New Zealand Engineering 1999 September

Construction

Flexibility Without Compromise

Three demands tugged in different directions to create a testing design puzzle for architects and engineers taking Christchurch’s $34m WestpacTrust Centre from the drawing board to reality.

The result is a work which yields to all three elements but compromises none.

Steering the thoughts of those who accepted the exacting undertaking were considerations of budget, acoustics and seismic protection.

Achieving an optimum solution in one sometimes threatened to encroach on others but, with perseverance, a balance was achieved and the Christchurch City Council, which commissioned the structure, is now the proud owner of the country’s largest and most versatile indoor stadium.

Adjoining Addington Raceway, the showcase of harness racing in the south, the WestpacTrust Centre is a unique building for all seasons in which functionality is the key.

Yesterday, it may have echoed with the whoops of dust-raising rodeo riders; today, it may be the gracious host to willowy-limbed skaters as they glide over glass-smooth ice; tomorrow, it may have given way to the aerial rough-riders of motocross as they launch themselves from wickedly undulating hillocks of dirt.

Multiplicity of use and ease of conversion were vital factors emphasised by Mike Kelly, executive director of NCC (NZ) Ltd which manages the facility, when the project was being planned.

The success of the public assembly industry, he asserted, was determined by simple issues - food and beverage, paid parking, merchandise and the ability to convert from event to event quickly, regardless of the type of event.

"You maximise your building and your seat yield by designing functionally adaptable buildings. Those are the operating issues which prevent buildings from becoming white elephants," he said.

"Turnaround is limited by all the things that have to happen between events - and cleaning and set-up of the building are significant parts of that activity."

The WestpacTrust Centre has concert seating for up to 8800 and can provide 4000 square metres of floor space for about 250 exhibit booths. It is 120 metres long, 105 metres wide, a maximum of 26 metres high and offers 13,800 square metres of gross floor space over four levels.

That it could be completed within such a conservative budget has captured the attention of engineers overseas who have been impressed by the many unique features the building offers.

Sited as it is near residential properties, curbing potential sound breakout was critical, particularly when the stadium became a venue for rock concerts.

Enclosing the sound source in a concrete shell would have been the most effective means of containing it but excessive concrete produces undesirable echo.

The acoustician would have preferred the roof to be covered in concrete but structural considerations precluded it. Clear span trusses which are 81 metres long take the roof’s weight, obviating the need for support columns which would have impeded view. A truss of such length could not economically support a concrete roof considering the spans are flexible and can move subject to temperature variation or wind load.

Structural engineers Ove Arup and Partners designed the roof to be seismically independent of the lower concrete-frame structure engineered by Holmes Consulting Group.

To reconcile the separate structures of the single roof and the segmented concrete underbelly, the trusses have bearings which stabilise the roof when needed but allow it to spread freely in other directions to minimise the load on the roof and its supports.

Unable to seal the 8000 square metre roof with concrete, the design team devised instead a layered structure using 30 mm-thick MDF particle board and fibreglass acoustic material. When sound is transmitted upwards, it enters these layers and is trapped, but a proportion is reflected. Sound needs a component of reinforcement for it to appeal to the ears.

Acoustics
Stronger sound waves that might have caused breakout are prevented from escaping by the addition of outer layers of noise-controlling, fibreglass batts and two layers of MDF board.

Building paper has been laid over the acoustic roof sections, constructed by the contractor, C. S. Luney Ltd, and covered with metal roofing.

When sound quality is such an important issue, even relatively small adjustments to the interior can affect the balance. Consequently, it was necessary to invest more than would otherwise be necessary when decisions were made on advertising display signage. Plexiglass, a robust, low-cost surface, proved to be acoustically unfriendly and NCC (New Zealand) Ltd contributed significantly to introducing a soft-skin, translucent material for the purpose.

"While much more expensive per sign, this material met the high acoustic standards required for the venue," Mr Kelly said.

Other more reflective signs had to be perforated to soften the surface.

Separating the complex into nine sections to minimise seismic action was a challenge to the building’s acoustic integrity but careful construction ensured it was not achieved to the detriment of the sound requirement.

There are three main seating blocks, four circulation pods, back of house and an entrance. The roof is supported by three independent structures and its structural steel trusses, which measure six metres deep at mid-span, can carry loads of up to 18 tonnes.

The complex includes more than 500 precast concrete elements, the largest weighing 32 tonnes.

The technical complexity of the building beneath its simple external skin may not be readily appreciated. Who would have realised, for instance, that the air conditioning can sense numbers in the building and adjust to suit?

Wiring the WestpacTrust Centre went far beyond the installation of 45 switchboards, tens of thousands of metres of cabling and more than 1500 light fittings. Being a purpose-built complex, much of the work was of a one-off nature involving special effects and high density lighting, remote controls and systems that are fed back to a building management system computer.

Much of the work was carried out 21 metres above the floor level. A two metre-wide catwalk with spurs for lighting, sound and television installations required complex electrical and data connection work, much of it controlled by remote switching.

The 20 corporate suites, which may be leased, are luxuriously appointed. Each is equipped with a wet bar and refrigerator, closed-circuit cable television, a telephone, air-conditioning, custom furniture and carpeting.

Size constraints tested the designers’ ingenuity. Working within only about 20 square metres, they had to accommodate 10 to 12 people as well as provide areas for dining, drinking and lounging.

Extensive use of mirrors, reflective surfaces, simple lines and monochromatic colours were employed to produce a sense of spaciousness. This was enhanced by zoning the suites into three separate activity areas, each on a different level.

From every aspect, the WestpacTrust Centre is a triumph of design and construction. It draws on the collective genius of several professions working in unison for a common goal.

Their accomplishment is a state-of-the-art facility which is a boon to Christchurch and the envy of cities throughout New Zealand and beyond.

Brian Cowley is a freelance journalist based in Seattle