Footbridges will become increasingly common in our dense, complex cities
As the physical environment of cities becomes more complicated we are going to see more bridges, quite a lot of them footbridges. It is happening already, even in relatively uncomplicated Britain. The pressure is to create bridges that are not only more credible architecturally, but that also solve difficult problems of access, location, ground conditions, site and climate. Techniker's Matthew Wells has recently completed several footbridges in Hong Kong with architect Anthony Ng. Ng once worked with Derek Walker on Milton Keynes. 'He's a cut above, ' says Wells. So is Wells, who is one of those rare people who is a formally qualified architect who practises as an equally qualified engineer - and has first-class degrees in both disciplines. Wells and Ng have collaborated for some time in Hong Kong, most recently on bridge designs, one in Tung Chung, a new town on Lantau Island, the other at Wong Tai Sin in Kowloon.
Hanging from the roof The Wong Tai Sin footbridge spans 38m across a busy Kowloon road, and carries tens of thousands of visitors from the mass-transit railway (MTR) station at ground level to the Taoism Wong Tai Sin temple complex, one of Hong Kong's most celebrated shrines, on the other side of the road. It also links the adjacent new shopping area with an extensive housing estate.
Wells says: 'There was this Rudolph Schindler-like building adjacent, and we were trying to echo that with big planes of glass and with the deck as slender as possible and all put together as a technically crude truss.' So the bridge is a simple air-conditioned transparent glass and steel truss structure.
The basic structure looks like an inverted classic three-element queen-post truss with a horizontal section in the middle. But it is actually a box girder stayed by paired Macalloy tension cables, propped by two inverted vertical outriggers at its two points of contraflexure. The cables run diagonally down from the ends of the girder and across the flat section between the ends of the two outriggers. The concrete deck is simply slung along the bottom of this assemblage.
The box truss forming the roof has a symmetrical airfoil section, its rolled edges created by the 460mm tubes forming the upper chord of the truss on either side. The flat curves of its top and bottom are maintained by shaped 10mm plate transverse beams - analogous to the ribs of an aircraft wing, here at 2m intervals. Four 15mm stiffening plates are welded between each rib, so the final unclad view of the beam is of an egg-crate that is cambered in section, with four outrigger legs.
The skin, which has additional longitudinal stiffening, consists, top and bottom, of shaped 15mm plate. The deck is hung from the ends of the transverse ribs on 120mm x 40mm thick hangars. The laminated glass is independent of the hangars, fixed just inboard of them in a deliberately over-deep aluminium glazing channel concealed in a groove either side of the ceiling and at ground level. There is a narrow external walkway at deck level, which serves for both cleaning and as a crash barrier. During the design phase, a similar bridge in Hong Kong was demolished by a driver with an illegally high load.
The bridge was assembled in a compound 100m up the road from the site. The steelwork had been fabricated in three sections in mainland China and brought in, on its side, by road. It was welded together upright on a series of trestles and all the elements attached including the concrete deck - but not the glass. Wells says: 'All that glass represents a serious superimposed dead load, and the structure was designed as a big banana which would settle as the glass was fixed from inside.' That explains the oversize of the glass fixing channels. The other reason for leaving off the glass was that, during the trip down the road and the final lift, the assemblage would briefly become a beam with a long cantilever at each end.
Temporary braces were fixed to counteract this, since even the considerable flexibility in the glass holding channel would not have coped. The lift took place one midnight, and involved a 200 tonne telescoping mobile crane and very experienced operators.
CREDITS CLIENT Upper Wong Tai Sin Housing Estate ARCHITECT Anthony Ng Architects ASSOCIATE ENGINEER Joseph Chow and Partners Critical fulcrum The Tung Chung bridge is actually a viaduct to whose mid point is attached a secondary bridge from a sports club - supported in its middle by a cluster of props in an elongated inverted pyramid array. Hence the office name, the Tung Chung Vee-Prop bridge. Tung Chung is a new town to the north-west of Lantau, the big island west of Hong Kong Island. More significant is the fact that Chek Lap Kok airport is just to the north across a bridge, and its link road to Hong Kong runs right through the town, dividing it into the social-housing quarter and the private quarter where the bridge is located.
Wells and Techniker had worked on the roof of a giant inhabitable bridge linking the two sides, but the current brief was to design connections between recently constructed housing blocks and a community leisure centre. The biggest problem was that not only had the whole town been settled on reclaimed land, but also the local foundations were reclamation fill on top of an old, filled, drowned river valley.
Anthony Ng had already got Sean Billings of Arup Facades, the facades equivalent of Arup's acoustic eminence grise, Derek Sugden, to design the glass canopies that are to be found above walkways all over the development. They are a welcome acknowledgement of the local monsoon-typhoon climate and were incorporated into the design of the Techniker bridges. These are real walkways rather than just paths, because the general principle behind the town design is to maintain a vertical separation between pedestrians and vehicles. So the Techniker solution ended up as a long viaduct of simply supported slabs spanning 5m between exceptionally long 6.5 metre columns linking two of the four residential blocks. The branch off to the adjacent sports club is 60m long, and its section can be described roughly as a pair of V-shaped beams connected below by a flat barrel vault. In fact there is a pair of 1m deep steel plates, braced by the outer sides of the Vs. The curved steel soffit between them aligns with the bottoms of another pair of thinner and shallower longitudinal steel plate beams.
The tall residential blocks have very deep pile foundations and are unlikely to move.
Not so the sports club, which was built on a raft and could be expected to subside up to an extraordinary 200mm. This means that the same could happen to the bridge and viaduct.
Piling would have been prohibitively expensive, and the idea of a raft was discussed but looked hardly viable. On this project the local engineer was Maunsell SouthEast Asia. Its engineers came up with an Archimedes-style answer: displacement foundations.Wells says:
'It's a very clever solution. You replace the earth you dig out with foundations of an equivalent weight. It acts very much like a pontoon bridge.'
With the support issue settled, there came the inevitable problem of differential settlement which, Wells says: 'occurs like a wave across the ground. We came up with the idea of supporting the viaduct on independent trestles which have articulated connections with each other and act rather like a caterpillar when the ground subsides. Our Vee-prop bridge meets the viaduct at its midpoint and is able to rotate at the prop position to accommodate any future settlements.'
Local building-control officers insisted on a pin joint in the branch bridge at a fifth point between the prop and the caterpillar viaduct.
Wells was a bit disappointed because joints are always expensive and need maintenance - and because it was decided to put in a balancing mock joint on the other side of the prop. His original thought was simply to allow the bridge to flex.
Wells is basically comfortable with the two bridges and very happy about the construction. He says: 'Both bridges were fabricated in mainland Guangdong province and the quality of welding is exceptional. The fabrication works are in a water city so you can, as we did to Tung Chung, get very big sections shipped over by boat.'
CREDITS CLIENT Newfoundworld ARCHITECT Anthony Ng Architects ASSOCIATED ENGINEER Maunsell SouthEast Asia