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Here's one we made earlier

A new grove of steel 'trees' is sprouting at Stansted Airport. Judith Cruikshank reports on how the construction team is applying the lessons learned on the original terminal to the latest phase.

Passengers were thin on the ground in 1991, when the vast, beautiful Norman Foster-designed terminal at Essex's Stansted Airport opened. Today it is a different picture. The glazed terminal building is still as impressive as ever, but it is much busier. A constant stream of cars deposit or collect travellers and on the west side of the building a huge white wall has replaced the glass.

Behind it, the terminal is expanding. Two more bays are being added to the original five as part of a £200M investment by BAA.

Extension of the building was envisaged in the original design and from the outset there was outline planning for a further three bays. The two under construction will add 11,500m 2tothe terminal. In passenger terms this means space for 72 more check-in desks and 300m 2ofretail space in which travellers can flex their credit cards.

An outstanding aspect of Foster's design is the tree structure which rises from the base of the building supporting the floors from its trunk. The branches then spread out to support the roof grid which acts like the leaf canopy. Four substantial steel tubes form the trunk, each topped by four steel branches.

Tensioned macalloy bars hold the branches in position. These are connected to the trunk structure by the aptly named Jesus nut and bolt assembly - the term comes from the aerospace industry, where it was first applied to the large nut that held the rotors onto early helicopters.

If this came loose, all the pilot could do was blaspheme!

The branches carry four gridline beams supporting the 18m 2diagrid which forms the structure of the roof shell. Each pair of trees supports a further roof shell between them, giving a 36m span between the centreline of the trees. In all, the extension comprises 12 trees and 44 shells.

Stansted was built on a greenfield site which meant the original team had 'half of Essex' to play with, as BAA's head of projects at Stansted, Russell Batchelor, puts it. The situation this time around was very different.

A busy terminal, a railway line, offices and the shuttle carrying passengers to the satellite terminal represented formidable obstacles.

Space was at a premium and there could be no interruption to airport traffic. And over-riding all these considerations was the desire for the construction process of Stansted Phase Two to show significant improvements over Phase One.

All the main suppliers for the second phase were familiar with BAA's way of working. They were selected early in the process, but had not all been involved with the original building.

Original 3D drawings were downloaded to Rowen, the new steelwork supplier. Glazing contract Schmidlin had worked on the first phase and was sourcing glass from the same Austrian factory. Working with these two, plus Prater, the roofing contractor, project manager Mace began the first run studies for the construction programme.

The key element was clearly the erection of the trees and roof shells. Analysis soon showed that this would take around three months, but preparing the diagrids for lifting would take six to seven months. So either considerable space had to be found for an accelerated programme of diagrid preparation, or the erection programme would have to align with the assembly, which would mean a general slowing down of the project.

On the other hand, if the amount of pre-erection work on the diagrids was increased so that effectively roof shells were being pre-assembled, the programme would automatically speed up at the post erection.

All the parties involved could see the benefits of pre-assembling the roof shells and lifting them into position fully completed. It would clearly save time and would also mean a dramatic decrease in the amount of working at height. In the first phase steelwork had to be repainted where it had been damaged by following trades and working at ground level it would be easier to control quality. But understandably, Rowen baulked at the idea of lifting a completed, glazed roof shell.

So the team swung into a programme to convince Rowen that it could and would be done safely. At this stage crane hirer Weldex was brought into the team to add its expertise. Finally, after a good deal of high level brainstorming it was decided to run a full scale trial at Rowen's Sutton in Ashfield factory.

The advantages were clear, says Mace production leader David Coulson. It would allow all the parties involved an opportunity to prove their methodology, to check the drawings from the original suppliers and to resolve any problems with materials and members of their own supply chains in advance of the main construction programme. And for Mace it would provide a realistic estimate of the time taken to assemble and erect a tree, complete with its roof shell. There would be real time data on which to base both programme and working methods.

The client gave approval and the test went ahead. An 18m 2roof shell complete with roof decking and glazed rooflight was successfully lifted into place.

The pre-assembly programme was on and Coulson estimates this saved a total of 12 man days on each shell. The benefits in terms of safety are probably inestimable.

Problems still remained, however. The production schedules of all the parties involved and their suppliers had to be co-ordinated and there was a nasty moment when one of Schmidlin's suppliers went into receivership. There was also the small problem of space to preassemble the 44 shells.

This was found on the north side of the airport, but moving the first complete shell from assembly area to site on the back of a lorry at night was a heartstopping moment. But everything went according to plan and Weldex's 250t Demag crane placed the shell safely onto its tree. As the project proceeded, production and erection rates were further improved with practice and - all the suppliers emphasise - quite remarkable co-operation between trades.

With growing confidence in the method the amount of preassembly increased. Shells over the monorail were positioned complete with soffit linings, which allowed a significant reduction in night working, and temporary works. Coulson reckons that in this area pre-assembly saved around £100,000.

Schmidlin's Paul Haslam says it will be difficult for him to move from a contract where genuine teamwork was in play to the 'traditional confrontational one'.

With luck, however, some of the Stansted attitude will rub off onto other clients and other project teams, to the benefit of both.

Cash draw

Construction of the Stansted exxtension started last April with the aim of completion by April 2002.

Currently the programme is running ahead of schedule and estimates are that it will complete three months early in December 2001. Client BAA is clearly delighted by the prospect of taking control of the new space earlier than expected. But doubly so, since the early delivery can be seen as a vindication of its newly revised Developed Project Procurement Process.

The headline is 'Eliminate Waste'. But that means more than a tidy site - important though that is. What BAA aims to do is streamline the entire construction process both organisationally and technically and to make it worth the contractor's while to do so through a system of rewards.

If, for instance, the Stansted extension comes in under target cost the difference will be split between the client and contractors on a fifty-fifty basis and all the suppliers will be rewarded proportionately to their role.

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