Prefabricated Elements

Even before the main contract was signed, Laing O'Rourke's chairman, Ray O'Rourke, ordered two of Europe's largest luffing-jib tower cranes, says Butler. The Comedil CTL650-45 rigs were later joined by a pair of smaller luffing cranes.

"We spent four to five hours a day planning the next 24 hours, making sure all the cranes were working all the time," says Severfield's Williams.

Because of their large capacity to lift and place preassembled elements, the big cranes were key to the contractor's strategy of maximizing off-site production. For example, Severfield fabricated each floor of the north core in three 16-m-long "tables" weighing 30 tonnes. They were made in Northern Ireland and fitted out in England before arriving on-site.

Laing O'Rourke also almost eliminated cast-in-place floors by developing a new, precast-plank system, says John Stehle, structural leader in the company's engineering excellence group.

After winning the contract, "we had to do a research-and-development program, get the client sign-off and actually make the planks," Stehle explains. The contractor started using the planks at level six, placing them at a rate of about 500 sq m a day, he adds.

With the last pieces of steelwork being placed in recent weeks, the drive for prefabrication appears to have worked. About 85% of the building on Leadenhall Street was made off-site, Butler estimates.