Aggressive environmental conditions and weight restrictions present an ideal location for a low maintenance 'plastics' bridge. Jon Masters reports.
Restricted to pedestrian use since 1995, the lift span of Bridgetown's Chamberlain Bridge in Barbados has proved an ideal candidate for replacement by Maunsell's fibre reinforced polymer (FRP) composite bridge technology when construction starts in 2002.
Hot climatic conditions combined with attack from saline water have caused serious deterioration of the wrought iron swing-bridge section. FRP provides a low maintenance and lightweight solution.
The Barbados Tourism Corporation (BTC) is very keen on the idea of a low maintenance solution, says Maunsell Structural Plastics (MSP) project manager John Cadei, because it is very difficult to prevent corrosion in Bridgetown's hot, saline environment. A lightweight replacement, with hydraulic rams to provide the raising mechanism, will also keep construction costs and mechanical and electrical equipment at a minimum, winning out in Maunsell's feasibility study over a replacement swing bridge along with bascule, rocking lift bridge and sliding retractable options in steel.
'The existing foundations would require significant strengthening to take any additional load. We realised the potential for composite materials to solve the client's problem.'
The Chamberlain Bridge's two masonry arches and wrought iron swing structure was completed in 1875. and the aggressive conditions have taken their toll on the iron truss and its lattice webs.
'Inspection of the Chamberlain Bridge showed advanced corrosion to the extent that one of the primary truss chords had rotted through and was providing no structural contribution at all, ' says Cadei.
As well as Bridgetown's two main bridges crossing the Careenage port inlet, BTC's $60M refurbishment programme includes bridges in Speightstown and St Lawrence Gap. The 1960s built prestressed concrete Duncan O'Neil Bridge is currently providing the only crossing for traffic in bridgetown.
The new FRP bridge is designed by AECOM-Maunsell, working as subconsultant to BTC's main architect, planner and design consultant, the Barbados-based Design Collaborative.
The composite material consists fundamentally of glass fibres set in a polymer resin. 'We were looking for three properties from the deck material.
Good fatigue resistance is needed as well as low weight and protection against corrosion.
FRP composites are not suitable for all situations, but they are ideal for these particular circumstances, ' says Cadei.
The corroded swing section forms one of the outer spans of the Chamberlain Bridge. The new structure, 12.5m in length and 8.9m wide, will have a FRP composite deck connected to a steel transverse box beam at the pivot end, protected against corrosion by a high specification paint system. The beam will be hinged to a new concrete substructure housing the inclined hydraulic rams, which will lift the span from beneath.
According to Maunsell's design team leader Will Duckett, construction will be fairly simple. The deck and hinge steelwork will be assembled from prefabricated components on adjacent land before being lifted on to the substructure.
The deck material will be Maunsell's modular advanced composite construction system (ACCS), developed by its specialist Maunsell Structural Plastics division.
Standard ACCS panels have a square cellular cross-section with walls varying in thickness from 2.5mm to 4mm. The panels are formed using pultrusion, which pulls the glass fibres soaked in polymer resin through a heated dye, which then cures the material. 'Pultrusion presents quality and cost benefits over the more basic and conventional alternative of contact moulding, ' says Duckett.
The primary structure of the Chamberlain lift bridge deck will be built up from standard ACCS panels bonded together to create what Maunsell describes as a supercellular structure. Vertical panels, running longitudinally along the deck and joined at 680mm centres between horizontal panels, will form the 760mm deep primary superstructure.
Heavy duty ACCS roadway panels with 12mm to 15mm thick walls will be laid transversely over the super cellular structure to distribute wheel loads. Footpaths will be built up with a further layer of standard panels laid in the longitudinal direction.
A tougher vinylester resin is being used to manufacture the roadway pieces than the isophthalic polyester resin of the standard ACCS panels.
'Careful selection of materials is very important, ' says Cadei. 'It must be considered in terms of the particular environment, manufacturing process and geometry.'
Maunsell's expertise in composite structures stems from a number of important projects.
The ACCS system was first applied to a real contract in 1987 for a bridge enclosure around steelwork of the UK's A19 Tees viaduct in Middlesbrough.
Further research resulted in a long term serviceable solution, initially for fully bonded composite footbridges. The world's first and longest span advanced composite footbridge based on pultrusion was built using ACCS at the Aberfeldy Golf Club in Scotland in 1992.
According to Cadei, the next phase in the development will involve around three months of testing at Britain's TRL (see box).
'A full scale composite road bridge is the next step. But trials are needed to build greater confidence in fatigue behaviour before a UK Highways Agency standard for composite road bridges can be contemplated.'
TRL trials A full standard for use of composites in road bridge construction is the focus of TRL trials for the UK Highways Agency on a fibre reinforced polymer (FRP) composite road deck designed by Maunsell Structural Plastics.
'This research will address concerns over the fatigue effects of repetitive high wheel loadings, ' says MSP project manager for the TRL testing, Will Duckett.
'Composites have very high fatigue resistance in the direction of the fibres, but less so against the fibres. This is more of a concern with pultrusion, which has a relatively high proportion of fibres in the same direction and is realistically the most cost effective method of forming FRP structural members for the construction industry.
The TRL is testing a 4m by 2m composite deck structure over a three month period using its Trafficking Test facility, developed with the HA for performance modelling of pavement structures and road joints. A typical deck section built from Maunsell's modular advanced composite construction system is being tested, because it is the only system readily available, says Duckett.
'A recent feasibility study on a UK road bridge refurbishment project showed that a composite deck, had it been used, would have produced the most cost effective solution on the basis of construction cost alone.
'The lightweight composite deck precluded the need for strengthening the weakened foundations. But it could only be assumed that the composite design would have worked in terms of fatigue resistance, which is why the HA has funded this latest research.
'The results will allow a draft specification to be drawn up and the HA is keen to find a suitable project for the next step of building a pilot composite road bridge.'