精东影视

Feature, 8 May 1992

Arch arrival

Passengers arriving at the UK鈥檚 first truly international railway station will alight at Waterloo on platforms basking in sunlight under a roof of stunning complexity. James Macneil describes how designers and contractors have overcome a constricted site to build the twisting arch roof and the new airport-style terminal.

Designed to provide a lasting impression for passengers arriving from, or leaving for, the Continent, Waterloo International Terminal is certain to produce high opinions of British roof construction.

Although no more than half-built, the three-pin arch roof spanning the platforms already dominates the site. Formed from blue, banana-shaped, bow-string trusses springing from stub columns on the platform slab鈥檚 edge and pinned together at the centre, the colonnade catches the eye from any angle. At the northern end, where the stainless steel and glass cladding has advanced several bays down the platform, the train shed roof resembles a 21st century greenhouse.

The 400m-long roof is the crowning glory of the 拢130m project to build the first London terminal for trains arriving from Europe via the Channel Tunnel. Construction manager Bovis Construction and client European Passenger Services 鈥� a subsidiary of British Rail 鈥� are working closely with the designers and trade contractors to conquer the vagaries of the site geometry within a tight 27-month programme.

Project architect Nicholas Grimshaw and Partners, working closely with structural engineer YRM Anthony Hunt Associates, designed the roof. Architect David Kirkland, responsible for the roof, says it 鈥渓ooks very strange and very complex鈥�.

But he explains: 鈥淣inety per cent of the design is down to site constraints and physical problems. Instead of fighting them, we鈥檝e let all the problems come together and dictate how the structure evolves.鈥�

Snug fit

Squeezed between the existing rail tracks and the site boundary, the terminal tapers and curves towards its southern end. 鈥淚t fits the site like a sock,鈥� says Kirkland. This funnel-shaped plan translates through the structure to the roof. The most northerly of the 36 arches, closest to Waterloo鈥檚 concourse, spans 48.5m while the southernmost arch only clears 32.7m.

The site constraints severely limited where the trusses could spring from. And, because the station has five platforms 鈥� two centrally-located sets of double tracks and a single track on the western edge of the building 鈥� a symmetrical arch spanning the full distance would not give enough clearance for trains on the western side.

To solve these problems and arrive at an unusual design solution in one swift manoeuvre, the designers decided to shift the central pin between the two trusses as far to the west as possible. 鈥淭his allowed us to get a tight radius truss on the western side that avoided the train envelope,鈥� explains Kirkland. It also helped restrict the building height 鈥� an important precondition as originally there were plans to build an office development over part of the terminal.

By the time the roof form was finalised, many of the structural problems were already resolved. YRM Anthony Hunt Associates director Alan Jones says: 鈥淭he shape isn鈥檛 something you can arrive at without any recourse to the structure. It is a very pure form that mirrors forces within it.鈥�

Each arch consists of two prismatic bow-string trusses connected at a central knuckle joint. The larger truss, which springs from the eastern side of the station, has two telescoping, hollow compression booms on its upper face and a single smaller solid tension bar beneath, to stop the truss spreading under load.

As the arch profile is asymmetric, the small truss is reversed, with two tension rods forming the outer chord and a single, internal compression strut.

Says Jones: 鈥淏ecause we had to set the roof up to suit the substructure, the geometry was just a nightmare. We had to build a full three-dimensional computer model of the whole 400m length, primarily to check the geometry because the setting out points were just a series of co-ordinates.鈥�

Standardisation 

Ease of manufacture and erection was not forgotten as the design evolved. 鈥淲e tried to build in as much standardisation as possible,鈥� says Jones. 鈥淭he central pin joint can take any orientation we want, and where the spans vary, we took the truss cross-section and scaled it down. These measures minimise the number of different truss designs required.鈥�

Steel fabrication and erection contractor Westbury Tubular Structures is working on two fronts. Like courting stick insects, pairs of mobile cranes perform delicate tandem lifts to tie each arch together, after painstakingly assembling the truss sections on the concrete deck. 鈥淓ach crew begins a new bay every two to three weeks,鈥� says Patrick Crotty, Bovis鈥檚 project director.

Briggs Amasco Curtainwall is chasing the steel erectors down the platform, fixing the cladding 鈥� a shimmering combination of corrugated stainless steel and toughened glass.

The roof is designed to daylight the platforms and give a view for passengers inside and public outside. The tops of the main trusses are glass-clad, with stainless steel spanning between, while the western side is entirely glazed on the inside chord of the smaller trusses, leaving the steelwork exposed. 

鈥淚t is like the main truss being the lid of a toy box, with the smaller truss propping it open and displaying the trains inside,鈥� enthuses Kirkland.

On the western elevation the glazing spans between the trusses. 鈥淭he spans counter-curve, twist, expand and contract in the space of a bay,鈥� says Kirkland. The solution was to mimic nature: 鈥淭he cladding system is like a snake鈥檚 scales; as the building twists and turns, the horizontal joints just overlap.鈥�

The gap between each layer of glass is sealed with a neoprene gasket resembling the wiper blade of a car. Another gasket, flexible in three dimensions, was developed for the vertical joints.

Glass beams

On top of the large trusses, where the glass spans up to 4.2m, a vertical glazing system is adapted for the task. 鈥淲e didn鈥檛 want to confuse the structure by adding more steel members to support the cladding so we used horizontal glass fins as beams to get the glass sheets to span,鈥� explains Kirkland.

The edge of the glazing steps along the plan profile of the truss, overlapping the stainless steel like the armour of an armadillo. 鈥淚nstead of cutting each glass sheet to an angle to fit the trusses鈥� plan profile, we decided to use standard sized sheets of glass,鈥� explains Kirkland. 鈥淭his reduces the cost of the glass by around 30%.鈥� 

The fixings for the glass panels are formed from three standard cast stainless steel components, bolted together in varying combinations to give all the connections required to fix the roof. 鈥淪tandardisation allowed us to use casting techniques to produce the fittings in a very economical way,鈥� says Jones. There are 299,000 components in the 10 000m虏 of structural glazing, which weighs 249 tonnes. The terminal also has 10 000m虏 of stainless steel cladding. 

鈥淭here was as much time spent on the cladding as on the steel structure,鈥� says YRM鈥檚 Jones. Briggs Amasco director Angus Glennan agrees: 鈥淣o way does this fit into the crinkly tin image. To devise a cutting schedule for the stainless steel sheet alone has taken a six-month CAD operation.鈥� 

To simplify their task, the design team used both physical and computer models throughout the development of the structure. It started with a brass model and progressed through an Intergraph computer model, created by YRM, to a 1:10 scale model of a single bay.

The final model was a full-scale erection of two bays of the actual roof at Westbury鈥檚 site in Yorkshire last summer. 鈥淎t that stage we were sure of the design but weren鈥檛 sure how easy it would be to erect,鈥� says Kirkland. 鈥淚n terms of saving time on site the mock-up was absolutely indispensable.鈥�

But, spectacular as it is, the roof is a relatively minor part of the terminal work, accounting for 10% of the project value. 鈥淚t is only the froth on the top of the building,鈥� says Grimshaw鈥檚 team leader, Neven Sidor. 

Beneath the glittering crown, a three-level 200m-long terminal extends from the Network SouthEast concourse at the northern end of the site. The remaining 200m of the platform sits on a viaduct.

The terminal is designed to operate in a similar manner to an airport. Passengers will wait in departure lounges before boarding the trains, and leave and arrive through immigration and customs control points. 

Immediately below the platform is the departures area and, below this, at ground level, is the arrivals hall. The single basement will be devoted to public carparking.

Fast track

Only 15 months after Bovis took full possession of the site, fit-out trades are beginning to move into the cavernous concrete shell. Mike Foulkes, EPS鈥檚 project director, says: 鈥淚t is quite an achievement to complete the concrete substructure in this time. We鈥檝e got another year to fit it out.鈥�

As with those on the platforms above, the trades inside the terminal are working away from the existing station. Glaziers erecting the expanses of structural glazing and blocklayers beginning the internal partitioning are the only evidence of the impending flood of service and finishes contractors into the reinforced concrete shell. 鈥淚t will then be a matter of sequencing the trades and geometric co-ordination between various services and fittings,鈥� says Crotty.

If the three-storey terminal building is overshadowed by the roof, it is unlikely many people are aware of what Crotty describes as the 鈥渉eart and lungs of the terminal鈥�. The labyrinth of brick vaults beneath the existing station is being cleaned, waterproofed and fitted out to provide 9500m虏 of support facilities and plant rooms. Already major plant items are in place and blocklayers, waterproofers and services engineers are working to convert the area into usable space.

Public information 

Under the full gaze of the public are the external infrastructure works. Including a footbridge, pedestrian subways and connection of essential services, these are carefully planned to minimise disturbance to the 250 000 passengers that pass through the adjacent Network SouthEast terminal daily. 鈥淲e have given pretty strong advice to the public as to what we are doing and how it will affect them,鈥� says Crotty. 

鈥淏ut it鈥檚 not purely altruistic,鈥� he confesses. 鈥淚t is hard enough to build a project of this size on a normal city site. With a site like this anything that reduces problems with the public is worth it.鈥� Bovis has anticipated the logistical problems involved and planned the work since early in the project. Says Crotty: 鈥淚t is getting more complex but it is not new to us. We have been tweaking and updating our outline plans for some time.鈥�

鈥淲e are buying in a better way,鈥� says Foulkes. 鈥淚t may seem bureaucratic but when you have 80 packages you have to have a firm approach.鈥�

All change for BR fast-track policy

Waterloo International marks a departure from British Rail鈥檚 traditional procurement methods. As a wholly-owned subsidiary of BR, a renowned traditional construction client, European Passenger Services may have surprised observers when it selected construction management.

Mike Foulkes, EPS鈥檚 project director, says although this is not the first time a BR project has used construction management, 鈥渂efore it has always been in partnership with a developer鈥�.

Foulkes says the project is an ideal application for construction management. 鈥淚t is a fast project, a lot of the design is done on the go. It would be difficult to find a main contractor with all the skills required for this site and the project needed an active client.鈥�

To ensure the construction management worked, Bovis and EPS have assembled all consultants and major trade contractors on site. 鈥淭he closer we get the client, designers and construction manager, the better the project runs,鈥� says Bovis project director Patrick Crotty. To this end, the client, construction manager, consultants and major trade contractors share a three-storey office perched on stilts over the taxi access ramp on the eastern side of the station. 

Bovis also set out to perfect the way it selected trade contractors. 鈥淭he classic problem of competitive tendering is that you end up with a lowest bidder who you are not sure can cope with the job and a higher bidder who you are confident of,鈥� explains Crotty. 鈥淭he way to avoid that is by prequalification.鈥�

Before a contractor is asked to bid, its operational abilities are thoroughly assessed. 鈥淚f you do this you do away with the questions that come up afterwards,鈥� says Crotty.