A bridge from the factory

A UK consulting engineer believes the construction industry could transform its quality and efficiency by adopting manufacturing techniques. It has developed two modular building systems in an attempt to lead the way, reports David Fowler

Construction is a branch of engineering which is stubbornly resistant to change and new technology. It has stuck to traditional methods, despite predictions by theoreticians over the years that working practices, costs and quality could be transformed if lessons were learned from production techniques used in other branches of engineering.

This year, though, two systems developed by a UK-based consultant look set to break into the mainstream, with potentially far-reaching consequences.

Maunsell, noted for its bridge and structural engineering expertise on projects such as the Second Severn Crossing, signed a deal in March with US engineer Strongwell, the largest manufacturer of pultruded composites, to make, market and sell Maunsell’s Advanced Composite Construction System (ACCS) in the US and Mexico.

And later this year, the first contract using a modular bridge deck system, which Maunsell claims could transform the economics of bridge construction, is planned.

The development of both systems was led by Peter Head, European regional chief executive at Maunsell. Head believes that nothing like ACCS, a modular construction system for bridges and buildings, has been seen in construction technology for over half a century. He describes the Strongwell deal as `one of the most significant licensing agreements between Europe and the US since Freysinnet introduced prestressed concrete. It’s a totally new construction system.’

Head believes the deal will propel ACCS, which has been used successfully in a handful of projects over the last decade, into the mainstream.

The system was developed as an enclosure for the A19 Tees viaduct in the late 1980s. The steel bridge was suffering from serious corrosion. Maunsell designed a lightweight composite deck to fit under and around the steel girders of the viaduct, to protect the structure from the elements and provide permanent access for maintenance.

But Head saw the potential for more than a one-off solution. The enclosure was conceived and designed as a modular system with a limited number of parts which could be assembled to build a wide range of structures. `It was always configured as a global construction system,’ says Head. `We always had it in mind for general construction use.’

For the Tees viaduct, Maunsell designed the system and a code of practice for its structural design and use, pre-empting a process which usually takes years of deliberation by expert committees. `The key was our ability to design and specify a total design entity,’ says Head.

Initially, the pultrusion industry was surprised by the complex production specification, which called for full traceability of materials. But it rose to the challenge. GEC Reinforced Plastics of Preston won the Tees contract, and its tooling costs were paid off on that project.

The system, which has won a Millennium Product award, has just three main components: a cellular `plank’ which acts as a deck or wall member; a connector, about 80mm square; and a toggle to lock components together.

The components are made from a thermosetting polyester resin with continuous glass fibre mat at the top and bottom of each section and unidirectional fibres elsewhere. Maunsell consulted experts such as Westland, the Ministry of Defence and British Rail Research in developing the system.

Manufacturing is highly automated: `We make a very small number of components to a very high standard,’ says Richard Irvine, managing director of Designer Composites Technology, the Maunsell subsidiary formed to market the system. The manufacturing process is fast: components are produced at a rate of a metre every two minutes.

ACCS uses a unique connection method. Adjacent components are brought together side by side, with epoxy adhesive applied to one face. A toggle is slid into a groove along the side of each component, holding them together in accurate alignment while the adhesive dries.

Factory approach

Head envisaged assembly of the sections on site would be carried out under factory conditions, and this persuaded Ciba-Geigy to become involved in developing the adhesive.

`They had virtually gone out of the civil engineering market,’ says Head. `The use of epoxy resin on site had got a bad reputation. We said: “We want a factory approach on site, with people working in controlled conditions.” Ciba-Geigy was happy with that approach and formulated a resin with the right viscosity.’ Importantly, no surface preparation is needed.

Since the Tees viaduct, the system has been displayed in the Science Museum and used on a number of projects. These include a 63m footbridge at Aberfeldy in Tayside, Scotland, stayed by a main-span cable and built by Head and a team of student volunteers in 1992.

The ACCS system was also used on a replacement for The Bonds Mill fixed-lift bridge over the Stroudwater canal, Gloucestershire. The new bridge was required to carry 38-tonne trucks needing access to an industrial estate; ACCS made it light enough to be built on the existing bridge’s foundations, which would not support a steel lift bridge. The project went from inception to completion in nine months, including a research and development programme into the effect of wheel loads on the composite structure.

As well as being used to enclose the Second Severn Crossing’s approach viaduct, ACCS was used for the two-storey site offices, which have been converted into a visitor centre.

In the US market, Strongwell anticipates using the system initially for prefabricated buildings. A market for composite building panels exists there, particularly for industrial applications where corrosion is a potential problem.

But Maunsell’s vision of bringing manufacturing techniques into the construction industry does not end there. It has gone on to develop the Spaces system, a modular, enclosed, steel space-frame system suitable for bridge decks and long-span roofs.

Spaces, a patented system, is based on the premise that a space frame is structurally more efficient than plate girders or box girders. But since the 1960s, steel bridge deck design has favoured box girders over trusses – the two-dimensional equivalent of a space frame – because of their lower maintenance costs. Trusses have a comparatively large and complex surface area which needs painting.

`But if it’s enclosed it doesn’t matter,’ says Head. The rate of corrosion on the Tees viaduct since enclosure has been reduced by 96%, to a negligible rate at which it would take 120 years for 1mm thickness of steel to rust away.

Maunsell concluded that an enclosed steel space frame with welded joints could be a cost-effective approach to bridge construction. Again, Head envisaged the system being constructed on site in factory conditions `with robots and people in white coats’ and the capability of making 100m span units at the rate of one a week. `It would be very quick and simple, with a total steel weight half that of a box girder,’ says Head.

To develop the concept, Maunsell brought together a group including British Steel Tubes and Pipes; River Don Castings (which supplies the complex joint between the structural members); fabricator Watson Steel; glass fibre maker Vetrotex; resin supplier Scott Bader; and DCT, its own subsidiary.

Over four years, the group developed the complete system in terms of the optimum layout, grade of steel, size and thickness of tube. The conclusion was that the 50% weight target could be achieved.

But the cost-effectiveness depended on the cost of welding the joints,’ Head says. And the geometry of space frame joints is typically extremely complex.

To solve the problem, Maunsell put together another consortium under the Department of Trade and Industry Innovative Manufacturing Initiative research programme, with three universities. This designed a robotic welder with six axes.

It then began tendering for bridge contracts, submitting designs based on Spaces. However, despite coming close on at least one occasion, it failed to win anything, which Head puts down to innate conservatism and vested interests in the construction industry.

That looks set to change this year. Maunsell was supporting Irish consultant Roughan & Donovan on a design for a cable-stayed crossing for the M1 Dublin-Belfast road over the River Boyne.

Poor ground conditions dictated that the bridge must be low in weight. Maunsell’s proposal for an enclosed space-frame deck has been accepted and a construction contract is due to be awarded later this year.

`The Spaces system will transform the economics of bridge building,’ Head predicts. He adds that it has very wide applications, including roofs for airport buildings, and is expected to be especially suitable for cable-stayed and suspension bridge decks and two-way spanning structures.

Head is convinced that to remain competitive, construction must adopt a Spaces-type philosophy of using innovative manufacturing methods: `We saw the complexity of what Laing had to do as temporary works for the Second Severn Crossing, and the difficulty the contractor had on the Storebaelt crossing [linking Denmark’s two main islands] in achieving the complex prestressed concrete detailing.

`The industry has gone down an avenue that it’s convinced is a cheap way of doing things. In fact this approach is highly risky and does not deliver the specified quality. The construction industry has to stop pretending.’