Chair Bars in Concrete: Purpose, Spacing, Quantity, and Cutting Length

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Chair bars are supports used to hold reinforcement in its specified position while concrete is placed. In slabs, rafts, and footings, they commonly support an upper reinforcement layer above a lower layer. Their size, shape, and spacing depend on the reinforcement arrangement and project requirements.

There is no single chair-bar diameter or spacing that applies to every slab or footing. Follow the structural drawings and project specifications. If these do not show the required support arrangement, ask the responsible engineer to confirm it before fixing the reinforcement.

What Is a Chair Bar?

A chair bar is a bent reinforcing bar or other suitable bar support that holds reinforcement at a required elevation. It may support an upper reinforcement layer directly or support a bar that carries the upper layer.

Bar supports help maintain the position of reinforcement and the specified concrete cover. CRSI describes supports as devices used to hold and space reinforcement; common types include concrete blocks, plastic chairs, and wire supports.

Chair bars are not automatically part of the structural reinforcement design. Their role is generally to support and position the reinforcement during construction, unless the project design specifically assigns them another structural function.

Where Are Chair Bars Used?

Chair bars or other reinforcement supports may be used in:

• Slabs with more than one reinforcement layer
• Raft and mat foundations
• Footings with top and bottom reinforcement
• Beam or slab reinforcement assemblies where the drawings specify supports

For a single reinforcement layer, another type of bar support may be more suitable. Select the support to suit the surface, reinforcement arrangement, required cover, and project specification.

How to Determine Chair-Bar Height

Determine chair height from the reinforcement section detail, rather than using a general formula based only on slab thickness and cover.

First establish the required elevations of the supported bars and the point where the chair bears on the lower reinforcement or support bar. Then dimension the chair between those contact points, allowing for the bar diameters and layer order shown in the detail.

The required height can change with:

• Slab or footing thickness
• Top and bottom concrete cover
• Bar diameters and layer order
• Whether the chair bears on a bar, a support bar, or another support
• The shape and bend dimensions of the chair

A generic subtraction such as “member depth minus two covers and two bar diameters” can give the wrong height if the bar layers or bearing points differ. The final chair should keep the reinforcement at the specified elevation and cover.

How to Determine Chair-Bar Spacing

Use the spacing shown on the structural or placing drawings. If no spacing is given, do not treat a common site rule of thumb as a design requirement. Ask the engineer or follow the project’s approved specification and construction method.

The required arrangement depends on the support type, reinforcement weight and layout, and the need to keep the bars stable during concrete placement. Supports should be numerous and strong enough for the reinforcement they carry and should remain stable during the planned work. CRSI’s RB4.1 standard specifically addresses the design, use, and material requirements of reinforcement supports.

Do not assume that chair spacing can be selected from bar diameter alone. Also, do not use chairs as walkways or as supports for concentrated construction loads unless the arrangement has been specifically designed for that purpose.

How to Calculate the Number of Individual Chairs

For a regular grid of individual chairs, first decide the maximum spacing and edge setback from the approved project arrangement. Then count the chair positions in each direction.

For a rectangular area:

• Number along length = number of chair positions along the length
• Number along width = number of chair positions along the width
• Total individual chairs = number along length × number along width

For equal spacing with a defined setback at both ends, the positions in one direction can be estimated as:

Number of positions = round up[(dimension − 2 × edge setback) ÷ maximum spacing] + 1

This is a layout-counting method, not a recommendation for chair spacing.

Example: Individual Chair Grid

Assume, for quantity-estimating purposes only:

• Slab dimensions: 10 m × 10 m
• Maximum grid spacing specified for this example: 1 m
• Edge setback: 0.5 m

The available distance between the first and last chair positions in each direction is:

10 m − 2 × 0.5 m = 9 m

At 1 m maximum spacing, this gives 10 positions in each direction. The estimated total is:

10 × 10 = 100 individual chairs

This example only shows how to count a stated layout. It does not establish 1 m spacing or a 0.5 m edge setback as suitable for other projects.

If the design uses continuous chair bars or support lines, calculate the number and length of those rows from the approved layout instead of treating them as a grid of individual chairs.

How to Estimate Chair-Bar Cutting Length

Prepare a sketch of the chair’s actual shape and mark its dimensions. For a simple inverted-U chair with two feet, the preliminary length along the bar’s centreline is based on:

Chair length ≈ 2 × vertical legs + top segment + 2 × feet

For example, suppose the dimensions along the centreline are:

• Two vertical legs: 0.30 m each
• Top segment: 0.30 m
• Two feet: 0.45 m each

The preliminary straight-segment total is:

2 × 0.30 + 0.30 + 2 × 0.45 = 1.80 m per chair

For 100 chairs, the preliminary total is 100 × 1.80 m = 180 m.

This is only a quantity estimate. The final cutting length must account for the specified bend shape, bend diameters, and the bar-bending schedule or fabrication rules used on the project. Do not use the example dimensions as a standard chair shape.

How to Select the Chair Type and Material

Choose a support that suits the required elevation, reinforcement arrangement, supporting surface, and project specification. Depending on the application, this may be a manufactured wire or plastic chair, a precast concrete support, or a specifically detailed bent-bar chair.

Do not assume that a particular bar diameter—such as 12 mm—or a range such as 16–25 mm for footings is universally required. The existing article does not establish those sizes as general code requirements. Where bent reinforcement bars are proposed, the project documents or responsible engineer should confirm the bar size, shape, and use.

Also confirm that the support will not reduce the required concrete cover or cause a conflict with other reinforcement.

Site Checks Before and During Concreting

Before concrete placement:

• Check that the top reinforcement is at the specified elevation.
• Check that the chair arrangement matches the approved drawings or instructions.
• Tie or secure supports as needed so they do not shift during the work.
• Confirm that required cover is maintained at faces and edges.
• Check that bars and supports do not obstruct concrete placement or consolidation.

Monitor the reinforcement during placement. If supports move, deform, or become unstable, stop and correct the arrangement before the reinforcement is covered.

Common Mistakes to Avoid

• Applying one chair spacing to every slab or footing
• Choosing chair-bar diameter from an unsupported rule of thumb
• Calculating height without checking the actual reinforcement section
• Treating a quantity estimate as a structural capacity check
• Using an unexplained Euler buckling calculation as the complete design method for a bent chair
• Assuming ACI 318 strength-reduction factors automatically apply to temporary bar supports
• Using unspecified scrap bar without confirming it is acceptable for the project

Frequently Asked Questions