D15 or D20? Choosing Tie Rod Diameter Without Oversizing
> In brief > - Working load comes first; diameter follows from it. > - Spacing is a consequence of the load, not a starting point. > - A smaller rod at a tighter spacing is not always the cheaper answer.
D15 and D20 dominate formwork practice, and the choice is usually made from habit rather than calculation. Oversizing wastes money and can create its own problems at the panel.
Our own production and test data are available on request — see how our factory works and contact the engineering team for the documents behind any figure quoted here.
D15 is a tie rod with a stress area of about 176 mm2, suitable for walls up to 3.5-4 m with normal pour rates. D20 refers to a tie rod with a stress area of about 314 mm2, used for high-pressure zones and thick walls. Tributary area means the area of the wall that each tie rod supports, determined by the panel layout. Cone and sleeve assembly is the hardware that fits into the wall to secure the tie rod, with size constraints based on wall thickness and cover. Thread damage means any nicks or wear on the rod's threads, which can reduce its effective section and load capacity.
Capacity difference in practice.
The stress area of a D15 rod is about 176 mm², and an Rd20 about 314 mm² — roughly 1.8 times the section. At the same class, the allowable tension scales with that ratio: if a D15 class 8.8 rod is certified in the 90-150 kN band, an Rd20 in the same class lands around 230-260 kN.
The useful rule: one D20 replaces roughly two D15 ties in capacity, so switching diameter changes the number of tie positions, not just the size of each one.
Standard basis: ISO 898-1
When D15 is the right answer.
Walls up to about 3.5-4 m with normal pour rates, where tie spacing of 0.6-0.8 m is practical. Panels with existing tie holes sized for D15. Projects where the wall thickness limits the cone and sleeve envelope. Reusable systems where the rod cost per cycle matters more than peak capacity.
When D20 earns its cost.
Thick walls where the pressure at the bottom zone is high. Fast pours or cold concrete, both of which raise lateral pressure. Single-sided formwork, where tie spacing is often dictated by the existing wall and cannot be tightened. Heavy slab and bridge formwork where fewer, stronger ties reduce congestion.
The factors people forget.
WALL THICKNESS AND COVER: the cone and sleeve assembly for a larger diameter occupies more of the wall section. On a 200 mm wall with 40 mm cover each side, the remaining envelope may not accept a D20 cone.
PANEL COMPATIBILITY: tie holes, washers and clamps are diameter-specific. Changing diameter mid-project can mean changing hardware across the fleet.
HANDLING: a longer, thicker rod is heavier, and site crews notice. Above about 4 m, D20 rods are a two-person lift and the productivity loss can outweigh the material saving.
REUSE AND DAMAGE: thicker rods are more tolerant of nicks and handling damage; a D15 rod with 0.5 mm of thread damage has lost a larger proportion of its section.
Standard basis: EN 13670 — Execution of concrete structures
A workable decision sequence.
- Take the design pressure at the base of the pour and the tributary area you can physically achieve with the panel layout.
- Calculate the required tension per tie, apply the code safety factor, and compare with certified capacities for both diameters.
- Check the wall thickness and cover against the cone envelope for the larger option.
- Confirm hardware compatibility across the fleet.
- Compare total cost per square metre of wall formed, including rods, cones, nuts and reuse cycles — not price per rod.
Mixed systems.
Using D15 in the upper zones and D20 in the lower zone of a tall wall is common and legitimate, because pressure falls with height. It requires clear marking on the layout drawing, or a crew will fit whichever rod is nearest.
| Diameter | Allowable Tension (kN) | Impact on Tie Positions and Cost |
|---|---|---|
| D15 | 90-150 kN | Fewer ties, lower rod cost, more positions |
| D20 | 230-260 kN | Fewer tie positions, higher rod cost, stronger ties |
| D15 | Suitable for walls up to 4 m | Practical for normal pour rates, tight spacing |
| D20 | For thick walls, high pressure | Better for fast pours, heavy formwork, fewer ties |
| D15 | Reuse and damage tolerance | More sensitive to thread damage, lighter handling |
| D20 | Reuse and damage tolerance | More tolerant of nicks, heavier handling |
References and standards.
- EN 14399-1 — High-strength structural bolting assemblies
- ISO 16047 — Hexagon head bolts and hex nuts
- High-strength bolting guidance
- EN 1090 — Execution of steel structures
- EN 13670 — Execution of concrete structures
- ACI 347 — Guide to formwork for concrete
About us: This article discusses the factors in choosing between D15 and D20 tie rod diameters. Our production and test data support the technical details. Our contact page provides access to the engineering team for further documentation, and the about page details our manufacturing capabilities. This article was reviewed by the Qianheng Formwork Engineering team.