The Condition of the Aberfeldy Footbridge after 20 Years in Service
摘要整理
Aberfeldy footbridge was a milestone in the development of FRP composites for construction when it was completed in October 1992. Whilst there are now longer FRP-decked bridges, the cable-stayed Aberfeldy bridge still has the longest span. It was something of a demonstration project for FRP construction, combining a pultruded GRP deck, towers and parapet system with aramid parallel-fibre cables. The bridge has now been in-service for close to 20 years in the Scottish Highlands, and during that time has received little maintenance. It is timely to return to the bridge and assess its durability. This paper describes the condition of the bridge, including its overall structural performance, the state of the GRP components, the performance of the parapet, and the structure’s dynamic response. The paper provides lessons for the design of future FRP composites bridges and structures. INTRODUCTION The construction of Aberfeldy Footbridge was a milestone in the emergence of fibre-reinforced polymer (FRP) composites as structural engineering materials. Over the last twenty years FRP composites have become commonplace as structural strengthening and many FRP bridges have been built around the world. When Aberfeldy Footbridge was opened on 3 October 1992, applications of FRP in structural engineering were in their infancy (Hollaway, 2010). Whilst it was not the first FRP composite bridge, the construction of a cable-stayed footbridge with a 63m main span was substantially longer than any previous FRP bridge. It received numerous awards, such as the Saltire Award for Civil Engineering Design 1993. Whilst many FRP bridges have been built since, to the best of the author’s knowledge, no all-FRP bridge has yet been built with a longer span. The Aberfeldy Footbridge was a demonstration project that tested innovative technology. Like all such projects, it is vital to review how the structure has performed, to learn which innovations have worked well, and to feed this into the design of future FRP composite structures. This paper examines the performance of the Aberfeldy Footbridge after 20 years in service. It updates a previous paper that was written after 10 years in service (Cadei & Stratford, 2002). An Overview of the Bridge The Aberfeldy Footbridge crosses the River Tay at Aberfeldy in Perthshire, Scotland, and is shown in Figures 1 and 2. The cable-stayed bridge is 113m long, with a 63m main span, and back spans of 25m each. The bridge deck and towers are made from glass FRP (GFRP); the stay cables are Parafil aramid parallel-lay ropes, and the parapets are made from GFRP sections (Harvey, 1993; Lee, 1993). Apart from the aluminium connections required to attach the stay cables to the deck, the structure of the bridge is made completely from FRP. The bridge connects the two halves of Aberfeldy Golf Club. The use of FRP materials was driven by the ease with which the bridge could be constructed. The lightweight components were erected by students from Dundee University without the need for cranes or other heavy construction plant, and this resulted in a far more economical bridge than would have been possible using traditional materials (Burgoyne & Head, 1993). Figure 1. Aberfeldy footbridge from the south (December 2010). Figure 2. Aberfeldy footbridge from the north (June 2008). The bridge deck and towers were fabricated using components from the Advanced Composite Construction System (ACCS), a cellular modular construction system made from pultruded GFRP, shown in Figure 3. The bridge deck arrangement is shown in Figure 4. It has 3 GFRP planks across its width, with edge beams to provide stiffness. These longitudinal components are supported on primary transverse beams that connect to the cables, and intermediate secondary transverse beams (Head, 1994). The ACCS components were bonded together on site using an epoxy adhesive, with a toggle that provided mechanical interlock until the adhesive had cured. The parapets (Figure 4) were fabricated from non-ACCS GFRP pultruded sections (which will be discussed in more detail below), and the bridge is surfaced using a former rubber conveyor belt. Plank 3-way connector Toggle connector Figure 3. The ACCS components used in the bridge deck and towers. (ACCS was developed by Maunsell Structural Plastics and is now produced by Strongwell Ltd. as Composolite). Figure 4. Aberfeldy footbridge from above and below the bridge deck (June 2008) Strengthening of the Bridge in 1997 The bridge was designed to a 5.6 kN/m pedestrian live load, and was not designed for the concentrated loads that result from motorised traffic. Consequently, the bridge was overloaded when it was crossed by a small tractor towing a trailer of sand, resulting in cracks in the top surface of the deck components (Cadei & Stratford, 2002). The bridge was strengthened during 1997 by bonding and rivetting GFRP plates onto the top surface of the deck (Figure 5). At the same time, the edge beams were strengthened to either side of each primary beam using prepreg CFRP sheets, to transfer the additional cable loads that would result from (for example) motorised golf buggies. Figure 5. GFRP plates retrofitted to strengthen the bridge deck (June 2008) Figure 6. CFRP strengthening retrofitted around a primary transverse beam (June 2008) IN-SERVICE PERFORMANCE APPROACHING 20 YEARS The team that designed and constructed Aberfeldy Footbridge has disbanded, and the bridge’s owner (Aberfeldy Golf Club) does not have expertise in structural engineering or FRP composites. Consequently, the bridge’s maintenance is best described as minimal, providing a useful case study into potential deterioration of FRP bridge construction. The condition survey below is based upon the author’s visual inspections of the bridge during visits in 2004, 2008, 2010 and 2011 (indicated by the dates in the figure captions). Overall Structural Performance 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 (Figure 7). Figure 7. Anchorage of a cable to the bridge deck (June 2008). Dynamic Performance Aberfeldy Footbridge is known for its lively dynamic response, due to its high live load (5.6 kN/m) to dead load (2.0 kN/m) ratio and slender proportions. Half of the dead load is ballast that was provided to improve the bridge’s aerodynamic stability. The bridge was not, however, designed to meet the allowable footfall response specified by the Highways Agency (Highways Agency, 1988), due to the bridge’s location on a private golf course (Cadei & Stratford, 2002). The dynamic performance of the bridge was characterised in 1995 (Pimental et al., 1995) and again in 2000 (Pavic et al., 2000). These studies determined the natural frequencies and damping ratios shown in Table 1 and graphically in Figure 8. The natural frequencies of the bridge were determined by the author in December 2011, and are reported alongside the previous data. Adverse weather conditions (heavy sleet and a temperature only slightly above freezing) prevented sufficient data from being collected to determine the higher natural frequencies and damping ratios in 2011. Table 1 – Dynamic response in 1995, 2000 (from Cadei & Stratford, 2002) and 2011. Frequency (Hz) Damping ratio for empty structure (%) Mode* 1995 200