Choosing Rebar Sizes for Concrete Beams and Columns

Rebar size for a concrete beam or column is not picked from a chart. The engineer first works out how much steel area the member needs, then chooses a bar size and count that supply that area, stay between the code minimum and maximum, and physically fit inside the concrete with the required cover and clear spacing. In Canada those limits come from CSA A23.3 (design) and CSA A23.1 (construction), and the bars themselves are metric sizes from 10M to 55M.

This post explains how that choice is made, so you can read a beam or column schedule and understand why it says 3-20M instead of 6-15M. It does not replace a design: every reinforced concrete beam and column needs sizes from the structural drawings.

Start with steel area, not bar size

A bar size on its own means little. What the designer needs is a total area of steel, in square millimetres, at each critical section. The bar choice is just a way of delivering that area.

So if a beam needs about 900 mm² of bottom steel, 3-20M (900 mm²), 5-15M (1,000 mm²) and 2-25M (1,000 mm²) all deliver it. Which one ends up on the drawing depends on everything that follows. The full chart, including 45M and 55M, is in our Canadian rebar size guide.

How beam bar sizes are chosen

The minimum

CSA A23.3 does not let a flexural member be lightly reinforced to the point where it would crack and fail suddenly. The simple rule for beams (Clause 10.5.1.2 in the 2004 edition) sets a minimum tension steel area of:

As,min = 0.2 × √f'c × bt × h ÷ fy

where bt is the width of the tension zone, h is the overall depth, f'c is the concrete strength and fy is the bar yield strength, all in mm and MPa. Footings and slabs use a different minimum.

For a 300 mm wide by 500 mm deep rectangular beam in 30 MPa concrete with Grade 400 bar: 0.2 × 5.48 × 300 × 500 ÷ 400 = about 410 mm². Two 15M bars (400 mm²) would fall just short; three 15M or two 20M would satisfy it. The standard also allows the minimum to be waived where the beam's resistance is comfortably above the load, which is the engineer's check.

The maximum

At the other end, A23.3 limits how much tension steel a beam can carry so that the steel yields before the concrete crushes. It does this through the depth of the compression zone (the c/d ratio), and the result is that a beam cannot simply be packed with bigger bars to carry more load. When the steel needed approaches that limit, the designer deepens the beam, adds compression steel or changes the concrete strength.

Fitting the bars across the beam

This is where bar size really gets decided. The bars in a layer have to fit between the stirrups with:

• Concrete cover to the stirrups, set by the exposure class in CSA A23.1. For interior beams in exposure class N it is 30 mm; exterior and corrosive exposures need more. See our concrete cover guide.
• Clear space between bars of at least the largest of 1.4 bar diameters, 1.4 times the maximum aggregate size, or 30 mm, so concrete can flow between them.

Take the same 300 mm beam with 30 mm cover, 10M stirrups and 20 mm aggregate, so the 30 mm minimum gap governs for 20M bars. The width left inside the stirrups is 300 minus 2 × 30 minus 2 × 11.3, about 217 mm. Four 20M bars take 4 × 19.5 = 78 mm of bar plus three gaps of 30 mm, 168 mm in total, which fits. Five 20M bars need about 218 mm, which does not quite fit, so the designer would go to a second layer or a larger bar. Larger bars need wider gaps (1.4 × 25.2 mm is about 35 mm for 25M), so four 25M is the most that fits in this width.

Fewer, larger bars are easier to place and concrete flows around them better. More, smaller bars spread cracking more evenly and develop in shorter lengths. Designers balance the two.

Development and anchorage

A bar only works if it is long enough beyond the point where it is needed. The development length grows with bar diameter, which is a real limit on bar size near supports, at cut-off points and in short members. The CSA A23.3 simplified formula (Clause 12.2.3) is:

ld = 0.45 × k1 × k2 × k3 × k4 × (fy ÷ √f'c) × db, not less than 300 mm

For a straight uncoated Grade 400 bottom bar in 30 MPa concrete, that works out to roughly 300 mm for 10M, 430 mm for 15M and 830 mm for 25M. Top bars in deep beams get a 1.3 factor because of the weaker concrete under them. If a large bar cannot develop in the space available, the designer hooks it or switches to smaller bars. Our post on rebar development length covers the factors and a full chart.

How column bar sizes are chosen

Columns follow a different set of rules because they mainly carry compression. CSA A23.3 sets these limits (clause numbers from the 2004 edition):

• Minimum steel: longitudinal bars at least 1% of the gross column area (Clause 10.9.1).
• Maximum steel: not more than 8% of the gross area, including lap splice regions (Clause 10.9.2). The standard notes that going above about 4% outside lap regions makes placing bars and concrete difficult, because laps double the steel at the splice.
• Minimum number of bars: four inside rectangular or circular ties, three inside triangular ties, six inside spirals (Clause 10.9.3).
• Maximum gap: the clear distance between adjacent longitudinal bars in a column cannot exceed 500 mm (Clause 7.4.1.3).

Example: a 400 mm by 400 mm column has a gross area of 160,000 mm², so the minimum steel is 1,600 mm². That can be 8-15M (exactly 1,600 mm²) or 4-25M (2,000 mm²). Four 20M bars (1,200 mm²) would not meet the minimum even though they look substantial.

The Reinforcing Steel Institute of Canada's standard practice suggests column bars should generally be as large as practical, with vertical bars in square and rectangular columns shop offset-bent so they lap cleanly with the bars above. Fewer large bars also mean fewer ties to support each bar.

Tie sizes follow the bars

Column ties hold the vertical bars in place and stop them buckling outward. A23.3 sizes them from the bars they enclose (Clause 7.6.5.1):

Tie spacing cannot exceed the smallest of 16 longitudinal bar diameters, 48 tie diameters and the least column dimension, and every corner bar and alternate bar needs the support of a tie corner. Our post on stirrup and tie spacing walks through the spacing rules with examples. Seismic design in ductile frames tightens these rules considerably, which is why columns in multi-storey work often look very different from house columns.

Practical factors that also steer bar size

Beyond the code checks, a few practical points push designers toward one size or another:

• Laps and splices. Laps get much longer as bars get bigger, and CSA A23.3 does not allow lap splices for bars larger than 35M except in limited compression cases. 45M and 55M bars need mechanical or welded splices. See lap length and splice zones for beam and column rebar.
• Bends. Bigger bars need bigger bend diameters, so hooks and corner bars take more room in a tight beam-column joint.
• Stock lengths. RSIC lists standard mill lengths of 12 m for 10M and 18 m for 15M and larger, which affects where splices fall on long beams.
• Consistency. Using one or two sizes across a job reduces the chance of a bar being placed in the wrong member. Many engineers keep beam and column bar sizes uniform where they can.
• Joint congestion. Where beams frame into a column, the beam bars, column bars and ties all meet. RSIC recommends sizing beam and column widths so horizontal and vertical bars do not end up in the same plane.

From the schedule to the bar list

Once the drawings set the sizes, the detailer turns each beam and column schedule into a bar list: every main bar, stirrup, tie and dowel with its mark, length and shape. That is the step where a fit problem shows up, such as five 20M bars in a beam that only fits four, so it pays to have it done carefully before anything is cut.

We detail, cut, bend and tag beam and column reinforcement from your structural drawings, including stirrups and ties, in sizes from 10M to 55M. Send us your drawings for a free take-off and quote, or see our rebar products for straight and fabricated bar.