Photo courtesy of Denton Corker Marshall. Photography: Morley von Sternberg
Structural challenges
The most complex engineering aspect of the project is the cantilevering fingers, which are designed as trusses. The high tensile forces produced at the top of each ‘finger’ and the high compression at the bottom are distributed through the composite steel deck floors to the main slipformed concrete core, via tension reinforcement where appropriate,
The component members of each finger are made of plated sections generally 450mm deep and 220mm wide. Flanges are typically of 30mm plate with 15mm webs.
The trusses are up to four storeys high, fabricated and welded at steelwork contractor William Hare’s works with a built in precamber. Each truss was cut through the diagonals into sections and was bolted together on site. Pods were also assembled on site and craned into place.
A row of 700mm hollow equilateral triangular columns rise the full height of the atrium, restrained at irregular intervals by the beams which also support the pods. They are geneally designed to run for three storeys between restraints, which means nearly 18m because of the courtrooms. Sliding movement joints were placed between the atrium façade support structure and the main structure to accommodate differential thermal movement between the façade and the columns.
The columns are made from steel plate of either 15mm, 20mm or 35mm thickness, depending on the load on the column, welded at each triangle’s vertex. One challenge for the steelwork fabrication was achieving a sharp edge detail desired by the architect at each corner of the columns. After mocking up a number of solutions a detail was selected in which rather than attempt to achieve a perfect point, the welded joint is squared off to 15mm wide.
Close cooperation between steelwork contractor and the rest of the design team from the earliest stage paid dividends. For example, after constructing a model in Strucad that included details such as the glazing system frame, areas of potential problems and missing information were identified and resolved before fabrication.