Chair Bar Design for Foundations and Slabs: Load Path, Checks and Worked Examples

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Chair bar design is the check that decides the diameter, head length and spacing of the supports that hold the top reinforcement mat in foundations and slabs. This guide shows how to check chair bars for construction-stage loads, head bending, leg buckling and top-bar sag, and how to select a chair bar diameter and spacing, with worked examples for a footing and a slab.

It picks up where our guide to chair bars in concrete stops. That guide explains what chairs are, how to find their height, how to count them and how to estimate cutting length, and it deliberately stops short of giving a diameter or spacing. This article shows how an engineer can arrive at those, and how to write them on the drawings.

Scope note: this is a design-practice method, not a code procedure. IS 456:2000 deals with the assembly and placing of reinforcement and its supports [VERIFY: check the current text of Clause 12, including any limit on support spacing and the placing tolerances], but it does not give a strength-design method for site-fabricated chairs. The loads and factors below are stated assumptions that you should adjust for your project.

Why does chair bar design need a check?

A chair is a temporary support. It does not carry load in the finished member, and once the concrete sets its job is done. During construction, however, it carries the weight of the top mat, the people walking on it, and the disturbance of concrete placement. If it fails, the top steel ends up in the wrong place, which means lost lever arm, reduced cover and a weaker member. For a general introduction to placing and supporting bars, see the CRSI guide to placing bars.

A chair can fail in five ways:

• Head bending: the horizontal part of the chair bends or sags under the top bars.
• Tilting or collapse: the chair falls over when someone steps on the mat or concrete pushes against it.
• Leg buckling: a slender vertical leg buckles under axial load.
• Top-bar sag: the top bars deflect between chairs and lose cover.
• Foot problems: the foot slips off the bottom bars or punches into the blinding.

For the usual bent-bar chair loaded by foot traffic, head bending and stability tend to govern. Leg crushing rarely does. That is why a chair can look generous on a simple buckling calculation and still sag in practice.

Construction-stage loads

1. Self-weight of the top mat

The unit weight of a bar in kg/m is d²/162, with d in mm. For a top mat with bars at 150 mm c/c in both directions, there are 6.67 bars per metre each way, or 13.33 m of bar per m². The table below gives the resulting weight. See our guide to the unit weight of steel bars if you need the derivation.

2. Construction live load

No code gives a construction live load for a reinforcement support, so treat this as a project assumption and write it down. Three levels are useful for comparison:

• 0.5 kN/m²: the mat is not walked on, and separate walkway boards carry the traffic.
• 1.0 kN/m²: occasional foot traffic.
• 1.5 kN/m²: the mat is used for access during fixing and concreting.

Our guide on chair bars warns against using chairs as walkways unless the arrangement has been designed for that purpose, and industry guidance on bar supports (see this ASPIRE article on ANSI/CRSI RB4.1) likewise cautions against overloading supports or using them to carry heavy equipment. The 1.5 kN/m² case is that design. If you do not want to design for foot traffic, specify walkway boards and use the 0.5 kN/m² case.

3. Load factor and placement effects

This article applies a factor of 1.5 to the combined weight and construction load. That is a judgement for a temporary stage, not a code value, so state it in your assumptions.

A uniform load does not capture concentrated effects such as concrete heaped near the discharge point, a pump hose dragged across the mat, or several workers standing together. Handle these with extra chairs in those zones or with protected walkways, not by inflating the uniform load.

Structural model

The checks below use deliberately simple models. They ignore two-way action of the mat and any fixity at the bends of the chair, both of which would reduce the demand, so the results lean conservative.

1. Load per chair: P = 1.5 × (w_s + q) × s_x × s_y, where w_s is the top-mat weight, q the construction load and s_x, s_y the chair spacings.
2. Load per leg: an equal split is only an assumption, so take the worse leg as 60% of P.
3. Head: a simply supported bar between the legs, loaded by two equal point loads of P/2 at its third points (two top bars bearing on it).
4. Top bar: a simply supported bar spanning one chair spacing, carrying the load on its own width.

Chair-level checks

Check 1: Head bending (usually governs)

For a head of span L_h with two third-point loads, the maximum moment is M = P × L_h / 6. The elastic section modulus of a round bar is Z = π d³ / 32. The limit used here is first yield, because yielding means permanent sag and lost cover:

Ratio = M / (f_y × Z) ≤ 1.0

Use f_y = 500 MPa for Fe 500 bar and no material factor, since first yield is already a serviceability-type limit.

Check 2: Leg buckling (screening only)

For a leg of area A, radius of gyration r = d/4 and effective length L_e = K × L, the slenderness is λ = L_e / r. The ideal elastic buckling stress is σ_E = π²E / λ² with E = 200,000 MPa, giving P_E = σ_E × A.

Use K = 1 if the chair is held against sway (tied to bars), and K = 2 if the top of the leg can sway. Treat P_E as an upper bound, not a capacity, because a bent bar has initial crookedness and eccentric load. As a practice margin, aim for leg demand below about one third of P_E at K = 2.

Check 3: Top-bar sag between chairs

Two things matter here. First, the sag under dead weight alone is permanent, so it must be a small fraction of the placement tolerance [VERIFY: IS 456 placing tolerances]. Second, the bar must stay elastic under the construction load so that the sag recovers. The simple-span formulas are δ = 5wL⁴ / (384 E I) and σ = M / Z.

Check 4: Stability and bearing (detailing)

• Tie each chair to the bottom bars so it cannot walk or tilt.
• Give each foot enough length to bear on at least two bottom bars, and make sure the chair does not stand on bare soil or polythene.
• Stagger the chairs so that they do not line up and form a plane of weakness.

The height of the chair comes from the reinforcement section detail, as explained in our chair bars guide. The cover requirements fix the levels the chair must hold.

Selecting chair bar diameter and spacing

The head-bending check gives the minimum chair-bar diameter directly: d = (32 M / (π f_y))^1/3. The table below applies it to chairs at 1 m × 1 m with a 300 mm head, for a mat that is walked on (q = 1.5 kN/m²) and for a mat protected by walkway boards (q = 0.5 kN/m²).

The table shows an important pattern: the construction load dominates and the weight of the top steel is secondary. Going from q = 0.5 to q = 1.5 moves the answer from 10–12 mm to 16 mm, while changing the top mat from 10 mm to 25 mm barely changes it. The construction-load assumption therefore matters much more than the top-bar diameter, and it is the first thing to settle with the contractor.

Even where the head check allows 10 mm, treat 12 mm as the practical minimum. It is a practice judgement, based on handling and stability on site, not a calculated limit.

Tall chairs in thick foundations. Leg slenderness becomes the other driver. Take an 800 mm raft with 50 mm cover and 25 mm bars in both mats. The chair is about 600 mm tall (800 − 100 − 4 × 25), and at 1 m × 1 m with q = 1.5 kN/m² the load per chair is P = 3.01 kN, so the worse leg carries 1.81 kN. A 16 mm chair passes the head check (ratio 0.75), but its legs are slender: with K = 2, λ = 300 and P_E = 4.4 kN, a leg ratio of 0.41, above the one-third practice margin. A 20 mm chair brings the head ratio to 0.38 and the leg ratio to 0.17, so 20 mm is the sensible choice. That choice rests on the slenderness judgement, not on a code limit.

Worked example 1 (foundation): 3 m × 3 m footing, 450 mm deep

Given: 50 mm cover top and bottom, 12 mm bars at 150 c/c both ways in both mats, chair height 300 mm (found from the section detail), chairs at 1 m × 1 m, head length 300 mm, Fe 500 chair bar. Construction load 1.5 kN/m², factor 1.5, worse leg 60% of P.

Result: the legs and the top-bar sag are fine, but a 12 mm chair at 1 m × 1 m fails the head check under a 1.5 kN/m² construction load. There are three ways to fix it:

1. Use a 16 mm chair at 1 m × 1 m (ratio 0.60). For this footing that is 9 chairs, using the layout-counting method in our chair bars guide with a 0.5 m edge setback.
2. Keep 12 mm and reduce the spacing to 0.8 m × 0.8 m. The load per chair drops to about 1.55 kN and the head ratio to about 0.91. That needs 16 chairs.
3. Keep 12 mm at 1 m and stop foot traffic on the mat by providing walkway boards, which brings the head ratio down to 0.55 (see the sensitivity table below).

With an assumed chair length of 1.8 m, options 1 and 2 use about the same quantity of steel, so choose on practicality. Fewer, stiffer chairs are usually easier to fix and to keep stable.

Sensitivity of the head check to the construction load (1 m × 1 m, 300 mm head, 12 mm top mat):

The same 12 mm chair goes from comfortable to failing as the assumed use of the mat changes. This is why the load assumption must be stated, and why the drawings should say whether the mat may be walked on.

Worked example 2 (slab): 200 mm slab with top and bottom mats

Given: a 200 mm slab with 25 mm cover top and bottom, a bottom mat of 12 mm bars at 150 c/c both ways, and a top mat of 10 mm bars at 150 c/c both ways. The chair height from the section detail is 200 − 2 × 25 − 2 × 12 − 2 × 10 = 106 mm, so about 105 mm. Chairs at 1 m × 1 m, head length 300 mm, Fe 500 chair bar. Walkway boards are provided, so the mat is not walked on and the construction load is taken as 0.5 kN/m². Factor 1.5, worse leg 60% of P.

Result: a 12 mm chair at 1 m × 1 m, about 105 mm high, passes the head check with a ratio of 0.51. A 10 mm chair also passes on paper (0.89), but 12 mm is preferred for margin and stability. The legs are short and stocky, so buckling is irrelevant. What differs from the foundation example is what controls the layout:

• The top bars, not the chair, limit the spacing. The 10 mm top bars are light and flexible. At q = 1.5 kN/m² the single-bar stress at 1 m span is about 302 MPa, and it reaches yield at a span of about 1.29 m. So 1 m is about the upper limit for this mat, however strong the chairs are. Also check the 1.6 mm dead-load sag against the placing tolerance for a 200 mm slab [VERIFY: IS 456 placing tolerances] and against any limit on support spacing in the standard.
• Tolerances are tighter in a thin slab. A few millimetres of sag matters more at 200 mm depth than at 450 mm.
• The walkway boards must not bear on the top mat. Support them on their own trestles or on the bottom mat or formwork. If they rest on the top mat, the construction load goes through the chairs anyway.

If the mat is walked on (q = 1.5 kN/m²): P rises to 2.37 kN and the 12 mm head ratio rises to 1.40, which fails. Use a 16 mm chair at 1 m (ratio 0.59), or keep 12 mm at 0.75 m × 0.75 m (ratio 0.79). On a 10 m × 10 m slab with a 0.5 m edge setback, using the counting method in our chair bars guide, 1 m spacing needs 100 chairs and 0.75 m spacing needs 169.

Recommendations for the drawings

• Write the chair on the drawing: bar diameter, head length, foot length, height and spacing. “Chairs as required” leaves the design to the fixer.
• State the construction-load assumption and whether the mat may be walked on. If it may not, specify walkway boards.
• Do not default to 12 mm. Check the head under the assumed load. For a mat that will be walked on, 16 mm is a better starting point than 12 mm. For a mat protected by walkway boards, 12 mm at 1 m is usually enough.
• In slabs, check the top bars as well as the chair. Light top bars can limit the spacing to about 1 m regardless of the chair size.
• Add chairs where load concentrates: near pump lines, discharge points and walkways.
• Tie and stagger the chairs. An untied chair that tilts is a failed chair.
• Consider proprietary supports where load data exists. Reinforcement supports have their own standard, CRSI RB4.1, Supports for Reinforcement Used in Concrete, and a manufacturer’s rated chair avoids a custom design. Some owners already require supports with sufficient load ratings; the Iowa DOT specification for reinforcing steel supports is one example.
• Get a qualified engineer to review any chair outside the assumptions below, such as very tall chairs, very heavy mats or unusual loads.

Assumptions and limitations

• The construction live load, the factor of 1.5 and the 60% worse-leg share are assumptions, not code values.
• The head is modelled as simply supported with two third-point loads. The real chair has some fixity at its bends, so this is conservative, but real load paths through a two-way mat can concentrate load on one chair.
• Euler buckling is a screening upper bound only. It ignores crookedness, eccentricity and the effect of the bends.
• First yield is used as the head limit. A chair that has yielded may still carry load, but it will have sagged and the top steel will have lost cover.
• The examples assume Fe 500 chair bar with f_y = 500 MPa. Using mild steel or scrap of unknown grade invalidates the numbers.
• The results are for chairs on a bottom mat. Chairs bearing on blinding or soil need an additional bearing check.

Frequently asked questions