Refurbishment of the historic Old High Court building in Wellington acquired additional scale and significance when it was determined that the Supreme Court should be housed in a completely new but adjoining building. Warren and Mahoney Project Architect, Nick Warring, says there were many iterations during the design process as the client worked through various options. “One of the overriding requirements, however,” says Nick, “was the courtroom had to be readily identifiable as a separate entity within the building, and from the outside offer a degree of transparency to the new seat of justice in New Zealand.

“The built expression of these requirements is evident in both the geometry and structure of the new building. While the main building is planned around the orthogonal grid of the rigid concrete frame structure, the courtroom is distinctly elliptical in shape and formed from a separate steel frame over its two storey height. Interior walls to offices and surrounding rooms are kept well back from the courtroom to ensure legibility of the elliptical form from outside the building.”

Eva Hogenhout of Holmes Consulting Group was the Project Engineer. “The steelwork comprises a series of 600mm wide truss-like vertical frames, spaced evenly around the perimeter of the court and designed to support both the interior and the exterior cladding. Concealed within the frames are heavier, cranked Universal Columns at each junction of the main concrete frame and the perimeter of the ellipse. These bent UCs actually support the ends of concrete beams, which cantilever up to 6.0m beyond the concrete column locations. The UB stubs and the cranked UCs form a part of the lateral load resisting system. The UB stubs were site-welded to cast in weld plates at the end of each concrete beam, in order to achieve continuity. The cranked UCs support the additional compression loads attributed to them as they prop the ends of the beams to ground floor level. Because of this connection, the steelwork forming the courtroom experiences the same lateral movement as the main structure.”

An earlier scheme provided for seismic separation between the courtroom and the concrete frame. Sliding joints were considered at each of the upper level perimeter skylights. Instead it was decided to tie in the courtroom steel framing, as this simplified the construction detail of the steelwork and the waterproofing. “Although construction tolerances for the courtroom structure,” says Eva, “are no more stringent than for other parts of the building, there was the potential for minor misalignment of steelwork to have resulted in accumulated displacement. To prevent this, the steel structure was surveyed at every girt cleat location several times during erection.

“The heads of the frames are joined and restrained by an elliptical CHS ring beam. This supports a roof-top skylight, while more elliptical steel frames support shading louvres and glazing below the upper skylight. Skylights have been designed around the perimeter of the courtroom where it projects through both the first floor and the roof levels. Curved steelwork again forms the bounding elements used to frame both the glazing and the gutters.”

The shape of the courtroom dictated that the material of choice couldn’t be concrete. At one stage glulam timber was considered but the superior strength to weight ratio of steel saw it prevail. Croucher & Crowder Engineering Services Ltd in Hawera was awarded the contract for the structural steel fabrication and erection. Project Manager Alan McIntosh says detailing the fabrication drawings “took more than four months of emails and phone calls to the engineer and architect including the unusual step of supplying the architect with a 3D model of the structure for checking the geometry of steelwork. Unless the drawings were correct with the geometry and shape of this steelwork, the structure would never work. A phrase often uttered throughout the fabrication process was ‘Trust the drawings!’.

“The main structure is fabricated using 24 identical elliptical truss frames and four rolled to a slightly different radius at the ends of the main structure. Once the first truss frame was assembled it was used as a jig in which to assemble the remaining truss frames. As the truss frames were completed they were then assembled into pairs as required in the engineer’s design These were then transported in pairs to our painting sub-contractors in Wanganui.

“On site a central load bearing scaffold tower was constructed to 1.5m below the finish height of the top elliptical ring. A total of 14 customised jacks were placed to support the top elliptical ring and provide any height adjustment required. The truss frames were craned into position, bolted to the ground and rested on the jacks at the top. They were then bolted / stitch welded together as required until the structure was complete. A survey of the truss frames was completed once the shape of “The Orb” was correct all the UB ties were installed locking the structure into place. We then handed over to Mainzeal Property & Construction Ltd to commence the fitting of the shaped timber girts and cladding.

“The spiral staircase is also an challenging part of the project and is still a work in progress with the detailing almost complete fabrication due to commence any time soon, with the plan being to deliver these stairs as three complete units and corkscrewing them down into the opening in the concrete slab.” The spiral staircase provides the main vertical circulation throughout the building. Warren and Mahoney had intended for the stair to be constructed in concrete, but Holmes Consulting Group advised that the weight would cause problems.

Eva Hogenhout again: “Being a free-standing helix, the spiral stair has no central post. It rises from the basement to roof level and is supported by the suspended in situ concrete landings at each level. In situ beams cast in the floor support the stringer loads back to the main frame locations. With an outer diameter of 4m and passing through approximately 450 degrees between each floor, the stair was a challenge in terms of both detailing and fabrication.”