Anchor bolts (anchor rods) tie a column's base plate to the concrete. The holes in the base plate are made deliberately larger than the rods so the column can be set despite small errors in rod placement, and heavy plate washers go over those holes. Edge distance has two parts: enough steel between each hole and the edge of the plate, and enough concrete between each rod and the edge of the footing or pedestal, which CSA A23.3 sets as at least 6 times the rod diameter for torqued cast-in headed anchors (2004 edition). The engineer chooses rod size, grade, embedment and layout, and those choices then drive the plate size.
This guide explains each of those details so you can read a base plate drawing, check a shop drawing and avoid the problems that show up on site.
Anchor rod basics
Anchor rod or anchor bolt? The terms are used interchangeably. CSA S16 uses "anchor rod". In practice they are threaded rods or bolts cast into concrete, with a head, nut, plate or hook at the bottom end to anchor them.
Grades. The Canadian Institute of Steel Construction (CISC) notes that ASTM F1554 covers three grades: Grade 36 (248 MPa), Grade 55 (380 MPa) and Grade 105 (724 MPa). Most rods for gravity columns are Grade 36. Higher grades are used for uplift, moment bases and other high forces. The drawings should state the grade, and higher grades should not be swapped for lower ones, or the other way round, without approval.
How many. CSA S16 requires columns to be fitted with at least four anchor rods, and where four non-collinear rods are not feasible, special precautions must be taken. CISC explains this is for erection safety: a column standing on its rods before the frame is connected needs to resist tipping in more than one direction.
The parts at the plate. A typical rod assembly at the base plate has a levelling nut under the plate (if levelling nuts are used), the base plate, a plate washer, a hardened washer where specified, and the top nut.
Base plate hole size: why the holes are oversized
A steel-to-steel bolted connection uses a hole only slightly larger than the bolt. That doesn't work for anchor rods. The rods are cast into concrete by a different trade, using a template, and they move during the pour. Even a well-placed rod may be a few millimetres off.
CISC addresses this directly: a standard bolt hole, or 1/16 in clearance, is not practical for anchor rods, because larger holes are needed to accommodate rod installation tolerances. The consequences:
- Holes are oversized relative to the rod. Published design references, such as the AISC Steel Construction Manual and AISC's design guide for base connections, list a recommended hole diameter and plate washer for each rod diameter. The designer picks the sizes and shows them on the drawings.
- Plate washers cover the hole. Because the hole is large, a normal washer would pull through. A thick square plate washer with a hole close to the rod size spreads the nut's force over the base plate.
- Shear needs a separate path. Oversized holes mean the rods won't touch the plate until it slides. CISC notes that where anchor rods have to transfer shear, appropriately designed washers with standard holes are field welded to the base plate in the erected position. Large shear forces are often taken by a shear lug instead.
So don't be surprised when the shop drawing shows holes well over the rod diameter. Do be careful about enlarging them further in the field without the engineer's approval, because that can affect the washer design and the shear path.
Edge distance in the base plate
The steel around each hole has to be wide enough to carry the forces in the rod without tearing out, and wide enough to fabricate cleanly. CSA S16 sets minimum edge distances for bolt holes, and the designer checks them with the oversized hole in mind.
The practical constraint is often clearance rather than strength:
- Nut and washer clearance from the column and its weld. The nut has to turn and the plate washer has to sit flat.
- Wrench and tool access, especially for rods inside the flanges of a W-shape column.
- Hole position on HSS columns. Rods normally sit outside the tube, so the plate must extend far enough past the column for the hole, the washer and the edge distance.
These are the reasons anchor rod layout usually sets the plan size of a base plate, and the plate size then sets its thickness. Our base plate size and thickness chart and column base plate design example show how that plays out.
Edge distance and spacing in concrete
The concrete side is designed to the anchorage provisions of CSA A23.3. We checked the 2004 edition, which sets minimum spacings and edge distances to prevent splitting (Clause D.9) unless reinforcement is provided to control splitting. The 2024 OBC references the 2019 edition, so confirm current values with your engineer.
| Anchor condition (CSA A23.3-04, Clause D.9) | Minimum |
|---|---|
| Spacing, untorqued cast-in anchors | 4 x anchor diameter |
| Spacing, torqued cast-in and post-installed anchors | 6 x anchor diameter |
| Edge distance, cast-in headed anchors not torqued | Based on concrete cover requirements (Clause 7.9) |
| Edge distance, cast-in headed anchors that will be torqued | 6 x anchor diameter |
| Edge distance, post-installed anchors without product test data: undercut / torque-controlled / displacement-controlled | 6 / 8 / 10 x anchor diameter |
Post-installed anchors may use smaller values from product-specific tests, and must also be at least twice the nominal maximum aggregate size from an edge.
Example. A 20 mm cast-in headed rod that will be torqued needs at least 6 x 20 = 120 mm to the nearest concrete edge. Four such rods on a 200 mm square pattern, each with 120 mm to the edge, need a pedestal at least 440 mm square for this rule alone.
These are minimums to prevent splitting. The real capacity checks (concrete breakout, pullout, side-face blowout) usually want more room, especially for rods carrying uplift. A rod close to a pedestal edge has much less breakout capacity than the same rod in the middle of a footing.
In pedestals, the 2004 edition of CSA A23.3 also requires ties around the anchor bolts near the top of the pedestal (Clause 7.6.5.8). See setting steel columns on footings and pedestals for how pedestals are reinforced.
Projection and thread length
The rod has to stick out of the concrete far enough to take, from the bottom up:
- The grout gap (or levelling nut and grout gap)
- The base plate thickness
- The plate washer and any hardened washer
- A full nut
- A few threads beyond the nut
Too little projection and the nut can't be fully engaged. Too much and the rod may foul a wall finish, a door or equipment. The projection and the threaded length are shown on the anchor rod drawings, and checking them before the pour is cheaper than fixing them after.
Layout tolerance and templates
Most anchor rod problems start with layout. A template drilled to match the base plate, fixed to the formwork at the right elevation and orientation, keeps the rods where they belong. Rotated templates, swapped column orientations and rods that move when concrete is placed are the usual culprits. Our guide to base plate layout and anchor bolt templates walks through it.
If rods do end up out of place, the engineer of record decides the fix. Cutting, bending or heating rods, or torching holes in the plate, should not happen without that approval.
Common mistakes
- Rods too close to the edge of a pedestal or wall, often after a pedestal was made smaller to save concrete.
- Hole sizes too tight on the shop drawing, leaving no tolerance on site.
- Missing plate washers, or ordinary washers used over oversized holes.
- Wrong rod grade substituted from what was on hand.
- No check before the steel ships, so the misfit is discovered with the column in the air.
Base plates drilled and detailed to suit
We prepare shop drawings in-house, cut and drill base plates to your details, and weld them to columns and posts in our CWB certified shop. Send the anchor rod plan with your steel drawings and we'll detail the plates to match. Engineering services are available for projects that need them.
Send your drawings for a quote, or see the services we offer.
Frequently asked questions
How much bigger should base plate holes be than the anchor bolts?
Noticeably bigger than a normal bolt hole, because cast-in rods are never placed perfectly. The Canadian Institute of Steel Construction notes that a standard bolt hole clearance is not practical for anchor rods. The hole size is chosen by the designer from published recommendations for each rod diameter and shown on the drawings, with plate washers to cover the oversized holes.
What is the minimum edge distance for anchor bolts in concrete?
It depends on the anchor type and design. In the 2004 edition of CSA A23.3, cast-in headed anchors that will be torqued need at least 6 times the anchor diameter to the edge, and untorqued ones are based on the concrete cover requirements. Post-installed anchors follow the product's test data, or larger default values without it.
How many anchor bolts does a steel column need?
CSA S16 requires building columns to have at least four anchor rods for erection safety, unless special precautions are taken where four non-collinear rods are not feasible. Small posts outside that scope still follow the designer's details.
What grade are anchor bolts?
Anchor rods in Canada are commonly specified to ASTM F1554, which has Grades 36, 55 and 105. Grade 36 is the usual choice for gravity columns, and the higher grades are used where the design needs more strength.
