A crane gantry girder under AISC 360 checks biaxial bending with a really simple linear interaction — no axial term at all, just the vertical wheel-load moment and the lateral surge moment added together against their respective capacities.

Every check above runs automatically inside the Structyze Gantry Girder module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.

The Example We'll Use

• Span 6 m
• Section 450 × 200 × 20 × 12 mm
• Maximum static wheel load 100 kN
• Vertical impact factor 25%, lateral surge 10% of wheel load
• Material ASTM A992/A572 Grade 50, F_y = 345 MPa

Working Through It

Both flange and web check compact (utilisation well under 1 on local buckling limits), so full plastic capacity is available about both axes. Factored wheel load W_u = 1.6(100)(1.25) = 200 kN; factored surge S_u = 1.6(100)(0.10) = 16 kN.

Using the single moving-point-load approximation: M_ux = 200(6)/4 = 300 kN·m (major axis), M_uy = 16(6)/4 = 24 kN·m (minor axis, from surge). Moment capacities: M_rx = 0.9(2.2243×10⁶)(345)/10⁶ ≈ 690.6 kN·m; M_ry ≈ 128.8 kN·m.

What Governs, and Why

Biaxial bending governs at utilisation 0.621 (300/690.6 + 24/128.8 = 0.434 + 0.186) — the vertical wheel-load term dominates, a typical split for a moderate 10% surge allowance. A heavier surge percentage, or a lighter section chosen mainly for vertical bending, would shift more of that margin onto the minor-axis term, the same pattern seen across every code in this series for gantry girders.

Where Structyze Takes Over

Every check above runs automatically inside the Structyze Gantry Girder module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.

For Gantry Girder and the rest of the full catalogue — 80+ components across RCC, Steel, Composite, Timber, Masonry and more, across 20+ design codes — buy the full Structyze desktop app or see everything it does on the Software page.

Quick FAQ

Why no axial term in this biaxial check?

A gantry girder under gravity and surge loading doesn't carry any significant axial force of its own — the biaxial interaction here is purely between the two bending directions (vertical wheel load, lateral surge), unlike a beam-column, which would add a P/P_r term to the same interaction.

What's not covered by this single moving-load model?

Most real overhead cranes have two wheels per rail on a bogie, not one — the single moving point load (M_max = PL/4) is exact only when the wheelbase is small relative to the span. A wheelbase comparable to the span needs the true governing load position found by moving the resultant until it and the nearer wheel sit equidistant from mid-span.

Final Thoughts

The vertical-versus-lateral split in a gantry girder's biaxial check is a clean illustration of how a modest secondary load (10% surge) can still claim a meaningful share of the governing utilisation once it's checked on its own weaker axis.