Timber prepares to take the stage

By - , Build 183

The move to low-carbon construction is putting a spotlight on the timber industry. How prepared is the industry to meet the demands facing it?

LVL roof forms at Nelson Airport terminal.

TIMBER HAS ALWAYS been at the heart of New Zealand house construction and still accounts for around 90% of the wall framing in new homes. The biggest change in this area in recent years is the uptake of engineered timber – laminated veneer lumber (LVL) now makes up around 10% of timber framing.

New trends in timber usage

While conventional framing timber is not likely to lose its crown any time soon, two powerful trends are driving the demand for timber in new directions in the 2020s – the move to low-carbon construction and the explosive growth of medium-density housing.

Move to low-carbon construction

The Ministry of Business, Innovation and Employment (MBIE), as part of its Building for climate change programme, has flagged that the time will come when applications for building consent will require whole-of-life embodied carbon data for the proposed building materials. In time, a mandatory cap will be imposed on embodied carbon, which will gradually be lowered.

Bio-based materials such as solid timber and engineered wood products (EWPs) have big advantages in a zero-carbon world because they have captured and stored (sequestered) atmospheric carbon dioxide as the timber was growing. This is very different from other materials whose manufacture typically releases greenhouses gases.

The move to low-carbon construction received a big push in December 2020 with the government decision that the public sector must move to carbon neutrality by 2025, including the buildings it leases and constructs.

Economic and Regional Development Minister Stuart Nash says government agencies can exert a strong influence towards greater use of lower-emissions building materials. One of New Zealand’s greatest natural resources is our renewable forest plantations, and use of timber can help achieve higher energy efficiency ratings for new builds.

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Growth in medium-density housing

The other trend is the boom in medium-density housing (MDH). In the last 7 years, there has been extraordinary growth in multi-unit dwellings as a proportion of all new dwellings consented. While the annual number of stand-alone houses consented in the year to October 2020 was actually down 0.5%, the number of townhouses, flats and units consented was 35.6% higher than a year earlier.

Together, the two trends point to pressure for the building industry to find ways of replacing higher-carbon materials with solid wood and engineered wood products such as LVL, glue-laminated timber (glulam) and the panellised construction of cross-laminated timber (CLT). Completely new products have also started to appear in New Zealand, such as wood fibre insulating panels where about 95% of the content is wood.

Resources to help industry

There are already good resources to support industry in its use of solid timber, many of them published by BRANZ, including the 2019 publication Multi-storey light timber-framed buildings in New Zealand: Engineering design, which covers buildings up to 6 storeys.

Other guidance is in development. Colleen Wade (former BRANZ fire scientist and now Director, Fire Research Group Ltd) is contributing to an international project developing a new guide for the fire design of timber buildings. Publication is planned for 2022. Another project under way within BRANZ – to be completed in 2023 – will provide guidance on designing and constructing timber-based hybrid structures with increased seismic resilience.

MBIE and Standards NZ are also moving on this, revising NZS 3604:2011 Timber-framed buildings – extending its scope to three full storeys for certain types of buildings to better support higher-density housing and response to climate change. Publication is planned for 2023.

Engineered wood products

Perhaps the biggest challenges for New Zealand lie in the area of EWPs. Despite its vast forest resources, New Zealand lags behind European countries in its use of wood in commercial and institutional construction. The Ministry for Primary Industries estimates use here of EWPs is growing by a paltry 1% per year.

There have been availability challenges for local materials. CLT producer XLam, for example, was founded in Nelson in 2010 but bought in 2015 by Australia’s Hyne Timber, and its CLT panel production shifted to Australia.

In many cases, lack of standardisation and lack of existing engineering resources have meant that problems must be solved and new solutions found as a project is under way. With the new Nelson Airport terminal, which was designed by Studio Pacific Architecture, Tectonus worked with Nelson Pine to develop the earthquake dampers for use in the LVL columns as the project proceeded. The architects took out the top award for commercial architecture in the 2020 NZIA awards for the terminal’s innovative structural and seismic design.

A survey in the building industry found that close to half of respondents saw barriers to the use of EWPs (see BRANZ Study Report SR453 Usage and uptake of engineered wood products in New Zealand). Cost, limited knowledge and issues around availability were frequently mentioned – the limited number of manufacturers, limited stocks held by suppliers and significant lead times required for some materials. This barrier was often mentioned in relation to CLT.

Expanding production

With CLT at least, 2021 will see new local availability, with the opening of Red Stag’s CLT plant in Rotorua. A 1.1 ha building – the largest timber building in New Zealand – it has the potential for three computer numerical control (CNC) lines, with automated computer control of machining tools and robotic handling. The facility will be able to produce panels up to 16.5 m × 4.5 m.

Also beginning production in early 2021 is the 3,500 m² Hector Egger NZ plant in Cromwell. A joint venture between the Swiss company and New Zealand partners, the factory will produce prefabricated timber panels and high-tech structures for residential and commercial buildings.

Red Stag is also working on finding engineering solutions for EWP under the Mid-Rise Wood Construction programme with the Ministry for Primary Industries. The programme encourages the use of New Zealand-grown timber, mainly in the form of engineered timber products in the construction of mid-rise buildings using prefabrication. It will share costs of design and construction documentation, with engineering solutions developed being made available to wider industry.

Red Stag is constructing Clearwater Quays in Christchurch with glulam beams and CLT panels. The 5-storey apartment block will be one of the tallest timber buildings in the South Island and is being used as a reference for the programme. Other commercial/ mixed-use timber buildings in metropolitan areas will follow.

The sky is the limit

There is potential for expanded use of EWPs in applications from additions and renovations to seismic retrofitting to tall buildings.

The comparatively light weight of some engineered timber products can give them a big advantage in building additions and renovations. At Monash University Business School in Melbourne, a large 3-storey function pavilion constructed of CLT was added to the 8th-storey concrete terrace of an existing building, supported by the existing foundations.

Timber also has growing possibilities in retrofit work to strengthen earthquake-prone buildings. Tests at the University of California San Diego, for example, found that CLT with elastic steel wires is an effective and affordable solution for seismic retrofits of low-rise timber structures (see CLT in seismic retrofits, Build 146). This work used technology developed by the Structural Timber Innovation Company (STIC) in New Zealand.

In New Zealand, the tallest mass timber building is the 9-storey Auckland City Mission being built with CLT walls and floors. Overseas, timber buildings of 18 storeys have been built.

In 2018, Japanese forestry company Sumitomo announced an ambitious concept for a 350 m, 70-storey tower made from 90% wooden materials. The hybrid wood and steel-braced tube structure is targeted for 2041, the company’s 350th anniversary year.

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LVL roof forms at Nelson Airport terminal.

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