To calculate the rebar in a concrete column, add up three things for each storey: the vertical bars (storey height plus the lap into the next lift), the ties (clear height divided by the tie spacing, plus one, times the length of one tie), and the dowels from the footing at the base. Multiply the metres of each bar size by its mass per metre for the weight. As a check, CSA A23.3 requires the vertical bars to be between 1% and 8% of the column's gross area, and 1% works out to about 78.5 kg of vertical steel per cubic metre of column.
The bar sizes, spacing and laps for any real column come from the structural drawings. The worked example below uses illustrative values to show the method.
The limits that shape column steel
CSA A23.3 sets minimum and maximum reinforcement for columns. The clause numbers below are from the 2004 edition; your engineer works to the edition the Ontario Building Code references.
| Requirement | Limit | Clause (A23.3-04) |
|---|---|---|
| Minimum vertical steel | 1% of the gross area | 10.9.1 |
| Maximum vertical steel | 8% of the gross area, including lap regions | 10.9.2 |
| Minimum number of bars | 4 in rectangular or circular ties, 3 in triangular ties, 6 in spirals | 10.9.3 |
| Minimum tie size | 30% of the largest vertical bar diameter for bars up to 30M; at least 10M for 35M and larger or bundled bars | 7.6.5.1 |
| Maximum tie spacing | Smallest of 16 vertical bar diameters, 48 tie diameters and the least column dimension | 7.6.5.2 |
| Ties at floors | First tie no more than half a spacing above the slab or footing, last tie no more than half a spacing below the lowest reinforcement above | 7.6.5.3 |
A note in the standard points out that more than 4% steel outside the lap regions can make it hard to place the concrete and the bars, especially at beam-column joints. That is why most columns sit closer to the minimum than the maximum.
Columns in seismic force resisting systems have further rules for tie spacing and hooks. Our post on stirrup and tie spacing works through the tie limits.
How much steel is in a column: the percentage method
When you only need a budget figure, the percentage method gives the vertical steel quickly:
- Steel area = percentage × column area
- Mass per metre of height = steel area in mm² × 0.00785 kg/m (since 100 mm² of bar weighs 0.785 kg per metre)
- Mass per cubic metre of column = percentage × 7,850 kg/m³
| Column | Gross area | Vertical steel at 1% | at 2% | at 4% |
|---|---|---|---|---|
| 300 × 300 mm | 90,000 mm² | 7.1 kg/m | 14.1 kg/m | 28.3 kg/m |
| 400 × 400 mm | 160,000 mm² | 12.6 kg/m | 25.1 kg/m | 50.2 kg/m |
| 500 × 500 mm | 250,000 mm² | 19.6 kg/m | 39.3 kg/m | 78.5 kg/m |
| 600 × 600 mm | 360,000 mm² | 28.3 kg/m | 56.5 kg/m | 113.0 kg/m |
Per cubic metre, 1% is 78.5 kg, 2% is 157 kg and the 8% maximum is 628 kg of vertical steel.
This method leaves out ties, laps and dowels, which add real weight. Use it to sanity-check a design or a bar list, not to order from.
Worked example: one storey of a 400 × 400 column
Column: 400 by 400 mm, 3.0 m floor to floor, 200 mm slab above, so 2.8 m clear height. Grade 400W bar.
From the (illustrative) drawings:
- Vertical bars: 4-25M. Area 4 × 500 = 2,000 mm², which is 2,000 ÷ 160,000 = 1.25%. Above the 1% minimum, and four bars meets the minimum count.
- Ties: 10M at 300 mm. The tie is at least 30% of the 25M bar (0.3 × 25.2 = 7.6 mm, and 10M is 11.3 mm). The maximum spacing would be the least of 16 × 25.2 = 403 mm, 48 × 11.3 = 542 mm and 400 mm, so 300 mm is inside the limit.
- Laps: the drawings show 740 mm laps for the 25M verticals. That matches the CSA A23.3 compression lap for 25M Grade 400 bar from our lap splice chart. If the engineer requires tension laps, they are longer.
- Footing dowels: 4-25M with a 90° hook resting on the footing steel, 525 mm into a 600 mm footing and projecting 740 mm to lap with the verticals.
Vertical bars
- One bar: 3,000 mm storey + 740 mm lap above = 3.74 m
- Four bars: 14.96 m × 3.925 kg/m = 58.7 kg
RSIC practice estimates lapped column verticals in square and rectangular columns as shop offset bent, so the bars from below step inward to lap inside the bars above. The offset adds very little length but needs to be on the bar list.
Ties
- RSIC practice, unless the drawings say otherwise, makes column ties 80 mm smaller than the column: 320 by 320 mm out to out.
- One tie: 4 × 320 = 1,280 mm, plus two 135° hooks at about 100 mm each for 10M = 1.48 m
- Count: the first tie is 150 mm above the floor and the last is 150 mm below the slab above, so (2,800 minus 300) ÷ 300 = 8.3, round up to 9 spaces, so 10 ties. RSIC practice adds one or two ties below the offset bend; add one: 11 ties.
- Total: 11 × 1.48 = 16.28 m × 0.785 kg/m = 12.8 kg
Footing dowels (base storey only)
- One dowel: 525 + 740 = 1,265 mm vertical, plus a 25M hook allowance of 400 mm = 1.665 m
- Four dowels: 6.66 m × 3.925 kg/m = 26.1 kg
Total for the base storey: 58.7 + 12.8 + 26.1 = about 97.6 kg. Each storey above with the same layout adds about 71.5 kg (verticals plus ties).
Multi-storey columns, laps and joints
- Each lift laps with the one below. Vertical bars usually run one storey plus a lap, so a four-storey column has four sets of verticals and four laps.
- Changing column size. When the column face above steps in by more than 75 mm, RSIC practice ends the lower bars near the top of the slab and uses separate dowels to lap with the bars above.
- Ties at the joint. Under a slab, ties run to within half a spacing of the lowest slab reinforcement (A23.3-04 Clause 7.6.5.3). Where beams frame into the column, the details show whether ties continue through the joint; A23.3 lets them stop just below the shallowest beam where beams frame in from all four sides.
- Large bars do not lap. CSA A23.3 does not permit lap splices for bars larger than 35M except in limited compression cases. 45M and 55M columns use mechanical or welded splices; see rebar couplers vs lap splices.
- Splice location. Column laps are usually just above the floor, but seismic frames often move them to mid-height. Our post on lap length and splice zones explains why.
Common mistakes in column take-offs
- Leaving out the laps. On a multi-storey column, laps are a large share of the vertical steel.
- Counting ties over the full floor-to-floor height instead of the clear height, or the other way round, without checking what the drawings show through the joint.
- Forgetting the extra ties at offset bends and the closer tie zones that seismic details require.
- Missing the dowels, which belong to the footing pour but are part of the column.
- Ignoring cross-ties. Larger columns with bars along each face need inner ties so every corner and alternate bar is supported and no unsupported bar is more than 150 mm clear from a supported one (A23.3-04 Clause 7.6.5.5).
Our guide to choosing rebar sizes for beams and columns covers how the bar sizes themselves are set.
From quantities to bar lists
A column take-off is straightforward once the details are clear, but it multiplies fast across a building. We prepare free take-offs from your drawings, then detail, cut, offset bend and tag column verticals, ties and dowels to match. Send your drawings for a quote, or see our rebar products.
Frequently asked questions
How much steel is required in a concrete column?
CSA A23.3 requires the vertical bars in a column to be at least 1% and not more than 8% of the gross column area, including lap splice regions. In mass terms, 1% is about 78.5 kg of vertical steel per cubic metre of column. Most columns are designed well below the 8% ceiling.
What is the maximum steel in a column?
8% of the gross area under CSA A23.3, and that limit includes the regions where bars are lapped. The standard notes that more than 4% outside lap regions can make placing concrete and reinforcement difficult, so designers usually stay well below the maximum.
How many bars are needed in a column?
At least four bars in a rectangular or circular tied column, three in a triangular tied column and six in a spiral column, under CSA A23.3. The actual number and size come from the engineer's design.
How do you calculate column ties?
Count the ties over the clear height between floors: start no more than half a spacing above the slab or footing, stop the same distance below the beam or slab above, divide by the spacing, round up and add one. Each tie length is its perimeter plus two hook allowances.
