The Aberfeldy FootBridge at 20

The Aberfeldy FootBridge at 20

Aberfeldy Foot Bridge, one of the first all-composite bridges ever built, is 21 years old.  On the occasion of its 20th birthday, it was assessed and found to be performing excellently in the large-scale structural issues, but suffering a bit in some of the details.  Good news and good lessons for future bridge builders.

It was built in the summer of 1992.  The Aberfeldy Golf Club in Scotland had only 9 holes, but they owned enough land for another 9 holes… on the other side of the River Tay.  The existing hump-backed bridge was not suitable for golfers and their gear to walk across, so the club sought an inexpensive bridge solution to enable expansion of the course.

They contacted Prof. Harvey at Dundee University, who shaped the project ina  way that would involve students as much as possible.  He arranged for design to be carried out by Maunsell Structural Plastics with student assistance.  The students then constructed the bridge during summer vacation, supervised by an engineer from Maunsell and another from a civil engineering contractor, O’Rourke, which managed the contract.

The bridge is 113 m long, with a main span of 63m, making it the longest span composite bridge in the world.  It is 2.23 m wide, and has two towers each 17.5m high, from which fan 40 cable-stays arranged in 2 planes.

The deck and towers are made of interlocking cellular GFRP pultrusions made by Maunsell, a structural system they had developed called the Advanced Composite Construction System (ACCS). Standard profiles were used; it was not a custom-made project.  Cables are Parafil ropes made from Kevlar aramid fiber.  The only non-FRP parts are the steel and concrete foundation, and the steel cable connections.

The pultrusions were assembled into the deck onsite, inside a tented ramp structure.  Tower legs were fabricated off-site, bonded together onsite and erected to the foundation. The pieces were joined with epoxy.   Each leg weighed only 1.25 kg (about 2750 lbs), and they were handled without cranes, using only a telescopic forklift truck.

A series of FRP crossbeams are attached at the cable-stays, and carry the deck. They were held in position by temporary wires during construction.  Once the main span deck was assembled, it was pulled across the span by winches.  Then the side spans were assembled, and then the entire span was lowered into final position on the cross-beams.  To protect the deck from damage by golfer’s spiked shoes, a “wear resistant surface” was added: a former rubber conveyor belt.

This construction process was made possible by the extreme light weight of the bridge.  This lack of weight, in fact, posed something of a problem.  The structure is “easy to excite” that is, to be set into vibration or undulation at certain frequencies.  (The effects of bridge undulation were demonstrated to dramatic effect in 1940 by the Tacoma Narrows Bridge in Washington state, a steel girder bridge affectionately known as Galloping Girtie.  See http://www.youtube.com/watch?v=3mclp9QmCGs for stunning footage of this historic event.)  Since the Aberfeldy FRP bridge was designed to carry a much higher load than its own dead weight, concrete was added to some deck cells to improve its vibrational damping.  In fact, half of the dead load of the bridge is concrete ballast.  The rubber conveyor belt was also chosen to improve damping.  The bridge has survived severe flooding and 140 km/hr winds, and is considered aerodynamically stable.

For unknown reasons, the original design did not take into account golf carts or other wheeled traffic.  As a result, vehicles have damaged the bridge in various ways over the years.  A small tractor was allowed to cross the bridge towing a trailer of sand, and the weight damaging the deck, cracking some planks.  In response, the bridge was strengthened in1997 with GFRP pultruded plates bonded to the topside of the deck, as well as carbon fiber-reinforced sheets applied to the deck’s edge beams on either side.

In 2012, Dr. Tim Stratford of the School of Engineering, University of Edinburgh published a condition survey of the bridge as 20 years, based on inspections performed in 2004, 2008, 2010, and 2011.  Stratford noted that the bridge has received little maintenance over 20 years, and serves as a good study for the durability of FRP structures.

“The primary structure of the bridge continues to perform well, with no visual signs of overall structural deterioration. Whilst a visual inspection will not detect every deficiency, FRP composites are brittle and hence structural deterioration usually results in cracks in the GFRP components. Careful inspections of the bridge deck have not revealed damage that is a result of structural deficiency, but impact damage has occurred that will be discussed below. The bridge deck has a smooth curve and there is no evidence of movement of the towers, deck, foundations or abutments. There are no slack cables, and there is no sign that the cables have pulled out from their anchorages.”

However, Stratford found that the bridge had worn in ways that might have been preventable.  There is damage to the handrail/parapet structure from impacts with golf carts, which might have been avoided or designed to withstand, had the designers had taken into account the possibility of golf carts on a golf course.

The FRP modules used to construct the parapets were different from those used for the deck and towers, and the handrail has not weathered well.  The ACCS pieces in the deck and towers have a veil, an outer layer of polyester fabric that traps resin at the surface, helps reduce moisture absorption, and protects against UV degradation.  The non-ACCS parapet pieces lack this protection, and as a result, the surface of the handrail has eroded, exposing fibers

There is also considerable growth of mold and lichen on the bridge’s surfaces.  It is thought that anti-fungal additives in the resin could have prevented this.

For more detailed information on this landmark bridge, see The Condition Of The Aberfeldy Footbridge After 20 Years Of Service by Dr. Tim Stratford (http://www.research.ed.ac.uk/portal/files/3582804/CGB_STRAT.pdf, and Aberfeldy Bridge – An Advanced Textile Reinforced Footbridge, C.J. Burgoyne and P.R. Head (http://www-civ.eng.cam.ac.uk/cjb/papers/cp25.pdf).

 

Images sourced as noted.