 This
year the theme for the David Winter Collectors Guild is on bridges. To
celebrate I felt that a little bit of history and technical background is
required.
A
bridge has one fundamental function - to allow loads to cross obstacles, mainly
waterways, without the need or when it is impossible, financially or practical,
to build embankments. They for the most part truly ubiquitous - a natural part
of everyday life.
A
bridge provides passage over some sort of obstacle. These may be a river,
estuary, lake, valley, a road, a railway line or other transport routes.
Generally the loads will either be vehicular traffic, pedestrians or animals.
There are five basic types of bridges: Beam, Arch, Cantilever, Suspension and
Movable.
Bridges can twist or bend under severe climatic conditions or
manmade forces which can have disastrous consequences. In order to prevent this
from happening bridges must be stiff enough to resist movement and each member
from which the bridge is made must be strong enough to withstand the load which
is placed upon it.
Beam
Bridges
A
beam or "girder" bridge is the simplest kind of bridge. In the past they may
have taken the form of a log or slab of stone placed across a stream but today
they are more familiar to us as large box steel girder bridges. There are lots
of different types of beam bridges. Despite being a cantilever bridge the Forth
Railway Bridge uses box steel girders at its approaches.
As they are relatively simple to construct beam bridges are
the first type to be constructed. Evidence has been found that they go back as
far as the Bronze Age.
In its most basic form, a beam bridge consists of a
horizontal beam that is supported at each end by piers. The weight of the beam
pushes straight down on the piers. The biggest drawback though with a beam
bridge is that it is not possible to create large spans.
Arch Bridges
Arch bridges are one of the oldest types of bridges and have
been around for thousands of years. Arch bridges have great natural strength.
They were originally built of stone, but the most common material is brick.
When Parliament allowed turnpike roads to be constructed, improvements in
design began to appear. Railway builders however, made great use of arches
often linking many together to form long viaducts to cross valleys. The first
ever stone railway bridge is the Causey Arch in Durham and its arch is over
100ft high. However it was the Romans who were the first to build arch bridges,
and some of their bridges and aqueducts still stand today.
These
days arch bridges are built of reinforced concrete or steel and the
introduction of these new materials allow arch bridges to be longer with lower
spans. The first iron bridge was Abraham Darby's Iron Bridge. It was opened in
1781 and was built to cross the River Severn at Coalbrookdale in Shropshire.
The
design of the arch, the semicircle, naturally diverts the weight from the
bridge deck to the abutments. Instead of pushing straight down, the load of an
arch bridge is carried outward along the curve of the arch to the supports at
each end. Called abutments they prevent the bridge from spreading out.
Basically all arch bridges are the same. It is there
innovative design and aesthetics which make them different. Arches are well
suited to crossing valleys and rivers and are one of the more beautiful kinds
of bridges.
Cantilever
Bridges
Cantilever bridges normally use pairs of cantilevers back to
back with a short beam bridge in between the cantilevers. Modern motorways have
cantilever bridges stretching across them, they have a cantilever coming out
from each side and a beam bridge in between them.
The
most famous cantilever bridge in the world is the Forth Railway Bridge. This
bridge was completed in 1964 and has a main span of 3,300 feet crossing the
Firth of Forth in Scotland. The intentions for the bridge was that it would
carry a railway engine and carriages up to 260 metres from the nearest support
which at the time was quite an ambitious project.
Huge pillars take up the compression which are held up by the
narrow top members. Attached to these are complicated struts and cross bracing
which withstand the forces so as to prevent buckling and twisting. The outer
cantilevers have counterweights at the ends to maintain balance.
The bridge was designed and built by Benjamin Baker in the
late 1880's and was one of the first cantilever bridges to be constructed and
is still regarded as an engineering marvel and is recognised the world over.
Suspension
Bridges
Suspension bridges in their simplest form were originally
made from rope and wood and they could be a nerve racking experience to use due
to the tendency to move and flex when used. Modern suspension bridges use a box
section roadway supported by high tensile strength cables and despite having a
stronger, more stable appearance the very nature of their construction means
that they still move and flex.
A suspension bridge suspends the
roadway from huge main cables, which extend from one end of the bridge to the
other. These cables rest on top of high towers and have to be securely anchored
into the bank at either end of the bridge. The towers enable the main cables to
be draped over long distances. Most of the weight or load of the bridge is
transferred by the cables to the anchorage systems. These are imbedded in
either solid rock or huge concrete blocks.
The
most famous suspension bridges are the Clifton Suspension Bridge that crosses
the Avon near Bristol, the First Severn Road Bridge and the Humber Road Bridge
which has world's longest centre span measuring over 4,600 feet. The Clifton
Suspension Bridge was designed by Isambard Kingdom Brunel in 1831 and is one of
his most spectacular achievements despite the fact that he never saw its
completion in 1864. The principal span of 702 ft seems even more daring because
of the 250 ft deep Avon Gorge that it crosses. In fact another famous bridge
builder, Thomas Telford, doubted the advisability of erecting a suspension
bridge of this size in such an exposed position.
In the
early nineteenth century, suspension bridges used iron chains for cables. The
high tensile cables used in most modern suspension bridges were introduced in
the late nineteenth century. Today, the cables are made of thousands of
individual steel wires bound tightly together. Light, and strong, suspension
bridges can span distances from 2,000 to 7,000 feet far longer than any other
kind of bridge.
Moveable
Bridges
Moveable bridges are mostly beam bridges except that they are
designed so that the deck moves in some way. Movable bridges are generally
constructed over waterways where it is impossible or prohibitive to build a
fixed bridge high enough for water traffic to pass under it. The most common
types of moveable bridge are the Bascuale and Swing bridges.
The
bascule bridge follows the principle of the ancient drawbridge. It may have one
complete span or have two halves that meet at the centre. The most famous
moveable bridge of this type is Tower Bridge in London whose lower centre span
is a double-leaf bascule type. The upper span does not move and allows
unimpeded travel for pedestrians while the centre span is raised.
The
swing bridge is usually mounted on a pier built midstream or on an island. For
narrower crossings they were built on one bank of the river. To allow vessels
to navigate past the deck of the bridge is swung parallel to the stream.
The biggest downside to moveable bridges is the disturbance
to the free flow of traffic. In an age when the waterways were used as a main
means of moving materials and goods these bridges were in constant use much to
the announce of those wanting to cross the river or canal.
A Brief History of Bridges
In
ancient times bridges were created at first by using a log thrown across a
stream then crude stones were rested on intermediate supports. Despite being
simple and easy to construct these types of bridge became a great obstruction
to river traffic. Later, arched structures of stone or brick were used.
However, it was the Romans who realised the full potential of the arch and they
created long, arched spans, many of which are still standing to this very day.
Very
early bridges would have been simple in construction but as building techniques
improved and the needs of keeping navigation on the river unimpeded the design
of bridges would have to take this into account. Arch bridges are the most
common as it allowed a greater height as well as strength. Then in the 18th
century, during the Industrial Revolution, bridges began to be built of cast
and wrought iron.
Early bridges were fundamental in the prosperity
of village life and the Industrial Revolution. If a village is near a river or
stream then the means of a crossing it becomes an important part of the
village's development. At first this crossing may have been just a ford if the
river or stream was small enough. For deeper, faster moving water a bridge is
the best answer. Usually villages were created along stage coach routes and so
to help with the construction of the bridge a toll would be charged.
As
traffic increased both on the rivers and roads bridges had to be built to
withstand the greater loads and traffic movements. As techniques improved and
the number and size of waterways, railways, and roads increased more inventive
bridge designs had to be found. Simple beam and arch bridges just weren't
practical to cross larger rivers and estuaries. Suspension bridges allowed much
greater spans to be achieved but for larger and heavier loads cantilever
bridges were needed.
From the first humble bridges made from just posts
and planks of wood crossing small streams to the monumental creations of the
20th century linking islands and crossing fast flowing navigable waterways
bridges are well and truly part of life.
Many of
the pictures used in this article have been supplied by my father who retains
their copyright. |
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