John Hilton, Aurecon’s Bridges Expertise Leader, discusses how the safety of temporary bridge works can be improved to reduce the risk of collapse.
Let’s look at bridge collapses around the world – Why do they occur? Where do they occur? 50 years ago, most bridge collapses occurred in service. These bridges often failed due to one or more of:
The world has moved on. Modern bridges statistically now perform far better in service. Today, if a bridge is to fail, collapse will in fact most likely occur during construction.
In the last two years, available information indicates around half of all bridge collapses around the world occurred during construction. We have seen this in South Africa, North America, China and Australia. Moreover indications are that, more often than not, these failures were design – not construction – related. Of the design related failures, the deficiencies have been invariably in the design of temporary works, rather than the design of the permanent works per se.
Temporary works for bridge construction comprises a whole host of different structures; support to excavations, piling platforms, formwork support, lifting gantries, temporary piers and support, cofferdams; essentially all the gear need to support and erect the bridge elements during construction, and generally removed at the completion of the work.
Today, more than ever before, there is a high degree of scrutiny and review for permanent works designs for bridges. For a start, they are invariably independently reviewed. In many instances, and certainly for major or particularly complex bridges, there is also an independent review undertaken by a design company independent of the designer, such as a proof engineer or independent reviewer – sometimes both. The requirements for this independent review are usually put in place by the Authority via a contractual obligation. Generally for temporary works design, this extra level of scrutiny and review is not required contractually, and is usually not undertaken. This is unfortunate, as failure of temporary works can often be traced back to fundamental elementary errors; errors that would be picked up by a normal verification/review process.
Safety factors are invariably lower for temporary works design due to the reduced time exposure to risk. Many temporary works elements are primarily supporting dead load, ie the self-weight of the structure or element, so the overall load factor is less than for ‘permanent’ structures with a 100 year design life. The loading on construction supports is of relatively short duration. A lower safety factor provides a reduced margin if the unexpected occurs, such as unanticipated foundation movement or unforeseen stress concentrations.
Bridge codes and specifications are generally written around design obligations for the permanent works, with much less emphasis on risks associated with temporary works design.
In light of recent failures, the safety factors for the design of temporary works for bridges should be reviewed to determine if they are appropriate. Current codes and specifications, as they relate to temporary works design, should also be reviewed.
At Aurecon, we have an in depth understanding of bridge codes and standards. Some of our bridge design experts are on the code committees that relate to bridge design and construction and know both the rationale behind code requirements, and also the direction where the rules and regulations are heading.
Aurecon recently designed extensive temporary works for the Sydney Harbour bridge upgrade for the New South Wales Roads and Maritime Services (RMS). Working over seven lanes of traffic with 270 000 vehicles crossing the bridge every day, and two busy railway lines, resulted in a high degree of risk and complexity. If anything was to go wrong, the implications would be substantial. An innovative, purpose-built bridge arch access system was built to minimise the risk of moving tonnes of steel across great heights to remote areas of the bridge. All went extremely well. RMS was delighted with the result and Aurecon received an award from WorkCover for the project.
Digital engineering – the ability to draw in 3D and turn a model into the full construction stage sequence – should make a big difference in the incidence of bridge failure during construction. Aurecon routinely designs bridges in 3D. We have found it to be very beneficial for clash detection and constructability. The digital bridge can be constructed element by element. This also provides all project stakeholders with a much better understanding of the structure and the way that it is to be put together.
The Barangaroo Reserve and the Cutaway project in Sydney involved the construction of a large structure – the Cutaway, with several hundred precast concrete girders and supporting elements forming the roof, all at differing levels and locations. The design and documentation was undertaken by Aurecon in full 3D. Our digital model was subsequently used during construction to assist in construction sequencing, crane set up, etc, and was much appreciated by the contractor.