Chair Bar Calculator

How to work it out

1. Chair count N = ⌈slab area ÷ chair spacing²⌉. At 1 m spacing it is one per square metre; at 0.8 m about 1.56.
2. Chair height h = slab depth − top and bottom cover combined. It is not the slab depth, or the top layer will protrude through the surface.
3. Bar length L = top flat + 2 × h × √2 for a triangular chair, where h × √2 is the leg length.
4. Total length = N × L; weight = total length × 0.00617 × d².
5. The top flat runs 0.15-0.25 m, long enough to support the upper mesh — shorter and it slips off.

Where it goes wrong

• Setting chair height to the slab depth — deduct both covers (typically 15 + 15 = 30 mm), or the top layer ends up with zero cover and rusts from the outside in.
• Multiplying area by a fixed factor — 1 m and 1.2 m spacing differ by 30%, so compute ⌈area ÷ spacing²⌉.
• Using Φ6 chairs — too soft, the top layer collapses underfoot; use Φ8 at minimum, and Φ10-Φ12 above 200 mm slabs.

Material and cost

Chairs are 3-6% of slab steel — a small quantity that must not be omitted. Φ8 runs about 1.6 per metre: a 100 m² slab at 1 m spacing and 120 mm deep takes about 100 chairs of 0.5 m, roughly 80 in material.

On site

• Tie or tack-weld chairs to the upper mesh; simply resting them there lets them wander during placing.
• Above 200 mm, or where waterproofing matters, steel section stools are more stable than bent bar chairs.
• Chairs are temporary works steel and settled against actual usage; they are rarely drawn, so keep pre-cover records.

Basis

• GB 50204-2015 Code for acceptance of concrete structures, Chapter 5 (bar position and cover)
• GB 50666-2011 Code for construction of concrete structures, Section 5.4
• 16G101-1 Standard drawings for flat-slab detailing