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Heritage trail

Bridges Conservation

Researching the book Conservation of Bridges was quite a challenge. Author Graham Tilly of Gifford & Partners reflects on the task.

Collecting material on bridge conservation turned out to be a fascinating as well as time consuming activity. The subject has not been comprehensively considered before, so the necessary background research was more than usually demanding. The work covered all types of bridge as well as dealing with their architecture and archaeology. It was therefore necessary to search through numerous publications for case studies and to draw on personal experience.

Some of the more fascinating inputs were through less formal routes, for instance an interest in archaeology led to finding that the Museum of London Archaeology Service had excavated a Roman culvert (now infilled) in the City of London.

The excavated section was 20m long and in pristine condition, the estimated total length is 800m. This is the earliest example of fully preserved masonry arch construction in Britain and convincing proof of its durability.

Travelling around on other activities provided opportunities to stop at unexpected bridge works to see what was going on. Site workers were always helpful and pleased to talk and their knowledge and enthusiasm impressed me - conservation work is evidently in good hands.

On one occasion I came across work being carried out on the tollhouses on Cleveland Bridge across the River Avon in Bath. The workman offered to show me around and I was surprised to discover that there were two floors between road level and the towpath beneath.

The middle floor gave a unique view of the superstructure and the different strengthening schemes carried out in the past, some representing good conservation, some not.

On another occasion I was visiting James Dredge's little known Victoria Bridge of 1836, a few miles downstream from Cleveland Bridge, when I met an American civil engineer who had come to Britain especially to see some of our suspension bridges.

We have a rich heritage of bridges in Britain, many on side roads and bridleways, that are of international interest but their importance often goes unnoticed locally.

During the work it came across very strongly that many of our historic bridges, even listed and scheduled ones, have been spoiled or damaged by utilities. It is a common sight to have ugly pipework attached to the spandrels of masonry arches.

In other cases the pipes have been buried beneath the road surface sometimes causing damage to the structure by trenching into the barrel in order to position them at sufficient depth.

Modern bridges have also been damaged. A post-tensioned concrete bridge that had been listed as a Grade 2 heritage structure had a gas pipe attached to the outside face of one of the edge beams some time after construction. This was an ugly addition that spoiled the appearance of the bridge but, more importantly, some of the holes drilled into the concrete for the bolted connections severed the post-tensioning tendons in different places.

The damage was revealed some years later by progressive cracking in the concrete. Every effort was made to save the bridge but its spare and economic design left no scope for making effective repairs and it was necessary to replace it. The only consolation was that the gas pipe was re-routed, leaving the new bridge uncluttered.

Parapet bashing is another modern activity that is despoiling historic bridges. Parapets are very important to bridges as they dominate the appearance of the side elevation and are the main feature when viewed from the roadway. Collisions with parapets can be caused by long vehicles on bends in the approaches and by vehicles approaching narrow bridges from opposite directions and reluctant to give way until too late. Parapet bashing is becoming commonplace, local authorities typically have to cope with one or more a month in their areas.

In half of the incidents it not possible to identify the vehicle and obtain insurance payment for the damage. Repairs can get expensive with the need to match the brick or stonework.

And repairs to stone balustrades and iron parapets present more difficult problems.

Most masonry parapets have inadequate impact strength so that bridge managers face the dilemma of whether and how to deal with them without destroying their originality. A variety of schemes has been devised, some very original indeed. An example is Magdalene Bridge, Cambridge, where Cambridge County Council has strengthened the cast iron parapet by fitting prestressing strand longitudinally tensioned between the pilasters at each end. The pilasters have reinforced concrete cores and are clad in stone to give an appearance similar to the originals.

The strand is cleverly positioned, leaving the original cast ironwork undisturbed.

In the past it has not been uncommon for historic bridges to be strengthened unnecessarily or demolished altogether.

This has been due in part to narrowly focused use of codes and a reluctance to take the structural analysis to a more advanced stage. Masonry arches are a case in point as they invariably have reserves of strength - as demonstrated in TRL's programme of collapse tests in the 1980's. Until recently it has not been possible to calculate these reserves with any certainty but this can now be achieved using discrete elements.

Winston Bridge in County Durham, a 32m span masonry arch set in a dramatic location across the River Tees, had been assessed in the normal way and found to be under strength.

A contract was placed to carry out strengthening work but was reassessed as part of the preparatory work using state of the art structural analysis and found to be strong enough. This project and the decision to do nothing was seen as the purest form of conservation and won an Historic Bridge Award in1991.

When researching historic concrete bridges I was impressed by their durability. By modern standards they should never have survived so long, as the concrete was poorly compacted having voids and honeycombing, and very low cover thickness to the reinforcement. Nevertheless the performance of many of these bridges has been impressive - some are now over 100 years old and many are still in everyday use.

Axmouth Bridge at Seaton, a 3-span mass concrete arch bridge built in 1877, survived 600mm settlement of one of its piers during construction which can clearly be seen along the line of the parapet. Over the years the concrete has weathered to an attractive appearance dominated by the exposed local aggregate. Nunn's Bridge at Fishtoft is the first insitu post-tensioned bridge in Britain and is in excellent condition after 55 years.

Concrete is sometimes painted to camouflage unsightly patch repairs. This is particularly unsuitable for historic concrete and has to be a last resort as the patina of age is destroyed and subsequent weathering is unsightly. Although modern paints and coatings are designed to be elastomeric and weather resistant, they can still crack and peel.

Moreover surface irregularities are emphasized by paint. It would be much better to design the repair materials to be compatible with existing concrete in composition and appearance.

This approach would benefit from research to develop mixes that combine conservation requirements with practical issues such as adhesion to the substrate and ability to cope with shrinkage. Basically it would be preferable for repairs to historic concrete to follow the same philosophy as masonry.

Conservation of Bridges was sponsored by the Highways Agency. Graham Tilly is the principal author and editor.

Alan Frost of Donald Insall & Partners wrote the chapter on architecture. The book is published by Spon Press. Price £55. ISBN: 00419 259104.

www. sponpress. com

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