The Auckland War Memorial Museum Grand Atrium development is an exciting project that presented Holmes Consulting Group with many structural engineering challenges and led to a number of innovative structural solutions.

The first challenge was architect Noel Lane’s proposal to suspend a four storied timber clad bowl structure inside the courtyard. Close collaboration with Noel Lane enabled an effective solution where the primary ‘megaframe’ structure compromises storey deep structural steel box trusses arranged in a cruciform configuration. The four structural steel truss towers house the main service shafts for the courtyard building including lifts, services risers, and a stair. The bowl structure is an independent structure hanging from the trusses and seismically separated from the surrounding floor plates.

Capping the development is a dome which spans 47 metres across the existing museum courtyard. Structural steel trusses radiating from the centre of the dome form the primary structure. These trusses are supported on a perimeter beam with a series of posts down to the new and existing roof structures.

One of the first tasks on site was to excavate down 10 metres within the existing museum courtyard to create two basement levels. An effective retaining solution was developed using a combination of rock anchors, piles and structural concrete beams. Piles were bored at 6 metres spacing adjacent to the existing building column pad foundations, with a perimeter concrete ring beam tying these piles together. The ring beam was anchored back under the existing building to provide support to the existing foundations and the top of the excavation.

Underpinning work adjacent to the existing access tunnel under the original building includes pairs of continuous post-tensioned beams clamped either side of the existing columns. This solution fits well within the limited space available between the underside of the ground floor slab and the existing ground level. The beams were designed so that they provided the necessary temporary support to allow excavation for the plantroom and truck dock to proceed. They also form the permanent support solution minimising the impacts on the truck dock space, with only one midspan column supporting the post tensioned beams.

A further challenge was a two level underground carpark constructed in the space between the rear of the existing museum building and an underground water reservoir. Retaining piles and a top down construction methodology were adopted to minimize ground deflections adjacent the reservoir structure.