How Is Structural Aluminum Design Done to Code Requirements?

Aluminum structures are designed per the Aluminum Design Manual, with allowable stresses for alloys like 6061-T6 and 6063-T5. Because aluminum's modulus is roughly a third of steel's, deflection and vibration usually control member sizing rather than strength. Welded joints use reduced heat-affected-zone strengths, connections favor bolting where possible, and the design checks the same buckling modes as steel at aluminum's lower stiffness. The one-line version: design aluminum like steel, then triple the deflection check and halve the weld strength. Engineers who internalize those two adjustments produce good aluminum designs; engineers who don't produce bouncy ones.

The 6000-series (magnesium-silicon) alloys — 6061-T6, 6063-T5 — are the structural workhorses: extrudable into complex shapes, weldable with the HAZ penalty, and strong enough for real structure. The T6 temper means solution heat-treated and artificially aged to peak strength; welding returns the nearby material toward the soft T4-ish condition. The 5000-series (magnesium) alloys like 5083 offer excellent weldability with less strength loss and dominate marine structures. Extrusion is aluminum's superpower: the Aluminum Design Manual's world assumes custom cross-sections are cheap, so aluminum members are shaped with integral stiffeners, interlocking joints, and optimized geometry that would be absurd in steel. Curtain wall mullions are the everyday example — complex extruded profiles doing structural, thermal, and water-management jobs at once. The design leverages the shape freedom while respecting the modulus penalty.

Aluminum forgives strength optimism and punishes stiffness optimism. The design practices that matter most are the ones that account for what makes aluminum different from steel, not the ones that copy steel practice.

• Size for deflection and vibration first: strength checks come second in aluminum
• Use heat-affected-zone strengths at every weld — or bolt the critical connections
• Isolate dissimilar metals: galvanic corrosion at aluminum-to-steel contact is the durability risk
• Exploit extrusion: custom profiles with integral stiffeners beat plain shapes on efficiency
• Check buckling at aluminum's stiffness: the same modes as steel, reached at lower loads