...steelwork contractor Rowen. "We… developed those designs with them to something that was buildable and worked with our manufacturing processes," he says.

The original folded plate roof design was structurally efficient but needed a huge number of site welds. The contractor "did not have [enough] confidence that they could have sufficient welders on site," says Mitchell. The all-bolted final roof is not "as elegant," she says. "But there is no point in designing an elegant structure that can’t be built."

The structure consists of 22 girders crossing the building at 18-m intervals. The central 117 m of each is an arching box girder, 80 cm wide and up to 3.8 m deep. These are prevented from flattening by pairs of 11.5-centimeter-dia ties between their ends.

Support. Tube steel sections of abutment converge on central hinge.

The roof, now over half built, has arches bolted to top girders of supporting "abutment" beams. Adjacent pairs of abutment beams are propped by four tube steel struts that converge at two splayed legs. These inwardly leaning legs rest on cast steel feet and are stabilized by near-vertical ties between feet and roof.

Tube steel sections of each abutment, some over 90 cm wide, converge on a single, vast hinge made of 25-cm-thick steel plate. Special castings cap the abutment’s tubes to form bearing surfaces.

Rowen in July 2003 began erecting the entire roof and supporting abutments in six synchronized lifts. With the fourth roof section erected last month, the final lift is due early next year.

Solutions. Roof now being constructed is not as elegant as the one proposed, but it can be built. (Photo above and below courtesy of BAA)

Low Ceiling

Mitchell credits Austin for early spotting of the potential difficulty of roof erection within a 43-m ceiling imposed by air traffic controllers. He did a "graphic cartoon" showing arches being assembled on the ground and raised with a sort of scissor device.

Austin’s basic idea took off. Over 10 hours this April, three 18-m-long roof bays, with a total weight of 2,500 tonnes, were raised 39 m in one piece.

Construction of each roof section starts with crawler cranes positioning temporary towers to assemble two abutments on either side of the building. Then, sections of four, 117-m-long arches are bolted together inside the terminal on low level props, and tie rods installed.

Steel towers with strand jacks at their tops are then rolled into place at each arch end. By simultaneously activating pairs of these jacks, the entire roof section rises, to be fixed with bolts to awaiting abutments.

"There was a lot of work done on the tolerances that would be necessary for [roof elements] to fit together," says Mitchell. For example, adjacent sections of the huge arch girders each have a big male/female connection. These ensure precise positioning, allowing workers inside the girders to bolt abutting diaphragms together.

Made in large fabrications, Rowen delivers steelwork directly to the site, bypassing T5’s two logistics centers. These are another aspect of BAA’s direct control. The policy "is about making sure we are looking at the entire cost of the supply chain," says materials and logistics manager Doug Black.

Transportation to and from site is constrained by existing airport facilities and tight limitations imposed by planning permits. So BAA operates the logistics centers, a few kilometers from the work, to control material and equipment deliveries just as they are needed.

BAA invested $33 million in building the Colnbrook center, which has 10 hectares of covered space and an a railhead for bulk goods. For its other center, Heathrow South, the company leased over 8 ha of existing warehouse space.

The centers are not only staging posts, but also assembly facilities intended to minimize site work. About a quarter of the rebar, for example, is being pre-assembled at one center and delivered when needed, says Wolstenholme. And nearly three-quarters of mechanical and electrical items are being assembled elsewhere, halving the 800 to 900 workers needed on site, he adds.

Now, only about a third of materials are being shipped via the logistics centers. "Because a lot of it is bulk loads, there is no point in double handling," says Black. Main contractor Laing O’Rourke Civil Engineering Ltd., Dartford, generates most of the bulk material flows and manages the logistics.

All of this effort is focused on controlling movements of people, materials and equipment through the site’s single gate, now handling some 200 transits each hour, says Black.

Such meticulous preparation for procuring what is one of Europe’s largest construction jobs has been worthwhile, says Wolstenholme. As evidence, he cites the prompt recovery of 12 weeks’ delay in substructure work that the project had accrued by mid-2003.

The unusual approach to managing construction risks appears to maintain harmony. Riley estimates 98% of disagreements are resolved on site, without senior executives. "We don’t believe we can do this kind of building without a fully integrated team," adds Forster.