King Street Bridge, Melbourne — A Weld Crack Brittle-Fractured a New Span in the Cold

At about 11:00 on the cold winter morning of 10 July 1962, a suspended span of the fifteen-month-old King Street Bridge over the Yarra River in central Melbourne sagged and broke up beneath a 47-ton low-loader that was within the bridge’s posted load limit; by chance no one was killed or injured. The cause was not overload, not a design under-strength, and not corrosion. It was a brittle fracture — a fast, low-energy crack that ran through all four main girders of the span — initiated at the toes of transverse fillet welds where cover plates had been welded onto the tension flanges, in a notch-sensitive high-tensile steel embrittled by hydrogen, locked-in welding stress, and a near-freezing temperature.

The bridge had been designed in 1959 by the consulting engineers Hardcastle & Richards for the contractor Utah Australia, fabricated in welded BHP high-tensile low-alloy steel by the subcontractor Johns & Waygood, and inspected for the Country Roads Board. The four suspended girders of the failed span, designated W.14-1 to W.14-4 and roughly 100 feet long, each carried thickening cover plates welded to the bottom (tension) flange to add section where the bending moment was highest. The transverse welds that closed off the ends of those cover plates were the fatal detail. They left a sharp geometric notch exactly where the steel was most highly stressed in tension, and in the heat-affected zone beside each weld the metal had been hardened, hydrogen-charged, and cracked during fabrication.

Brittle fracture needs three things together: a flaw, a tensile stress, and a steel cold and notch-sensitive enough to run the crack instead of yielding around it. The King Street span supplied all three at once. Pre-existing cracks sat at the weld toes; the cover-plate ends concentrated the tension; the high-carbon, high-tensile plate had poor notch toughness and was sitting at a temperature near its transition into brittle behaviour. When the heavy low-loader drove onto the western carriageway and raised the live-load stress, the cracks ran. The span dropped about a foot. The forensic verdict, established by the 1963 Royal Commission chaired by Sir George Barber, was unambiguous: brittle fracture from defective welds at the cover-plate terminations, in a steel and a detail that should never have been welded that way without preheat and toughness control.

The failure was, in retrospect, built in. The cracks at the weld toes had existed since fabrication and were not found by the inspectors of either Johns & Waygood or the Country Roads Board. Neither the contractor nor the fabricator fully grasped that high-tensile steel demanded a different welding discipline from ordinary mild steel — controlled heat input, preheat, low-hydrogen practice, and crack inspection. The bridge did not so much fail under traffic as wait for the first cold morning and the first heavy load to arrive together.