When Samson Corporation Limited, decided there was an opportunity in the urban renewal taking place in Auckland’s Karangahape Road precinct, the developer commissioned RTA Studio as the architects. The brief was for an iconic commercial office building with retail at street level and self-contained car parking.

Richard Naish, Design Director of RTA Studio, responded with a highly original concept, designing a project that consisted of five towers arranged around a plaza, which would be eye-catching; each tower is built up of shoebox shapes that have been deliberately offset. “We conceived the building as a cross-contamination of two street conditions; the decorative and highly considered heritage facades of Karangahape Road and the haphazard, organic condition of the service orientated Cross Street. Therefore, the towers of stacked boxes are screened and set back from Karangahape Road but fully exposed and rising over Cross Street.

“As the structure for this strategy was always going to be challenging, we collaborated closely with the structural engineer and explored many options of steel frame configurations that would serve both the architectural aspirations and the engineering requirements.”

The engineering consultants chosen for the project were Brown & Thomson; Oisin Frost says that in his opinion, “this was one of the most complicated buildings I’ve ever had the pleasure of working on. The adopted structural solution was chosen to satisfy tight architectural constraints. The tower office spaces generally required column-free, open-plan floor plates with net-lettable floor areas maximised. Consequently, structural elements had to be located within a relatively tight building envelope, which in this case meant member depths were limited to 360mm for columns and either 460 or 530mm for beams.

“The structure consists of two five-storey and three six-storey towers springing from a ground floor podium transfer level over an in-ground, four-level basement car parking structure. The towers have generally been fabricated with primary steel frames that support suspended composite concrete floors, double cavity external timber stud walls all covered by a rain screen. The portal legs are seated on a network of concrete beams at podium level that transfer loads into the regular beam/column grid supporting the basement car-parking levels below.

“It’s reminiscent of haphazardly-stacked shipping containers,” says Oisin, “each tower being an assembly of open-ended boxes on top of each other.

Towers 3, 4 & 5 facing Cross Street have been constructed using offset portal frames at regular centres in the lateral or short direction, with tension/compression braced frames in the longitudinal direction. The upper levels of Towers 1 & 2 facing Karangahape Road share many of these characteristics, but their lower levels are more conventionally braced by flanking, precast boundary walls and a heavily reinforced in situ concrete shear core.”

Where possible standard Steltech column and beam sections were used, but liberal use was also made of customised sections at lower levels. This was because of the magnitude of the forces and moments, which made it necessary to achieve high member stiffness to ensure that deflections remained within acceptable[r1] limits. Oisin Frost again: “This was a unique structural system that lent itself to the optimisation of erection procedures. We called for full height, pre-fabricated portal legs to be installed first, followed by the primary and secondary transverse framing. Standard bolted connections were used wherever possible. The flooring system was installed last, floor by floor.”

The steel constructor’s subcontract was won by Jensen Steel Fabricators Limited. Greg Jensen: “To make certain that we could meet the construction programme, we formalised a joint venture with Manukau Welders Ltd. As the programme accelerated two more SCNZ members also made valuable contributions: Grayson Engineering Ltd and Auckland Steel Ltd. That’s one of the strengths of our association.

“To pre-fabricate the continuous portal legs,” says Greg, “and match the stagger between floor levels, we shop-welded column and beam cranks. Though labour intensive, the shop welding avoided what would have been even more expensive onsite welding.”

The composite flooring system was prompted by the need for easy-to-handle materials that would satisfy fire and acoustic ratings while providing the necessary in-plane rigidity to transfer lateral forces. Steve Stickland, Product Manager for ComFlor Building Systems, says: “The engineers at Brown & Thomson wanted a proprietary profiled metal decking system, specifying a clear span of up to five metres and strict depth limitations. ComFlor 80 met the specs and was expertly laid by Composite Floor Decks Ltd.”

Ironbank is clad in 5mm thick steel, a first for Thermosash Commercial Ltd who installed it. The steel went through a pre-weathering process offsite.

The five towers provide some 4,500m² of commercial and retail space, with parking for 95 vehicles tucked below street level by a car stacking machine.