Rigid Airfield Pavement Edge Stress Screening Calculator

Frequently asked questions

Can this calculator be used for actual FAA airfield rigid pavement design?

No. Full FAA airfield rigid pavement design for dual or dual-tandem gear configurations requires FAARFIELD (layered-elastic/3D-FEM analysis with cumulative fatigue damage per AC 150/5320-6E/F/G) — multi-wheel superposition and coverage/fatigue life have no closed-form equation. This calculator computes only the classical Westergaard edge stress for a single wheel, as a rough preliminary screening indicator.

What do the radius of relative stiffness (l) and equivalent radius of resisting section (b) represent?

The radius of relative stiffness (l) reflects how the slab's own stiffness, thickness, and the subgrade support work together to spread a load. The equivalent radius of resisting section (b) adjusts the tire's actual contact radius for the slab thickness when the contact radius is small relative to the slab (a < 1.724h), per Westergaard's edge-loading formula.

Why is the confidence rating always low?

The confidence is fixed at low because this is explicitly a single-wheel screening formula only — it cannot capture real dual/dual-tandem gear load superposition or fatigue life from repeated aircraft passes, both of which materially affect actual airfield pavement performance and require FAARFIELD analysis.

Does this account for load transfer at the joint, the size of the slab panel, or temperature curling?

None of the three. There is no field here for dowels, joint spacing, panel dimensions or a temperature gradient, so what you get is the load-only stress on a free edge — one slab carrying the whole wheel with nothing across the joint helping it. Each omission runs a different way. A doweled joint or a tied shoulder moves part of the wheel into the neighbouring slab, so at a protected edge the true stress sits below this figure and the number is on the safe side. The classical edge case also assumes a slab that is large compared with the radius of relative stiffness, and the breakdown prints that radius so you can check it: at the default inputs it reads 1,041 mm (41 in), and a panel that is not several times that is outside the assumption the formula was derived under. Temperature is the one that can run against you — a gradient through the slab bends it with no aircraft on it at all, and depending on which way the gradient runs that stress can add to this one rather than cancel it. Treat the output as one component of the edge stress, not the total, and have the combination modelled properly before anything is built to it.

How do I set the subgrade modulus k and the tire contact radius correctly?

k is the one field on the page carrying a unit that the metric/imperial toggle does not touch. Wheel load switches between kN and kips, slab thickness and contact radius between mm and inches, the concrete modulus and the answer itself between MPa and psi — but k stays in MPa/mm in both systems, because the site has no stress-per-length dimension to convert it with. Plate bearing results are often quoted in pci, and the 0.054 MPa/mm default is about 199 pci, so divide a pci figure by roughly 3,684 before it goes in the box. The field's range, 0.02 to 0.15, spans about 74 to 553 pci, so any three-digit figure you are about to type is a pci number heading into a MPa/mm box. The page catches the typing but not the consequence: type 200, leave the field, and it clamps the entry to the field maximum and tells you it has done so, then computes on 0.15. That returns 1.99 MPa where the 200 pci you actually meant, 0.054 MPa/mm, gives 2.24 MPa. Eleven percent low, and close enough to a real answer to survive a skim past the notice. Convert the figure yourself and retype it; never let the clamp stand in for the conversion. Use the k at the top of whatever the slab is actually cast on, which where there is a subbase commonly means the composite value on top of it rather than the raw subgrade below. The contact radius is a different kind of trap: it is the radius of an equivalent circle, a = sqrt(P / (pi × p)) for a contact pressure p, not a dimension you measure off a tire. That makes it a pair with the wheel load, and nothing on the page enforces the pairing. The defaults — 89 kN (about 20 kips) on a 115 mm radius — imply a contact pressure of roughly 2.14 MPa (311 psi). Push the wheel load to 200 kN and leave the radius alone and you have quietly specified a 4.81 MPa (698 psi) tire. Recompute the radius from the load and the contact pressure — usually taken as the tire inflation pressure — every time you change either one.

The stress came out below my concrete's flexural strength — does that mean the slab works?

No. This page produces a demand and stops there: it never sees the concrete's flexural strength (modulus of rupture), and it never counts passes. A slab can sit well under its rupture stress for one wheel pass and still crack once enough of them have gone over it, and repeated loading is a first-order part of how airfield slab thickness gets set, not a refinement on top of it. The check you actually need puts this stress against the specified flexural strength with an allowance for the number of repetitions, and this page holds neither half of that. Where the number does earn its keep is comparing options, because the sensitivities behave the way the slab does. Thickness dominates everything: at the default wheel, taking a 300 mm slab to 400 mm drops the edge stress from 2.24 to 1.28 MPa, while sweeping k across its entire allowable range — 0.02 to 0.15, a 7.5-fold change in support — moves it only from 2.48 to 1.99 MPa, and the elastic modulus across 20–35 GPa moves it about 6% with Poisson's ratio under half a percent — all three enter the formula only inside the logarithm, which is why they are so weak. A stiffer concrete slightly raises this stress for the same reason; what it does to the slab's strength is not part of this calculation at all. So a marginal figure here is a thickness conversation before it is a subbase one. And do not try to cover the missing wheel of a dual gear by entering the whole gear load — the stress is exactly proportional to the load, so that just multiplies the answer; it superimposes nothing.