A view of the Forth Road Bridge from South Queensferry.
© John G. Fender 2011
The Scottish Region visited the Forth Road Bridge on 23rd August 2007 and were given a detailed presentation on the Bridge and tour of the control room.
The presentation, by Alastair Andrew, General Manager and Bridgemaster, ranged from the the construction of the bridge to the present day challenges facing the Forth Estuary Transport Authority (FETA). The story of the bridge begins in 1947 when the government decided that a bridge be built and set up the Forth Road Bridge Joint Board to build and operate the bridge. The Joint Board would also be responsible for the maintenance of the bridge once completed.
Two firms of consulting engineers, Messrs. Mott Hay and Anderson and Sir Freeman Fox and Partners, were involved in the design of the bridge. A consortium of Sir William Arrol & Company, The Cleveland Bridge & Engineering Company and Dorman Long ( Bridge and Engineering) was established to build the bridge. Construction work began in 1958 and the bridge was opened in 1964. It cost £11.5 million to build and this amount included the approach viaducts and 8 miles of dual carriageway road. An additional 8 miles of other roads and some 24 minor bridges brought the overall cost to £19.5 million.
The bridge is 2,512 metres long with a main span of 1,006 metres. The bridge is 33 metres wide with a two lane carriageway in each direction, plus a footpath on each side. The main span has a clearance of 44.3 metres above the mean high water. The bridge towers are 153 metres high and the bridge is supported by two main suspension cables of 590mm diameter. These cables are made up of 11,618 individual wires. The spans are attached to the main cables by 192 sets of hanger ropes. When the bridge opened traffic volumes were far lower than today and in the first year some 4 million vehicles used the bridge. Today the number of vehicles using the bridge is over 22 million a year. This makes it one of the busiest bridges in the country and during every working day it is operating at or over its design limit.
When the bridge was designed, the design was based on a 22 ton lorry. Since then lorry weights have increased, for example to 38 tonnes in 1983 and to 44 tonnes in 2001. The bridge has a design capacity of some 2,880 tonnes but is presently carrying 5,620 tonnes. This has required strengthening work on the main towers and trusses to cope with the increased loads. The bridge's vehicle parapets were built before there was a British Standard established for them and recently tests have been carried out to determine if the parapets meet the current criteria for containment. If the tests are unsuccessful, all 6 miles of steel parapet will require to be replaced. The main bearings and deck expansion joints were designed for a 30 year life span, but are now 45 years old and need replacing. Consultants have already been appointed to advise FETA on this work.
The method used to paint the bridge has had to change over the years as the removal of old paint, by sand or shot blasting, must now be done in controlled conditions to contain the spent material and paint particles in order to protect the environment. The road surface is one and a half inches thick and the original surface lasted for 25 years before needing to be replaced. The surface consists of Trinidadian mastic asphalt mixed with graded aggregate. This now only lasts 8 years before needing replaced due to the increased volumes of traffic and wear caused by the use of "super single" high pressure lorry tyres. These tyres lead to longitudinal cracking of the surface. Resurfacing has to be undertaken by hand as there are no machines capable of laying the type of surfacing used, as it is laid straight onto the steel decking of the bridge.
The cost of the maintenance for the bridge is currently met from toll revenue and when FETA was set up to replace the former Joint Board, the new authority put forward a proposal for a variable toll of up to £4, but the application for this co-incided with a by-election in Fife and chancellor Gordon Brown during the election campaign announced that there would be no such increase in tolls. The SNP announced as part of their 2007 parliamentary election campaign that tolls would be scrapped altogether and now following the Scottish Parliamentary election results when the SNP gained power, the tolls are now about to be scrapped. The revenue raised from the tolls pays for the maintenance of the bridge, around £12 million per year, but once the tolls are removed, the Scottish Executive will have to provide the funds needed to maintain the bridge.
However, the main challenge facing FETA is the condition of the main cables. Problems of corrosion had been discovered in some older American bridges and in 1998, the US government set up a working party to examine how best to approach the problems of examination and analysis. Draft guidelines were issued in 2002. In April 2003 site investigations of the main suspension cables on the Forth Road Bridge began and the findings surprised everyone. The main cables are made up of 11,680 galvanised wires and it was found that the galvanising had been lost and black pitting of wires was found. A very small number of broken wires was also discovered. Tiny cracks were also found in the wires and this was attributed to the method of making the wires using a 5 foot diameter mandrel.
The discovery of corrosion has lead to the conclusion that the bridge safety factor will drop below 2 around 2014 and at that point it will have to be closed to heavy traffic and possibly be closed completely by 2020. This means that the bridge has a life of only 7 years if the corrosion cannot be halted. Acoustic monitoring, consisting of 15 microphones on each cable was fitted in to the main cables in 2006 and since then this has detected 10 wires breaking. The solution chosen to try to halt the corrosion is to dehumidify the main cables. This technique was developed in Japan to protect their suspension bridges and involves applying an airtight coating to the main cables, then pumping in dried air to remove moisture. This can be done as there is a tiny space between each of the wires making up the main cable. Moist air then is removed and this should stop the corrosion. It would be possible to replace the main cables, but this would require extensive disruption to traffic over a prolonged period and involve short closures of the bridge.
The discovery of the problems has led to new proposals for a replacement crossing and although there have been proposals for a new bridge before, these came to nothing. A study commissioned by SESTRAN has been undertaken for a replacement crossing and proposals include a bridge and tunnel. A public consultation is being undertaken and details can be found at the website www.forthreplacmentcrossing.info. A decision on a new crossing, whether it be a bridge or tunnel needs to be made this year so that construction can get underway to allow completion of the new crossing by 2014 when the existing Forth Bridge may have to have traffic restrictions introduced if the action being taken to halt the corrosion in the main cables is unsuccessful. However, it will not be known if the de-humidifying treatment has worked until 2009 - 2010.
A tour of the control room followed and members were also afforded the opportunity to put questions to Alastair Andrew on many aspects of the bridge's operation and maintenance as well as on the challenges that were highlighted in the presentation.
The Scottish Region would like to thank Alastair Andrew for his presentation and assistance with this article and also to the staff on duty who were most helpful with the visit.
Further information on the Forth Road Bridge and up to date information on roadworks can be found on the Forth Estuary Transport Authority Website at www.feta.gov.uk
Report by John Fender
Report by John Fender.
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