```html Large Marine-Aluminum I-beams (16 in deep): Availability, Weight, Cost, Shipping, and Rough Load Capacity

Large “standard” I-beams for a seastead triangle frame (marine aluminum, ~16 in deep)

Important context: The very long I-beams you see in US highway overpasses are almost always hot-rolled structural steel shapes (or plate girders), not aluminum extrusions. Aluminum I-beams exist, but “standard” availability and economical lengths are much more limited.

1) Can you get 50–80 ft extruded I-beams in marine aluminum, ~16 in deep?

Structural caution for seasteads: An open I-beam has poor torsional stiffness and can be fatigue-sensitive under wave-induced cyclic loading. Many marine structures prefer box sections / tubes / trusses because they handle torsion and fatigue details better. You should have a naval architect/structural engineer set the load cases (wave slam, global bending, fatigue, corrosion allowance, connection design).

2) Rough weight of a 16 in deep aluminum I-beam (example estimates)

Because “16 in high, 6 in wide” does not uniquely define an I-beam (flange thickness and web thickness matter a lot), below are two representative 16" x 6" extruded-style geometries to give you a realistic weight range. Aluminum density ≈ 0.0975 lb/in³.

Example section (approx) Dimensions used (inches) Area (in²) Weight (lb/ft) Weight @ 40 ft Weight @ 60 ft Weight @ 80 ft
Light 16x6 I-beam Depth=16, flange width=6, flange t=0.50, web t=0.375 ~11.6 ~13.6 ~544 lb ~816 lb ~1,088 lb
Heavier 16x6 I-beam Depth=16, flange width=6, flange t=0.75, web t=0.50 ~16.3 ~19.0 ~760 lb ~1,140 lb ~1,520 lb

Real catalog sections could be lighter or heavier than this, but these are reasonable “order of magnitude” numbers.

3) Very rough cost estimate (beam only, not including engineering, splices, coatings, fasteners)

Pricing varies wildly with alloy/temper, whether it’s stock vs custom extrusion, quantity, and certification requirements. For small quantities, aluminum structural shapes are often priced more like a fabricated product than a commodity.

Ballpark example (beam-only, ignoring tooling):

Example beam Approx weight Beam-only cost @ $8/lb Beam-only cost @ $15/lb
Light example @ 40 ft ~544 lb ~$4,350 ~$8,160
Light example @ 80 ft ~1,088 lb ~$8,700 ~$16,320
Reality check: If you only need a handful of beams, the “true” cost is often dominated by (1) custom die/minimum run, (2) shipping/handling, and (3) connection/splice fabrication, rather than raw metal value.

4) Shipping to Anguilla (practical considerations + rough ranges)

Anguilla is typically served via regional Caribbean logistics (often transshipment through a larger hub). Exact routes and rates change constantly; the numbers below are only “order of magnitude”.

Why <= 40 ft usually wins

Very rough freight ranges (China → NE Caribbean → Anguilla)

Recommendation: For cost and logistics, design around container-friendly lengths (commonly ≤ 20 ft or ≤ ~39 ft), then use engineered splices (bolted flange splice plates, fishplates, or truss nodes) designed for fatigue and corrosion protection.

5) Rough “working load” of a simply-supported 16 in aluminum I-beam with uniform load

This is a simplified beam calculation. It does NOT include: lateral-torsional buckling, local buckling, connection eccentricity, fatigue, dynamic wave loads, impact/slam, corrosion allowance, weld heat-affected-zone strength reduction, or code-required safety factors for a marine structure. Use it only for initial sizing intuition.

Assumptions for the Light example section (16" deep, 6" flange width, tf=0.50", tw=0.375"):

5A) Strength-limited uniform load (bending only)

For a simply supported beam with uniform load w (lb/ft) over span L (ft): max moment M = w L² / 8. Using the above section and stress assumption gives an allowable moment of about 97,000 ft-lb.

Span L (ft) Total uniform load capacity (lb/ft) limited by bending (includes beam self-weight) Minus beam self-weight (~13.6 lb/ft) → remaining for payload (lb/ft) Total payload over span (approx)
40 ~485 lb/ft ~471 lb/ft ~18,800 lb
50 ~311 lb/ft ~297 lb/ft ~14,850 lb
80 ~121 lb/ft ~107 lb/ft ~8,560 lb

5B) Deflection-limited uniform load (often the real limiter for long aluminum spans)

Many designs limit deflection to about L/360 for “floor-like” structures (criteria vary a lot for marine platforms). For uniform load, maximum deflection is: Δ = 5 w L^4 / (384 E I).

Span L (ft) Deflection limit used Total uniform load (lb/ft) that meets deflection limit (includes beam self-weight) Minus beam self-weight (~13.6 lb/ft) → remaining payload (lb/ft) What this means
40 L/360 ~108 lb/ft ~94 lb/ft ~3,760 lb payload over 40 ft if you keep deflection “tight”
50 L/360 ~55 lb/ft ~41 lb/ft ~2,050 lb payload over 50 ft
80 L/360 ~13 lb/ft ~0 lb/ft (negative) At 80 ft, the beam’s own weight is about enough to exceed L/360 deflection
Key takeaway: A 16" deep aluminum I-beam spanning 50–80 ft is usually governed by deflection (and marine dynamics/fatigue), not just bending strength. If you truly need 50–80 ft between supports, you typically move toward:

6) Practical guidance for your triangle seastead concept

If you want, I can refine the numbers

If you provide any of the following, I can produce a tighter estimate (still preliminary, but much less “hand-wavy”):

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