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What is conservation? Saving an ancient building from falling to pieces is clearly conservation. Routine repair to a recently build structure that is in constant use is clearly not. But the division between repair and conservation is by no means clear.
The subject is so complicated and subjective that little more can be done here besides ask some questions and provide some examples.
Should we conserve anything at all?
For what reasons should we conserve things?
Should we conserve simply on the grounds of age?
Should we conserve simply on the grounds of elegance?
Should we conserve an early example its type?
Should we conserve an early example by an important builder?
Should we conserve to satisfy a group of enthusiasts?
Should we conserve to foster tourism?
Should we conserve because the structure is still in use and still needed?
How should we conserve?
By exact replication using original materials?
By sensitive use of modern techniques?
Is the appearance important?
What if the old structure is too small or too weak for modern traffic?
Should we remove it and build a new one in the same place?
Should we build a new one close by, which may well spoil the view of both?
What if it is physically impossible to build a new one elsewhere?
How far can we go in enlarging an old structure for modern use?
There are no easy answers to these questions, and people can become quite emotional about such things. Then there are questions such as the following.
Who should decide whether conservation should be attempted?
Should the local populace be consulted?
If so, should their opinions be respected?
Who should pay for conservation?
The local community?
The enthusiasts who want the conservation?
The users of the structure?
Who should decide how the conservation should be done?
The local community?
Local community leaders?
A government agency?
The following paragraphs refer to some examples of bridges that are the subject of conservation.
Telford's Over Bridge
Over the river Severn from Gloucester lies the village of Over. Thomas Telford built an unusual bridge here. It would have been more economic to build his standard 150 foot cast iron bridge, but the worthies of Gloucester would have none of it: iron was not wanted, even though the bridge is invisible from the city. During much of the 20th century the bridge carried the A40 trunk road over the Severn, but the volume of traffic meant that it became too narrow. Because the ground is not very suitable for arches (Telford's bridge sank at the centre by ten inches.) a new steel bridge was built close by. The new bridge affords a very good view of the old bridge, but prevents a distant view of it. More details can be found in the page about Severn arch bridges. Gloucester city's web-site does not appear to mention the bridge as a tourist attraction or indeed as anything else, though it is very unusual in being an arch with cornes de vache construction.
This book is so comprehensive that it could be read as an interesting book about bridges in general - Conservation of Bridges - Graham Tilly - Spon Press - ISBN 0-419-25910-4.
This bridge is conserved not only because of its age but because it is extremely unusual, employing propped beams in timber. More information can be found in the page about Wye bridges.
Completed in 1599, Wilton bridge still carries the A40 road over the river Wye near Ross-on-Wye. It was widened on the upstream side by means of a series of concrete beams resting on the extended cutwaters. The downstream side retains its original appearance.
Holt Fleet Bridge
This bridge is one of several fine cast iron bridges over the river Severn. These are around 170 years old, and were built for much lighter vehicles than those we now use. That they can be used at all is testament to their design and construction. This example has been strengthened unobtrusively be encasing the arch ribs in concrete. More details can be found here.
Some changes to town bridges are invisible because they involve a bypass and bridge which are far from the original site. These solutions are of greater benefit than a simple bridge improvement, because they help to keep long distance traffic out of the town. A good example is found at Bridgnorth on the river Severn, where a fine concrete bridge takes the A458 over the river, downstream of the town.
Very old bridges are often widened in order to cope with modern traffic flows. Attempts are usually made to minimise the effect on the appearance of the bridge.
One method is to build another section on one side, matching the form of the original as closely as possible, and facing it if possible with matching stonework. This method leaves one side unaltered. If only one side is visible to the public, that should be the untouched side.
Another way leaves most of the bridge as is, but widens the roadway by some kind of cantilever method, or by extending the cutwaters. Again, altering only one side is the preferable option.
In principle, a second bridge can be built alongside or nearby, but in towns and cities, access to the river is unlikely to be available.
Some examples are listed below.
Bridgnorth bridge over the river Severn.
Near Monmouth over the river Wye, a multi-arch bridge.
In older times, people were not so squeamish about altering things and knocking them down, and it is not unusual to find ancient bridges containing a wide variety of arches, as at Hereford.
Near Monmouth, of two unused railway bridges, one was removed and one remains, and it is possible to walk over it.