Timber reaches new heights

This Issue This is a part of the Commercial buildings feature

By - , Build 135

Exciting new possibilities for cross-laminated timber were a hot topic at last year’s world conference in timber engineering. Easy to erect, cost-effective and ideal for low- to mid-rise building, will it work in NZ?

Cross-laminated timber panels ready for dispatch.
Cross-laminated timber used for two floors and roof on a steep site in Nelson, February 2013.
Cross-laminated timber panels ready for dispatch.

FIRST DEVELOPED IN EUROPE where the technology has been in widespread use in low- to mid-rise construction for several years, cross-laminated timber (CLT) is now receiving attention here in New Zealand.

With and against the grain

Like other engineered timber products, the process to manufacture CLT bonds and compresses several layers of solid timber together to form a single unit. Where CLT differs, however, is that the grain of each alternate layer runs at 90° to the previous layer, increasing the strength and rigidity of the timber in both directions.

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Suitable for walls and roofs

Where a glulam or laminated veneer lumber (LVL) product typically results in a single construction element, such as a beam or column, the criss-cross nature of CLT means it forms a complete panel, making it suitable for wall, floor and ceiling components in a larger structure.

‘A glulam beam might form part of a portal frame over a hall or gymnasium or some other open area,’ says Graeme Beattie, BRANZ Principal Structural Engineer. ‘The beam might be pretty hefty, but it is still a single, long linear element. Cross-laminated timber, on the other hand, is actually a replacement for the entire wall. That places it in a different class of product.

‘As a flat panel, it provides very good bracing resistance within the panel itself and within the larger structure if the panels can be connected together well enough.’

In this regard, CLT is more akin to prefabricated framing units and precast concrete tilt slab than to traditional stick construction.

A CLT product is usually manufactured using three, five or seven layers, with thicknesses ranging from 20–45 mm per layer and a total thickness of up to 250 mm, depending on the manufacturer’s press and the financial viability of the product. Each layer can be a different thickness, although the thickness of each layer is usually symmetrical around the core.

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Alternative to concrete and steel

Although it consumes large quantities of timber, CLT can make use of lower-grade material in the core and inner layers, producing a much stronger and higher-quality product than the materials used in its manufacture. It can also offer greater thermal mass, better acoustics and improved airtightness if designed and installed correctly.

Timber proponents point to CLT as a serious alternative to concrete and steel in low- to mid-rise construction applications and the obvious answer to rapidly increasing demand for the speed and quality of prefabricated building components.

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Could be ideal for new Christchurch CBD

The Christchurch rebuild has been cited as the perfect proving ground for CLT in New Zealand, where low- to mid-rise, aesthetically pleasing, seismically robust and rapidly completed buildings are de rigueur. The sustainability and carbon sequestration of timber technologies is also appealing.

‘There are two quite distinct approaches to commercial construction,’ says Robin Jack, Director of XLam NZ Ltd. ‘The first is the concentrated-load approach, which uses a primary loadbearing structure of columns and beams. The second is the distributed-load approach, which uses internal and external walls to form the loadbearing structure.

‘It would be unusual to use a distributed-load structure for a commercial office building because you cannot easily change or repurpose the building’s internal spaces as it has all these loadbearing walls that can’t be moved.’

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Best for fixed-wall buildings

This is why CLT tends to suit honeycomb structures, such as apartments, hotels, schools or retirement villages, where fixed internal walls distribute the load down to the foundations. However, in a traditional commercial building where steel and concrete columns and beams create a large, clear space with movable partitions, CLT panels can still be used as roofing, mid-floor diaphragms and shear walls.

‘Cross-laminated timber has a place in both types of building, but where a design calls for internal partitions that are unlikely to move and there is no need for a primary loadbearing structure, a building can be constructed very quickly and cost-effectively using CLT panels,’ he says. ‘For those kinds of buildings, I think CLT has a huge and immediate future.’

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10 storeys high and climbing

In much of Europe, building codes limit the height of wooden buildings to 4 storeys, so the use of CLT technology has typically been restricted to apartment buildings, small office complexes and schools. No such limitation exists in the United Kingdom, making way for London’s Graphite apartments, a 9-storey apartment complex built using CLT from the second storey up. The structure’s 150 mm thick and 9 m long spruce panels were manufactured in Austria, shipped to the UK and screwed together on site to form the exterior and interior walls, floors and roof.

Melbourne’s new Forté apartments push the envelope to 10-storeys, making it the tallest wooden apartment building in the world. This is an honour it may not hold for long as CLT manufacturers expect the technology to take buildings to 13, 14 or even 15 storeys within the next couple of years. Plans for a 17-storey office building are already under way in Norway’s far north.

Melbourne’s Forté apartments – a retail and residential mix building, and currently the world’s tallest CLT construction.

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Seismic risk complicates matters

While New Zealand can boast the first manufacturer of CLT in the southern hemisphere and examples of CLT structures are appearing, the technology is not yet well understood by the building industry.

Although the performance-based New Zealand Building Code does not prescribe against the use of CLT or any other technology as a commercial construction technique, conditions in New Zealand are quite different to those in London, Melbourne and Scandinavia.

‘We are still in our infancy when it comes to CLT structures in New Zealand because of our seismic risks,’ says Graeme Beattie.

‘While it would be fairly straightforward to create an elastically designed 2- or 3-storey CLT structure that complies with the Building Code, going any higher would require a connection system between panels that is proven to withstand the forces from a sizeable seismic event.’

Robin Jack agrees, but adds that many good connection systems are in use overseas and it remains to be seen if and how these can be adapted to New Zealand conditions.

‘Up until now, no one has manufactured CLT in New Zealand using local timber species, so naturally there is not a lot of experience, but it is being accumulated over time,’ he says. ‘It may be that we do not need to develop a whole new array of connection systems. We may simply need to better understand what is already available – how to specify and use them with our particular products and understand any limitations they might have.’.

Cross-laminated timber used for two floors and roof on a steep site in Nelson, February 2013.

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More testing needed on fire risk

Fire performance in a timber structure is always a concern – however, CLT is known to be more fire resistant than stick timber construction, for example. When fire attacks the flat CLT panels, it tends to char the surface, which acts to insulate the lower layers and slow the spread of the fire. Fire testing of CLT in overseas laboratories has produced impressive results, and local testing is expected to verify similar performance under local conditions.

‘Whatever building system you use, you have to engineer a commercial building for fire performance, and CLT is no different,’ says Robin. ‘Like all building materials, it has its own unique advantages and disadvantages, but it is critical that any CLT building is properly designed and you know what you’re doing during construction.’

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Immediate potential

At this stage, these are concerns typical of an emerging technology, and investment in local research is already under way to better understand how CLT will perform in a New Zealand setting.

‘The industry is actively looking for answers to all these questions, but I doubt that you will see 9- or 10-storey commercial CLT buildings going up in New Zealand in the next few years. The technology is much more likely to start in apartment buildings, retirement villages and other 2-, 3- or 4-storey buildings of that kind and gradually move into the commercial space,’ says Robin Jack.

Meanwhile, Graeme Beattie says there are no barriers in the New Zealand Building Code for the use of CLT as a commercial construction technique. ‘As long as the fire and seismic performance can be proven to be up to scratch, there is nothing to prevent anyone from using it. That is the whole point of having a performance-based Building Code.’

Barring a major unforeseen obstacle, it does seems likely that CLT technology will become a viable alternative in New Zealand’s mainstream commercial construction industry.

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Cross-laminated timber panels ready for dispatch.
Cross-laminated timber used for two floors and roof on a steep site in Nelson, February 2013.