Mesh, spacers, chairs and tying
Tying wire for a mat of bars
- where
- bar crossings per m²
- bar spacing each way (mm)
- number of ties
- area of the mat (m²)
- share of crossings tied (%)
- total wire length (m)
- wire used per tie (mm)
- wire mass (kg)
- wire diameter (mm)
- density of steel (kg/m³)
- coils needed
- coil weight (kg)
Explain this equation
Estimates the number of ties and the weight of tying wire for a mat of bars, so enough coils go to site.
ni is the number of bar crossings in each square metre, set by the spacings sx and sy. f is the share of crossings you tie, and T the number of ties over area A. lt is the wire used in one tie, Lw the total length, dw the wire diameter, mw the wire weight, Kc the coil weight and Nc the coils needed.
Crossings per m² are 10⁶ divided by the area of one grid square. Wire weight is length times cross-sectional area πdw²/4 times the density of steel, 7850 kg/m³; the 10⁶ converts mm² to m². The ⌈ ⌉ brackets round up to whole ties and coils.
Sense check: 16 gauge (1.6 mm) wire weighs about 16 g per metre, so an 8 kg coil is about 500 m. Tying alternate crossings of a 200 mm grid over 100 m² is 1250 ties, about 4 kg at 200 mm a tie.
Watch out: cages, laps and edges are usually tied at every crossing.
Estimates the number of ties and the weight of tying wire for a mat of reinforcement, so you take enough coils to the job.
Enter your values to see the result.
Good to know
Bars at sx and sy centres cross 10⁶/(sx·sy) times per m². The share tied comes from the specification and BS 7973-2: laps and edges are usually tied at every crossing and the body of a mat at alternate crossings. Annealed 1.6 mm (16 gauge) wire weighs about 0.0158 kg/m, so an 8 kg coil is about 500 m. Pre-cut ties are commonly 6" or 8" (150–200 mm); measure what a double-wrap or figure-of-eight tie uses if you tie from the coil. Allow more for heavy bars and cages.
Common mistake
Tying too few crossings in mats that will be walked on during the pour, so bars shift and lose spacing and cover.
Limits
Regular mats. Cages, columns and walls with links tie at every link-to-bar crossing. Stainless or galvanised wire must match the reinforcement where required.
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
- BS Fixings, Black annealed tying wire 16 gauge (1.6 mm), 8 kg coil, about 510 m
- Heaton Manufacturing, Tying wire, wire ties, pullers and nips (annealed tying wire in 8 kg and 1.5 kg coils; 6" and 8" pre-cut ties; figure-of-eight and saddle ties)
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.