A key milestone is final erection of the giant roof to the main concourse. Dave Parker reports from T5A on preparations for the 'big lift'.
Ever since work started on the T5 project back in July 2002, the only sign of the mammoth endeavour visible to motorists on the nearby M25 has been the 'crane forest', the multi-coloured jibs of probably the largest, densest and busiest complex of large cranes ever seen in the south of England. Soon, however, a much more permanent landmark will slowly heave over the eastern horizon. The unmistakable waveform roof of T5 Concourse A is beginning to rise.
A complicated, tricky and testing operation is needed to get all the giant steel box section rafters into their final position. 'In fact, this is more on the scale of major bridge engineering, ' says T5A project leader Andy Smith.
'We'll be lifting the central arch sections of four rafters, their connecting beams and three bays of roofing from ground level up nearly 30m to their final position on the roof abutments, a total lift of 2,500t. And we'll be doing it six times in all.'
T5 numbers are always impressive. Designed by Richard Rogers Partnership, T5A's roof will have a clear span of 156.6m between abutments, with tapering cantilever 'tusks' extending the roof at each side to a total width of 176m. Rafters, fabricated steel box girders, are at 18m centres, and vary in depth from 800mm to 3.8m.
Circular hollow section (CHS) members up to 914m diameter make up the complex, widespreading abutment trusses. Total steel tonnage in roof and abutments is 18,500t; when complete before the end of this year the roof will be just under 400m long.
One key factor in the final choice of construction method was the height limitation of 43m, imposed to avoid interference with Heathrow's many radar systems.
Another was safety: a method that minimised work at height was obviously preferable. And, of course, there was the universal T5 pressure to prefabricate as much of the structure off site as possible.
Early in the design development, the decision was taken to separate the roof and facade structure from the internal superstructure.
Structural engineer Arup designer Steve McKechnie says this option offered several benefits.
'In the short term, it means an earlier watertight envelope, so the internal structure can be built in a weatherproof environment. Quality will be improved, and disruptions minimised, ' he says.
'In the longer term, the separation of the two structures beneath the clear span of the roof means BAA can make major changes to the internal layout with much less disruption than normal.'
Prefabrication potential was somewhat limited by the sheer scale of the main structural members. Steel fabricator Watson Steel Structures could only transport sections up to 30m long and 56t in weight. Conventional construction would have seen these sections joined to form the major structural sections by an army of welders, an option the T5 team preferred to avoid wherever possible.
Arup's solution was to transform the central 117m upward curving sections of the roof rafters into perfect bowstring arches with all joints permanently in compression.
Pairs of unobtrusive 115mm diameter locked coil strands tie the ends of the arch together - permanently. Individual rafter sections are fitted with male and female 'Lego-style' shear connectors.
These arches could easily be assembled on the ground floor slab from sections supported on temporary props situated deliberately over node points in the underlying substructure. Achieving accurate assembly under these conditions is relatively straightforward.
Another key decision was to erect three bays of the centre section at a time, link them with the secondary beams, and install specially developed roofing 'cassettes' supplied by Hathaway Roofing. Complete roof cladding and services can then be installed at lower, safer heights, before the lift begins.
More complex temporary works are needed for the 'angel truss' abutment structures and the distinctive tusks that extend the roof into an upward curve at each side. 'These have to be erected first, before the main centre section is lifted, ' explains sub project leader leader Frankland.
'It is vital that the roof eaves are in line. It is also vital that there is sufficient tolerance and adjustment built into the design for the final connections between the centre sections, the abutments and the tusks.'
Massive high strength friction grip bolts connect the tusks - which arrive on site in one piece - to the central arches. More than 250 bolts are needed at each end of each arch. The angel trusses, with their 'arms', 'legs' and 'wings' linked by complex nodes fabricated from steel plate up to 250mm thick, also demanded radical solutions.
'They start out as pin-jointed structures, with the CHS sections fitted with cast steel connections, ' explains McKechnie. 'This allows everything to be adjusted and connected without welding before the joints are locked to allow load transfer.'
Specialist consultant the Rolton Group was called in to design the temporary works scheme with the team that would make this construction strategy practicable.
'The weight is not the problem, ' says chairman David Rolton.
'Modern strand jacking techniques have coped with much heavier loads than we have here.
The real challenge was the design of the temporary works for the abutments.'
As each angel truss supports the ends of two rafters, only two pairs of the abutment support frames were needed. The solution was a tubular framework liberally equipped with jacking points to allow adjustment during erection and make load transfer to the foundations very controllable.
Transfer trusses within the frames ensure that all loads are taken out to substructure nodes.
The clever bit is that the complex support frames are designed to split apart into two sections very easily. These can be quickly moved, repositioned and reassembled ready for the next roof section.
By comparison, the actual lifting frames are relatively simple structures, essentially pairs of large diameter steel tubes straddling the rafter sections and carrying a pair of strand jacks on their upper crossheads. Stability comes from the abutment supports, and again, the eight frames are designed to be simple to move on to their next lift.
On paper, this is an elegant and ingenious solution. It copes with the T5 project's most pressing imperatives: the need for speed, and the all-enveloping logistics straitjacket. However, the stakes were too high and the technology too advanced for BAA to commit to this strategy without some demonstration that the design team's basic roof erection concept would actually work.
Off-site trials of tricky erection procedures are not new to BAA.
What took place at Severefield Rowen's plant in Yorkshire over three months last year, however, was by far the biggest such exercise ever. It cost BAA around £4.1M pounds - money well spent, says T5A production leader Gareth Lewis.
'Of the £4.1M, £3.25M went on the actual components of the roof, abutment and facade, as well as the temporary works, ' he says. 'All these will be dismantled, brought down to Heathrow and re-used in the actual roof itself. And as a week's delay on the roof would cost us at least £300,000, the trial was a real bargain.'
In Yorkshire, the trial - dubbed the Abutment First Run Study (AFRS) - had a formidable list of objectives. The design of the abutment trusses and nodes had to be finalised, assembly and erection techniques had to be proved and improved, actual construction tolerances established, the health and safety requirements sorted out. And the whole team had to be trained.
'Basically we're applying the same philosophy we pioneered at Stansted, ' says Watson roof construction manager John Calland. 'The most complex part of the roof structure is taken off the critical path and tested with the whole team involved.
'We learned an awful lot, especially about the nodes and the tolerances needed.'
Significant improvements to the supports for the 'torso' node, where the angel's arms, legs and wings all meet, was one of the key benefits; in fact, the torso wasn't changed at all. The trial also showed up serious production difficulties with the first version of the cast 'foot' nodes.
Corus and William Cook Cast jointly developed a special jig that made machining the three bearing faces on the feet much easier. And erector Steelcraft came up with the idea of hinged splice plates for the crucial connections between the central arch and the abutment truss.
Overall, the team came away from the trial reassured and confident that the giant undertaking that lay ahead of them was definitely a goer. The real thing would be somewhat more complex, of course. The ground floor slab, off which the roof must be built, is relatively thin, and all serious loading, from the temporary works and the army of heavy crawler cranes alike, has to be taken down into the basement substructure through its steel frame.
This means careful planning and accurate placing of crane mats and temporary bridging beams. More than 20 cherrypickers swarm on the slab, threading their way between a dozen or more crawler cranes of up to 80t capacity. Steel sections weighing up to 56t are lifted straight off their delivery lorries and erected. Roofing cassettes which form 80% of the roof build are lifted straight off their delivery lorries and onto the roof - a classic example of the whole T5 approach, says Smith.
Final preparations are almost complete. Early next month, when the forecasters confidently predict a three-day window of favourable weather, the first roof section will lift off on its one-day journey to the top of the abutments. The actual strandjacking operation falls to specialist subcontractor Fagioli PSC, the company that at almost the same moment will be engaged in the equally high profile lift of the Wembley Arch. The T5 team is far from overawed at the coincidence, pointing out 'you could fit 48 football pitches into T5A once all the floors are completed.'
As the various stages of roof assembly follow closely behind substructure construction, so facade and the internal specification will begin as soon as possible after the first roof bay is in position.