EC2 Prestressed Spun Pile Check

What it checks

A prestressed high-strength concrete (PHC) spun pile is a hollow circular section, centrifugally cast and pretensioned. This tool computes the section properties of the annulus, works through the prestress chain from jacking to the effective long-term force, builds the axial-moment interaction diagram, and checks slenderness and second-order effects.

A spun pile is not an ordinary reinforced concrete column with a hole in it. The prestress changes the cracking behaviour fundamentally, the concrete is typically C60 to C80 rather than C30, and the section is hollow - all three change the answer, which is why a general RC column check is not a substitute.

The prestress chain is where the capacity is decided

The force in the strands at the moment of jacking is not the force acting in service. It reduces through elastic shortening when the strands are released onto the concrete, then through relaxation of the steel, and then through creep and shrinkage of the concrete over time. The effective prestress after all losses is typically 20 to 25 per cent below the jacking force.

That matters because prestress is what keeps the section uncracked under moment. Overestimating the effective force overestimates the cracking moment and therefore the stiffness that governs deflection and driving behaviour. The tool works through each loss explicitly rather than applying a single lump-sum percentage, so you can see which loss dominates for your pile.

What is different from a solid RC column

The hollow section is efficient in bending because the material is at the perimeter where it earns the most second moment of area, but it has less concrete area for axial load and thin walls that limit how much moment can be developed before the compression face is critical.

The prestress also means the section starts in compression, so it stays uncracked to a much higher moment than an equivalent RC section - which is exactly what a pile needs, since a cracked pile in aggressive ground has a durability problem as well as a stiffness one. But the same prestress reduces the available compression capacity for axial load, so the interaction diagram is shifted rather than simply scaled.

Formula reference

Annulus area

$A = \dfrac{\pi}{4}\left(D^{2} - d^{2}\right)$

Annulus second moment of area

$I = \dfrac{\pi}{64}\left(D^{4} - d^{4}\right)$

Initial prestress

$P_i = A_p\,\sigma_{p,i}$

Elastic shortening loss

$\Delta\sigma_{el} = \alpha_e\,\sigma_{cp}$

Modular ratio

$\alpha_e = \dfrac{E_p}{E_{cm}}$

Concrete stress from prestress

$\sigma_{cp} = \dfrac{P}{A}$

Cracking moment

$M_{cr} = \left(f_{ctm} + \sigma_{cp}\right)W$

Slenderness

$\lambda = \dfrac{l_0}{i},\quad i = \sqrt{I/A}$

Assumptions and limits

This is a structural section check. It says nothing about geotechnical capacity - shaft friction, end bearing, negative skin friction, group effects and settlement are a separate assessment, and for most piles the ground governs the working load rather than the section.

Driving stresses are not analysed. A pile can be perfectly adequate in service and still be damaged during installation: driving generates both compressive and tensile stress waves, and tensile driving stress in a long pile through soft ground is a common cause of cracking. A driveability analysis is a separate exercise.

Prestress losses depend on the concrete age at transfer, the curing regime and the ambient conditions, all of which are estimates rather than certainties. Splices between pile segments, the pile shoe and the connection to the cap are detailing matters this tool does not cover.

FAQ