Mesh, spacers, chairs and tying
Chair height for a top mat
- where
- chair height standing on the formwork or blinding (mm)
- slab or base thickness (mm)
- nominal cover to the top bars (mm)
- top mat bar diameters (mm)
- chair height standing on the bottom mat (mm)
- nominal cover to the bottom bars (mm)
- bottom mat bar diameters (mm)
Explain this equation
Works out the height of chair needed to hold the top mat of a slab or base at the right cover, either standing on the formwork or blinding or standing on the bottom mat.
h is the overall thickness. ct and cb are the nominal covers to the top and bottom bars. dt1 and dt2 are the two bars in the top mat and db1 and db2 the two bars in the bottom mat. Hs and Hb are the chair heights for the two ways of standing the chair.
The chair has to reach the underside of the lower bar in the top mat. Working down from the top face, that point is the top cover plus the two top bars below the surface. If the chair stands on the bottom mat, the bottom cover and both bottom bars also come off.
Sense check: a 250 mm slab with 40 mm covers and 12 mm bars throughout needs a 186 mm chair on blinding or 122 mm on the bottom mat.
Watch out: pick the nearest standard chair at or just under the answer. A taller chair loses top cover.
Works out the height of chair needed to hold the top mat of a slab or base at the right cover.
Enter your values to see the result.
Good to know
The chair supports the underside of the lower bar in the top mat. From the soffit that is the thickness less the top cover and the two top-mat bar diameters. If the chair stands on the bottom mat, take off the bottom cover and the two bottom bars as well. Order the nearest standard chair at or just below the figure, never above it, or the top cover is lost.
Common mistake
Forgetting that each mat is two bars thick, so the chairs are one bar diameter too tall and the top steel finishes in the screed zone.
Limits
Flat slabs and bases with two-layer mats. Where links, laps or crossing beams change the build-up, check those points separately. Continuous chairs should be tied and supported as BS 7973-2 describes.
Sources
- BS 7973-2:2001 Spacers and chairs for steel reinforcement and their specification. Fixing and application of spacers and chairs and tying of reinforcement
- IStructE / The Concrete Centre, Standard method of detailing structural concrete (4th edition)
- BS EN 1992-1-1:2004 Eurocode 2 (Table 3.1 fctm, fctk,0.05; 3.1.6 fctd; 4.4.1 cover; 8.4 anchorage, Eq. 8.2–8.6; 8.7 laps, Eq. 8.10–8.11)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Always work to the engineer’s drawings and bending schedules, BS 8666, BS EN 1992, BS EN 1090-2, BS 5531/the BCSA National Structural Steelwork Specification and manufacturers’ data. Lifting must be planned under LOLER by a competent person.