High-girder spans torn out by gale-force winds while a passenger train was crossing. 75 dead. The bridge had been designed without explicit wind-loading and with under-strength cast-iron columns.
Bouch's design assumed a static wind load of 10 lbf/ft² (≈ 480 N/m²). On 28 December 1879, gusts reached 100+ mph (≈ 2200 N/m²) — over four times the design value. The lattice-girder columns failed in lateral buckling, and the train and central spans plunged into the Firth of Tay.
Wind load underestimated by a factor of ~4 — a cardinal error of stress-budgeting. Compounded by cast-iron column connections (brittle, no ductility) and lateral-torsional buckling slenderness ratios that would today require χ_LT < 0.4 — the columns had no chance.
Lateral-torsional buckling (Cb factor) — flags slender column sections under wind-induced moment.
Open Steel Section Design