The six-storey BNZ Building was purpose-built to consolidate 1,000 bank staff previously spread over a number of sites around Wellington. Beca was commissioned to provide structural, building services, fire and environmentally sustainable design (ESD) services as well as the tenancy fitout for the BNZ.
The 25,000 m2 building is situated on reclaimed land, which required significant ground improvement work prior to construction. Vibro-replacement, involving the installation of several hundred gravel columns, improved a section of the site to help mitigate the effects of liquefaction in a major seismic event. In addition, water seepage became a problem during the piling process, and Beca assisted Tonkin & Taylor Ltd to specify a polymer-based additive to firm up the drill holes – the first time this method had been used in New Zealand.
The building is composed of three low-rise office towers connected by two full height atria, another first for New Zealand. All levels except level five are completely open to the atria, reflecting the tenant's requirements for transparency, lightness and space. The piers are joined together by pedestrian bridges, which also serve to seismically link the separate building towers. Of concrete-filled, steel box construction, the bridges are connected to the piers by articulated connections involving steel pins, reducing the requirement for expensive seismic joints.
As the largest clear floor span for any office in the Wellington region, the 17-metre floor spans required extensive floor vibration modelling.
The seismic frames are set back from the façade which allows floor to floor glass giving ample light and views across the harbour.
The open-plan, three-piered design required quite specific building services engineering. The HVAC system includes a variable air volume (VAV) ventilation system. The building's high level of natural light is complemented by a Digital Addressable Lighting Interface (DALI) system, with sensors automatically adjusting the artificial lights based on daylight availability.
Eight large fans on the atria roofs assist with general air circulation and help extract smoke in the event of a fire. Fire design required complex smoke spread and egress modelling.
Through ESD features, Beca's building services team helped the building achieve a 5-Star (New Zealand Excellence) GreenStar rating since completion in May 2009.
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