Chris Wise, the man who put the wobble in our walk on the Millennium Bridge, has designed another. But don’t worry, he’s sure that this time you’ll be able to jump up and down to your heart’s content.
The engineer who designed the wobbly bridge is back – with another footbridge. Chris Wise, the former Arup man who designed London’s Millennium Bridge, has his own company, Expedition Engineering, and it has designed a footbridge for Stockton-on-Tees – hopefully without its infamous forerunner’s wobble. “This is the first slender bridge I’ve done since leaving Arup,” he says.
Understandably, Wise has taken every precaution to make sure the Northbank Bridge will be rock solid. “We have tried to learn from the Millennium Bridge – and every other bridge built in the past five years,” he says. “This bridge couldn’t be built but for the Millennium Bridge – so every cloud has a silver lining. If something goes wrong, and is studied very intensively, that knowledge feeds back into the gene pool so you can benefit.”
This isn’t just about understanding what makes bridges wobble, but includes advances in computer analysis and modelling, and a better understanding of which materials are the most durable on footbridges. So what new ingredients and methods has Wise employed to take bridge engineering forward?
The Northbank Bridge looks as if it will be technically very advanced. It will consist of two very slender arches that blend into each other in a single, sinuous shape, and which supports the deck by means of a series of cables. This fluidity is reinforced by the bridge’s asymmetry – the larger arch spans two-thirds of the 120 m and the other 60 m. Although the middle section falls short of the 144 m central span of London’s Millennium Bridge, Wise was aware that this new bridge could suffer from the same pedestrian-induced wobble.
“We wanted to make bloody sure it didn’t happen again,” says Wise. “This is the second time I’ve been down this route in the past seven years.” Thankfully for Wise, a repeat performance is far less likely, given engineers’ understanding of a phenomenon that goes by the name of synchronous lateral excitation. “I prefer wobble,” chips in Wise.
The wobble came to national prominence in June 2000 when the Millennium Bridge started moving very slightly from side to side. Because pedestrians perceived a slight sideways movement they adjusted their step to fall in with the sway, as this is naturally more comfortable. Because so many people simultaneously adjusted their step to the bridge’s natural frequency, the swaying became more pronounced. Again, people compensated their step to accommodate the increased movement and within a very short time some people were having to hold on to the balustrade for support.
Happily, Wise can now predict with far greater ease whether or not a wobble will afflict a design. “Our understanding of people’s interaction with bridges has grown immensely since the Millennium Bridge,” he says. The analysis tools available to engineers were far more limited five years ago. “With the Millennium Bridge we tried to second-guess every load condition it would be subjected to. We analysed something like 150 conditions but missed condition number 151, which says it wobbles.”
State-of-the-art computer analysis means it is much easier to analyse a variety of pedestrian-induced wobble scenarios. “We have looked at 10,000 different horizontal and vertical wobbles,” Wise says. The computer model analyses what happens when there is anywhere from one to 1080 people on the bridge – the maximum number it can accommodate – in a vast number of different permutations. This includes getting them to walk, jump, run on the spot and all rush from one side to the bridge to the other. The analysis tool compares the 10,000 different loads generated by the theoretical pedestrians and maps this against the natural frequency of the bridge to see whether it will wobble. “The chances of you missing one of these are much less,” says Wise. He will also have the analysis checked by two other experts, just to be on the safe side.
It was just as well that Expedition did do the sums, because they showed that the bridge needed to be fitted with dampers – initially the team thought they were not needed. “What we thought wasn’t a problem actually was, as we didn’t have enough damping for the vertical condition,” Wise says. The footbridge will probably have three five-tonne mass dampers under the deck of the main span to dampen vertical and lateral movement, and two five-tonne dampers under the small span to tackle vertical movement. The dampers will be attached to the bridge with large springs at the sides so they can move semi-independently of the deck.
The growth of 3D computer modelling has greatly simplified design. “Another thing that has happened in the past five years is that people are drawing straight to 3D. Wise says 3D modelling enabled engineers to work out how to build the footbridge much earlier on. Ed McCann, Expedition’s associate director and environmental engineer, says this has had a profound effect on engineering. “You can take structural design closer to the edge, reduce the uncertainty and end up with a more rarefied design.”
Expedition has also looked at other footbridges built in the past five years to see how they have lasted. They found many bridges had suffered from vandalism, accelerated decay and neglect. “The surface of the Hungerford Bridge in London is cracking up, the approach decks bounce and the lighting doesn’t work,” says Wise.
A plan to use steel mesh at Stockton was dropped after the team found the wire mesh covering the angled arch at York Millennium Bridge had got damaged. “We took out the wire mesh because if someone gives it a boot it becomes permanently dented,” says McCann.
“The Millennium Bridge has a stressed wire handrail,” McCann continues. “It was pointed out to us that thieves could cut this at each end and nick it. We have developed a crimp detail that means they can only take 12 m at any one time.”
On the positive side, it should be much easier to build Northbank Bridge than the Millennium Bridge as there are more specialists now. “With the Millennium Bridge, there weren’t the people who could build that sort of structure – at the time they specialised in either motorway bridges or huge suspension bridges,” says Wise. “We scoured the place for contractors. There were only one or two who could offer a combination of technical understanding and very high quality architectural finishes in a marine environment.”
The tender to build Northbank Bridge goes out within the next two months and it is scheduled for completion at the end of next year. Then the people of Stockton will dish out the ultimate test – a strident march en masse across their bridge.
How the Stockton bridge works
The twin-arch design of the Stockton bridge was adopted for sound technical, as well as aesthetic, reasons. According to Wise, simple beam bridges are good at handling off-centre and patch loads but use lots of material. A bridge supported by an arch is more efficient in terms of material utilisation but is not so good at handling eccentric loads. The solution at Stockton is intended to combine the best of both worlds. "We wanted the efficiencies of an arch and the benefits of a beam, so we have linked the two arches together – which is very unusual," says Wise. Using two arches rather than one means that the spans are shorter and, because the arches are linked, if one side is heavily loaded it can transfer some of the loads on to the other arch. This means it acts as a counterbalance, "like a curved seesaw", says Wise.
Two cables on either side of the deck will tie the ends of each arch together and stop them from moving apart. Because these cables will be under tension they will help prevent the bridge from swaying. Vertical loads will be carried by the arch using steel cables attached to the deck. The deck itself will be made from panels of precast concrete, selected in part for their ability to dampen movement.
The interaction between the precast deck units and the deck cables means the deck acts as a single beam. "The deck of the Hungerford Bridge in London is twice the thickness," says Wise. The precast concrete sections will be cast sequentially on the river bank to ensure they fit neatly together. They will then be clamped onto the deck cables - just like the Millennium Bridge in London.
The Arup approach to designing footbridges - five years on
Because engineer Arup had to solve the Millennium Bridge wobble, it is pretty knowledgeable when it comes to designing long-span footbridges these days. According to Angus Low, a director of Arup who specialises in bridge engineering, the firm starts by assessing the number of people who will use the bridge, so it has a starting figure for how much damping might be needed.
Once the bridge-use figure has been established, the mass of the part of it that could wobble is worked out. The natural frequency of the bridge is also established. Finally Arup has to work out how much inherent damping is in the bridge. This is trickier than it sounds as, surprisingly, poorer quality steel tends to have better damping characteristics than good quality steel. "The quality is improving, which is worse for us," says Low. "I rang up Corus and said 'can you tell me the damping characteristics of your steel?' and they gave me the flat answer 'no'."
Finally, Arup uses the knowledge accumulated from its Millennium Bridge experience to calculate if its design will wobble. "It's a fairly simple calculation," Low says. He adds that virtually all long-span footbridges need some form of additional damping. "Long-span footbridges have always suffered from synchronous lateral excitation," he says. "There are several long-span suspension footbridges in Scotland and everyone knows they rattle around. The only thing that has changed is society’s expectations."
Tees Valley Regeneration through English Partnerships
Speirs and Major