Auckland HomeGround wood building for city’s most vulnerable

Tall mass wood building earthquake risk required engineers to pioneer a solutions

The Auckland City Mission HomeGround has been described as ground-breaking and leading edge, which is only a starting point for a truly impressive 9 level mass wood building recently completed in the centre of Auckland.

Back in 2007 the Auckland City Mission ran a design competition to create a new complex on its central-city Hobson Street site. Won by Stevens Lawson Architects, a vision was born for a new multi-storey facility that would provide integrated wraparound services and accommodation for some of the city’s most vulnerable people.

The dream is now a reality with work completed for the nine-storey Auckland City Mission HomeGround, the tallest building to use cross-laminated timber for its core structural strength in New Zealand.

It is revolutionary in its use of Cross Laminated Timber (CLT) on such a large scale, and is undoubtedly the answer to building sustainably, adding value to a natural, renewable resource through technology – as well as fast, safe, economical and good for the planet.

However, as anyone who has been on a construction site knows, when a new material or method is introduced to site it adds dimensions of uncertainty and risk. All of these can add time and cost to a project.

It is true that the timber product supplied by XLam has been used on numerous smaller projects around New Zealand, but it had never been used in a project of this size. Because the structure above Level 2 was all timber, the team put serious time and effort into understanding it, by many meetings with XLam to gain specialist knowledge of the system and the construction methodology.

As the build progressed the team began to know the product and the system and became more confident with the methodology involved. This was a highlight of the project for the team being exposed to how it works, how it affects different elements of design and architecture, and seeing how the team dealt with all the new systems involved.

CLT designed buildings have been at the forefront of design in recent years across Australia however New Zealand’s unique Volcanic landscape has meant that a more cautious approach has been taken, hence the landmark status of this project. The understanding of the seismic requirements in a timber structure of this size in a country with a known earthquake risk has meant that the engineers have had to pioneer their way to a solution.

Fire rating requirements were a related challenge, requiring an understanding of how the key requirements for the timber structure differed from the conventional steel and concrete, and understanding how Council was treating the fire design and the implications their calculations had on the design, and in turn cost.

To address the challenge, the team went through the design options and reviewed the acceptable solutions, followed by a cost analysis for presentation of the most cost-effective options to the team. This allowed the consultancy team and the contractor to identify which option presented the most favourable methodology, resulting in a solution that was the most cost and time effective for the client.

The procurement process was another key process within this project, by gaining an early start tendering a P&G Margin contract to get a contractor on board, and then tendered trade packages until they had a 60% lump sum before commencing on site.

This was a fairly unique process but used out of necessity, and enabled working on an open book basis to procure the remaining 40% of trades as and when the design progressed.

The open book basis ensured the contractor had obtained competitive market pricing on each trade through the tender process, and assured the client was provided with confidence of an accurate cost forecast during progress through the trades.

The cost-focused and innovative procurement approach allowed insight during the design stages to review and provide cost conscious alternatives to the client and engage in active talks with the Contractor as to how to better the buildability, which ultimately resulted in cost savings.

Basic statistics for the mass timber project were 9 levels of timber structure including lift shaft and staircases, using 2,358 cubic metres of CLT delivered in 72 truckloads. Construction team 6 to 10 people assembling floors, stairs, and wall panels at a rate of 2 weeks per floor. The team also subcontracted the installation of steel members. 

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