Wooden it be nice?
Why aren’t light timber frames used for mid-rise buildings in New Zealand?
It’s a mystery, to be honest. I guess part of it is the perceived risk. It hasn’t been done before here and it’s presumed there will be a cost to learning how to do it.
But in fact, the lessons have been learned overseas. Along with hundreds of actual buildings, there are robust design standards and guides, including from places that have the same seismic issues as us.
Isn’t timber being used now to build high-rise towers – why are we talking mid-rise?
Yes – it’s important to make the distinction between two types of construction.
Timber has been in the headlines a lot recently with companies like Lendlease in Australia constructing the world’s tallest timber building — a 45-metre, nine-storey office tower in Brisbane.
But these high-rises use engineered timber products, such as cross-laminated and glue-laminated timber, to provide structures with the strength of traditional steel. They’re very appealing for commercial tenants — the buildings are constructed much faster and tick a lot of boxes in terms of sustainability, because the timber is plantation timber.
But engineered timber can be as costly as steel — although, if constructed without a concrete component, is lighter, which means you can save on foundations.
However, for savings on a scale that could ultimately save home-buyers significant money, you need to look at traditional light timber frames – of the kind we use for houses, which can go to about five storeys. The frames are also plantation timber, a renewable resource that captures carbon dioxide as it grows.
How much could New Zealand home-buyers potentially save?
It’s hard to say what the cost savings would be for home owners, but I don’t think it would be unreasonable to talk in the range of 10 per cent. The use of light timber frames in social housing overseas is a testament to their affordability.
We recently did an analysis for a client in the middle of constructing a traditional, five-storey steel braced-frame apartment block. We calculated they could have saved about 10-12 per cent on the super-structure, had they gone with timber, and cut out about 30 per cent of the piles, given the reduction in weight. These savings would be on top off those from reduced construction time and reduced labour demands. However, for a range of reasons, the client decided against this option.
Overseas, industry practice has meant structure savings have been achieved in the order of about 20 per cent. Cost of onsite labour and construction time can be cut by 50 per cent — as a lot of the materials can be prefabricated and walls and floors panellised (turned into “cassettes”). So the time it takes to get a building water-tight can be cut dramatically.
Wood is cheap – but isn’t it noisy too?
Noise is probably the prime concern for developers considering these projects. People think it will be like living in a cheap two-storey timber home, where you can hear everything that’s going on upstairs.
But there’s plenty you can do, and mid-rise timber buildings have had no trouble meeting more stringent noise regulations overseas. Floors in mid-rise timber frame buildings can be very acoustically effective — you just have to put a little more effort into detailing them. There are tried-and-true solutions incorporated overseas.
In Canada, for example, they typically provide a gypsum concrete screed over the timber floor in the order of 40mm. This not only increases acoustic performance, it provides mass to limit vibrations in the floor and give it a more solid feel.
Resilient rails on the wall and ceiling also reduce sound transfer; floating floor systems are another option.
What about shrinkage?
All buildings move to some degree. Timber is a natural product, and as it dries out, it shrinks. Lengthwise, shrinkage is usually small and doesn’t impact building design – but for timber elements loaded horizontally or “across the grain” these effects must be considered for the framing components in wall and floor design, and their interface with rigid elements like claddings and services.
Again, it’s all about the detailing. You would generally have “movement joints” where the wall meets the floor at each level, allowing downward movement as the timber shrinks.
Mechanical “take-up” devices, which ratchet down as the building moves, are used overseas to compensate for shrinkage, and reduce “slop” in the system and thereby the lateral movement created by earthquakes.
Are these buildings strong enough to withstand earthquakes?
Timber is often a better choice if earthquakes are more likely. First, in an earthquake, the heavier the building, the more force needed for the building to shift with the ground (inertial force). Second, the more rigid the building, the more it shakes.
Timber buildings are light and flexible, two qualities that reduce this “seismic load”, allowing them to move with the earthquake better and limit damage.
Ductility is another consideration – the ability of a building to sustain damage in a dependable way. Light timber-frame buildings use a flexible plywood shearwall system to resist lateral forces. In an earthquake, “ductility” is achieved by the nails in the plywood slowly working themselves out of the timber framing. The plywood is fixed to the timber wall, but if it gets overloaded, the nails will tend to work their way out. Other elements of the shearwall system – diaphragms, foundations and chord elements, for example – are designed to remain elastic and not suffer damage.
Using international design standards, after “Serviceability Limit State” earthquakes and wind events, no structural damage should occur and only readily repairable cosmetic damage could be expected. Following “Ultimate Limit State” earthquakes, there may be delamination of the plywood linings, requiring repair or replacement, but the building must remain standing.
When it comes to designing for seismic events, foundation costs can be a significant saving – timber buildings can withstand earthquakes to the same degree as heavier, stronger buildings, while putting much less demand on foundations. So you don’t need to spend as much there.
What about fire?
Again, it’s about detailing. Often even in steel or concrete-framed apartment buildings, light timber framing is used for the partition and inter-tenancy walls. Light timber-frame buildings overseas meet their codes for fire. Don’t forget the Grenfell fire in London was attributed to cheap, highly flammable cladding – not a timber frame. It’s about ensuring all the timber elements are protected, generally with fire-resistant plasterboard, and that the frame cannot be exposed in any way. This guards against the potential for rot too.
Where to now for light timber-frame construction in New Zealand?
Well, we’ve got to keep talking to architects, developers and builders about the benefits, and dispelling the myths.
Thomas Leung, the structural engineer we’ve partnered with in Canada to enhance our expertise, has a huge track record. Thomas literally helped write the book on the subject – a tome called the “Mid-rise Wood-Frame Construction Handbook”, published by FPInnovations. Their design techs assisted us with our recent cost analysis for our client who wanted the numbers on a timber alternative.
Through collaboration, we have the expertise. We have a mature construction industry, and we have the material – plantation timber – in abundance. Using all that to address housing affordability makes sense. The first cab off the rank will reap the benefits in terms of marketing and future projects.