| William Guild | Octobre 2019 |
Climate change is redefining the way we are thinking about the built environment. Timber may provide a solution.
We are running out of sand! Who would have thought? As it turns out, we use a lot of it in our buildings. Sand is used predominantly in concrete, which is the second most consumed commodity on Earth after water.
Concrete was the miracle material of the Twentieth Century. It could be moulded into any shape, was much cheaper than stone and could be reinforced with steel to provide strength in both tension and compression. Today, we use it to build everything from our houses to our airports. However, cement, the binding agent in concrete, is extremely energy intensive to produce and emits vast quantities of carbon dioxide. So much in fact that it represents around 8% of the world’s total CO2 emissions, which is more than emissions of vehicles and aviation combined. If we want to avoid depleting our beaches and reduce the amount of carbon dioxide in the atmosphere, concrete can no longer be the primary material of choice in the coming years. How then to build in the future?
In addition to the environmental challenges, humanity is facing an unprecedented demographic boom. It is estimated that around 68% of people will live in an urban environment by 2050. This puts huge pressure on cities to build new homes for the rising number of inhabitants, which is prompting many places such as London to densify the existing urban fabric. But can this be done without relying on concrete?
Forests are Back
As carbon dioxide is released into the atmosphere at an alarming rate, governments and environmentalists are looking to the forest as a possible solution. Trees sequester carbon in the air and release oxygen. This process is part of a wider carbon cycle where living organisms later release the carbon back into the environment when they die. The cycle was balanced until humans discovered fossil fuels buried underground. By extracting and burning these hydrocarbons, vast quantities of excess carbon were released into the atmosphere, far more than the natural cycle could cope with.
Over the past few years, a growing number of initiatives to replant forests across the globe have emerged. According to the Economist, countries in the European Union have reforested an area the size of Portugal in the last 20 years. This came about as a result of a decline in the area used for agriculture. In Asia and Africa, some interventions have been far more radical. The ‘Great Green Wall’ in the Sahel and the Three North Shelterbed Project in China and Mongolia are strategies aimed primarily at combatting desertification but also have an important role to play in absorbing carbon dioxide. In June, Ethiopia broke the world record for the number of trees planted in a single day – 350 million trees in 12 hours – and hopes to plant around 4 billion trees over the summer.
However, planting trees does not solve the problem on its own. When trees reach maturity – 120 years for an oak – they begin to decay at a faster rate, releasing more carbon into the environment. Eventually, forests reach a balanced carbon state with as much CO2 being absorbed as it is released. This means it no longer acts as a ‘carbon sink’, making it redundant in the battle to reduce greenhouse gasses. Managing the growth of forests is therefore a vital part of the process. Felling mature trees gives way to younger individuals and provides us with a source of timber, a material we can use in our buildings.
Wood: traditional material, new concept
Timber, which is effectively fossilised carbon, has been used by man for thousands of years. It was readily available and much soften and lighter than stone. However, the advent of concrete and steel brought with it the possibilities of building taller and mitigating against the risk of fire. This led to a revolution in architectural and engineering terms with the consolidation of the modernist movement, which spread around the globe during the Twentieth Century.
Today, with the pressing concerns of climate change, an increasing number of architects worldwide are looking at new methods of timber construction as an alternative to concrete. Originally invented in Austria, Cross Laminated Timber (CLT) was developed to fill a gap in production left by the shrinking paper industry. Sheets of softwood are glued together in layers at right angles to one another to create stratified panels similarly to plywood. While the material qualities of wood are preserved, its mechanical performance is considerably enhanced. Panels can be used as either wall or floor units arranged in a cellular configuration thus creating rooms, corridors and stair cores. A variety of ‘mass timber’ alternatives such as Glue Laminated Timber (GluLam) and Laminated Veneer Lumber (LVL) can be used for framed construction similarly to concrete and steel.
Unlike concrete, however, the natural properties of timber provide buildings with better moisture and temperature control. It is even suggested that the use of wood creates healthier environments. This may have important implications in the design of future hospitals for example. From a structural perspective, mass timber performs equally as well as concrete and steel for small to medium buildings. For instance, T3 Minneapolis by Canadian architect Michael Green is a 7-storey office building that has a structure entire composed of CLT and GluLam. Where the public is still to be convinced is fire safety. With people in Britain still reeling from the Grenfell Tower tragedy, it is understandable that timber buildings do not come across as an acceptable alternative. In fact, CLT and other mass timber components tend to perform better than reinforced concrete when exposed to fire. Wood naturally chars, which creates an inflammable layer a few millimetres thick. Beyond this the timber does not lose its structural integrity. Conversely, the steel reinforcement in concrete tends to lose its strength as it is heated causing the concrete around it to crack and, in some case, fail all together.
Though mass timber products have been around for 30 years now, it has predominantly been used in low-rise schemes such as houses, industrial warehouses and office blocks. It is understood that laminated timber is strong enough to support taller structures but government regulations in most countries with regards to fire prevention do not permit the construction of such buildings. In some places, however, the mindset is beginning to change. Acton Ostry’s Brock Commons student accommodation in Vancouver became the tallest CLT building to date standing at 18 storeys. The design was given planning exemption to test the material and set a precedent for designs in the future.
Now under way in Sweden, the Skelleftea Culture Centre by White Arkitekter will stand at 69m (19 storeys). The prefabricated components including the GluLam frame and the CLT cores and floor plates are considerably reducing the time spent on site and make for a much safer environment. Furthermore, the building is much lighter than a concrete framed alternative – around 30% less weight has been achieved at Dalston Works in London for example – meaning less foundation work. In this regard, the construction process also requires far fewer shipments of material, thus further reducing the carbon footprint of the building. These projects set an important precedent for the industry and provide tangible evidence that timber can be a suitable alternative to concrete in the race to expand our cities.