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University of Queensland: A double-skin tubular arch bridge system

23 December 2019

According to the ‘National State of the Assets 2018 Roads & Community Infrastructure Report’ commissioned by the Australian Local Government Association, the need for investment in infrastructure today exceeds $30 billion and roughly one in five local timber bridges are in poor condition.

The direct costs of replacing bridges including materials, labour and equipment are apparent. But the cost and safety risks of closing roads, often for months, as well as associated traffic disruptions are just as considerable in urban, regional and remote locations alike. Formwork, a site presence of months or years, and labour-intensive processes all add to the costs for stakeholders.

Rapid bridge replacement using pre-fabricated components is a worthy solution to the disruption and cost of traditional bridge repair. The team at the University of Queensland (UQ), School of Civil Engineering is tackling the challenge with an innovative composite construction technique for building lightweight bridge components. The design team included Brisbane-based company RocketC, global engineering consultancy Arup and the Hong Kong Polytechnic University under the stewardship of Dr Dilum Fernando, Associate Professor, UQ.

The novel double-skin tubular arch (DSTA) bridge system, including double-skin tubular (DST) beams, columns and arches was designed around an inner steel tube, an outer fibre-reinforced polymer (FRP) tube, and a concrete layer between the two tubes. It’s this strong compressed layer of concrete that allows the bridge to bear such heavy loads, while the tensile forces are carried through the steel. The tubes can be placed concentrically or eccentrically, depending on A double-skin tubular arch bridge system the specific needs of stiffness and strength. They also serve as stay-in-place formwork to eliminate the use of temporary formwork.

Dilum Fernando working on his double-skin tubular arch bridge system at The University of Queensland.

A single-lane railway overpass bridge prototype was designed and fabricated at the UQ Structures Laboratory. The length and mass of the DSTA bridge were constrained to 12.5m and 23 tonnes respectively, to allow easy transportation to site on a standard heavy vehicle (National Heavy Vehicle Regulator, 2016) and lifting with a mobile crane. The width of the bridge was selected to be equal to the standard railway track gauge used in Australia, which is 1.435m. Advanced numerical modelling approaches, as well as simplified design approaches, were developed to design the systems.

Construction was carried out in five phases:

  1. fabrication of steel and GFRP segments,
  2. assembly of GFRP segments around steel of arch and columns and welding, (c) sealing of GFRP tubes and concrete casting for the arch and columns,
  3. assembly of GFRP segments around steel of beam segments and welding, and
  4. sealing of GFRP tubes and concrete casting for the beams.

The GFRP tubes were filament-wound (with 8 layers of GFRP, having fibre angles of ±82º C to the tube axis) and then cut to size.

For the fabrication of joints, two different processes (namely the wet-layup and pre-preg processes) were used. The quality of the joints formed using pre-preg GFRP was found to be better than that of joints made using wet-layup GFRP. However, both types of joints showed no signs of damage during the loading test. While the pre-preg process gave better quality, it required heating for curing of the pre-preg, and the process was found to be more demanding than the wetlayup process. However, with better design of a heating box, the construction efforts could be greatly reduced. The concrete mix design used was found to be suitable for casting of concrete in DSTA bridge sections, and no air voids were observed.

The hybrid DST members have the benefit of excellent corrosion resistance, ductility and strength/weight ratio. During testing, the joints showed excellent performance against static and dynamic loading. Advanced numerical modelling approaches, as well as simplified design approaches, were developed to design the systems.

Ease of construction and reduction of embodied energy make the DST option an attractive alternative to traditional bridge members. An economic feasibility study using a typical example of an urban level crossing found that by reducing the time spent onsite to the order of a few days, instead of months or years, substantial cost savings could be made. The study concluded that material costs would be higher than those incurred for a traditional pre-stressed concrete bridge, but the total cost of the DSTA bridge would be in the order of $80million as opposed to $200million for the traditional design.

Dr Fernando, from the School of Civil Engineering, said: “The bridge is three times as strong as conventional reinforced concrete bridges, yet only a third of the weight. It can be pre-fabricated and transported on a semi-trailer and erected without the need for specialised heavy lifting equipment in about 72 hours. This means bridge building projects that previously caused up to six months of disruption can now be completed in just three days.”

For this reason and the science involved, the research team received the inaugural World Innovation in Bridge Engineering Award in Portugal last year. “The award recognises our design as a game-changer, coming in ahead of designs involving more than 200 authors from 50 countries,” said Dr Fernando.

The $50,000 prize was announced and sponsored by the Faculty of Engineering at the University of Porto (FEUP) and BERD, a project, research and engineering firm specialising in bridges.

This article first appeared in:

Connection Magazine

Issue 51: December, 2019

Author: Kerryn Caulfield

For this and more stories, please download the latest copy of our Connection magazine.

Issue 51, December 2019