|World Heritage Bridges
Bridging rivers, gorges, narrows, straits, and valleys always has played an important role in the history of human settlement. Since ancient times, bridges have been the most visible testimony of the noble craft of engineers. A bridge can be defined in many ways, but Andrea Palladio, the great 16th century Italian architect and engineer, hit on the essence of bridge building when he said "...bridges should befit the spirit of the community by exhibiting commodiousness, firmness, and delight." In more practical terms, he went on to explain that the way to avoid having the bridge carried away by the violence of water was to make the bridge without fixing any posts in the water. Since the beginning of time, the goal of bridge builders has been to create as wide a span as possible which is commodious, firm, and occasionally delightful. Spanning greater distances is a distinct measure of engineering prowess.
In terms of engineering, bridges are discussed by design or type (beam, arch, truss, cantilever, suspension, or moveable); length (usually expressed in terms of clear or overall span); and materials (stone, wood, cast and wrought iron, and what we use today - concrete and steel). The purpose of this contextual essay is to provide parameters of value and significance so that we can focus our attention on those bridges - globally - that best illustrate the history of bridge building, and to encourage their preservation.
What is a World Heritage bridge? The World Heritage Committee states that to be of World Heritage status a monument or site must be of outstanding universal value. It must illustrate or interpret the heritage of the world in terms of engineering, technology, transportation, communication, industry, history, or culture. World Heritage industrial sites and monuments must meet one or more of the following criteria and pass the test of authenticity:
Represent a masterpiece of human 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;
Be an outstanding example of a type which illustrates a significant stage in bridge engineering or technological developments.
A World Heritage bridge, like other properties, must meet the test of authenticity in design, materials, workmanship, or setting (the Committee has stressed that reconstruction is only acceptable if carried out on the basis of complete and detailed documentation of the original artefact and to no extent on conjecture). The criteria of authenticity may apply to Japanese bridges like the Kintaikyo spanning the Nishiki River in Iwakuni or Palladio's bridge over the River Brenta at Bassano a Grappa near Venice (Italy). In the same context, some bridges have been moved when unable to function at their original location. It is not unusual in the USA, for example, to relocate a metal truss bridge to a less travelled road when it can no longer handle the traffic; the same probably holds true for other countries. This is within the functional tradition of some bridge types and should not be viewed as a negative factor in determining the integrity of a relocated structure.
The definition of authenticity is in the process of being expanded to include intangible values such as a bridge that embodies the spirit or character of a people or place, as New York City is embodied in the Brooklyn Bridge, San Francisco in the Golden Gate, London in Tower Bridge, Sydney (Australia) in the Harbour Bridge, or Bosnia-Herzegovina in the recently destroyed Stari Most in Mostar.
Bridges nominated for World Heritage listing also must have legal protection and management mechanisms to ensure their conservation. The existence of protective legislation at the national, provincial, or municipal level is therefore essential and must be clearly stated in the nomination. Guidelines for nominations state that each property should be compared with properties of the same type dating from the same period, both within and outside the nominating State Party's borders.
For the purpose of this contextual essay, bridge design and construction is dealt with chronologically by material and by type. In addition to the obvious evaluation factors as age, rarity, integrity, and the fame of the builder, consideration also is given to the substructure (piers, abutments, foundation), the superstructure (beam, arch, truss, suspension, and combinations thereof), the materials of construction (their strength and properties), the evolution of construction techniques, and whether the bridge advanced structural theory or methods of evaluating material behaviour.
Bridges discussed in this essay illustrate important types or technological turning points and are listed at the end. Some, like the Pont du Gard (France) and the Iron Bridge (UK), are already inscribed on the World Heritage List. Others may be candidates for listing given adequate study, comparison, and evaluation. Not every potential World Heritage bridge candidate is cited. It is the job of TICCIH and its member countries to identify and make a case for outstanding bridges so they can be appreciated and protected like the great architectural and natural monuments already designated.
The first bridges were natural, such as the huge rock arch that spans the Ardèche in France, or Natural Bridge in Virginia (USA). The first man-made bridges were tree trunks laid across streams in girder fashion, flat stones, such as the clapper bridges of Dartmoor in Devon (UK), or festoons of vegetation, twisted or braided and hung in suspension. These three types - beam, arch, and suspension - have been known and built since ancient times and are the origins from which engineers and builders derived various combinations such as the truss, cantilever, cable-stayed, tied-arch, and moveable spans.
The essential difference among types is the way they bear their own weight - the "dead load" and the "live load" - a person, the railway train, wind, or snow that is applied to the bridge. The weight of beam, truss, and girder bridges bears directly downwards from their ends on the ground, piers, or abutments. Arch bridges thrust outwards as well as downwards, acting in compression. The cables of suspension bridges act in tension, pulling inwards against their anchorages.
If two or more beam or girder spans are joined together over piers, they become continuous, a form favored by European engineers, who had the mathematical knowledge to analyze the indeterminate stresses introduced by such systems. A case in point is the Town lattice truss invented by Ithiel Town, an American, in 1820, which is a rare instance of reverse techno- logical transfer. The form originated in the USA, but was widely adopted in Europe, especially in iron railway bridges. The lattice fell into disfavor in the USA, where a preference existed for statically determinate bridges of heavy timber, whose forces were easier to calculate.
A more complex form of the beam is the truss, a rigid self- supporting system of triangles transferring both dead and live loads to the abutments or piers. A more complex form of the girder is the cantilever, where trussed and anchored ends of the girder support a central span. They were favoured for deep gorges or wide fast-flowing streams where false work, a temporary structure, usually of timber, erected to assist in the construc- tion of the permanent bridge, is impossible to build. The three principal types - beam, arch, and suspension - often were combined in a variety of ways to form composite structures, the type selected depending on the nature of the crossing, the span required, the materials at hand, and the type of load anticipated - pedestrian, vehicular, railroad, or a channel of water as in aqueducts.
Other than the clapper bridges of England and similar spans surviving in other countries, bridges dating from prehistoric periods are rare. Bridges of twisted vines and creepers found in India, Africa, and South America, the ancient cantilevers of China, Kashmir, and Japan, if any survive, or the wooden arches of Japan may be candidates for World Heritage listing because they perpetuate primitive ingenuity and craft technology that is important to recognize. Since some of their materials cannot be original, these structures will have to pass the test of authenticity.
In 51 BC, during the Gallic War, Caesar attested to the construction of narrow wooden bridges by Gallic builders over wide rivers as the Loire, Seine, and Allier of 600ft (200m) span, used by pedestrians and domestic animals. The stone vault probably first sprang forth in Anatolia and the Aegean region of Asia Minor (central and western Turkey) in the 2nd millennium BC for short spans in civic construction. The Mesopotamian civilizations introduced the first major development of brick vaulting in the royal palaces, and also probably the first important arch bridges in the 6th century BC.
Fig. 1 Ponte Saint-Martin (c 25 BC) near Torino (Italy).
The greatest bridge builders of antiquity were the Romans. They applied a civil engineering repertoire on an unprecedented grand scale and achieved impressive results. Roman engineering introduced four significant developments to the art of bridge building that never had been prominent before: the discovery and extensive use of natural cement, development of the coffer dam, perfection and widespread application of the semi-circular masonry arch, and the concept of public works (Figure 1).
In these important respects, the Roman engineer vastly improved upon the efforts of his predecessors. Public water supply was the most significant aspect of Roman civil engineering: nothing like it had been achieved before nor was it to be emulated until the 19th century. Structural evolution achieved by Roman engineers is manifest in aqueducts, dam construction, and highway bridges that relied on the development of concrete, and a growing awareness of its strength.
The Romans mixed a cement, pozzolana, found near the Italian town of Pozzuoli (ancient Puteoli), with lime, sand, and water to form a mortar that did not disintegrate when exposed to water. It was used as a binder in piers and arch spandrels, and mass-formed in foundations. Coffer dams (temporary enclosures built in river beds to keep the water out while the foundations were established) were made by driving timber piles into the river bed, removing water from the area enclosed, and then excavating the soft ground inside. Despite the use of coffer dams, Roman bridge foundations typically were not deep enough to provide sufficient protection against scour. Most of the Roman bridges that survive are those built on solid rock such as the Pont du Gard aqueduct (c AD 14) near Nîmes (France), the Alcantara Bridge (AD 98) on the Spanish-Portuguese border, and the aqueduct at Segovia (AD 98), which are three of the most famous surviving Roman bridges and aqueducts. Scholars have researched Roman bridges and aqueducts for many years, so it should be possible to arrive at a well reasoned selection of Roman-built bridges for World Heritage listing.
Bridges of Asia
Figure 2 Phra Phutthos (12th century), Kompong Kdei vicinity (Cambodia), was constructed at the end of the 12th century during the reign of Jayavarman VII. With more than twenty narrow arches spanning 246ft (75m), this is the longest corbeled stone-arch bridge in the world. Institute of Asian Culture, Sophia University, Tokyo, Japan
Bridge building in Asia extends back earlier in time than in Europe. Because structural concepts of suspension, cantilever, and arch were first developed there with great sophistication, every effort should be made to identify surviving examples (Figure 2). China was the origin of many bridge forms: Marco Polo told of 12,000 bridges built of wood, stone, and iron near the ancient city of Kin-sai. The first chain-link suspension bridge, the Panhogiao or Panho Bridge (c 206 BC), was built by General Panceng during the Han Dynasty. In 1665, a missionary named Kircher described another chain-link suspension bridge of 200ft (61m) made up of twenty iron links, a common bridge type built during the Ming Dynasty that was not adapted until the 19th century in America and Europe. China's oldest surviving bridge, and the world's oldest open-spandrel segmental arch, is the Zhaozhou Bridge (c AD 605), attributed to Li Chun and built south-west of Beijing in Hebei Province during the Song Dynasty. Its thin, curved stone slabs were joined with iron dovetails so that the arch could yield without collapsing. This technique allowed the bridge to adjust to the rise and fall of abutments bearing on spongy, plastic soils and the live loads of traffic.
Following the decline of the Roman Empire with its many engineer- ing achievements, beam, arch, suspension, and cantilever bridge building flourished in China while languishing in Europe for nearly eight centuries. Chinese bridge builders experimented with forms and materials, perfecting their techniques. Selected examples, found in the countryside and parks, may be candidates for World Heritage listing.
Other fine bridges survive in Iran, such as the Bridge of Khaju at Isfahan (1667), with eighteen pointed arches, carrying an 85ft (26m) wide roadway with walled, shaded passageways, flanked by pavilions and watch towers. This magnificent bridge, combining architecture and engineering in splendid functional harmony, also served as a dam, and included a hostelry where travellers found cool rooms for rest and refreshment after hot desert crossings (Figure 3).
Picturesque bridges, such as the Kintaikyo at Iwakuni (1673), with its five wooden arches intricately wedged, slotted, and dovetailed together, are found in Japan. The superstructure of this bridge has been rebuilt for centuries (the central three arches every 18- 22 years, and the side spans every 36 years), maintaining the fine craft tradition of the bridge keepers for centuries (Figure 4). Shogun's Bridge (1638), crossing the Daiya-gawa River in the sacred City of Nikko, is the oldest known cantilever. The bridge was badly damaged in the typhoon of 1902, rebuilt, and exists today bearing foot traffic. It consists of hewn stone piers pierced with rectangular holes that permit the insertion of tightly fitting cut-stone struts, two anchor spans, timber beams jutting out in cantilever form, and a suspended span.
Figure 3 Bridge of Khaju (1667), Isfahan (Iran), combining architecture and engineering in splendid harmony, functioned as a bridge, dam, and a resort for thirsty travellers coming off the desert.
Figure 4 Kintaiko (1673), Iwakuni (Japan), with its five wooden arches intricately wedged, slotted, and dovetailed, has been faithfully rebuilt for centuries. Each generation of craftsmen has carefully replicated the joinery techniques and materials of their predecessors.
The revival of bridge building in Europe following the fall of the Roman Empire was marked by the spread of the pointed arch westward from its origins in the Middle East. The pointed arch typically was a Gothic architectural form important structurally in the development of palaces, castles, and especially the cathedrals of western Europe, but not very important for bridges. Medieval bridges continued such multi-functional traditions as the Isfahan Bridge in Iran. Chapels, shops, tollhouses, and towers adorned fortified bridges such the 1355 Pont Valentré at Cahors (France) or the Monnow Bridge (1272, 1296) at Monmouth, Wales (UK), which were built with defensive ramparts, firing slits, and drawspans.
Christian religious orders formed after the fall of the Roman Empire greatly assisted travellers by building bridges. In western and central Europe, religious groups managed popular financial institutions, with Papal sanction, both for bridge construction and for hospitals. The influence of these groups lasted from the end of the 12th to the early 14th century, and their perseverance ensured the construction of major bridges over wide rivers as the Rhône and the Danube.
The bridge over the Rhône at Avignon (1187), for example, a wooden deck on stone piers, was built by such an order under the inspired vision of a young shepherd, later canonized as St Bénézet for his accomplishment. The four surviving arches, dating from the bridge's rebuilding around 1350, rank as one of the most remarkable monuments of medieval times in view of the 101-110ft (31-34m) elliptical arches with radii varying at the crown and haunches.
As the Middle Ages drew to a close, stone arches of remarkable spans were built in mountain valleys where rock abutments provided solid foundations for spans in excess of 150ft (50m), such as the Vieille-Brioude and the Grand Pont du Doux in France.
Renaissance and Neo-Classical bridges
The great era of medieval bridge building was followed by the Quattrocento, the transition period from the medieval period to the Italian Renaissance, when the confidence and unbounded enterprise of engineers was manifested in bridges like the 1345 Ponte Vecchio, an early Florentine bridge in Italy, designed by Taddeo Gaddi, that carries a street of goldsmiths' shops on three segmental arches. This was followed by the technical efficiency and artistic advancement of Renaissance ideals of civic order during the Neo-Classical period of the 17th and 18th centuries, represented by long span and multiple stone arches: eg Santa Trinità (1569) in Florence, the Rialto (1591) in Venice, and the Pont Neuf (1607) in Paris. These bridges, which are among the most famous bridges in the world today, are all on the World Heritage List, although only as components of historic town centre inscriptions. Renaissance engineers had learned much about foundations since Roman times, though they rarely were able to excavate deeply enough to reach hard strata. They had, however, perfected techniques of spread footings - wide timber grillages resting on piles driven into the river bed upon which stone piers were laid. In the foundation of the Rialto Bridge, designer Antonio da Ponte drove six thousand timber piles, capped by three stepped grillages so that the abutment stones could be laid perpendicular to the thrust lines of the arch. Though built on soft alluvial soils, the bridge continues to support a street of jewellery shops enjoyed by tourists four centuries later.
The end of the Italian Renaissance witnessed a new vision of bridge construction. More than merely utilitarian, bridges were designed as elegant, grand passage-ways that were part of the visual perspective of the idealized cityscape - major accents to the totally redesigned merchant and capital cities. No country attempted to advance this concept more than France at the end of the 16th century, where a national transportation department of architects and engineers was set up, responsible for designing bridges and roads (Ponts et Chaussées). This corps of specialists gave the Neo-Classical period a range of monumental and elegant bridges on rivers as the Loire (Blois, Orléans, Saumur) and the Seine in Paris. This model spread all over Europe, producing large monumental urban bridges in capitals such as London, Saint Petersburg, and Prague.
In Italy, Bartolomeo Ammannati evolved a new form for the Santa Trinità Bridge - a peculiar double-curved arch whose departure from an ellipse was deliberately concealed by a decorative escutcheon at the crown. Its 1:7 rise-to-span ratio resulted in an elegantly shallow, long-arch span widely adapted in other bridges of the Renaissance. The bridge was reconstructed using original stones recovered from the river following demolition during World War II.
By the mid-18th century, masonry bridge building had reached its apogee. French engineer Jean-Rodolphe Perronet designed and built the Pont de Neuilly (1774), the Pont de Saint-Maxence (1785), and the Pont de la Concorde (1791), the latter completed when Perronet was eighty-three. Perronet's design goals were to slim down the piers and to stretch arches to the maximum. The Pont de la Concorde still represents the perfection of masonry arch construction, even though sceptical officials forced Perronet to shorten the unprecedented centre span of the bridge to 92ft (28m). Long, elegant, elliptical arches, piers half their former widths, special machinery for construction, and the introduction of an architectural motif used until the 1930s, the open parapet with turned balusters, completed this outstanding bridge. Widened in the 1950s, its original appearance was carefully maintained. Another masterpiece of the French Classical style is the Pont de Bordeaux of nineteen arches, more than 1640ft (500m), completed in 1822.
In the United Kingdom, a young Swiss engineer, Charles Labelye, was building the English equivalent of Perronet's bridges. On his first bridge, Westminster (1750) over the Thames, he developed the caisson, which made it possible for pier foundations to be built in deep, fast-flowing waters. To solve a problem that had confounded bridge builders since Roman times, Labelye used huge timber boxes constructed on shore, floated into position, and slowly sunk to the bottom of the river by the weight of the masonry piers being laid above. Fifteen semicircular arches, incrementally diminishing in length from the centre and rising in a graceful camber, set a high engineering and architectural standard that stood for over a hundred years.
England's other great bridge designer during this period, John Rennie, built the first Waterloo Bridge in 1811. Its level road and arches lasted until 1938. Rennie's next great bridge was Southwark Bridge (1819), also over the Thames in London, which was built not in stone but in the new miracle material of the 19th century - cast iron. It had three arches whose central span of 240ft (73m) dramatically demonstrated the potential of the new material.
Figure 5 Pontypridd Bridge (1756) over the Taff in South Wales (UK), had to be rebuilt several times until its builder, William Edwards, got the correct rise-to-span ratio to ensure that the 140ft (43m) arch would not collapse after removal of the falsework.
Wooden bridges are some of the most ancient. The first Roman bridge, the Pons Sublicius (c 621 BC), was a wood-pile structure over the Tiber in Rome, extending pedestrian access to the Aventine Hill. The earliest detailed description of a wooden bridge, a timber-pile structure over the Rhine constructed in 55 BC, was written by Julius Caesar in his De Bello Gallico. The best extant model of this type survives today over the Brenta at Bassano a Grappa, near Venice. It was built by Palladio in 1561, destroyed in 1945, and reconstructed identical to the original in 1948.
By the mid-18th century, carpenters working in the forested regions of the world further developed the timber truss bridge. The most famous were two Swiss brothers, Johannes and Ulrich Grubenmann, who built bridges at Schaffhausen, Reichenau, and Wettingen that combined diagonal struts and trusses to produce remarkably long spans for their time. The Schaffhausen Bridge (1757), over the Rhine in northern Switzerland, had two spans, 171ft and 193ft (52m and 59m) respectively, which rested lightly on an intermediate pier when loaded. It was burned by the French in 1799 during the Napoleonic Wars. One of the few Grubenmann bridges to survive is Rumlangbrücke (1766), with a span of 89ft (27m).
European engineers visiting the New World during the 19th century marvelled at the spans achieved by American timber bridges. Especially noteworthy was Louis Wernwag's 340ft (104m) arch truss of 1812, the "Colossus," over the Schuylkill in Philadelphia, the longest spanning bridge in the world at the time. Covered bridges, sheathed in wood to keep the structural timbers from deteriorating, are an icon of the American landscape. Outstanding spans that survive today include the Cornish-Windsor Bridge (1866) over the Connecticut River and the Bridgeport Bridge (1862), whose clear span of 208ft (63m) makes this gateway to the California goldfields the second longest single span. According to the National Society for the Preservation of Covered Bridges Inc, some 800 wooden covered bridges survive in the USA, more than in any other country (Figure 6).
Figure 6 Bridgeport Bridge (1862), clear-spanning 208ft (63m) over the South Fork of the Yuba River near Grass Valley, California (USA), has two parallel trusses based on the Howe patent of timber and iron rods, flanked by solid wooden arches cut to the curves and reflected in the exterior siding. It is the second longest covered wooden bridge span in the USA, after the Blenheim Bridge (1855) in New York State, which is 210ft (64m).
Regardless of the capability of advanced societies like the Romans to build bridges in stone, the material for the ages, its cost always remained a problem. Wooden bridges were an economic alternative important to every civilization during all historic periods from prehistoric times to the first American settlement, from classical Rome to the European Enlightenment, including China, Japan, and south-east Asia. Wooden bridges have played a major role in the history of human development. The architectural varieties and structural types - girder, arch, suspension, truss, pontoon, and covered - were numerous. By virtue of the nature of their material, extant examples are scarce, as is the historic record. Nature, acts of God, war, and arson have decimated wooden bridges throughout time. A special global effort should be initiated to identify, access, and protect wooden structures of all kinds. A group of experts should be convened in the USA and in other parts of the world where timber bridges survive to recommend a selection for nomination to the World Heritage List.