With virtually no internal columns, the building's vertical and horizontal loads are carried by exposed perimeter mega-frames of large, fabricated box sections. The heaviest, 24-m-long column weighs 42 tonnes.

Mega-frame diagonals are fixed together with bolts up to 7.6 centimeters in diameter at prefabricated nodes on every seventh floor. Nodes are about four meters long and made with steel up to 18 cm thick, weighing as much as 21 tonnes. The self-supporting core tower, including some 3,000 tonnes of steelwork, is fixed to the rear of the main frame.

"When we started the bid we knew if we didn't get the structure right, the building wouldn't work," says Andy Butler, project director with prime contractor Laing O'Rourke Construction Ltd., Dartford, U.K.

Fine-Tuning the Frame

A key innovation at the bidding stage dealt with the lateral displacements the steelwork frame would experience during erection because of its lack of symmetry, says Butler. Arup had anticipated pre-cambering the vertical elements to precise dimensions. Displacements would be progressively eliminated as the building neared completion.

But with such an intricate structure, the approach would have created geometrical challenges, claims Butler. With guidance from Chicago-based Skidmore, Owings & Merrill, "we thought we'd be a little smarter," he explains.

Instead of pre-cambering the columns, the contractor made the mega-frame diagonals a few centimeters short and packed the ends with spacer plates. Then, when each mega-level was in place, the contractor stretched the diagonal with jacks to achieve their correct lengths.

Level by level, "we pulled the building into shape," says Butler.

At the start, the method seemed "incredibly complicated," remembers Karl Williams, senior site engineer with the steelwork subcontractor Severfield Watson Structures Ltd., Bolton, U.K. However, the operation in the field "happened within millimeters of the analysis," he says.

Severfield Watson started its $104-million subcontract on-site in July 2011. By that September, "we were putting columns [weighing 30 tonnes] in the basement," says Alex Harper, the company's contracts director. Altogether, the firm placed some 18,000 tonnes of steelwork in more than 11,250 elements. Made of steel plate up to 13 cm thick, the steelwork came in lengths up 28 m and weighed as much as 56 tonnes.