Historical introduction

Therefore, from early history bridges have been installed to avoid people getting their feet wet when crossing a stream or to create a short cut over or around some obstacle.

Perhaps the first ideas for bridges came from fallen trees, rock falls or hanging creepers across rivers or streams. With the development of tools it became possible to cut trees and stone slabs and to place them where required to form bridges. However, the span was generally limited by the size of the tree or weight of slab that could be transported. Also with the skill of making ropes from natural fibres it became possible to construct suspension bridges of small span.

The Romans were innovative engineers and architects. They developed trestle timber bridges and as masters of the stone arch constructed such magnificent works as the aqueducts in Segovia, Spain and Pont du Gard in France.

Ironbridge, which spans over the River Severn in Coalbrookdale, is recognised as the first major structure in cast iron.

Recognised as being the first major structure in the world constructed of cast iron, Ironbridge is supported by five iron arches made up of ten half-ribs each cast as one piece at Darby's Coalbrookdale foundry. The bridge was commenced in 1779 and opened on New Year's Day 1781. Following a conceptual design by architect Thomas F. Pritchard, the detailed design, fabrication and erection was carried out under the direction of ironmaster Abraham Darby III. Detailing of the bridge was carried out by Darby's foreman pattern maker, Thomas Gregory, and this is thought to have resulted in the similarity between carpentry joints and the mortise and tenon joints found in the ironwork. Three hundred and seventy-eight tons of cast iron were used in the bridge's construction which is thought to have consumed several months production from the furnaces existing at that time.

Thomas Telford, founder of the Institution of Civil Engineers, designed many significant bridges using the new high strength materials cast and wrought iron, notably Pont Cysyllte Aqueduct (1795-1805) near Llangollen, Menai Bridge (1818-1826) between Anglesey and mainland Wales, and many less well-known examples such as Mythe Bridge near Tewkesbury (1823-26).

Pont Cysyllte Aqueduct carries the Llangollen Branch of the Ellesmere Canal in a 3.6m wide sectional cast iron trough, of 305m overall length. Nineteen spans are supported on cast iron arch ribs 13.6m long spanning between the stone piers that rise up to 39m above the River Dee. This was Telford's first excursion into canal engineering and he worked under the supervision of William Jessop.

At the time of its construction, Telford's Menai Bridge, which fully exploited the tensile strength of wrought iron for the suspension chains, was, at 579 feet (176.5m), the longest span in the world. The individual links 9 feet (2.7m) long were calculated to have breaking load of 87.75 tons (874.3 kN). However, prior to use in the bridge they were tested to a load of just 35 tons (348.7 kN), equivalent to a stress of 11 tons/square inch (170 N/mm2), to avoid damaging the links. This proof load was double the expected stress in the links under maximum loading (5.5 tons/sq. in. or 85 N/mm² ).

Mythe Bridge, over the River Severn to the west of Tewkesbury, was considered by Telford to be rather special. The elegant 170 foot (51.8m) single span arch has six cast iron ribs, each divided into 8 segments, which were cast in the Shrewsbury works of William Hazledine. Loads are transferred from the deck of cast iron flanged plates to the arch ribs by "X" pattern spandrel bracing.

Railway builder Robert Stephenson is renowned for the Newcastle High Level Bridge (1849), and two innovative tubular railway bridges one at Conwy (1848) and the other; Britannia Bridge, spanning the Menai Straits (1850).

The Newcastle High Level Bridge (1849) is a bowstring arch with both road and rail decks one above the other.

The tubular bridges, designed to carry the Chester and Holyhead Railway, "...marked a significant advance in the art of bridge building" and could be considered as prototypes for modern box girder bridge construction. The 122m long, single span, Conwy bridge consists of two wrought iron tubes, rectangular in section, and weighing 1300 tons each. These were fabricated, on shore, by riveting together individual plates of wrought iron. Then the tubes were floated into position on pontoons before lifting onto their supports.

The Britannia Bridge consisted of two main spans of 140m, with twin tubes weighing 1800 tons each, and two side spans of 70m. The construction and method of erection was similar to the Conwy bridge with the main tubes of the Britannia Bridge being assembled on the Caernarfon side of the Straits before being floated out and slowly raised 30m. Once all four spans of the Britannia Bridge were in place they were connected longitudinally at the supporting towers to create a continuous beam and therefore a more efficient structure. Sadly, in 1970 a fire in the timber roof protecting the wrought iron tubes caused major damage and the original tubes were replaced by arches springing from low level. At the same time a road deck was added above the railway tracks but the original towers and their embellishing lions were retained.

Isambard Kingdom Brunel (1806-1859), perhaps the best known of British engineers, created many beautiful bridges using a wide range of materials. His iron bridges include the Royal Albert Bridge (1859), at Saltash near Plymouth and the Clifton Suspension Bridge high over the Avon Gorge in Bristol (which was actually completed in 1864, five years after his death).

Brunel used a hybrid structure for the Royal Albert Bridge, which has two main spans of 455 feet (138.7m) carrying a single rail track. The deck of each span hangs from a parabolic tubular arch (of elliptical section 16 feet by 9 feet (4.9m x 2.7m)) tied by wrought iron suspension chains made from links 20 feet (6.1m) long. This forms a closed structural system, with the outward thrust of the arches being balanced by the inward pull of the suspension chains so that the supporting piers carry predominantly vertical loads. Many of the chain links were originally made for the Clifton Suspension Bridge but were used here when that project was delayed due to financial problems.

At the age of only 25, Brunel won a competition for the design of a bridge over Clifton Gorge in Bristol. Although construction work commenced in 1836 it came to a halt in 1842 when the client's money ran out. Following Brunel's death in 1859, as a memorial to his great achievements, Clifton Suspension Bridge was completed in an amended form. Some of the chains used in the bridge came from Brunel's Hungerford Footbridge, London which, although only completed in 1845, was removed to make way for Charing Cross Railway Bridge.

In the USA perhaps the greatest pioneer of bridge engineering was John Augustus Roebling (1806-1869), designer of many innovative bridges. These included a suspension/cable-stayed bridge to carry a railway line over the gorge below the Niagara Falls (1855) and the Brooklyn Bridge (1883) in New York (completed by his son Colonel Washington A. Roebling).

The bridge over the Niagara River designed by John Roebling was the first suspension bridge to carry a railway. It was a double deck bridge of 250m span with the upper deck for trains and the lower deck for road traffic. Loads were carried by a hybrid system of a timber Howe truss hung from a combination of suspension cables (with 3640 wires per cable) and stays radiating from the pylons. In 1896, due to further increases in loads on the bridge (which had already reached 2.5 times the originally design loading) a replacement arch bridge was constructed.

Roebling's best known work was the 486m span bridge crossing New York's East River between Brooklyn and Manhattan. He died shortly before construction was commenced in 1869 but the work was taken over by his son Washington Roebling (1837-1927), who completed the work despite becoming paralysed due to the effects of "caisson disease" caused by working under compressed air in the deep foundations of the bridge.

Gustav Eiffel, who is probably best known as the designer of the Eiffel Tower in Paris, designed many elegant trussed bridges including the 165m span, parabolic-arched Garabit Viaduct (1884) in the Massif Central of France.

The main span of the Garabit Viaduct is an elegant example of a two-pinned, trussed parabolic arch which supports a trussed multiple span girder. To construct the arch, which reaches over 120m above the river level, the two halves were built in sections, cantilevering from their bases and tied back to the previously installed approach viaducts.

The first major steel structure in the United Kingdom was the Forth Rail Bridge, designed by Benjamin Baker (1840-1907), partners in the consulting engineers Harrison, Barlow, Fowler and Baker, and opened officially in March 1890. At the time this was the largest bridge structure in the world.

The bridge's structure is that of a continuous trussed girder that has been broken down into smaller parts by the introduction hinged joints at points of contraflexure (zero bending moment) along its length. This produces three massive balanced cantilevers supporting two short suspended spans providing two clear spans of 1700 ft (521m).

The structural principle is admirably demonstrated by the "Human Cantilever" used by Baker to explain the bridge in a lecture to the Royal Institution in 1887 (Mackay, 1990, p16). Here the Japanese engineer Kaichi Watanabe (seated on the central suspended span which acts as a simple beam in bending) is supported by tension in the arms of the two men sitting on the chairs and compression in the sticks thrusting against the seats of the chairs. Through similar forces in the men's arms and the sticks remote from the middle span, his weight is counterbalanced by the two piles of bricks suspended from ropes. Horizontal components of the forces in the men's arms and the sticks cancel (as they are equal and opposite). However, to resist the vertical component of the tension forces in their arms, the men's bodies are in compression, transferring the load down to the chairs, which also support the vertical component of the forces in the sticks.

As Baker commented "The chairs are representative of the circular granite piers. Imagine the chairs one-third of a mile apart and the men's heads as high as the cross of St. Paul's, their arms represented by huge lattice steel girders and the sticks by tubes 12 feet (3.65m) in diameter at the base, and a very good notion of the structure is obtained."

The Sydney Harbour Bridge, which spans 503m across the harbour entrance, has become a major landmark.

Since the Forth Railway Bridge many large-span girder and arch bridges have been constructed around the world. One of the best known is the 1650 foot (503m) span Sydney Harbour Bridge, which was built by the British company, Dorman Long and Co. following their successful bid in a design competition in 1923. Their design closely resembling the draft proposal made by engineer John Bradfield.

Construction of the Sydney Harbour Bridge was commenced simultaneously from both sides of the harbour with a cantilevered method of erection in which the arch truss segments were successively tied back by a total of 128 cables, 70mm in diameter. The bridge contains 50,300 tons of steel of which 37,000 tons are in the main arch.

Robert Maillart (1872-1940) contributed much to the aesthetic development of modern bridges with his slender and elegant forms built in reinforced concrete between 1900 and 1940. For example the three-pinned arch of Salginatobel Bridge 1930.

Maillart was a master of the relatively new material reinforced concrete. He understood the importance of and interaction between calculation, construction and detailing but gave the greatest attention to the form of his structures. In many of his bridge designs the form is derived from the funicular shape of the arch or expresses the bending moment diagram of the structure. Other notable examples of his bridges are Rossgraben Bridge (1932) and Schwandbach Bridge (1933). (see Billington, 1979 and 1990)

In the 20th century bridge spans became ever longer and the principle of suspension cables became ever more refined, culminating with the designs for the (First) Severn Bridge (1961-66) and the Humber Bridge, (1973-81) which with a mainspan of 1410m was, until recently, the longest span in the world.

Completed in 1966, the main span of the Severn Bridge is 990m with two side spans of 305m suspended by inclined hangers from main cables that are carried over pylons consisting of two slender steel box columns linked by deep cross beams. The 3m deep steel box girder deck has an aerofoil section designed to minimise lateral wind loading.

The Humber Bridge has a main span of 1410m and side spans of 280m and 530m (i.e. the total length between anchorages is 2220m). The form of the 155.5m high towers appears lighter than that of the Severn Bridge as, although there are three rather than two, the cross beams are not so deep. A similar trapezoidal box girder was used for the deck. This was assembled in a yard on the riverside and towed on pontoons for lifting into position from existing deck level and welding to previously installed units.

For medium spans cable-stayed bridges have taken over from suspension cables as the main bridge form and this is reflected in the design of the Faro Bridge, Denmark, completed in 1985, the Queen Elizabeth Bridge, Dartford, completed in 1991 and the Second Severn Crossing, completed in 1996.

The navigation channel of the Faro Bridge (Designer: Cowiconsult; contractor Monberg and Thorsen) has a main span of 290m and two side spans of 120m each. Queen Elizabeth II Bridge, Dartford, (Engineer: Kværner Construction, Cleveland Structural Engineering Ltd and H. Homberg and Partners) has a main span of 350m with two side spans of 181m and the Second Severn Crossing (Engineer: Sir William Halcrow and Partners, SEEE of France; Architect: Percy Thomas Partnership) has a main span of 456m.