Seastead Leg – Lateral Wave Load vs. Material Strength

This page presents a simplified static‑bending analysis for a single leg of the seastead when a sideways wave tries to tip the platform. The leg is assumed to be a solid plate of marine‑grade aluminum (6061‑T6) with a thickness of 0.5 in. The “thick” part of the leg that faces the incoming wave is taken as 4 ft wide (the forward part is about 4 ft, the aft part is thin).

1. Material properties

• Alloy: 6061‑T6 (marine‑grade aluminum)
• Yield strength σ_y ≈ 35 000 psi (≈ 240 MPa)
• Ultimate strength σ_U ≈ 45 000 psi (≈ 310 MPa)
• Seawater weight density γ = 64 lb / ft³ (≈ 10 050 N / m³)
• Gravitational acceleration g = 32.2 ft / s²

2. Cross‑section geometry (rectangular plate approximation)

• Width (effective depth) b = 4 ft = 48 in
• Thickness t = 0.5 in

Section modulus for bending about the axis through the thickness:

3. Allowable bending moment

Using the yield criterion:

If the more conservative ultimate strength is used:

4. Relating moment to a uniform side load

A leg behaves like a cantilever of length L = 19 ft. For a uniformly distributed load w (lb / ft) the maximum moment at the fixed end is

Insert the numbers:

5. Wave‑generated side load (linear wave theory)

Linear (Airy) wave theory gives the horizontal hydrodynamic pressure on a vertical member as a triangular distribution. The resultant force per unit length is

where

• γ = 64 lb/ft³ (seawater weight density)
• H = wave height (ft)
• D = effective projected width of the leg (≈ 4 ft for the thick part)

Solving for H when w_wave = w_max:

Using the yield‑based load:

Using the ultimate‑strength load:

6. Summary of results

7. Practical considerations

• The analysis assumes a uniform side load and linear wave theory. Real sea states produce highly varying, often impulsive, loads; drag and inertia terms can multiply the static force by 2–5×, especially for breaking waves.
• The leg is not a plain flat plate; it is a NACA‑foil shape with internal ribs, bulkheads, and often foam filling. Those features raise the effective stiffness and strength far above the simple plate result.
• Marine aluminium structures are prone to fatigue, stress‑corrosion cracking, and buckling. A thin 0.5‑in skin would need regular inspection and probably additional stiffeners or a thicker gauge for long‑term reliability.
• Typical design practice uses a safety factor of 2–3 on loads and 1.5–2 on material strength. Applying a factor of 2 on the above wave heights still limits the allowable wave to ≈ 0.5–0.7 ft, far below many storm‑wave heights.
• If the design must survive waves of 1 ft or more, the leg should be thickened, reinforced with internal webs, or constructed as a box‑girder core to increase section modulus dramatically.

8. Bottom line

A 0.5‑inch‑thick aluminium leg with the geometry described can only carry roughly 30–40 lb per foot of side‑load before the material yields. A modest wave of about a quarter‑foot (≈ 3 in) would generate that load under ideal linear‑wave conditions. In the open ocean, wave heights of 1 ft and above are common, so the leg as described would be at risk of failure unless additional structural reinforcement (stiffeners, thicker plating, internal foam‑filled webs, or a box‑girder core) is added.

For a more accurate assessment, a finite‑element model of the actual foil‑shaped leg, together with a Morison‑equation based hydrodynamic analysis that accounts for drag, inertia, and wave steepness, should be performed.