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And it's an awesome sight. It soars over everything and everybody. To get a feel for the size of the task that has just been completed, perform the following thought experiment. First, imagine a steel football pitch. Now, put your fingers behind the goals and push until it bows gently around the centre line, then hoist the whole thing 30 m above the ground. That is, in effect, what the project team has just done. And this, remember, is just a small part of the roof; there are another four equal-sized sections (and one smaller bit) that have to be raised to form the final article 鈥� albeit a month later than planned.

The construction of this roof is one of the most complex and critical tasks on the whole of the Terminal 5 project. At its high point, it towers 39 m above the apron, held aloft on 11 pairs of giant steel supports. However, the airport's radar is in operation 2 m above this point 鈥� a height constraint that was so inflexible that Mike Davies, Richard Rogers Partnership's project architect, refers to it as "the cursed 39 m figure". As well as constraining the design, the curse also prevented the use of cranes to assemble the roof.

To get round the height problem, the roof team opted to use a new and untried method of erection: this involved assembling the roof sections flat on the ground and then jacking them into place. Each of the five equal sections measures 117 脳 56 m. There is an additional single section 117 脳 18 m long, which fits on the end of the terminal. This method of construction had the advantage that most of the work could take place on the ground, making assembly a relatively easy and safe option.

At the moment, this first arched section hangs suspended from the eight pairs of cables used to hoist it into position above the terminal's ground-floor slab. The focus of the work now is on connecting this to the four steel abutment towers that will support the roof permanently, high above the passenger concourse. Once this has been done, the forest of temporary steelwork holding the abutment structures in place will be moved ready to begin the construction of the next set of abutments.

The roof is being erected from south to north 鈥� the same direction that the terminal's substructure is constructed. So, while the first roof section has been installed at the southern end of the building, at its northern end, construction of the main terminal's substructure and basement is still taking place. Because of the four-week delay in the raising the first roof section, construction of the second is practically complete and ready to be lifted into place. And adjacent to that, assembly of the third roof section now under way.

At the moment, the whole lifting operation takes three weeks. In time, the team hopes to reduce this. The aim is to complete the installation of the roof by the end of the year. Bill Frankland, head of the roofing team at Laing O'Rourke, is not prone to gushing shows of emotion. How does it feel to have successfully completed one of the most critical tasks in the construction of the new airport terminal? "We're very pleased," he says.

Installation of the first roof section has taken a month longer than expected 鈥� the knock-on effect of a delay in substructure work caused by bad weather two winters ago. In fact, the heavy rains pushed parts of the substructure back 14 weeks, so the teams have reclaimed 10. The roof team have to finish their part of the work by the end of the year. "If the roof is delayed, the entire T5 programme is delayed," says Andy Smith, BAA's project leader for the main terminal building. So, no real pressure there, then 鈥�

Here's how the roof was raised
Step 1 Construction of the bowstring trusses
The first part of each roof section to be constructed is its supporting structure. The 2500-tonne weight of each roof section is carried on four enormous bowstring trusses, which span the entire width of the terminal in one leap. The trusses are set at 18 m centres. Secondary steelwork spans between the four curved top members to stitch them together and form the three-bay section that will be hoisted into position.

The top member of the trusses is formed from a huge, curved box-section beam. This is assembled from truck-sized steel sections, prefabricated and bolted together on site. Steve McKechnie, an associate at structural engineer Arup, says: "We designed the steel to maximise off-site prefabrication. This removes the safety hazard of site welding as well as the quality risk of trying to weld under site conditions." To ensure the beam is assembled with the correct amount of curvature, it is assembled on a line of giant steel columns set out on the terminal's ground-floor slab. The columns increase in height from the edge of the floor slab until they reach a maximum in centre of the beam.

With the rafters assembled, the bow's string is added. A pair of 115 mm diameter steel cables is strung across the beam from one leg of the arch to the other 鈥� however, the cable is not tensioned until the roof's supports have been constructed.

Step 2 Assembling the roof supports
The roof is held aloft on a series of figure-shaped constructions called abutment structures. These are formed from sections of circular hollow section steelwork bolted to a central "torso". Two tubular steel legs support the torso, and two tubular arms reach upward from it to support the roof. "The knuckle [that is, the torso] where everything comes together has had years of team input so that it is Tonka Toy-simple to assemble 鈥� the whole structure is held together by bolts," says Mike Davies, the project architect with Richard Rogers Partnership.

The abutments face each other across the width of the terminal. Each abutment supports two bowstring trusses, one on each arm, so two abutments are needed on each side to support the four trusses.

To hold the abutment's limbs in place during construction, a forest of temporary steelwork is used as a giant jig. "This steelwork is more of a machine than a supporting framework," says Bill Frankland, roof project leader at Laing O'Rourke. It can be used to manoeuvre steelwork into position during erection. This steelwork also supports the curved tusk-shaped extension to the trusses that form the curved edge of the roof. The temporary steelwork will be dismantled and reused on the assembly of each subsequent abutment.

To ensure the roof erection proceeded smoothly on site, the arch-supporting abutments were trial-assembled at steelwork contractor Severfield-Rowen's base in North Yorkshire in "a first-run study". The trial generated about 100 tweaks to the design and erection methodology and construction tolerances.

As well as proving to the design team that the erection method was workable, the study gave the steelwork erectors valuable practice. Severfield-Rowen is using the same team on site that practised on the trial abutment. The temporary steelwork used in the study was also reused on T5.

Step 3 Covering the roof
While the abutments were under construction, the roof covering was installed. Once again, prefabrication featured in its construction. The roof was covered in pre-assembled roofing cassettes, each 6 脳 3 m, developed by Hathaway Roofing. These contain the perforated lining layer that will form the underside of the roof, a structural deck, a sound absorbing layer and a fleece layer. "We've preassembled 80% of the roof covering off-site," says Hathaway's Ian Coverdale. The only part not installed is the standing-seam covering and the thermal insulation. This can only be installed once the bowstring trusses have been bolted to the tusks, after the roof is in position.

However, even though the roof covering and insulation is not actually fitted, it is still stacked on top of the roof and tied down ready to hitch a ride when the roof is raised.

Step 4 Installing the lifting towers
The roof is hoisted into position using a series of cables and modified strand jacks. There are eight lifting towers needed for each roof section. Each lifting tower is constructed from two steel tubes, positioned so as to straddle the end of each truss. A crossbar links the tops of the two tubes. Mounted on top of this crossbar is the strand jack. Cables from the strand jack drop down to a beam inserted beneath the truss ends.

Step 5 The lift 鈥� part 1
The roof is raised into position in two stages. First the ends of the trusses are lifted 200 mm to allow the tie that spans between ends of each bowstring truss to be tightened. "We tensioned the bowstring truss by putting in 3000 tonnes of pre-stress," explains McKechnie. As the tie was tightened it increased the curve in the enormous roof, lifting the centre sections of the steel beams clear of their supports. "Pulling on the tie-wire made 2500 tonnes of steel lift up in the centre," says McKechnie.

It took two days to sequentially pre-stress all four rafters.

At the same time as the truss is being pre-stressed, the team took the opportunity to check all the equipment ahead of the main lift. "For us, this is the lift proper because it provides the first opportunity to properly load all the equipment," says Frankland. Once everything has been surveyed and signed off, the lift proper can commence.

The lift 鈥� part 2
Before the first roof section could be lifted the roof team had to wait for a suitable "weather window" says Frankland. It is not the roof lift that needs the window; the process has been developed so that the lift can go ahead even in a raging storm. The calm conditions are needed for the operatives working from cherrypickers that nursemaid the jacks.

The roof is lifted 30 m progressively in 450 mm increments. During the lift, the roof is monitored to ensure it does not go out of alignment. The maximum each bowstring truss can move relative to its neighbour is 40 mm. It took 10 hours to raise the first section of roof into position. "The excitement only starts as the beam nears its abutment connections," says Frankland.

Step 6 Attaching the roof
Once in position the roof section is connected to the abutment structures, which is what is currently happening on site. The beams are bolted to the tusks and the arms are connected to the central arch rafters. Once all connections have been made, the weight of the central roof section will be progressively taken up by the abutment structure as the strand jacks lower the 2500-tonne central section gently into position. Finally, the temporary steelwork is split apart and rolled into position ready to begin work on the next set of supports.

Step 7 After the lift
Once the roof is in position. Hathaway's team will install the roof insulation, untie the standing seam roofing sheets and complete the installation of the covering. When the second roof section has been raised, the empty roof bay between the two sections will be stitched together with secondary steel beams and the installation of the roof cassettes on this section can commence. As soon as the temporary steelwork is clear of the first section, the follow-on trades will rush to start on the construction of the interior and to begin the cladding installation.