Seastead Leg Structural Analysis: Lateral Wave Load Capacity

Seastead Leg Structural Analysis

Objective: Determine the uniformly distributed lateral force a single leg can withstand before structural failure (yield/ultimate), and estimate the wave height required to generate that force.

Disclaimer: This is a preliminary engineering estimation based on simplified beam theory and linear wave theory (Morison equation). It does not account for dynamic amplification, slamming loads, stress concentrations at connections, fatigue, corrosion, or complex hydrodynamic diffraction effects. A full Finite Element Analysis (FEA) and Computational Fluid Dynamics (CFD) study by a certified naval architect is required for final design.

1. Leg Geometry & Assumptions

Parameter	Value	Notes
Length (L)	21.5 ft (258 in)	Total leg length
Submerged Length (L_sub)	10.75 ft (129 in)	Lower 50% (lower half in water)
Chord (c)	8.5 ft (102 in)	NACA 0035 foil chord
Max Thickness (t_foil)	35% chord = 2.975 ft (35.7 in)	NACA 0035 profile
Wall Thickness (t_w)	0.5 in	Marine aluminum plate
Trailing Edge	Blunt (last 0.5 ft cut off)	Fits 8.9 ft container height
Support Condition	Fixed at top (welded to triangle frame), Free at bottom	Cantilever beam
Load Distribution	Uniform (evenly along leg)	Analysis per user request

Parameter

Value

Notes

Length (L)

21.5 ft (258 in)

Total leg length

Submerged Length (L_sub)

10.75 ft (129 in)

Lower 50% (lower half in water)

Chord (c)

8.5 ft (102 in)

NACA 0035 foil chord

Max Thickness (t_foil)

35% chord = 2.975 ft (35.7 in)

NACA 0035 profile

Wall Thickness (t_w)

0.5 in

Marine aluminum plate

Trailing Edge

Blunt (last 0.5 ft cut off)

Fits 8.9 ft container height

Support Condition

Fixed at top (welded to triangle frame), Free at bottom

Cantilever beam

Load Distribution

Uniform (evenly along leg)

Analysis per user request

2. Material Properties (Marine Aluminum)

Alloy / Temper	Yield Strength (F_y)	Ultimate Strength (F_u)	Density
5086-H116 (Typical Marine)	30 ksi (30,000 psi)	40 ksi	0.096 lb/in³
6061-T6 (Alternative)	40 ksi	45 ksi	0.098 lb/in³

Alloy / Temper

Yield Strength (F_y)

Ultimate Strength (F_u)

Density

5086-H116 (Typical Marine)

30 ksi (30,000 psi)

40 ksi

0.096 lb/in³

6061-T6 (Alternative)

40 ksi

45 ksi

0.098 lb/in³

3. Cross-Section Properties (Simplified)

The NACA 0035 foil is modeled as a thin-walled closed section. The primary bending resistance comes from the top and bottom skins (flanges) separated by the foil thickness.

Note: This ignores the contribution of the leading/trailing edge skins, making it a conservative (lower bound) estimate.

4. Maximum Bending Moment Capacity

M_yield = F_y × S = 30,000 psi × 1,820 in³ = 54,600,000 in-lbs = 4,550,000 ft-lbs

M_ultimate = F_u × Z = 40,000 psi × 2,730 in³ = 109,200,000 in-lbs = 9,100,000 ft-lbs