It's 18 months late, but the first new crossing over the Thames in more than 100 years opened for good yesterday. And, as Nick Morrison discovers, the North-East provided a steady hand.

ITS design may have been an innovation - a shimmering blade of light across the river - but when it came to stopping it from wobbling, the answer came from our Neolithic ancestors. Fitting huge dampers to the Millennium Bridge in London so it could absorb movement may have seemed straightforward, but getting the dampers along a bridge which was only four metres wide was another matter.

"We went back to basics," says engineer Brendan White. "We looked at mechanical means of pulling three tonne weights, but there were no fixed points on the bridge to attach a winch. In the end, the simplest way to do it was to think back to the Stone Age. We managed to find special trolleys to put the dampers on and then we pushed them by hand."

The problem of manoeuvering the dampers into place was just one of a series of obstacles faced by Cleveland Bridge in carrying out remedial work on the crossing. The first new bridge across the Thames in more than 100 years - and the only one in London solely for pedestrians - the £18m structure had opened to a wave of delirium in June 2000.

Just two days later it was closed, after it began to rock alarmingly from side to side. Although the designers said it was safe, pedestrians were forced to hold onto the safety rails to keep themselves steady.

Engineers Ove Arup looked at a number of possible solutions, including limiting the number of people allowed on the bridge at any one time, before settling on the option of fitting dampers. And this is where Darlington-based Cleveland Bridge came in.

With a long tradition of bridge building to their name, from the Tyne Bridge to the Sydney Harbour Bridge, the company was asked to take part in a preliminary test, fitting one of the three tonne dampers to see what effect it had. This proved successful, and Cleveland Bridge later won the contract to install the full damping system, worth around £5m.

"We had no design responsibility for the structure or the damping system, we were there to figure out a way of getting these very large weights across a very flexible bridge, and install them below the deck," says Brendan, who was appointed project manager for the work.

"It was a fantastic opportunity for us, to work on such a high-profile project, and it is helping to raise our profile nationally."

And for Brendan it was a personal landmark as well. An engineer, he has been with Cleveland Bridge for three years, and the Millennium Bridge work was his first job as project manager. "I jumped at the chance when I was told we were interested in it, and it was a bit of a scoop for me. There were a few sleepless nights, but I got a lot of support," he says.

Setting up an office on the doorstep of Tate Modern, the team arrived on site in May last year. And their tasks included lifting the dampers over a busy footpath without putting pedestrians at risk and making sure the deck was replaced every night to prevent injury to those using it as their own private bridge while it was officially closed.

The work itself saw a total of 58 tuned mass dampers fitted in pairs, out of sight underneath the deck. Mainly working on springs, and weighing between one and three tonnes each, the dampers absorb energy to limit the bridge's movement up and down.

Another 37 viscose dampers - similar to car shock absorbers and developed for space shuttle launch pads - were also fitted under the deck, and are the only part of Cleveland Bridge's work which is visible. And between them was installed a criss-cross bracing of hollow steel tubes, the only element manufactured in Darlington, to support the viscose dampers.

While the bridge's design, from an idea sketched on the back of a napkin, may have been daring, it was also responsible for many of the logistical problems. Not only did the four metre width require traditional methods to transport the dampers, but its lightweight construction, of aluminium and stainless steel, meant the heavy dampers also had to be carefully manoeuvered so as not to damage the structure. The team had the added difficulty of working over a busy waterway, but completed the project without accidents or safety incidents.

But, although Brendan had just a small team, no more than 15 on site at any one time, including Ron Hall, who succeeded him as project manager in December, the solutions to their difficulties were a team effort.

"I had a lot of older guys working with me, with a vast amount of experience, and we would tap into their ideas and what they thought we should do," Brendan says. "I'm a firm believer in using the knowledge people have to come up with ideas. If you have 15 guys you have 150 years of experience as a minimum. There were a lot of obstacles in our way, and the difficult part was often figuring out in what order we were going to do things. We didn't have a great deal of time to plan the project and there was a lot of pressure on us to deliver on time, but we didn't experience anything we couldn't overcome."

The bulk of the work was completed towards the end of last month, when 2,000 volunteers were recruited to walk across the bridge. And the result showed Cleveland Bridge's efforts to be a success.

Despite the dismay which greeted the bridge's original closure, Brendan says the reaction to their presence was positive throughout their contract - an enthusiasm confirmed when it reopened yesterday.

"I think people were genuinely pleased to see something starting to happen to the bridge, with it having stood there for almost a year with nothing happening," he says. "Everyone was waiting for it to reopen and it will have a lot of use, not just people going between St Paul's and Tate Modern, but also people going from the City to south of the river.

"Throughout the summer we were constantly bombarded with questions from the public about when it was going to be ready." But while the crossing may always be saddled with the tag of the wobbling bridge, he is in no doubt of its new-found stability.

"I think it is a fantastic structure. It is in the right place, and from an engineering point of view, being able to work underneath it was amazing," he says. "When we first moved on site we noticed the movement straight away. But as we started to install the bracing and the dampers we could feel it becoming more solid. All structures have to move, otherwise they snap, but this will not wobble like before.

"It is always going to have the reputation as the bridge that wobbled, but to be able to see what stopped the wobbling was quite fascinating."