Anchor bolts are steel rods set into concrete so a column, post, sill plate or piece of equipment can be bolted down to the foundation. They come in two broad families: cast-in anchors that are placed before the concrete is poured, and post-installed anchors that are drilled and fixed into concrete that has already hardened.
This guide explains the main types, the ASTM grade used for structural anchor rods, how a steel column is actually set on its anchors, and the mistakes that cause trouble on site.
What Anchor Bolts Do
An anchor bolt has one job: to move force between a steel or wood member and the concrete below it. Depending on the building, that force can be:
- Uplift (tension). Wind trying to lift a roof, or a braced frame trying to pull a column base out of the footing.
- Shear. Wind or seismic load trying to slide the base sideways.
- Erection stability. Holding a freshly set column upright before the beams around it are connected.
Gravity load, the weight pushing down, mostly goes through the base plate and grout straight into the concrete. The anchors are there for everything that is not simply pushing down, which is why they cannot be treated as an afterthought.
Cast-In Anchor Bolts
Cast-in anchors are set in position, usually with a plywood or steel template, and the concrete is poured around them. They are the standard choice for steel columns and heavy equipment because the engineer controls the embedment and the anchorage from the start.
Common cast-in shapes
- L-bolts (bent or hooked). A rod bent 90 degrees at the bottom. Common for wood sill plates and light framing.
- Headed anchor bolts. A rod with a forged or heavy hex head at the bottom, often with a plate washer. The head bears against the concrete and gives more reliable pull-out resistance than a hook.
- Threaded rod with a nut and plate. An all-thread rod with a nut and plate at the bottom does the same job as a headed bolt.
- J-bolts. Similar to an L-bolt with a curved hook.
Post-Installed Anchors
When the concrete is already in place, anchors are drilled in afterward. These are what most people find in a hardware store under the name “concrete anchor bolts.”
| Type | How it holds | Typical use | Watch out for |
|---|---|---|---|
| Wedge anchor | A clip at the tip expands against the hole wall as the nut is tightened | Racking, equipment, ledgers, light steel | Needs sound concrete and enough edge distance |
| Sleeve anchor | A sleeve expands along its length | Lighter fixings, masonry and concrete | Lower capacity than a wedge anchor of the same size |
| Adhesive (epoxy) anchor | Threaded rod or rebar bonded into a cleaned hole with resin | Heavy loads, retrofit, close to edges | Hole cleaning and cure time are critical |
| Screw anchor | Hardened threads cut into the concrete | Quick fixings, temporary work | Follow the manufacturer’s hole size exactly |
| Drop-in anchor | An internally threaded shell set with a setting tool | Overhead hangers for pipe and conduit | Shallow embedment |
Post-installed anchors used for structural loads are normally products with published test-based design values, and the manufacturer’s installation instructions become part of the design. Drilling a hole the wrong size or skipping the hole cleaning step can cut capacity badly.
ASTM F1554: The Standard for Structural Anchor Rods
Cast-in anchor rods for building columns in the US are usually specified to ASTM F1554, the standard covering straight, bent, headed and headless anchor bolts, also called anchor rods. It defines three strength grades, named for their minimum yield strength in ksi: 36, 55 and 105. Buy the current edition from ASTM directly if you need the exact wording, because the grade tables and supplementary requirements are what a fabricator works from.
| F1554 grade | Minimum yield strength | Where it tends to be used |
|---|---|---|
| Grade 36 | 36 ksi | The common default for column anchor rods |
| Grade 55 | 55 ksi | Higher tension demand; a weldable version is available as a supplementary requirement |
| Grade 105 | 105 ksi | High uplift, such as braced frames and tall poles |
A few things about its scope are worth knowing on site. The standard is intended for anchoring structural supports such as building columns, sign and light poles, and bearing plates to concrete. It also covers all-thread rod used for anchoring. It does not cover mechanical expansion anchors, powder-actuated fasteners, or anchors made from deformed rebar. Zinc coating requirements are included, which matters when anchors are galvanized; see how galvanizing protects steel for what the coating does and does not do. Galvanizing also behaves differently once the steel is cast into concrete: the American Galvanizers Association reports that galvanized steel in concrete tolerates four to five times the chloride concentration of black steel, and that zinc corrosion products migrate into the pores instead of building up pressure and spalling the concrete (checked September 2026).
F1554 is a different standard from the structural bolts used in beam connections. Our bolt grades guide covers Grade 5, Grade 8 and the A325 and A490 families used for steel-to-steel connections.
How a Steel Column Is Set on Anchor Bolts
For a steel column, typically a W-shape or H-beam or an HSS tube, the sequence usually goes like this:
- Set the anchors with a template. The template holds the rod pattern, spacing and projection while the concrete is poured. The pattern has to match the holes in the base plate.
- Survey the anchors. Before steel arrives, the as-built positions and projections are checked so problems are found before a crane is on site.
- Level the base. The column is set on leveling nuts under the base plate, or on steel shims or a leveling plate.
- Plumb and connect. The column is plumbed and top nuts are snugged so the column is stable.
- Grout. Non-shrink grout is packed under the base plate so gravity load bears on the full plate.
- Final tightening. Nuts are tightened to the level the engineer specifies.
Base plate holes are made oversized compared with the rod diameter because cast-in anchors are rarely placed perfectly. Large plate washers go over those holes under the top nuts. The oversize allowance is a design decision; it is not a license to slot a hole on site.
What OSHA Requires for Column Anchorage
Because a newly erected column relies on its anchors to stay upright, federal steel erection rules set minimums. OSHA 29 CFR 1926.755, Column anchorage (checked September 2026) requires that:
- All columns are anchored by a minimum of 4 anchor rods (1926.755(a)(1)).
- Anchor rods are not repaired, replaced or field-modified without the approval of the project structural engineer of record (1926.755(b)(1)).
- Before a column is erected, the controlling contractor tells the steel erector in writing if its anchor rods have been repaired, replaced or modified (1926.755(b)(2)).
That four-rod minimum is why you rarely see a two-bolt column base in modern steel buildings, even for lightly loaded posts.
Common Anchor Bolt Problems
Rods in the wrong place
Templates move during the pour, or rods get bumped by a vibrator. The fix, whether that is a new plate, an adhesive anchor or a redesigned base, has to come from the engineer of record, as the OSHA rule above states. Bending rods to make them fit or cutting them off and welding on a new stub are the classic field fixes that cause problems.
Too little projection
If a rod does not stick up far enough for the leveling nut, the plate, the washer and a fully engaged top nut, the connection is not complete. Checking projection before the steel arrives is cheaper than fixing it with a crane waiting.
Edge distance and spacing
An anchor close to the edge of a pier or slab can break out a cone of concrete well below its own strength. Anchor capacity in concrete depends on embedment, edge distance, spacing and the concrete itself, not only on the rod grade. That is structural design work, and a licensed structural engineer has to size and check it.
Welding to anchor rods
Not every anchor rod material is intended to be welded. F1554 makes weldable Grade 55 a supplementary requirement, which is a hint that weldability should not be assumed. Any welding to anchor rods should be specified by the engineer. The gusset plate guide shows how quickly connection details can go wrong when they are treated as simple hardware.
Frequently Asked Questions
What is the difference between an anchor bolt and an anchor rod?
Nothing, in practice. ASTM F1554 uses both terms for the same product. Steel design documents tend to say “anchor rod” to avoid confusion with structural bolts.
What grade of anchor bolt is used for steel columns?
ASTM F1554 Grade 36 is the common default, with Grade 55 or 105 used where tension demand is higher. The drawings govern; the engineer specifies the grade.
How many anchor bolts does a steel column need?
OSHA 1926.755 requires at least four anchor rods for every column. Design loads can call for more.
Can I use wedge anchors instead of cast-in bolts?
Sometimes, for lighter work or retrofits, but it is a design decision. Post-installed anchors have different capacities and installation requirements, and a licensed structural engineer should approve them for any structural use.
What if the anchor bolts are in the wrong place?
Stop and call the engineer of record. OSHA does not allow anchor rods to be repaired, replaced or field-modified without that engineer’s approval.
Reference information only, not engineering advice. Disclaimer.