and the underside of the plate, which is filled with non-shrink grout after the column is plumbed. The gap is shown on the drawings; it allows for levelling and small errors in the concrete elevation.
