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INFINITY CROSSES THE TEES WEALTH CREATION of the vehicles moving across FORCES TO the bridge. The self-weight of a OVERCOME pedestrian bridge is much less Arches work well under than the moving loads – people uniform loads as they stay in – making the bending forces in compression. However, when an arch a problem. the loads are non-uniform, Pedestrian bridges are often bending is introduced to the subject to changing forces arch. In bridges that carry roads because of the way people this isn’t too great a problem as walk across them, and because the self-weight of the structure pedestrians tend to group in is much more than the loads non-uniform ways when they to allow water traffic underneath DESIGN the bridge, meant that it had CONSIDERATIONS to have a longer span to allow The design of the Infinity a gradual rise and fall in height. Footbridge was guided by Lifts, which would have allowed many considerations including a shorter length, were dismissed the location of the central because they would have pier, maintenance problems broken the linearity of the path experienced on local bridges and increased construction and and striking a balance between maintenance costs. rigidity, cost, and thinness These constraints led to of form. main spans of 60 m and 120 m, INFINITY The design of the bridge also one of the longest spans for had to accommodate existing a pedestrian bridge in the uses of the river. A 1,000-metre UK. Starting out as a rough rowing course runs diagonally up sketch made during a train the river on the proposed site of journey back from the site, CROSSES THE TEES the bridge, making a symmetrical the bridge is an asymmetrical bridge with a single support double tied-arch with one arch out of the question because twice the height and length Stockton-on-Tees’ Infinity Footbridge opened In 2003, the Royal Institute of the area. It also called for a the pylon would have sat in the of the other. Between them British Architects and English dramatic structure that would middle of the rowing lane. sits a curved inverted ‘saddle’, to the public in 2009 as the centrepiece Partnerships ran a competition represent value for money An alternative, crossing the river which aesthetically maintains a of the region’s regeneration. That same to design a new pedestrian and have a 120-year design life. in a single span, was too costly. sinuous flow to the arches and bridge across the River Tees A “Ford Mondeo…with alloy Expedition and Spence instead structurally reinforces them, year, the Institution of Structural Engineers in Stockton. The crossing wheels” as a local resident chose to place the foundation making the ultra-light bridge awarded the designers of the bridge, was to form part of the wider put it. off-centre, a decision that led to much stronger, and less prone regeneration projects which Expedition Engineering the bridge’s unique design. to wobble. Expedition Engineering, its highest accolade, encompassed a large part of and Spence Associates won The key social factor that The asymmetrical shape the Supreme Award for Structural Excellence. the north-east of England, and the competition with an influenced the design was the of the bridge eventually led specifically a swathe of post- attention to detail that belies fact that the bridge would to its name. An entrant to a Professor Chris Wise FREng, Expedition’s industrial land in the centre of the minimalism inherent in the connect existing walkways competition run by the Northern director, and Tim Harris, an Associate at Stockton itself. bridge. The site of the new on either side of the river Echo newspaper noted that, The competition called bridge was more or less where and would be the centre of when the water was calm its Expedition, talk about how this addition to for something that would Margaret Thatcher announced, a new pedestrian route from reflection formed a figure of demonstrate the quality and in 1987, what was then the Stockton railway station to the eight or the infinity symbol the north-east landscape came to be. The central support. Compression forces are taken from each of the innovation of the development biggest urban regeneration regenerated area. That, and the on the horizon. It was then arches through the arms to tie cables connected to the central one and become a centrepiece of programme in Europe need for a minimum deck level dubbed the ‘Infinity Footbridge’. © Morley von Sternberg 30 INGENIA INGENIA ISSUE 43 JUNE 2010 31 INFINITY CROSSES THE TEES WEALTH CREATION … most bridges have bulk added to them to make them sturdier, which has the side-effect of making them ‘fatter’. The design principle for the Infinity Footbridge was for it to have a very thin profile and for this to happen it had to be incredibly well tuned. stand on them. To deal with this are 32 sections, each 7.5 m in kind of loading, most bridges length and 5 m wide. The deck LATERAL EXCITATION have bulk added to them to itself is extremely light, lending Shortly after the Millennium Bridge over the Thames was opened A study of the bridge put this down to the sideways forces exerted make them sturdier, which has further need for a rigid structure. in London in June 2000, it famously started to sway from side by pedestrians on such a thin structure. To counteract it, dampers the side-effect of making them Made out of concrete to to side when large crowds of people walked across it. Dubbed were added along its length to absorb these forces and keep the ‘fatter’. The design principle for offer the bridge a ‘firmness’, the the “Wobbly Bridge”, the Millennium Bridge had succumbed to bridge stable. the Infinity Footbridge was for deck sections are nevertheless previously little-known problem: synchronous lateral excitation. Being another thin pedestrian bridge, the Infinity Footbridge it to have a very thin profile and only 125 mm thick with 380 mm It was found that the movement was caused by sideways was designed with this problem in mind. Expedition decided to for this to happen, just like a deep edge beams. The sections pedestrian loads combining to match the horizontal frequency of integrate into the bridge seven five-tonne tuned mass dampers musical instrument, it had to slot together and are held in the bridge. So-called synchronous lateral excitation occurs when along its underside – six on the longer span and one on the be incredibly well tuned. compression along their length, a random event, such as pedestrians matching step, causes slight shorter side. These are essentially heavy blocks of steel supported Expedition decided to adding to the bridge’s horizontal sideways movements of the bridge – when people place their feet, on springs and connected to ‘dash pots’. These dash pots are reduce the bending in the stiffness. they exert a small sideways force on the ground. It then becomes like a car’s shock absorbers, filled with a viscous fluid that resists bow by including a back span. Balfour Beatty, the contractor, more comfortable for the pedestrians to walk in synchronisation the movement of the blocks and cancels out the lateral forces For one span to deflect down, cast all of the deck sections with the bridge movement (hence, ‘synchronous excitation’). placed on the bridge by pedestrians. Each of the dampers is also the other must be lifted. The onsite. Each section was cast This instinctive behaviour means that the lateral forces exerted by tuneable, so they can be tweaked to correspond to the vibrations continuity across the central in a single piece, with all fittings, the pedestrians match the resonant frequency of the bridge, and are of the bridge. support means that each span channels and other features in phase, so the sideways motion of the bridge is increased. As the Again, the recesses for these dampers were moulded into the provides stiffness to the other. shaped into the moulds. These amplitude of the motion increases, so does the total lateral force. concrete deck to maintain the clean lines of the bridge. By using two tied arches, included drainage channels, overlaid with the two-span beam spouts and gargoyles for water principle, the Infinity Footbridge run-off, support for service ducts has managed to keep bending and fittings for the handrails. to a minimum and the section The long edge of each section sizes as light as possible. For is aerodynamically shaped like the arches to act together, like an inverted aeroplane wing to the two-span beam, they must prevent the deck ‘fluttering’ in have continuity over the central high winds. On other bridges of support. This is provided by the a similar type, these ‘nosings’ and reflex curve. other fittings are often bolted The arch was fabricated onto the deck afterwards, which by Cleveland Bridge from increases costs and maintenance. weathering steel. The arches Balfour Beatty are now applying themselves split either side of many of the concrete casting the deck below, which gives techniques it developed sideways stability. while working on the Infinity Footbridge to its other projects, such as the use of a wooden DECK CONSTRUCTION shed on wheels to form movable Over the river, the deck hangs on-site prefabrication yards. One of the five-tonne tuned mass dampers hung under, and within the Workers from nearby Cleveland Bridge helped carry out live pedestrian from the arch on 30 mm diameter To prevent sway, the deck Pairs of longitudinal cables run either side of the deck units. These act as ties to profile of the deck of the Infinity Footbridge © Tim Harris, Expedition testing of the bridge © Tim Harris, Expedition high-strength steel cables.

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