Special New Year post by James Wimberley and Michael O’Hare
The Finnish wood products company Metsä have commissioned a concept design from Canadian architect Michael Green for a clone of the Empire State Building in their proprietary engineered wood. He has also done designs for the Reichstag and the Colosseum, and filed a more serious proposal for a 35-storey wooden complex in Paris. His TED talk.
These flashy thought experiments are reminiscent of the concept cars unveiled at motor shows. A wood skyscraper would need absurdly massive columns at ground level. But it is certain that large, long-lived and elegant structures can be built in wood.
The Hōryū-Ji Temple in Japan was built in 607 CE – earlier than any freestanding Saxon stone church in England. Japan is prone to earthquakes; the very heavy overhanging tiled roofs and sliding vertical joints, developed by intelligent trial and error, damp the vibrations.
Image credit Wikimedia
In London, a seven-storey block of public housing flats, Bridport House, was completed in 2011, with all wood structural elements above ground. The fine Richmond Olympic Oval, built for the Vancouver Winter Olympics in 2010, is spanned by 100m composite wood-steel arches, holding up an all-wood roof with 12.8m spans on the secondary axis.
Wood is a good structural material. It is a natural composite, similar to fibreglass or the carbon-fibre panels used in fighter wings and luxury cars. Chains of cellulose, a polysaccharide, provide the fibres. The epoxy matrix is replaced by a brown gunk called lignin, more effectively as it forms a waterproof chemical bond with the cellulose. The basic structure evolved in an arms race between plants to carry their leaves above their rivals, requiring gastight tubes to carry dissolved mineral salts up by osmosis and sugars down. Wood is basically a bundle of lignin/cellulose tubes, strong under compression, stiff and elastic against lateral wind loads, and light for its strength – greater than carbon steel by weight. The tallest trees in the world, the 100+ metre coast redwoods, reach the equivalent of 30 stories.
The stiffness, elasticity and warp resistance can be increased by laminating, as in the curved composite bow of the Mongols and modern competition archery. Lamination also allows fabrication of elements of arbitrary size.
So these modern structures depend on engineered wood, in one of its many forms. One representative type is glulam, made out of overlapping lengths of timber stuck together with the grain running the same way with high-strength glue. New Zealand engineers are experimenting with prestressed glulam beams for six-storey buildings. The new wood technologies allow fabrication of elements in factories under controlled conditions, and of sizes no longer constrained by the source logs. We can take it that six-storey buildings are entirely feasible, and that is >99% of all of them. Another is plywood, layers of veneer with their grains at right angle to each other, making a dimensionally stable panel with myriad uses.
There is no architectural reason why a substantial part of new construction for low and medium-height buildings could not be shifted from steel and concrete to wood. Climate policy strongly suggests that this should be encouraged.
To review the pluses:
- Trees fix carbon as they grow; carbon makes up about half the weight of wood. If you burn the wood in a biomass power plant, the scheme is carbon-neutral over the cycle. But if you turn the wood into buildings or furniture, and maintain them, you sequester the embedded carbon for another century or more. That is the horizon of our climate problem – in 2116 our descendants will have better technology or none.
- The standard structural material for low-rise buildings, concrete, is very carbon-intensive. Wikipedia gives 410kg CO2 per tonne of concrete at a 14% mix. Cement-making, largely for concrete, is responsible for 4% of global carbon emissions. Over half of this is from the chemical reaction of calcining, so using renewable energy for heating can only go so far.
- Wood framing lends itself easily to thick insulation, as in the Nordic world.
- Wood is a nicer material than concrete: it retains a fractal complexity of surface detail, and comes in a range of warm colours. Laminated wood lends itself at least as well as concrete to the flowing curves Oscar Niemeyer advocated for sound Brazilian reasons.
- The Achilles’ heel of wood construction has historically been making connections that develop the full strength of the material; the mortise and tenon joints of post and beam framing are elegant carpentry but extremely inefficient. A family of relatively new fasteners like these and these have enormously increased the potential of structural wood.
- Fire, termites, and damp. Fire is the least of these worries. The Great Fire of London in 1666, which destroyed most of the mediaeval half-timbered city, had an official death toll of six, and that was largely down to the dithering of the Lord Mayor. Large wood beams char from the outside, forming a slow-burning and insulating layer of charcoal, and do not suddenly lose their strength from heating like steel. Smaller wood sections as in houses burn faster, but most house fires are mainly fuelled by the contents. Architects and building owners do need to take damp, fungi and insects seriously: the chemical defences of trees depend on their being alive.
- Unsustainable forestry. The sequestration claim is true of sustainably managed second-growth forests and new plantations – but not of old-growth tropical and temperate forests clear-cut by slash-and-burn loggers, a vandalism that emits large amounts of carbon from the burning of the waste wood. Metsä claim their sources are kosher, and we can be comfortable with the New Zealanders. But the guardian of the principal global certification scheme, the Forestry Stewardship Council, has been harshly criticised by other environmental organisations as wimpy. A tightening up of standards and inspection has to be part of the policy. We also need to keep the overall volume of wood used within a sustainable limit: which is made easier by the steady decline in demand for paper, mainly sourced from boreal birch and conifer forests and tropical eucalyptus plantations.
- Traditionalism. This applies particularly in Europe, less to the USA where most single-family houses and apartment blocks up to five stories are still built on wood frames. The English children’s story of the three little pigs exhibits the prejudice: proper houses to keep out the extinct wolves are made of bricks. James’ two houses in Spain and southern France have no wood in their structures, even the roofs. Historically, the wolves went long before builders ran out of timber and were forced to switch. Treeless Flanders and the Netherlands switched earlier. Many European children grow up thinking, quite wrongly, that wood buildings are quaint, picturesque, flimsy firetraps. This isn’t a reason at all.
- It’s marginal in the greater scheme of things. True. But going carbon-neutral requires attention to everything. The beauty of the full decarbonisation goal in the Paris Agreement is that it immediately generates the full list of problems to be solved and new technologies needed. The top of the list is obvious, and under way, if not fast enough. Efficiency: check. Cutting out coal for power generation: check. Rolling out solar and wind generation: check. Electric vehicles: check. The LLNL energy flowcharts show that for the US, electricity and transport between them use two-thirds of primary energy, so these are the big ticket items.But we cannot ignore the problems lower down the list, some of which are technically more difficult and interesting: deforestation, aviation, shipping, steelmaking, sequestration, and cement. So let’s get started on all of them.
- There are schemes for carbon-neutral or carbon-negative cement. This is a tall order and may not be feasible on the required scale at reasonable cost. The footprint from the firing of the clinker is large; you would have make the energy supply all-renewable also. This should certainly be explored, but is no reason as yet not to pursue wood building too, which we can do tomorrow.
For a given building, timber construction uses much less energy and emits far less CO2 than concrete over its supply chain from forest to building site. What scale of benefits could we expect from a policy to shift construction from concrete to wood? And what sort of policies are needed?
First, the shift will be partial. Because of the damp issue, concrete will still be needed for foundations, and possibly floors, but much less as the superstructure is lighter. Gustavsson et al (ref 1) analysed two recent four-storey wood-framed buildings in Sweden and Finland. On a life-cycle analysis, and assuming sustainable forestry, they estimated the carbon savings from wood construction at 30-130 kg C/m2. That is a disappointingly wide range, but let’s take a midpoint value of 80 kgC/m2. In the USA non-residential construction (the residential is mostly wood-framed already) is about 200 million m2 a year. If we can shift 20% of that from concrete to wood, that would be 3.2 million tonnes of carbon, or 0.16% of US emissions. For a parallel analysis, see ref 2.
Assuming we want to do this, what policies should be put in place?
- City building departments need to learn about modern wood construction and approach the permitting issues with an open mind. Allow or commission a showcase wood building.
- Ancillary trades also need to learn. Plumbers and electricians need to be comfortable with structural wood and its constraints. Large random holes in structural elements are not a good idea. But generally, wood makes their jobs easier.
- Including a virtual carbon tax in regulations and tax policies. This was a crucial and little-heralded step in the greening of the Obama administration. If a real carbon tax is politically impossible, at least let us assume one in carrying out cost-benefit analyses of proposed regulations. Tweaking property taxes so that the builders of concrete structures pay a carbon tax premium, and those of wood structures get a sequestration rebate, will not be earth-shaking. Suppose you switch a building from 120 tonnes of concrete to 20 tonnes plus another 20 tonnes of wood. A carbon tax and sequestration premium of $60 a tonne would make a one-off difference of $1200. Not earth-shaking, but combine it with incentives for energy and locational efficiency, and the incentives for low-carbon practice would add up to a decent nudge.
Our proposal will not appeal to romantic greens. It relies on high technology like glulam, fierce chemical treatments against insects, and technocratic forestry. We have most sympathy on the last point. The rigorous monocultures of say the French Office National des Forêts work against biodiversity and the aesthetic richness of complexity and surprise. We don’t recommend that any forest should be run solely with an eye to timber harvest. There is plenty of room for multiple objectives, negotiation and compromise, within a programme for more wood in our lives.
Post dedicated to Mike O’Hare’s father, inventor of a type of diagonal plywood: Lap-ply001
1: Gustavsson L., Pingoud K. and Sathre R., (2006). Carbon dioxide balance of wood substitution: Comparing concrete- and wood-framed buildings, Mitigation and Adaptation Strategies for Global Change, 11: 667 – 691
2: Perez-Garcia J., Lippke B., Comnick J. and Manriquez C., (2005). An assessment of carbon pools, storage, and wood products market substitution using life-cycle analysis results, Wood and Fiber Science, 37 Corrim Special Issue, 40 – 148
Note: this is James’ entry in the 2016 Masdar Engage Blogging Contest. Entirely off my own bat, not as representing the RBC blog, and implies no endorsement of any policy of Abu Dhabi other than its support for Masdar, which does good work.