By Lachlan Haycock and Joseph Harding
“There was an air of unreality about the disaster”, wrote the Hobart Mercury of the Tasman Bridge collapse, which occurred 50 years ago this month.
On 5 January 1975, the SS Lake Illawarra collided with the pile capping of two of the bridge’s piers, causing a 127-m length of roadway to fall into the water and across the ship’s deck.
The bulk carrier, carrying 10,000 t of zinc concentrate upriver, had veered off course partly due to a strong current and partly due to the captain’s negligence.
Following the initial impact, four passenger vehicles drove off the edge, and two stopped halfway in a now-infamous image from the disaster. Twelve people, including five motorists and seven crew members, died.
The bridge, a vital link between Hobart’s CBD and the town’s eastern suburbs, left 1000s needing to either board a ferry or drive to the next available river crossing, Bridgewater Bridge, located more than 20 km north. Widespread social impacts were felt in the years following, with the Australian Institute of Criminology reporting effects on transport costs, employment rates, and access to hospitals and schools.
For engineers, the story offers a vital reminder of a bridge’s design having an impact beyond simply the technical, but also on the social.
Construction started on the 1396 m-long girder bridge in 1960. It superseded the existing Hobart Bridge, which boasted a unique design: it was a curved floating concrete bridge with a lift span to allow ship passage. Alan Knight, Chief Engineer of the Public Works Department, realised that a curved alignment would not require midstream anchors. It would be as strong as an arch when the tide flowed out and strong in tension (like a suspension bridge) when the flow reversed.
The structure was mostly removed in 1964 and was awarded an Engineering Heritage National Marker from Engineers Australia in 2015.
The Tasman Bridge project, meanwhile, made heavy use of precast construction techniques, with components gathered in two casting bays adjacent to the bridge. In all, 150,000 t of concrete was mixed. Concrete for the eastern viaduct’s 70-ft-long beams was compacted by external vibrators before being bolted to the moulds; this was supplemented by poker vibrators.
Meanwhile, the concrete columns for the eastern viaduct were constructed in situ. According to a documentary by the Department of Public Works Tasmania, the beam units were water-cured for one week before being stacked on both sides of the casting bed until required for jointing and erection into position.
“Each pile was formed within a 54-in-diameter tube made of ⅜-in steel plates,” the documentary reported. “The 30-ft sections of tube were welded together as the tube was sunk into the riverbed. The cutting edge at the bottom of the tube was a thicker plate to avoid damage and collapsing of the tube.”
Upon completion, the Tasman Bridge was touted as the longest prestressed concrete bridge in the country, and ambitiously likened to the Golden Gate and Sydney Harbour Bridges.
With the bridge being a vital link between two shores, engineers promptly acted to repair the bridge. The Joint Tasman Bridge Restoration Commission outlined the approach in a different documentary.
“Three 43-m-long spans between piers 17 and 20 had collapsed. Piers 18 and 19 had been demolished down to silt level. The span between piers 16 and 17 [had] moved 150 mm on its rollers towards the gap, moving the top of pier 17 with it. The columns were cracked. Pier 20 was severely damaged, the [fallen] span had broken away half the pile cap, and and some of the piles were exposed.”
Engineers considered constructing a single span to cover the gap, but decided against this in favour of considering three alternative configurations:
1. Rebuild the bridge in its original configuration by replacing the missing two piers and three concrete spans.
2. Erect a single pier at the midway point between piers 17 and 20.
3. Install a new pier at the original pier 19 position only, with steel-box girders installed between the new pier and the existing pier 17.
Engineers opted for the third option, as this required the least removal of debris – and, therefore, the shortest construction time. Construction on the new section began later in 1975, and the repaired structure was reopened to traffic in October 1977.
The collapse of the Tasman Bridge wasn’t the last time such an event – when a collision with a bridge pylon has triggered catastrophe – has occurred.
Following the collapse of the Francis Scott Key Bridge in Baltimore, Maryland, in March 2024, researchers from the Polytechnic University of Valencia and the University of Vigo suggested taking a new approach to bridge maintenance. Their approach includes:
Updating standards. “Current safety codes and standards still lack guidance on how to deal with the increasing risks of material deterioration and more frequent extreme events. Most codes focus on the design of new bridges, not on evaluating the safety of old ones. Methods of evaluation also rely on historical data and trends to characterise loads due to traffic, vehicle impacts and environmental conditions. Yet, these vary widely through time and might not reflect future conditions.
Repairing broken bridges. “The huge backlog of bridge repairs must be addressed urgently. In the United States alone, more than 46,000 bridges are considered structurally deficient, and some 178 million trips are taken on these bridges every day. As well as being cheaper than rebuilding, extending the lifespan of bridges brings environmental benefits. These include reductions in resource consumption, waste generation and carbon dioxide emissions.”
Curbing hazards. “Engineers and bridge managers must develop a better understanding of the three stages that lead to structural collapse. These are: occurrence of a hazard; initial failure of a component owing to damage; and the propagation of failures through the structure. Various risk reduction measures are effective at each stage and can be used together to prevent collapse.”
Muhammad Hadi FIEAust, Professor in Structural Engineering at the University of Wollongong, told create that, in an ideal scenario, bridges would be constructed without any in-water supports.
“In my opinion, it would be better for us, as engineers, to avoid the problem entirely,” he said. “In this case, avoiding the problem means isolating it, placing the piers either in shallow water or on land, to avoid such accidents. I understand this is a more expensive approach, and I understand it takes more time and effort.
“But remember: we are designing bridges for 100-120 years, and they will be used by many people. So it will be much better to spend the money now, to make things safer for everybody else. It may be more expensive, but there is no possibility that any ship will hit the bridge.”