Load per tie = design pressure × horizontal spacing × vertical spacing. At 38.4 kPa with ties at 600 × 600 mm, each tie carries 13.8 kN. A tie rated 50 kN safe working load is then at 28%, and could go up to about 1,140 mm square spacing before it is overloaded.
Ties = ⌈L ÷ spacing H⌉ × ⌈H ÷ spacing V⌉
Load per tie = p × spacing H × spacing V
Utilisation = load per tie ÷ safe working load
Max spacing = √(SWL ÷ p) (square grid)
Tie length = wall + 2 × form + 2 × projection
Worked Example
A 12 m × 3 m wall, 250 mm thick, poured at 1.5 m/h at 20 °C, ties at 600 × 600 mm rated 50 kN:
- Design pressure (ACI 347) = 38.4 kPa
- Ties = 20 × 5 = 100
- Load per tie = 38.4 × 0.6 × 0.6 = 13.8 kN → 28% of 50 kN
- Max square spacing = √(50 ÷ 38.4) = 1,142 mm
- Tie length = 250 + 2 × 18 + 2 × 150 = 586 mm
Maximum Square Tie Spacing (mm)
| Pressure |
SWL 30 kN |
SWL 50 kN |
SWL 90 kN |
| 30 kPa |
1,000 |
1,291 |
1,732 |
| 40 kPa |
866 |
1,118 |
1,500 |
| 50 kPa |
775 |
1,000 |
1,342 |
| 70 kPa |
655 |
845 |
1,134 |
This limit is for the tie only. Walers, studs and plywood often govern at a closer spacing.
Frequently Asked Questions
How do I calculate the load on a form tie?
Multiply the design lateral pressure by the area each tie supports. At 40 kPa and 600 × 600 mm spacing, that is 40 × 0.36 = 14.4 kN per tie.
How far apart should form ties be?
Far enough apart that the load per tie stays below the tie's safe working load, and close enough that the walers and plywood are not overstressed. The maximum square spacing for the tie alone is √(SWL ÷ pressure).
Where do I find a tie's safe working load?
In the tie manufacturer's data. It already includes a safety factor on the ultimate strength. Never use the breaking load.
References
- ACI 347R-14 §4.2 lateral pressure; tie safe working loads per manufacturer (ACI 347 recommends a factor of safety of 2.0 on ultimate for standard ties)
