50 years in the life of a bridge: maintenance works on the Forth Road 1,006m-span

Author: Barry Colford, chief engineer and bridgemaster, South Queensferry, Scotland The Forth Road Bridge is a long-span suspension bridge and was opened in September 1964. At that time, the bridge had the fourth-longest main span in the world (1,006 metres). The bridge crosses the Firth of Forth, some 15km west of Edinburgh, and is a vital link in Scotland’s strategic road network. Its deck supports a dual two-lane carriageway without hard shoulders or strips. There is a separate footway/cycletrack on either side. About 25 million vehicles now cross the bridge each year. The historic importance of the structure to Scotland was recognised in 2001 when it was classed as a Category A listed structure. The bridge had been a tolled crossing since opening. However, in February 2008, tolls were removed, following an Act of the Scottish Parliament which removed tolls from all of the country’s tolled bridges. Funding for maintenance and operation of the bridge now comes via a Scottish government annual grant.

Details of the construction

The bridge has a main span of 1,006 metres and the side spans are each 408 metres long. The orthotropic deck on the main span is made up of a stiffened steel plate overlain with mastic asphalt, on a waterproofing layer. The deck to the side spans is a reinforced concrete slab with a similar surfacing detail to the main span. The decks on both the main and side spans are supported on steel stringer beams that span between large steel cross girders. These cross girders are supported by two longitudinal stiffening trusses, which in turn are supported by the main cables. The main cables transfer the loads from the bridge to the main and side towers and also down to the north and south anchorages. The anchorages are tapered rock tunnels, filled with concrete and post tensioned. Linking the stiffening trusses to the main cables are 192 sets of steel wire rope hangers. [caption id="attachment_20518" align="alignright" width="300"]

Suspended span stiffening truss[/caption] The main towers are of steel box construction and are formed from three fabricated steel boxes that are joined by cover plates to provide a five cell structure in plan. The legs of each tower are connected by cross members at the top, and just below deck level, and by diagonal stiffened box bracing above and below the deck. The approach viaducts to the bridge are themselves large steel and twin box girders with a reinforced concrete deck slab. The 11-span south viaduct is 438 metres long and the six-span north viaduct is 253 metres long. The design and construction of the bridge is described in the Proceedings of the Institution of Civil Engineers ₍₁₎.

Summary of major improvement works

In the 35 years after opening several significant capital projects have been carried out to replace, strengthen or improve elements of the structure. These projects were necessary due to changes in traffic loading; design code changes; deterioration of components and a risk assessment of shipping impact. These projects are listed below:

Strengthening of viaduct box girders £1,145,485
Main tower wind bracing strengthening £2,916,312
Main tower strengthening £12,742,999
Construction of pier defences £9,914,357
Hangar replacement £9,706,998

Between year 2000/01 and 2013/14 a total of more than £104 million was spent on a further programme of strengthening and improvement works. The schemes carried out, with a value more than £1 million, include:

Main span resurfacing overlay £1,068,893
Reconstruct toll plaza £1,323,532
Main span surfacing south £3,614,494
New gantry and runway £3,134,495
Main tower painting £7,497,980
approach road upgrade £16,633,843
Main cable acoustic monitoring £1,182,215
Main cable first internal inspection £5,472,439
Toll equipment replacement £8,331,885
Viaduct bearing replacement £18,650,176
Main expansion joints replacement £3,060,480
Anchorages Investigation £5,198,514
Main cable dehumidification £11,532,067
Main cable rp't/augmentation study £1,023,873
Cable band bolt replacement £4,374,281

Main cable work

One of the major issues to affect the bridge over the past 15 years has been the condition of the main cables. The two main cables are each made up of 11,618 high tensile galvanised wires, 4.98 mm in diameter. These are compacted together in parallel and wrapped circumferentially with a lower-strength galvanised round wire. Unfortunately, one of the disadvantages of this form of construction is that there is no way to inspect the main bridge wires without taking the wrapping wire off and opening up the cables to allow an inspection. [caption id="attachment_20521" align="alignright" width="300"]

Internal inspection methods[/caption] Following work carried out on the main cables of bridges in the USA, a decision was taken in 2003 to open up the main cables on Forth Road Bridge in order to carry out an internal inspection. This first internal inspection work was completed in 2004 and 2005 and a total of 10 18-metre-long panels (a panel is the length of main cable between vertical hangers) at various points on both cables were opened, inspected and rewrapped. In general, eight samples of individual wires - each six metres long - were taken at each panel, and cut to a length of 254mm for tensile testing. Tests to determine the degree of deterioration of the zinc coating and tests on water samples were also undertaken. To inspect a panel the wrapping wire firstly had to be removed. Strict containment had to be in place for this work as red lead was present. Initially, eight wedge lines were opened around the cable using brass chisels and then driving in hardwood and plastic wedges. This allowed inspection to take place down to the middle of the cable as shown in Figure 4. Much to the surprise of the inspection team, fairly extensive corrosion and wire breaks, although these were relatively small in number, were found in some panels. Given the relatively young age of the bridge these results gave cause for concern. Based upon the results of this limited intrusive inspection of the main cables, it was concluded that the loss of strength of the cables, based on the worst section uncovered, was about eight per cent. It was predicted that if deterioration could not be halted, the cable could lose 13 per cent of original strength by 2014 and 17 per cent by 2019. The results of the first internal inspection of the main cables at Forth were not only significant for the Bridge Authority but also for the wider bridge community. The results suggested that there were serious doubts over the use of paint systems to try to protect the cables of suspension bridges. As a consequence of the work at Forth, the owners of Severn and Humber bridges in England, instigated a programme of cable inspections. [caption id="attachment_20523" align="alignright" width="300"]

Main cable broken wires[/caption] At Forth, it was concluded that if the rate of deterioration due to corrosion could not be halted consideration would have to be given to the possibility of introducing loading restrictions on the bridge in or around 2014. It was clear that action was required to try to halt or limit the deterioration and to try to monitor the cables and the following works were commissioned: • Installation of acoustic monitoring on both cables; • Installation of a dehumidification system on both cables. An acoustic monitoring system was installed in 2006 in order to monitor future wire breaks within both cables and to provide information when selecting panels to open up for the next internal inspection.

Dehumidification

Following discussions with other bridge operators in Europe, Japan and the USA, a decision was taken to install a system of dehumidification on Forth Road Bridge. This work involves pumping dried air into the cables at various points having first wrapped it in an airtight neoprene membrane. As already described above, the main cables are made up of 11,618 parallel wires that have been compacted. However, within the cross-section of the cable between the contact points of the wires there are voids and at Forth these voids make up 20.5 per cent of the cross-sectional area. The key objective in dehumidification is to fill the voids with air with a low relative humidity. [caption id="attachment_20525" align="alignright" width="300"]

Acoustic monitoring[/caption] It is considered that a piece of galvanised wire in a chamber with a relative humidity of less than 40 per cent will not corrode. Therefore, if air with a relative humidity of less than 40 per cent can be introduced into the cable and surround all the wires, then the capacity for further corrosion is likely to be negated. [caption id="attachment_20529" align="alignleft" width="300"]

A dehumidification outlet vent[/caption] The air that is introduced within the cables at Forth is at a very low pressure (some 3,000 pascals) via inlets spaced along each cable. The air vents then travel along the cable, either 160 metres in the main span or 200 metres in the side spans. A typical outlet is shown in Figure 6. The works to complete the dehumidification of both cables was successfully completed before the target end date of October 2009 and the system is monitored and has produced the expected slow fall in the relative humidity within the cable over time as shown below. It should be stressed while it is considered that there is good reason to have confidence that dehumidification could slow down or halt corrosion there is no body of evidence yet available to allow an assurance to be given that this will work on Forth. While it is fairly certain that dehumidification will stop further corrosion, it remains to be seen what will happen to already cracked and damaged wires. A further internal inspection of the main cables was carried out in 2012 to verify that the dehumidification system was protecting the cables and to determine the estimated current and future strength of them. This inspection showed that the rate of strength loss appeared to be reducing. However, future inspections will be necessary to confirm these initial assumptions. In 2008, there were a number of issues relating to Forth Road Bridge that were causing concerns not only to the authority but also to the government and the wider public. These were the condition of the main cables and the likely disruption to users during any replacement or augmentation of the cables; the unknown condition of the anchorages and the major deck resurfacing and joint replacement work being scheduled.

As a consequence of these issues, the Scottish government decided to commence the process to build a new crossing across the Forth at Queensferry to ensure resilience in the transport system on the east side of Scotland.