the evolution of bridge design

This article may seem a bit over the top for railway modellers but if a freelance railway be being built, it may be of use to understand how bridge structures have evolved over the years.

The historical development of bridges is the field which best illustrates the progress of structural engineering from ancient times up to the present century. In particular the development in steel bridges shows how design has moved from intuition to advanced structural analysis.

When the new material, iron, and later steel, was ready for use in larger structures there already existed a quite highly developed technology in bridge building, namely for bridges in timber and bridges in stone.

Thus, the building of steel bridges was founded at the beginning on the then well-tried principles and construction methods of timber and stone bridges. Stone bridges provided the arch type while wooden bridges demonstrated mainly fine-structured trusses. According to the typical material properties of cast iron - the first type of iron available - iron bridges were first built as arches. Later, when steel was available, which is capable of acting in tension, various structural systems were developed on the basis of the principles of wooden trusses.

Arches transfer distributed vertical loads to the foundation mainly by compression. Due to the specific material properties of masonry they are basically the appropriate form of structure for stone bridges.

Pont du Gard built in 18 BC

In the Middle Ages the construction of flat arches was developed in order to build lighter bridges and larger spans.

"Pont de la Concorde" in Paris of 1791

In 1779 Abraham Darby III, an English iron founder, succeeded in building the first iron bridge in Coalbrookdale. The Coalbrookdale Bridge was constructed as an arch bridge like the examples in stone before, however, the arch was structured in 5 light ribs following the constructional principles of wooden structures. The bridge has a span of about 30 m and is still in use. Such cast iron bridges soon became common structures in Britain and were exported to other countries.

Another method used cast iron tubes for the compression member of the arch. This economical system was widely used, an excellent example being the "Pont du Caroussel" in Paris, which was built by Polonceau in 1839 with three spans of 48m each.

"Pont du Caroussel" in Paris

The largest cast iron arch ever built was the "Southwark Bridge" by John Rennie over the Thames in London (1819) with a span of 73 m

The "Mississippi Bridge" in St. Louis was built in 1874 by J.B. Eads. He used tubular members partly of iron and partly of steel to form the latticed arch of 159 m in span. It was the first bridge he built and surprisingly became the largest arch span in the world.

Gustave Eiffel also used trussed construction. He was the first engineer to develop the preparation of steelwork design up to full detailing and drawing of every element or single rivet. His most beautiful bridge was the "Viaduc de Garabit" in the South of France, built in 1884 with a span of 165 m .

Viaduc de Garabit

With the development of steel the size of structures increased. The largest arch spans were built in the years up to 1930

the "Bayonne Bridge" in New Jersey by O.H. Ammann in 1931 with a span of 504 m

 
the "Sydney Harbour Bridge" by R. Freeman in 1932 with a span of 503 m.

iron in the first period of bridge building could only be used in compression. It was not until more than fifty years later when larger bridge structures were built that bending structures were adopted using the newly developed wrought iron, and later steel, which were capable of acting in tension as well. At that time there already existed a highly developed technology of building such bridges in timber, in particular trusses of various shapes and systems. Since constructional steelwork at the start used a great deal of this knowledge a short overview is given below of the development of wooden bridges.

The heyday of bridge building in timber took place in the second half of the 18th century. From that time on the development of wooden bridges moved to the USA, where - due to the lack of trained carpenters - simplified structures came into use. Standardized and prefabricated elements and simple connections were made with unskilled labour, but nevertheless produced large bridge structures, especially for the railways. The main types of bridges resulting were trestle bridges and truss bridges.

The latter - among others - comprised patented systems like the widely used crosswise-pretensioned truss girder.

During the first half of the 19th century, steel bridges were frequently designed as trusses, particularly in the USA. This was mainly due to their economical load-carrying behaviour. However, in Europe this same development was interrupted by a short period, when tubular bridges were made of large plated girders.

To cross the Menai Strait, Stephenson decided to build a bridge in the shape of two rectangular tubes (each 4,4 m wide and 9 m deep) through which the two railway tracks ran.

The Britannia bridge carried the railway traffic well for 120 years until 1970 when it was damaged by a fire.

A second bridge of this type, but with somewhat smaller spans, was built by Stephenson over the River Conway at the same time.

Latticed girders were copied in steel, resulting in fine-mesh lattice girders since flat sections can resist compression forces only with reduced buckling length.

The largest beam bridge of this type in Europe, the "Dirschau Bridge" over the River Weichsel (Vistula) in Germany, was completed in 1857.

The first iron truss bridge to be made of struts was the "Grandfey Viaduct" near Fribourg in Switzerland, opened in 1862. Although similar in type to the wooden trestle viaducts in the USA, it was the first true modern trussed girder with appropriate compressive members.

A typical truss bridge of that time was the Danube Bridge near Stadlau in Vienna. It was built in 1870 as a continuous beam with five spans of 80 m each. The picture shows the process of launching.

Eiffel built a great number of truss bridges for the railway in France and Portugal. Eiffel's largest bridge of this type was the bridge over the Tardes near Evaux, with a main span of 105 m (72 + 105 + 72 m), built in the same period.

Truss bridges of the parallel-girder type were built in great variety, especially for the railways in Europe. The Rhine Bridge near Maxau in Germany, built in 1938, is a good example.

The objective of obtaining an optimum distribution forces in trusses led to new shapes of girders, the parabolic-truss girder with a curved upper chord and the fish-belly type girder with both chords curved in opposite directions.

The first Pauli girder, built in 1857, was the railway bridge over the Isar near Grob hesselohe (Germany) with spans of 53 m .

A gigantic bridge of the fish-belly type was the "Saltash Railway Bridge" near Plymouth, also known as the "Royal Albert Bridge". Completed in 1859 and having two spans of 139 m each. The builder was Isambard Kingdom Brunel.

Fish-belly type girders of Pauli's design had a lot of constructional advantages and were used in German bridges again and again. For example the second "Dirschau Bridge" over the River Vistula had six spans of 131 m.

A similar type of bridge was the double bow girder bridge, called "Lohse Girder" after its originator. The structural system, somewhere in between the fish-belly type and the tied-arch type, consisted of two trussed chords connected with vertical members. The most important examples are the five Elbe Bridges near Hamburg built in the period from 1872 - 1892.

Parabolic truss systems were also widely used, particularly for railway bridges. The "Lek Bridge" near Culenborg in the Netherlands had the longest span of such girders for a long time. It was built in 1868, using steel for the first time in bridges.

A special type of truss structure is the cantilever bridge. By making the truss girder deeper at the piers, cantilevers may be built far into the middle of the span. This technique is of great importance when bridging deep or rough water.

One of the greatest cantilever bridges is the "Firth of Forth Bridge" in Scotland. When built in 1883 - 1890 with main spans of 521 m, it gained the world record for the longest span bridge.

The bridge, which today is considered to be a unique and gigantic construction, is a masterpiece of engineering work. The depth of the truss above the piers is 106 m, the main tubular members are 3,7 m in diameter, and the whole bridge used 42.000 tons of steel and at times required up to 4.600 workers at the site to undertake the complex method of construction.

The "Hooghly River Bridge" in Calcutta, built in 1940 with a span of 455 m, is the fourth largest cantilever bridge. Although a late example of this successful type of bridge.

The bowstring arch through truss bridge was patented in 1840 by Squire Whipple. Thrust arches transform their vertical loads into a thrust along the arc of the arch. At the ends of the arch this thrust (at a downward angle away from the centre of the bridge) may be resolved into two components, a vertical thrust equal to a proportion of the weight and load of the bridge section, and a horizontal thrust. In a typical arch this horizontal thrust is taken into the ground, while in a bowstring arch the thrust is taken horizontally by a chord member to the opposite side of the arch. This allows the footings to take only vertical forces, useful for bridge sections resting upon high pylons.

After the exceptional example of the Britannia Bridge, plate girder bridges remained within spans of about 30 m. Fresh impetus was given by the development of welding in constructional steelwork. The use of welding began in about 1925.

A tied arch bridge acts like a beam structure, which is assisted in carrying load by an arch behaving similarly to a curved upper chord of a truss, while the deck girder acts like the lower chord. Arch and deck girder are simply connected by hangers and form a structure which has considerable constructional advantages compared to true trusses when bridging wide single spans or carrying heavy loads.

Such bridges were frequently used in the past, especially for heavy railway bridges. The first long span bridges were built in Hamburg over the Southern Elbe (1899) with four spans of 100 m.

Before discussing the Roeblings in detail, some remarks should be made concerning the use of suspension bridges for railways. The first attempt was made in 1830 by building a chain bridge over the River Tees near Stockton for an extension of the Stockton-Darlington line. The free span was 86 m, the calculated live load 150 tons, but disappointingly under less than half of the load the deflections were intolerably high. The Brooklyn Bridge was also designed to carry railways.

After more than 100 years since it was opened, the Brooklyn Bridge is still in use.

Nowadays, most rail bridges are constructed in pre-stressed concrete. They may be effective but are rarely as interesting!