Why not simply use the tie spacing from the previous project?

Because spacing is only valid together with the pour rate, the concrete temperature and the height of the lift it was computed for. Copy a layout across projects and one of those three inputs has almost always changed, usually in the direction that increases pressure.

What is a realistic allowable load for a D15 tie rod?

Manufacturers publish different figures depending on whether they quote the yield load or a permissible working load. Always compare on the same basis: if the datasheet gives a yield load, divide by the project safety factor before comparing it with the calculated load per tie.

Does the cone change the capacity of the assembly?

The rod is rarely the weakest element. The cone, the embedded anchor or the washer plate behind the waler often governs, and an assembly is only as strong as its weakest part, so the check must cover the whole load path rather than the rod alone.

Sizing D15 and D20 Tie Rods: Pressure to Spacing

How to turn concrete lateral pressure into tie spacing and rod selection - the arithmetic, the safety factor and the checks usually skipped.

In brief

Lateral pressure sets the load, the load sets the spacing.
The rod capacity comes from proof-tested values, not a table.
Compare against allowable load, not against a marketing figure.

Tie rod selection is often done backwards. A layout is inherited from the previous job, a rod size is chosen to match it, and the calculation is reconstructed afterwards to justify a decision already made. Reversing that order costs about ten minutes and removes most of the risk.

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.

Lateral pressure is the force generated by fluid concrete before it stiffens, typically estimated as the full hydrostatic value for the height of the lift. Tributary area refers to the area of formwork supported by a single tie rod, calculated as the horizontal spacing multiplied by the vertical spacing. Yield load means the maximum load a tie rod can withstand before it starts to deform permanently. Permissible working load is the safe load a tie rod can carry, usually the yield load divided by a safety factor. Safety factor is a multiplier applied to the yield load to determine the permissible working load, ensuring a margin of safety.

Step 1 - Establish the lateral pressure.

Lateral pressure is generated by the fluid concrete before it stiffens, and it never exceeds full hydrostatic pressure. A first, safe estimate is therefore the full hydrostatic value for the height of the lift: density multiplied by height. Taking 25 kN/m3 for normal reinforced concrete, a 3 m lift gives 75 kPa at the base. Real pressure is lower because concrete stiffens as it is placed, and lower still when the pour is slow or the concrete is warm, but the hydrostatic figure is the conservative ceiling and it is the right number to start from.

Standard basis: EN 13670 — Execution of concrete structures

Step 2 - Load carried by one tie.

Each tie supports the pressure acting on the tributary area it serves, which for a regular grid is simply the horizontal spacing multiplied by the vertical spacing. Lay that against pressure and the load per tie appears directly.

Pressure at base	Tie spacing 0.8 x 0.8 m	Spacing 1.0 x 1.0 m	Spacing 1.2 x 1.2 m
50 kPa	32 kN	50 kN	72 kN
75 kPa	48 kN	75 kN	108 kN
100 kPa	64 kN	100 kN	144 kN

Step 3 - Compare with the allowable load of the rod.

The calculated load must be compared with the permissible working load of the assembly, and this is where most procurement errors occur. Datasheets are quoted in different terms: some publish a yield load, others a permissible load already divided by a factor. If the figure is a yield load, divide it by the project safety factor before comparing. As an indication, a D15 system is commonly rated around 150 kN and a D20 around 230 kN in yield terms, which is why D20 is chosen for high lifts and heavy pour rates rather than for convenience.

Wall formwork with walers and tie rods before pouring

Standard basis: EN 14399-1 — High-strength structural bolting assemblies

Step 4 - The checks that are easily missed.

The cone, the embedded anchor or the washer plate, not the rod, is frequently the governing element.
Spacing is measured between ties as installed; a misplaced tie enlarges the tributary area of its neighbours.
At the edge and at corners the tributary area is not halved by symmetry, it is truncated, so pressure finds fewer ties than the grid suggests.
Ties in the lower zone carry more pressure per tie when spacing is constant, so a uniform grid is never uniformly loaded.

What to state in the RFQ.

Give the supplier the four inputs, not just a part number: height of the lift, pour rate, concrete temperature at placement, and the intended tie spacing. With those, capacity can be confirmed against the actual duty rather than against a catalogue page, and any mismatch between the rod and the rest of the assembly is caught before the pour rather than during it.

When you are matching a specification, the D15 / D20 high-strength tie rods page lists thread forms, grades and surface treatments alongside these figures.

References and standards.

About us: This article covers the calculation and selection of D15 and D20 tie rods based on concrete lateral pressure, which our team checked against our production and test data. Our contact page provides access to the engineering team for detailed inquiries, and the about page documents our factory processes and capabilities. This article was reviewed by the Qianheng Formwork Engineering team.

Sizing D15 and D20 Tie Rods: Pressure to Spacing