A base plate is the flat steel plate welded to the bottom of a column that spreads the column’s load onto the concrete below and gives the anchor rods something to hold. It sits on a layer of grout, it is bolted through to the foundation, and it is sized so the concrete under it is never asked to carry more pressure than it can take.
That sounds simple, and the plate itself is simple. What goes wrong with base plates is almost always the setting: rods in the wrong place, a plate set without a proper leveling method, or grout that never fully fills the gap. This guide covers what the plate does, what decides its size and thickness, and how it is set in the field.
What a Base Plate Does
A steel column carries a lot of load through a small area. A W8x31, for example, has a steel cross-section area of 9.13 square inches inside an 8 in by 8 in outline (AISC Shapes Database v16.0), while the concrete footing below is far weaker per square inch than steel is. Set the bare column end straight on concrete and the concrete would crush under the flanges. The base plate fixes that by spreading the load over a much bigger area.
Course notes on column bases from the Czech Technical University describe the assembly as three parts working together: the column, the base plate and the anchoring assembly (CTU Prague, Column Bases lesson notes, checked October 2026). In practice the plate has three jobs:
- Spread compression. The column load passes through the plate into the grout and concrete at a pressure the concrete can carry.
- Hold the column down. Uplift from wind, or the overturning of a column under construction, is resisted by the anchor rods pulling on the plate.
- Transfer shear. Sideways load at the base goes into the foundation through the rods, through friction under the plate, or through a shear lug welded under it.
The anchor rods themselves (grades, embedment, cast-in versus post-installed) are covered in our guide to anchor bolts and how columns are anchored to concrete. This article stays with the plate.
What Decides Base Plate Size
Plan size (length and width) and thickness are separate decisions, and a structural engineer makes both from the actual loads.
Length and width
The plate has to be big enough that the bearing pressure on the concrete stays under the concrete’s allowable bearing stress. It also has to be bigger than the column’s footprint, with room around the flanges for the anchor rod holes, nuts and washers, and for a wrench to turn them. For a wide flange column that footprint is the depth by the flange width, which you can look up for any shape in our wide flange beam sizes chart. In practice the rod layout and the edge distance on the concrete pier often set the plan size before bearing pressure does.
Thickness
Thickness comes from bending. The concrete pushes up on the whole underside of the plate, but the column only pushes down over its own footprint, so the part of the plate sticking out past the column bends upward like a short cantilever. The CTU notes model it exactly that way: the plate is treated as a cantilever from the column face, and the plate’s elastic bending strength is set equal to the moment from the bearing pressure. The result is that the required thickness grows with the overhang and with the bearing pressure, and falls with higher plate yield strength.
Two practical consequences follow. A wide plate with a big overhang needs to be much thicker than a snug one, so enlarging a plate “to be safe” without making it thicker can leave it too thin. And when uplift puts the anchor rods in tension, the plate also bends between the rods and the column, which is a separate check.
Stiffeners
Most base plates are plain flat plates. The CTU notes say column bases are generally designed with unstiffened plates, and stiffened plates are used where the base has to transfer high bending moments. Stiffeners are triangular plates welded between the column and the base plate. They make the plate act much stiffer, but they also add welding and get in the way of the nuts, so they only appear when the design needs them.
Pinned Bases and Fixed Bases
Most building columns are designed with a pinned base. The plate holds the column in place and carries vertical load and shear, but it is not relied on to stop the column from rotating, so frame stability comes from bracing or moment frames elsewhere. A fixed base is designed to resist moment as well: the rods are spread wider, the plate is thicker, and the foundation has to take the moment too.
The same idea applies to beam-to-column joints, which we explain in moment connection vs shear connection. A base that looks heavy is not automatically a fixed base. What matters is what the engineer designed it to do.
| Feature | Pinned base | Fixed (moment) base |
|---|---|---|
| Carries axial load and shear | Yes | Yes |
| Designed to resist column moment | No | Yes |
| Anchor rod layout | Usually close to the column | Spread wider to create a lever arm |
| Plate | Thinner, often unstiffened | Thicker, sometimes stiffened |
| Foundation | Sized for vertical load and shear | Also sized for overturning moment |
How a Base Plate Is Set in the Field
The anchor rods are cast into the concrete first. The column arrives with the plate already welded on in the shop, the plate holes drop over the rods, and the column is brought to the right elevation and plumbed before the gap underneath is grouted.
Leveling: four accepted methods
Federal construction rules require that columns be set on “level finished floors, pre-grouted leveling plates, leveling nuts, or shim packs” able to carry the construction loads (OSHA 1926.755, Column anchorage, checked October 2026). Each has its place:
- Leveling nuts. A nut and washer on each rod under the plate. Quick to adjust and common on light and medium columns. The nuts carry the column until the grout cures, so they limit how much can be built on top beforehand.
- Shim packs. Steel shims stacked under the plate to the right height. Simple and stiff; the shims stay in and get grouted around.
- Leveling plate. A thin plate set and grouted to exact elevation before the column arrives. Common for small columns; the column plate then sits directly on it.
- Finished floor. For light columns a level, finished concrete surface can serve directly.
Oversized holes and plate washers
Anchor rods cast into concrete are never in exactly the right place, so base plate holes are made larger than the rods to give some adjustment. A hole that big is too large for a normal washer, so a thick steel plate washer goes over each hole under the nut. AISC publishes recommended hole and plate washer sizes for each rod diameter in its Steel Construction Manual. If a hole has to be enlarged or a rod bent to make it fit, that is a change to the anchorage, and OSHA 1926.755(b) says anchor rods shall not be repaired, replaced or field-modified without the approval of the project structural engineer of record.
Grout
Once the column is plumb, the space under the plate is packed with non-shrink grout. The grout is what actually carries the load into the concrete over the long term, so it has to fill the gap completely with no voids. Large plates often get a grout hole near the center, and sometimes vent holes, so grout can be placed and air can escape. The CTU notes cite the Eurocode rule that where the grout layer is thicker than 50 mm (about 2 inches), its strength should be at least that of the concrete foundation.
The four-rod rule
For steel erection, OSHA 1926.755(a) requires all columns to be anchored by at least four anchor rods, and the rods, the column-to-base-plate weld and the foundation must be designed to resist a 300 lb eccentric gravity load placed 18 inches from the outer face of the column. That rule is about stopping a freshly set column tipping over while ironworkers climb it. Our overview of structural steel erection covers the rest of the sequence.
Base Plates for Tube and Pipe Columns
Square, rectangular and round HSS columns get base plates the same way, with the tube welded all round to the plate. Tubes are common for posts, canopies and smaller buildings because they look tidy and resist buckling well in both directions. Wall thicknesses and outside sizes are in our HSS and square tube sizes chart. A sealed tube on a plate should not trap water, so outdoor columns are usually capped at the top and detailed so moisture inside can escape.
In houses, the adjustable or fixed steel posts under a basement beam usually have small factory plates top and bottom instead of a full engineered base. Those are a different product, covered in our guide to lally columns.
Common Base Plate Problems
- Rods out of position. The most common one. Fixes range from slotting the plate to new post-installed anchors, and every option needs the engineer’s approval.
- Voids in the grout. A plate bearing on a few high spots instead of a full grout bed concentrates load and can crack the grout or concrete.
- Nuts never tightened after grouting. Leveling nuts hold the column up during erection, but the top nuts still need to be snugged once the grout has cured.
- Corrosion at the base. A plate sitting in water at grade, or a column base buried in a slab without protection, rusts where you cannot see it.
- Welding to the rods. Rod material is not always meant to be welded. Do not weld nuts or plates to anchor rods unless the engineer specifies it.
Base Plate FAQ
How thick should a steel base plate be?
There is no single answer. Thickness depends on the column load, the concrete bearing pressure, how far the plate overhangs the column and the plate’s yield strength, plus any uplift on the rods. A licensed structural engineer has to size it for the actual loads.
Why are base plate holes bigger than the anchor rods?
Rods cast into concrete are never perfectly placed, so oversized holes give room to adjust. A thick plate washer covers each hole under the nut. AISC lists recommended hole and washer sizes for each rod diameter.
What goes under a base plate?
Non-shrink grout, placed after the column is leveled and plumbed on leveling nuts, shims or a leveling plate. The grout carries the load into the concrete, so it must fill the gap completely.
How many anchor bolts does a column base plate need?
For structural steel erection, OSHA 1926.755 requires at least four anchor rods per column. The engineer may require more, depending on uplift, shear and moment.
Is the column welded or bolted to the base plate?
The column is welded to the plate, usually in the fabrication shop. The plate is then bolted to the foundation through the anchor rods. Weld sizes are shown on the shop drawings with standard weld symbols.
The Short Version
A base plate spreads a column’s load onto concrete and ties the column down. Its plan size comes from bearing pressure and the rod layout, its thickness from cantilever bending, and its performance from how well it is leveled and grouted. Base plates, anchor rods and their foundations must be designed by a licensed structural engineer, and any field fix to the rods needs that engineer’s approval first.
Reference information only, not engineering advice. Disclaimer.