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
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.
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
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.
structures including trusses and plate/box girder
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
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
The latter - among others
- comprised patented systems like the widely used crosswise-pretensioned
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
development of plated girders - Robert Stephenson
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.
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.
bridges (parallel girders)
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.
Pauli girder, Saltash
Bridge, Lohse girder (fish-belly or parabolic
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
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
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.
cantilever bridges, Gerber beams
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
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
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.
railway suspension bridges
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!