Pier Redundancy - The Vital Ingredient

Current developments in bridge design standards could significantly enhance the safety of road bridges above railway tracks, where both road and rail users are protected by the benefits of pier redundancy.

The pier redundant bridge concept requires that the bridge superstructure does not collapse with any one pier removed. It also typically provides for controlled weight, restricted use of the bridge by emergency vehicles such as ambulances and fire trucks with a pier removed.

For Australians the significance of pier redundancy in rail bridges was tragically illustrated in the Granville train crash of January 18, 1977. Of the 83 people who died on that fateful day, most lost their lives to the bridge which collapsed on the derailed train after it knocked out the supporting pier. A pier redundant design could have saved many lives.

Fortunately, pier redundancy offers a cost-effective and elegant form of protection against such deadly bridge collapses. Modern designs such as the Berri Bridge* and the Hindmarsh Island Bridge* over the River Murray in South Australia already demonstrate the viability of pier redundancy for bridges over navigable waters. The benefits of these virtually indestructible bridges can be applied equally well to railways.

At the same time, an unprecedented regulatory emphasis on safety provides the impetus to adopt concepts such as pier redundancy. There is a worldwide trend towards zero tolerance for human trauma in every area of modern endeavour. For example, Sweden has adopted a "vision zero" approach to road trauma which requires that the road environment be designed to allow for human error, with safety taking priority over cost. The same trends can be seen in workplace safety, where workplace injuries are seen as avoidable and unacceptable.

Leading the way locally in bridge safety is Transport South Australia with its safety provisions for bridges over navigable waterways. Both the Berri and Hindmarsh Island Bridges adopted the pier redundant concept specifically to meet the South Australian requirements. As far as can be determined, the Berri Bridge completed in 1997 was the first significant application of pier redundancy.

The same provisions can be applied for railway bridges, and the benefits of pier redundancy are now being considered by the committee for the new Australian Bridge Design Code. For bridges over railways, the Code stipulates a preference for a clear span between abutments - but where this is not achievable, the preference is for pier redundancy in the event of a collision. Other preferred options include supports of light construction with independent deflection walls but not integral deflection walls.

In the past, the general practice has been to design the structure to withstand an impact on the piers. Alternatively, the piers could be protected by auxiliary structures designed to absorb the collision impact energy. But the resulting piers and foundations tend to be very big and expensive. Auxiliary deflection structures are also expensive and can present impact hazards of their own. Perhaps more to the point, such structures can still fail when the impact is great enough, no matter what their strength or design.

The significance of bridge safety in the railway context can be seen by comparing the Granville disaster with the collapse of the Tasman Bridge in Hobart two years earlier. In 1975, two piers collapsed along with 127 metres of bridge superstructure. Although the collapse claimed the lives of five vehicle occupants on the bridge and seven crewmen on the SS Lake Illawarra below, the collapse of a much smaller bridge at Granville claimed more than seven times that number of lives.

At Granville, 83 people died and 213 were injured. Most of the casualties occurred when the superstructure collapsed, crushing the train carriages after the supporting steel trestles were demolished by the derailed locomotive.

It was a tragic example of where pier redundancy could have saved lives. This was recognised in the report on the formal investigation of the disaster by Judge J. H. Staunton, who effectively defined the concept in his recommendations.

Judge Staunton observed that the then Rail Authority planned to strengthen the trestles of similar bridges with concrete infills so as to better withstand glancing blows from trains. "However it must be recognised that it is of dubious value to strengthen bridge piers to the extent that they can withstand the most severe blows from locomotives," he said. "The consequence of this course of action may lead to the survival of the bridge structure but could cause a great deal of damage and possible loss of life through telescoping of rolling stock of the train."

Judge Staunton preferred an alternative concept to strengthen the decks of such bridges so that removal of a trestle would not cause the deck to collapse. "Clearly a single span bridge is the preferred type of bridge in almost every overpass situation. However it is recognised that factors of cost and physical clearance cannot always make this a practical course of action."

In short, Judge Staunton was laying the notional foundations for pier redundancy as a preferred concept for multi-span bridges.

Australia is not unique in suffering tragic consequences from the impact failure of a railway overpass. At Eschede in Germany on 3 June 1998, a derailed rail car on the Inter City Express train demolished a supporting pier and brought down the bridge on two of the rail cars. In addition, seven following carriages slammed into the collapsed superstructure, in an extreme concertina fashion, with the carriages lying parallel to the superstructure.

Designing and building bridges will always involve setting priorities. Future bridge owners will demand safety first, followed by best investment solutions, good aesthetics and low environmental impact. Whatever the possible or probable changes in design loadings, safety must come first. Its embodiment in design codes and government regulations is a natural part of our future.

The pier redundant concept has proved to be both safe and economical for use over navigable waters, and it is equally suitable over railways. Today's composite steel bridge structures mean that the safest design may also be the most economical.

By Frank Rapattoni

* View information on Berri Bridge and Hindmarsh Island Bridge:
Berri Bridge Arrives
Safety First for Hindmarsh Island Bridge