Seongsu Bridge — A Fatigue Crack at a Bad Weld Dropped a Span, Killing 32

At 07:38 on Friday 21 October 1994, a roughly 48-metre suspended span of the Seongsu Bridge over the Han River in Seoul tore loose between piers 10 and 11 and fell into the water during the morning rush, killing 32 people and injuring 17; the cause was not an overweight truck on that morning, not an earthquake, and not a barge strike, but a fatigue crack that had grown for years at the toe of a defective partial-penetration weld in a non-redundant truss connection. The vertical member that hung the suspended span from the cantilever arms had been joined to the lower chord by butt welds fusing only 2 to 8 millimetres of an 18-millimetre section. Under fifteen years of cyclic traffic the crack at that under-fused root advanced until the member parted, and with no second load path the span dropped.

The Seongsu was a Gerber (cantilever) truss: long anchor arms reaching from the piers, carrying a separate suspended span slung between them on hanger connections. The arrangement is efficient but fracture-critical by construction — the suspended span hangs from a small number of connections, and the loss of one with no redundancy releases the whole span. The failed detail was exactly such a connection, and its welds left voids and shallow penetration that acted as built-in starter cracks at the point of highest cyclic tension.

The forensic record was damning in its specificity. A Seoul District Prosecutor’s Office white paper, published 13 July 1995, named poor welding of the vertical members as the direct cause, and radiography of the structure found 110 of 111 examined connections riddled with weld defects. The bridge had carried on the order of 160,000 vehicles a day — far above its design assumption, many over the legal weight limit — accelerating the fatigue. Compounding it, the Seongsu had never received a detailed inspection in fifteen years, because Korean practice reserved deep diagnostic inspection for structures over twenty years old. Seventeen people — Seoul officials, the maintenance contractor, and original builder Dong Ah Construction — were convicted, and the disaster forced the Special Act on Safety Management of Structures through the National Assembly in January 1995, the foundation of modern Korean infrastructure inspection.