Why Bridge work is a real balancing act

The £31-million project to strengthen and widen the Tamar Bridge is a balancing act in more ways than one.

Being a suspension bridge, it is crucial that the structure remains balanced at all times.

This means major works, for example, the placing of new steel support structures and the removal and replacement of road decks, have to be carried out on the opposite halves of the bridges at the same time. The weight either side of the towers must stay fairly equal.

The 31-month project is also a balancing act between making steady progress on the alterations and allowing traffic to flow as smoothly as possible. With up to 40,000 vehicles using the bridge each day, the task is not an easy one.

The project managers aim to keep three lanes open at peak times with two lanes open at other times. Up to 16 overnight closures of the bridge are also planned.

Residents

A further balance has to be struck between the heavy construction activity involved in the project and the needs and concerns of the residents living in the shadow of the bridge. This is an aspect which Saltash residents are all too aware of after recent infuriating experiences caused by work on the Brunel rail bridge.

The team behind the project has taken steps to try to ensure good lines of communication with local residents through meetings, newsletters, correspondence and a 24-hour telephone information line (01752 365426).

Measures to reduce nuisance from noise and dust are also in place.

Team

One of the project's leading engineers is 34-year-old Tim Wood from Cornwall County Council.

He has been involved with the bridge since 1997 when he supervised the erection of maintenance gantries. He then became part of the project management team for the strengthening and widening scheme.

The team involves staff from the County Council and Plymouth City Council, the co-owners of the bridge; Cleveland Bridge Ltd, the main contractors for the scheme; and Hyder Consulting Ltd, the consultants.

Tim said: "I am enjoying working with an excellent team on such an interesting and complex project.

"This is the biggest project I've been involved with in my 16 years with the County Council.

"The scheme is essential for Cornwall. If a weight limit had to be imposed on the bridge it would stifle development in South-East Cornwall."

At the end of the project, the bridge will have five lanes instead of the present three. The two new lanes will run on cantilevered decks on each side of the bridge. The one on the northern side will be for local traffic, especially buses, from Saltash and the other will be for pedestrians and cyclists.

The strengthened and widened bridge is designed to take four carriageways of traffic plus the pedestrian and cycle lane.

The existing concrete road deck will be removed and replaced by a lightweight steel deck covered with tarmac. Steelwork will be strengthened right across the bridge and the suspension cable system will be upgraded.

Tim said: "To maintain the balance of the bridge, we will cut out and remove a section of the concrete deck at each end of the bridge at the same time. We are planning to take them up in 30ft x 10ft pieces, with the work starting in September 2000.

"Detailed mathematical analysis has been carried out to ascertain how much can safely be taken away at any one time.

Critical

"The balance is very critical. The bridge reacts like a washing line and the cables would sag asymmetrically if the weight was not evenly distributed.

"We are using computer modelling of the site operation to counter any problems or delays that occur."

Traffic will be diverted onto the new cantilevered lanes before sections of the old deck are removed.

As part of the project, the approach roads are being made wider, with the slip road at Saltash leading straight on to the bridge, thus removing a dangerous junction.

Anti-climb

The fences on the new cantilevered decks will be higher than the existing parapets and will be faced with anti-climb mesh.

All work, including landscaping and a bus priority system at the Plymouth end, is due to be completed by November 2001. By then, the bridge will only be slightly heavier than it was at the start of the project thanks to the lightweight steel deck.

The work on the bridge is designed to last for at least 120 years.

Wind tunnel tests

Wind tunnel tests have been carried out on the design of the bridge to see how it will behave in high winds. They have revealed that the bridge will show no increase in movement due to wind.

The tests also looked at the affect of various parapets which will become part of the detailed design.

Busy visitor centre

Exactly 37 years after the Queen Mother officially opened the Tamar Bridge, a ground-breaking ceremony took place to mark the start of work on the strengthening and widening of the structure.

A plaque was unveiled on the day, April 26, and the project's visitor centre was officially launched.

The centre, which is open on weekdays, displays models and plans showing the full scheme and gives information on current operations and the history of the bridge.

It is situated in the car park on the Plymouth side.

Hundreds of local people, as well as those from other parts of the country and abroad, have so far visited the centre. People who remember the original construction project in 1959-61 have shown particular interest.

Cubs from Saltash have viewed the displays and school groups from South-East Cornwall are likely to be making visits.

A video narrated by John Andrews of West Country Television describes the project using computer animation and includes archive footage of the Queen Mother opening the bridge.

Schools and other groups can arrange a visit by ringing Lea McIver on 01752 360066

Work behind the scenes

Even before the army of road cones took up strategic positions on the bridge and the first pneumatic drills hammered into the concrete, a huge amount of work had already gone into the strengthening and widening scheme.

A structural assessment in the mid-1990s confirmed that parts of the bridge wouldn't be strong enough to take the 40-tonne lorries that Europe would soon be allowing on our roads.

Indeed, it was thought that a 17-tonne weight restriction would eventually have to be imposed if no strengthening work was carried out.

So the bridge's co-owners, Cornwall County Council and Plymouth City Council, took the decision through the Tamar Bridge and Torpoint Ferry Joint Committee to investigate the best way of making the structure stronger.

The consultant engineering firm, Hyder, was called in. Its specialist structural consultants considered the options and liaised with Cleveland Bridge Ltd which was appointed as the main contractor for the scheme a year before work actually started on site.

Professionals from the committee, consultants and contractors worked together to develop the design of the bridge and to schedule the detailed programme for the works. A partnership approach has been adopted and emphasis is being placed on innovation.

The scheme to widen the bridge at the same time as the strengthening took place evolved because of the need to keep traffic moving.

A new Tamar Bridge Act was passed so the scheme could proceed. A Private Bill was deposited in the House of Commons in November 1996 and became law in July 1998.

Surface signs of the vast project

One of the first signs that the strengthening scheme was under way in April was the clearance of trees and vegetation near the approaches to the bridge.

The clearance preceded the construction of concrete retaining walls which will enable the approach roads to be widened. The wall on the south side of the bridge in Saltash has been faced with limestone masonry in keeping with the local conservation area.

Water and gas mains have been diverted at either end of the bridge, and work has started on constructing the cantilevered lanes which will run on each side of the bridge.

Scaffolding has been erected under the bridge to allow access for strengthening work involving steel sections running the full length of the structure and the fixing of additional metal plates.

During the first overnight road closure, large steel girders were positioned under the sections at each end of the bridge. These sections are to be substantially strengthened with latticed steelwork similar to that under the rest of the bridge.

A lot of temporary equipment has been attached to the bridge to help with the process of putting steelwork into place.

And two substantial depots are being built, one adjacent to the project's offices in the bridge car park and the other next to the approach road in Saltash. These will be used to further assemble steel delivered from Cleveland Bridge's works in Darlington to form large sections for the cantilevered lanes and new main deck. When the project is complete, the steel-framed depots will be dismantled.

In November, weather permitting, platforms are to be erected at the top of the two towers in preparation for the positioning of new cables.

The additional cables are needed to cope with the extra weight on the bridge during construction and with the advent of heavier vehicles.

Tamar Bridge – the facts

In 1961 the Tamar Bridge was the longest suspension bridge in the UK.

The central span is 335 metres (1100ft) with side spans each of 114 metres (374ft).

The original bridge construction project took two years and cost £1.8-million.

The main contractor for the present scheme, Cleveland Bridge Ltd, performed the same role in the 1959-61 scheme.

The original bridge used 27,577 tonnes of steel, 98,420 tonnes of concrete and 20.7km of main cables. The new scheme will use 4,300 tonnes of steel, 2,628 tonnes of concrete and 2km of cable as well as welds which would be 32.5km long if placed in a straight line.

A total of 18,579 cubic metres of material was excavated for the original bridge scheme. Another 3,286 cubic metres is being excavated for the present project.

A maximum of 140 people will be working on the strengthening and widening project at any one time.

The number of vehicles crossing the bridge each day in the early 1960s averaged 4,000, 10 times less than today.

The maximum weight of vehicles using the bridge in 1961 was 25 tonnes.

The bridge is closed to pedestrians and cyclists during the present project. A free shuttle bus is operating and during the night closures a free pedestrian ferry service is being provided, weather permitting.

Local children have been invited to design a Christmas card featuring the Tamar Bridge project.

The noise from expansion joints and other moving parts on the bridge should be reduced through work being carried out as part of the project.

There is a website about the project on the internet at http://www.tamarbridge.org.uk">www.tamarbridge.org.uk