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Bridge brilliance | A history of innovation

bridge illustration main feature 23

Bridge design has come a long way: from crudely pushing a wooden log over a stream to today’s longest bridge – China’s 35km Hangzhou Bay Bridge.

Much of this change took place over 6,000 years where a handful of innovations led from humble brick arches to today’s reinforced concrete megastructures.

The ancient civilisations of Assyria and Egypt could have been the first to introduce bridge building. Stone arches can be found over the entrance of the great pyramid of Ghizeh, Egypt (3200 to 4200 BC) but they were not true arches as they were made of single sloping stones meeting over an opening.

The first real evidence of bridge-building technology can be traced back to Babylonian society in about 4000BC. Outside Babylon was a great reservoir, holding overflow from the river Euphrates. The river was spanned in the centre of the city by a single brick arch, one furlong (220m) long and 10m wide. Brick was the only building material.

About this time in China, masonry arches were also known to have existed, documented at around 2000BC. Arches can be seen in the Great Wall of China (completed 214BC) for carrying it over streams and rivers.

Other early evidence of bridges is in passing references, often in the grander description of a military victory. The earliest record of a timber bridge is from Herodotus describing one built over the Eurphrates in Babylon about 783BC. With stone piers, connected with wooden platforms, it was removed at night to prevent thieves entering the city. One description was 11m wide and 201m long, and the piers were built on a sandy river bottom with arches of hewn stone fastened together with iron chains and melted lead.

Roman arch

The next giant leap in bridge building came from the Romans, who incorporated the true semicircular arch into their designs in about 500 to 600BC. Many of their best examples are still standing up to 2,500 years later – how many bridges built today will boast a similar lifespan?

The Romans’ expertise often derived from a tendency to place liability for bridge failures on their builders – money for construction was withheld until 40 years after construction.

The Alcantara Bridge, built over the Tagus River in Spain is a prime example. The mortar-less stone bridge was built around 100AD. It has been ruined four times since then – by wars, not by the elements. With six semi-circular granite arches, the largest spanning 28.8m, it measures 181m in total and its deck sits 62m above water.

Alcantara brdge

Alcantara brdge

The Alcantara Bridge — built without mortar — has stood over the Tagus River in Spain for about 1900 years.

Dark Ages

The Dark Ages (500AD to 1100AD) played host to a severe decline in the art of bridge building. Bridges often had steep gradients at the ends and unequal spans – and were generally inferior to those of the Romans.

In the 11th and 12th centuries the religious order The Brothers of the Bridge began a programme of bridge construction. Benedictine monks established houses at river crossings for comfort and safety, protecting vulnerable travellers against bandits, and preservation of the bridges and their passengers became a sacred duty. The monks also picked up skills in bridge design, construction and maintenance, and were in hot demand across Europe.

Perhaps the most famous of the Brothers’ bridges was built in 1186 to cross the Rhone at Avignon, southern France. Built with 22 spans – the longest 34m – and low arches, it inspired the aesthetics and engineering of many bridges to come.

The most accomplished bridges of the period gained the dubious honour of a “Devil’s Bridge”. There were dozens of these bridges that seemed unfathomable to the layperson. According to legend engineers turned to the Devil for help with construction. The Trezzo sull’Adda Bridge, built in Lombardy, Italy 1377, was such a design. With its single arch span of about 75m it held the record for largest masonry span for 500 years, until 1903 and the Adolphe Bridge in Luxembourg was built. The Trezzo bridge only lasted 39 years, however – when the city was under siege in 1416 the city’s leaders deliberately caused it to collapse by weakening the abutments.

Renaissance

Bridge building became more active in France than any other country between 1500AD and 1750AD, chiefly under the direction of the Brothers of the Bridges. The tendency was for thinner piers, and the use of flatter arch forms to create wider spans, without excessive rises, or steep roadway grades. Interestingly, around this time the first dredging machine was used to build the foundations of a bridge at Maastricht over the Maas river in the Netherlands, in 1635.

At the start of 18th century, it becomes evident in France that public bridges were in dangerous condition and needed rebuilding. In 1715 the first “Department of Bridges and Roads” was established. The “National School of Bridges and Roads” followed in 1747 – probably the world’s first school aimed at producing civil engineers.

Advances were made in England as well; Westminister Bridge, begun 1738 and finished 1750, marks the beginning of a new era, with caissons instead of cofferdams used for the first time. It also brought in a new era of “caisson disease”, or decompression sickness, causing countless deaths to workers.

Pont saint bénezet

Pont saint bénezet

The remains of the Pont d’Avignon — a landmark of bridge building achievement.

Around the late 18th Century the principle of the truss is rediscovered; it was first discovered by Italian architect Palladio in 1560 – for use in roofs, not bridges.

Iron

For a relatively short time in history cast iron was in fashion, and the Coalbrookedale bridge (built 1776) is a foremost example. With a span of 33m, centre rise of 15m. It contains 378t of metal and was the first bridge in any country entirely composed of iron.

Cast iron is relatively brittle and less reliable than steel, but suffers little to no corrosion from rust (unlike wrought iron and steel). Accordingly, the few examples that did remain standing, have stood a long time. But cast iron bridges died out from about 1875, with the introduction of the more ductile wrought iron and steel.

Structural steel, with a safe working strength 20% greater than wrought iron, came into general use about 1890, and in less than five years became the only metal used in bridges. First use of steel in a bridge globally was much earlier in 1828, for the chain bridge in Vienna, over the Danube canal, with a span of 101m.

Suspension bridges

Suspension bridges had been built throughout history, with examples seen in ancient remote villages of China, Japan, India, Tibet and Mexico and South America, using twisted vines or hide to attach to the bridge and nearby trees. But the first modern suspension bridge arrived in 1801, and bridged between Uniontown and Greensburg, Pennsylvania with a span of 21m.

The next big development, also in Pennsylvania, was the first wire cable suspension bridge, opened 1816, with a single span of 124m, a width of only 300mm, and cables comprising six 9mm-diameter wires. At this time the concept of resonance is unknown; the collapses of many of these bridges was put down to “wind” or “marching soldiers”.

Scientific rigour

Scientific and exact computation of stresses in bridge frames originated in the United States in 1847, with Squire Whipple’s A Work on Bridge Building, then Herman Haupt’s The General Theory of Bridge Construction published in 1851. Before this, bridge members were proportioned according to the judgement of experienced builders. Prior to 1860, railway companies designed their bridges in their own offices and manufactured parts in their own shops, but in the following 10 years, those who had become proficient in the art of bridge building organised contractors and engineering companies to take contracts and manufacture their bridges, frequently to their own patented designs.

Coalbrookedale bridge

Coalbrookedale bridge

The Coalbrookedale Bridge was the world’s first bridge made entirely of cast iron.

And as rail travel exploded, this rigour was sorely needed in the industry – Statistics from 1888 showed bridges on American railroads were failing at a rate of 25 per year, or 1 per 8,050km of railroad built.

Concrete

One of the first examples of a concrete bridge is still standing in Amalfi, Italy. It was built in the 6th Century. At 1.5m wide, 3m above water and 7m long, the structure is still in good condition more than 1400 years later. But the Romans used concrete mainly as a “filler” material, and it was not until the late 19th Century that exposed concrete was first used. The Grand Maitre Aqueduct (1850-65) in Farance is a prime example, it conveys water from the Vanne River the full 151km to Paris thanks to a series of concrete arches.

Conventional concrete bridge design spread from Switzerland, to America where the railroad companies renewed old steel bridges with concrete, often (like the Romans) facing the concrete structures with stone.

Reinforced

Brunel might have been the first person to experiment with reinforced concrete arches, building a semi-arch of brick 18.3m long with hoop iron bond, which supported itself through cantilever action.

Then in 1867, French gardener Jean Monier, decided to strengthen the large cement flower pots and urns he made with a single layer of wire mesh. In just the next few years he extended this idea to tanks, bins and arches, protecting his inventions with German patents.

With the patents spreading around the world quickly, the first example of a reinforced concrete bridge built in England spans the Waveney at Homersfield in 1871. It has a 15.2m span, a rise of 1.5m, and uses more than 100t of concrete. Definitive knowledge of reinforced concrete’s behaviour was a stumbling block for the industry. But with more tests and research it became the dominant force in bridge building, continuing through to the more architecturally-focused designs of today.

Grandmaitreaqueduct

Grandmaitreaqueduct

Source: Lehigh University Library

The Grand Maitre Aqueduct was one of the world’s first great uses of exposed concrete.

Box Girders

In 1845 engineer Robert Stephenson got the job to oversee the railway bridge over the Menai Strait at Anglesey, connecting the railway between London to Holyhead. He envisioned a radical system of prefabrication, where the horizontal elements could be broken into three sections, floated out, and lifted up into position on piers.

British engineer/mathematician team of Sir William Fairbairn and Eaton Hodgkinson were called upon. The pair had a good reputation, previously working elsewhere in Britain on railway bridges and finding a formula for which a beam will fail as a function of its length.

Their first thought was for cylindrical tubes, thought to be the ideal form. But various experiments led to rectangular boxes. More experiments were scaled up and up, again and again, over about six weeks, and with £6,000 assistance from the railway, up to a one-sixteenth model with girders 23.8m long. Eventually the rectangular box shape became large enough – about 4.5m wide – that, rather than run the trains on top, they ran them inside.

The innovation had an immediate impact – the spans of 140m on the bridge, known as the Britannia bridge were almost 15 times longer than the longest wrought iron span so far.

Few other rectangular, tubular iron bridges were ever built, due to more economical materials. But as a forerunner to the modern box girder, the Britannia is a landmark of engineering.

During the 20th century, box girder design evolved into lighter weight, even longer span steel structures, often used in suspension bridges, notably the Severn suspension bridge, built in the 1960s and and Hong Kong’s Tsing Ma bridge, buit in the 1990s.

Britannia Bridg

Britannia Bridge

Britannia Bridge, forerunner to the modern box girder.

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