The Helix Bridge

The Helix Bridge is one of the few, if any, bridges in the world to be named after its structure. In addition to its uniqueness of structure and form, it was designed to respond to its particular setting at the opening of the Singapore River to Marina Bay, a large inner harbour on which much of the city is now focussed. The concept derived initially from the desire to curve the plan of the bridge so that it sweeps down onto promenades either side. This ‘arc’ was also means of curving the pedestrian bridge away from an adjoining new vehicular bridge, also designed by the same team, while enabling the two to connect at a mid-point. The brief required the Helix Bridge to be canopied for shade and shelter in the tropical climate. The notion of a tubular cross-section, which allowed the canopy and deck to be integrated, evolved from this requirement. The team found that a double spiral structure would utilise up to five times less steel than a conventional box girder bridge, and equally became excited about the prospect of such a structure making an iconic statement about Singapore as a ‘green’, walking city. Having selected the design in a 36 entry international design competition, Singapore’s Urban Redevelopment Authority decided that it should be constructed entirely in stainless steel. This decision enabled the tubes, struts and ties to be finely crafted, the multiple connection joints being designed to appear organic. These details, and the helix form overall, led to the bridge being popularly compared to the DNA molecule, with its connotations of ‘life-giving’ and ‘sustainability’. The great intrigue of the structure is derived from its ability to curve in plan and section while maintaining continuity of its dual spiral over the 280 metre length. This achievement was the result of a process of highly sophisticated 3D computer modelling, through which the proportions and connections were progressively refined. To accentuate the ribbons of structure, LED lighting is incorporated along both spirals, and in ground lighting embedded to highlight the combination of glazed and perforated steel canopy segments. The final pieces of the design are a series of ovular-shaped cantilevered viewing ‘pods’, each with a capacity of 100 or so people, that extend out on the bay side to create ‘ring side’ viewing for water events. These decks reinforce the design intent of the bridge to optimize pedestrian experience of the bridge as new urban place as well as of its role as a vital connector between Singapore’s major existing and emerging urban precincts. Engineering Data The Helix Bridge is 280 metres long. It: • weighs 1,700 tonnes, and utilises 5 x less steel than a conventional box girder • comprises entirely straight members, and there are a total of 14,800 members • encompassed 33 load cases and 77 load combinations in design (1,139,600 design calculations per iteration) Geometry Mechanics The main load carrying system comprises: • two sets of counter-spiralling tubes • six outer helix tubes at 5.4m radius • five inner helix tubes at 4.7m radius • 273 CHS tubes rotating at 1 revolution per span • interconnecting frames at every 2.7 metres At midspans between piles, tension is in the bottom, and compression is in the top of the helix. At the viewing ‘pods’, the pattern is reversed. Due to the rotational geometry of 1 revolution / span, almost all the load is taken by only two sets of tubes!


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