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| The Gatwick Air Bridge at night |
Gatwick Air Bridge
Case Study
Not many pedestrian footbridges provide access to island piers, and give serious plane-spotting opportunities to passengers while maintaining the operational status quo of an airport and its airlines. The new 197m air bridge at London Gatwick Airport linking the north terminal with a new satellite terminal does all these things.
The bridge had to provide sufficient clearance to the 19.4m tall Boeing 747-400. However the size and shape were also dictated by the need to maintain the lowest overall height and minimize obstruction to ground radar. Bridges involving masts or tall piers were thus eliminated.
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| The first 747 to pass under the bridge |
A team comprising Arup as bridge engineer and lead designer and WilkinsonEyre Architects was charged with realizing the design brief. The bridge is fully enclosed with all services inside, so external maintenance is minimized.
Detailed Analysis and Design
A truss in the roof plane, and the concrete deck slab at floor level, provide lateral stability. Their edges are connected by vertical glazing support bars that complete the structural system while minimizing interruption of the glazed façade. Since all the deck elements contribute to the structural stability, a full 3-D skeletal model of the deck was developed using Oasys GSA, with the triangular plated box modelled with 2-D shell elements and the legs modelled as simple sticks.
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| GSA Model |
The construction sequence was developed with the contractor early in detailed design and included in a single analysis model: the bridge was 'built on the computer' in seven stages, including the addition and later removal of temporary supports. Normally, you would need a different model for each stage, making it difficult to combine the results. GSA enables all these difference stages to be incorporated into the one model, greatly simplifying the analysis effort.
The stresses locked in by this process were significantly different from those obtained by applying the full dead load to the completed structure. These stresses were then combined within the model with various live load, temperature, wind, and impact loadcases, and the results enveloped to give the design load effects. This model was also used to confirm the bridge's buckling stability and to determine the natural modes of vibration.
Dampers
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| GSA 3-D analysis of the 'knee' junction |
Dynamic analysis using GSA indicated that if more than 1200 people crossed the bridge at one time (the capacity of three fully-loaded 747s), it could exhibit excessive lateral response due to dynamic instability, like the London Millennium Bridge. Tuned slosh dampers (TSDs) were therefore installed.
How it was Built
Airports are extremely busy environments, even at night when runways are shut. To build anything above a taxiway without disrupting operations requires a high degree of planning and technology and the collective contribution of many individuals and organizations, especially when it involves building a 2700 tonne structure complete with internal and external glazing.
The principle was simple - design, build the bridge 1.5km from the site, move, and then erect - but rarely is this scale of prefabrication seen outside the offshore industry and certainly it was a pioneering process for a UK airport environment. This sequence would have been impossible to execute without modern moving and lifting equipment, and utilizing the structural analysis capabilities of Oasys GSA.
See an Oasys User Forum presentation on the Gatwick Air Bridge.
