Heritage bridges include 'obvious' structures such as the Pont du Gard and Segovia Aqueduct; Pont Avignon, Ponte Vecchio, Santa Trinita, Ironbridge, the Pulteney Bridge in Bath, England, and Roman bridges in Cordoba, Salamanca and Merida in Spain, and Trier in Germany. The list even includes the Old Bridge in Mostar, Bosnia-Herzegovina which was rebuilt after destruction through the help of an international scientific committee. The committee was established as a symbol of reconciliation, international cooperation and to recognise the coexistence of diverse cultural, ethnic and religious communities. The reconstructed bridge was inscribed in 2005, and is one of only six bridges which are listed as structures in their own right.

Although so few bridges have been inscribed as individual structures, many have been acknowledged because they form part of historic centres or cultural landscapes which have been listed. For example, the Roman bridges in Cordoba, Merida and Trier are mentioned in the evaluation by the International Council on Monuments & Sites, whereas in the case of Salamanca, the documentation justifying inscription of that city does not mention the bridge.

Although bridges are embedded in historic city centres or large cultural landscapes, they may not necessarily be of outstanding universal value. Despite this clarification, many of these embedded bridges are distinctive structures, and if included, World Heritage bridges would number in the hundreds. For example, 29 historic bridges including the Pont Neuf, Passerelle des Arts and Pont de la Concorde are included in the nomination for the banks of the Seine. Other more recent bridges, such as the Passerelle de Solferino would complete the 'family' of bridges within the boundaries of this nomination - certainly something for the Parisians to consider.

Similar examples would include the aqueducts on the Canal du Midi; the decorative bridges in Dessau-Wörlitz Park, Germany; Ponte Pietra and Ponte Castlevecchio in Verona; the hundreds of bridges spanning the canals of Venice, or St Petersburg; the bridges in the Spanish city of Toledo; viaducts on the Semmering Railway in Austria; India's Darjeeling-Himalayan Railway and Nilgiri Mountain Railway which include bridges and viaducts; the Blue Wonder steel bridge, part of the suspension cable railway in the Dresden-Elbe valley in Germany; the Charles Bridge over the Danube in Prague or the decorative bridges in the Summer Palace and Imperial Garden of Beijing. The most recent World Heritage bridge is Puente Vizcaya transporter bridge in Portugalete, Bilbao, inscribed this summer - Spain's first engineering World Heritage monument.

World Heritage properties must meet one or more of the following criteria and pass the test of authenticity in design, materials, workmanship or setting. They should represent a unique artistic achievement, a masterpiece of the creative genius; have exerted great influence, over a span of time or within a cultural area of the world, on developments in engineering theory, technology, construction, transportation and communication; or be an outstanding example of a type which illustrates a significant stage in bridge engineering, material or technological developments.

Reconstructed bridges are acceptable if the reconstruction was carried out on the basis of complete and detailed documentation of the original artifact, not conjecture. This criterion of authenticity was applied to the reconstructed Old Bridge in Mostar. It would similarly apply to Kintaikyo Bridge, over the Nishiki River in Iwakuni, Japan, or Palladio's Bridge over the River Brenta at Bassano a Grappa near Venice. Clearly, bridges and aqueducts surviving from the Roman Empire have been rebuilt many times over millennia.

In the same context, some bridges have been relocated when unable to function at their original site. It is not unusual in the United States, for example, to relocate a metal truss bridge to a less-travelled road when it can no longer handle modern traffic, and there is no reason to believe that this does not apply elsewhere. This is within the functional tradition of some bridge types and should not be viewed as a negative factor in determining the integrity or authenticity of a structure.

The engineering achievements of Isambard Kingdom Brunel are being celebrated throughout the UK this year (Bd&e issue no 43). English Heritage chairman, Sir Neil Cossons, announced that segments of Brunel's Great Western Railway presently on the UK's tentative list would be proposed for listing in 2010. This would add some of the UK's most significant bridges to the World Heritage List, such as the Wharncliffe Viaduct and Maidenhead Bridge, and other major railway engineering works such as Paddington Station, Box and Middle Hill Tunnels, the bridges and right-of-way through Bath, Temple Meads Station, and the Great Western Dock in which the iron-hulled SS Great Britain is berthed.

In addition to GWR features, two other Brunel-designed bridges - Clifton Suspension Bridge and Royal Albert Bridge - are likely to be considered according to the recently published consultation booklet, written by Keith Falconer who is coordinator of the World Heritage steering group within English Heritage. The Forth Railway Bridge in Scotland, designed by Sir John Fowler and Benjamin Baker is on the UK's tentative list and is scheduled to be put forward for inscription in future years. Thomas Telford and William Jessop's Pontcysyllte Aqueduct, the great iron trough and stone pier structure built in 1805 across the River Dee in Wales has also been submitted to Unesco.

The inclusion of so many engineering and industrial sites on the UK's tentative list has attracted criticism from some sectors, but both Forth and Pontcysyllte also have strong support, particularly from local councils in Scotland and Wales.

In the United States, there are 21 World Heritage sites - nine cultural and 12 natural. There are no bridges designated though two, the Brooklyn and Eads bridges, are on the tentative list, which presently is being revised. While the UK revised its list 16 years ago, America's tentative list is currently being updated for the first time in 24 years. The deadline for applying is 1 April 2007 and applications must be made by property owners to the National Park Service, which is responsible for compiling the USA's list. This means that the cities of New York, St Louis and San Francisco must apply in the next six months for the Brooklyn, Eads and Golden Gate bridges to be included on the tentative list. Preparation of World Heritage documentation would come later.

Why would an owner want an historic property designated as a World Heritage? To be compared with other international treasures such as Stonehenge, the Taj Mahal or the Grand Canyon has a certain cachet, in addition to legal protection and possible funding should repairs be required. But World Heritage designation also comes with responsibility. Part of the application process requires that owners certify, to the best of their ability, that they are capable of maintaining the property in a manner characteristic to the values it was originally nominated for. In the US, this has meant that, until now, all 21 World Heritage sites are publicly owned, primarily by the National Park Service.

Everyone agrees that the Brooklyn Bridge is eligible for World Heritage listing, including engineers in the New York City Department of Transportation which is the owner and party responsible for taking care of the structure. But the requirement that owners guarantee to look after the structure in a certain way can cause concern; some bridge owners are afraid that inscription would tie their hands if the bridge needed repairs.

In the case of historic bridges, it is almost certain that repairs will be required, and care has to be taken in design and execution of such works. The Brooklyn Bridge for example has had extensive repairs and rehabilitation over 123 years of service. However, the engineers responsible for taking care of the bridge have always made these repairs in a sensitive manner, carefully maintaining the bridge's visual appearance and the engineering intentions of John and Washington Roebling.

In order for the New York Department of Transportation to apply for the inclusion of Brooklyn Bridge on the tentative list, specific instruction would have to come from the Mayor's office or through one of the municipal commissioners. This would apply to the Eads, Golden Gate or any other bridges in the USA, and the likelihood is that most engineers will also have other favourites. The application process is not onerous, but competition is stiff since any country is limited to having ten cultural properties on its list. Although there is nothing to stop a country revising its tentative list at any time, experience shows that this is unlikely to happen unless a bridge owner can make a very strong case for having missed the previous revision deadline.

In addition to Puente Vizcaya and the other bridges mentioned above, one could argue that the George Washington or Verrazano Narrows bridges, some of America's greatest modern suspension bridges could be considered. Others might include Charles Ellet's Wheeling Suspension Bridge (1849) or the precursor to Brooklyn, Roebling's Delaware Aqueduct (1847), one of the oldest suspension bridges in the world. The suspension aqueduct has undergone extensive rehabilitation by the National Park Service, but retains the major engineering elements - the suspension towers and cables - and, the wooden canal truck has been faithfully replicated to John Roebling's original design. Others may include a selection of America's covered bridges of which there are 800.

One way round the restriction on the number of nominations would be to classify bridges by material, or bridge type, or families or groups of bridges, similar to the way the UK is conceptualising the Brunel monuments. Engineers could evaluate a selection of city bridges such as those found in Pittsburgh, Los Angeles, Cleveland, London, Paris, Prague, St Petersburg or Chicago.

GETTING LISTED

The World Heritage Convention is a Unesco treaty that was established in 1972 and is intended to ensure the proper identification, protection, conservation and preservation of the world's cultural and natural heritage. Since the first signature in 1972, more than 180 countries have ratified the convention. The World Heritage Committee, which was established by the convention in 1976, is an intergovernmental group of 21 member countries which meets annually; the World Heritage List and the World Heritage Fund were also established by the same convention. Each country can make nominations to the committee, and is encouraged to ensure the participation of a wide variety of stakeholders, including local and regional governments, local communities, non-governmental organisations and other interested groups. In the case of bridges, this would include engineers and engineering professional associations. But as yet it is unlikely that any engineer or engineering organisation has nominated a historic engineering monument, much less a bridge.

Member countries must first submit a tentative list of properties to the committee, at least a year before they intend to make their actual nominations. Nominations to the list will not be considered unless they have already been included on the tentative list, which each country is encouraged to revise and resubmit every ten years.

Decisions by the committee are based on objective and scientific evaluations by its two advisory bodies; the International Council on Monuments and Sites for cultural sites and the World Conservation Union for natural resources. These comprehensive evaluations enable the committee to make its decisions based on carefully-prepared documentation; thorough and consistent procedures; evaluation by qualified experts and if necessary, the use of expert referees. The main function of the committee, in cooperation with the nominating party, is to identify, on the basis of tentative lists, cultural and natural properties of 'outstanding universal value' which can be protected by the convention and to inscribe those properties on the World Heritage List. Although historic bridges have been nominated, the singular biggest failure of the nominating party is a contextual comparison with similar bridge types not only within the nominating country, but in other parts of the world.

The current World Heritage Committee consists of representatives from 21 of 182 member countries, and has the final say on inscription. It holds an annual meeting in June or July at which it reviews the status of inscribed properties. It has the power to recommend placing a site on the danger list if it is not being properly maintained. Such properties are monitored and financial assistance from the fund may be allocated if necessary. Currently, there are 830 World Heritage sites in 138 countries, consisting of 644 cultural sites and 162 natural ones.

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