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On the right track

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MetalWorks Transport

Expo Station on Singapore's new rapid transit line offers a dramatic and original vista for new visitors to the country

Visitors flying in to Singapore can now take the new Changi Airport rapid-transit line to reach the city. Expo Station is the first stop on the line; its dramatic roof structures - a 200m-long titanium hull and a 38m-diameter stainless-steel disc - will for many visitors be their first visual impression of Singapore.

The strong forms impart a symbolic value to the station, announcing its presence as the urban gateway to a dynamic modern city. But unlike other well-known structures with titanium cladding they were not designed as sculpture - they are the logical expression of functional, structural and environmental constraints. They also act as a conspicuous marker and direction finder on the relatively flat hinterland of the airport. This area is now being developed and includes the new Singapore Exhibition Centre. As part of its brief the new station had to accommodate the vast numbers of visitors to the Exhibition Centre - at peak times an estimated 17,000 passengers will pass through the turnstiles.

The metro runs into the station on a pair of elevated tracks that rest on cast-in-situ concrete viaducts. The platform, an arched concrete structure, is set between the tracks at upper level and is reached from the concourse at ground level by ramps, escalators and lifts. The process is the reversal of Foster's underground station at Canary Wharf, and the numbers of people involved demanded easy access and clear orientation within the station. A strong visual connection between upper and lower levels has been achieved by cutting out a long section from the floor that divides the platform from the concourse. Passengers can glimpse trains passing overhead as they enter and buy their tickets.

The platform and concourse are sheltered by the hull-like roof that takes the form of a sweeping toroidal curve. Supported only by two pairs of elegant, V-shaped columns along the sides, the hull is a steel diagrid 200m long which cantilevers 37m at both ends to a delicate tapering edge.As Singapore has one of the warmest and most humid climates in the world, the concourse has open stainless-steel screened sides instead of solid walls, which allow the space to be naturally ventilated; fresh air is drawn in through the screens to replace warm air, which rises and is expelled through rooflights. In addition, the screens give a feeling of openness, and suggest that the roof is floating over the platform and concourse.

The ticket-hall roof overhangs part of the platform roof to create a unique visual counterpoint. Supported on four raked steel columns, fire-protected and clad with stainless steel-panels, it is a 38m-diameter disc with a soffit of satin-finished stainless-steel panels that radiate from the centre and tilt slightly upwards at the curved edge to reflect the daytime sky.

At the centre of the disc is a circular rooflight glazed with light-diffusing glass. It directs natural light to a cylindrical glass and stainless-steel lift shaft immediately below.

The top of the stainless-steel tubular shaft is fitted with uplighters which illuminate the disc at night. The roof is covered with an aluminium deck for easy maintenance.

The materials used to construct the station reflect the harsh qualities of the local environment: it is exposed to marine seaborne salts, very heavy rainfall and baking temperatures combined with 90 per cent humidity.

In addition, the number of passengers using the station dictated the use of durable, low-maintenance materials with a long life.

The client wanted quality, not only in design but also in materials. The client specified a light-grey granite floor for the concourse, the platform and surrounding paved areas.

The choice of roof materials also has an environmental significance. The stainlesssteel soffit reflects daylight into the concourse, while the titanium cladding deflects the sun's rays, thus helping to create a microclimate on the platforms that is up to four degrees cooler than the outside temperature.

When it comes to transport, Singapore demonstrates the future. Drivers are charged for using the roads to encourage them to use public transport; this, in turn is efficient, clean and well-maintained - chewing gum is banned from the country.

The new Expo station makes public transport look positively glamorous.

CREDITS

CLIENT Land Transport Authority, Singapore (LTA)

ARCHITECT Foster and Partners

ASSOCIATE ARCHITECT PWD Consultants Singapore Land Transport Authority, Singapore

CIVIL ENGINEER Land Transport Authority, Singapore

MECHANICAL AND ELECTRICAL ENGINEER Land Transport Authority, Singapore

STRUCTURAL ENGINEER Ove Arup & Partners

QUANTITY SURVEYOR Davis Langdon and Seah

LIGHTING Claude Engle

LANDSCAPE ARCHITECT Cicada Singapore

A steel diagrid roof structure lined with steel panels

The concourse roof structure is a curved, toroidal diagrid of steel UB members that are connected by prefabricated nodes to a welded steel frame. Structurally it is so stable that a span of 70m between the two central columns provides ample support, while at the ends it is balanced by the 37m cantilevers.

The roof is covered with titanium sheet, chosen for its light weight, potential durability and ability to reflect the sun's rays. The sheets were cut on site with a rolling machine which was specially developed to taper their edges in order to accommodate the toroidal shape. These sheets were jointed with rolled seams.

The soffit of the roof is lined with a series of triangular stainlesssteel panels which are fitted between the diamond grid of the steel structure. Stainless steel was chosen for its ability to resist the harsh environment - although sheltered, the panels are exposed to the atmosphere.

Some panels are of stainless steel polished to a mirror finish, giving magical reflections of moving trains. Other panels are formed of a series of polished stainlesssteel dowels and are backed with acoustic-absorbent material.

Natural light is brought into the platform areas by means of diamond-shaped rooflights set at the apex of the diagrid; they are fitted with triangular louvres formed of stainless-steel dowels which match the adjacent solid panels and filter direct sunlight.

The bull-nose edge of the roof was fabricated in the factory from a thicker, 1-2mm gauge of titanium.

Gutters and downpipes are not used - none would be large enough to cope with Singaporean downpours. Instead, rainwater simply discharges along the two sides of the roof and drains into highcapacity land drains.

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