Although wood is no stranger to the construction industry, over the last century it has remained confined to residential and low-rise buildings. For more than a 100 years steel and concrete have been the main structural frame material for tall buildings, but slowly but steadily this appears to be changing as wooden tall building sprout everywhere and, as it is, the near future holds a good lot of wooden skyscrapers around the world. In Europe, for instance, the Council on Tall Buildings and Urban Habitat (CTBUH) recently released the study Tall Timber: A Global Audit on built, under construction and proposed towers. The report found that 21 timber buildings with a height of over 50 meters are set to be completed by 2019.
Boosted by demand for greener buildings with smaller carbon footprints despite a higher cost for materials, and by remarkable recent advances in the technological capabilities of wood this new trend has been gaining ground over the las few years and in countries like Canada, tall wooden buildings are changing the cities’ skyline. According to CTV News, “momentum builds to reduce the heavy carbon footprint of concrete and steel in construction as the urgency of the battle to combat climate change grows”. At an international level, there are also moves towards a greener built-environment –defined as “the human-made space in which people live, work, and recreate on a day-to-day basis”. As a token, a series of revisions to the International Building Code state that wood, including a new fortified wooden building material called “mass timber,” can safely be used as the main support structure in buildings as tall as 18 stories.
Architect Michael Green, principal at Vancouver-based Michael Green Architecture, who six years ago called for a global era of wood-framed skyscrapers at a TED conference, says that “For me, it all comes back to the carbon story. It all comes back to choosing renewables to build our cities”. Michael Green Architecture is the studio that designed the 35 storeys high Baobab building for Paris, the world’s tallest proposed building so far, which is scheduled for completion in 2019. It will be made from a hybrid timber and steel structure.
Drexel College of Engineering Professor Abi Aghayere, Ph.D., explains that technological developments in heavy timber construction have also helped to encourage architects and builders to embrace wood as a more sustainable and ecologically sound renewable material fit for building. “The biggest development in heavy timber construction is “mass timber” or, to be specific, a product called cross-laminated timber (CLT), which has allowed the wood to be used for taller buildings because it is very effective in resisting fire in comparison to standard lumber”.
What is cross-laminated timber?
It was developed in Germany and Austria and has been used in Europe since the 1990s. It has already been used successfully in Europe and Canada for tall buildings as high as 14-18 stories.
Professor Abi Aghayere explains that CLT consists of stacked layers of typically low-grade wood, glued together to form a solid material, with adjacent layers oriented perpendicular to each other. “A 3-ply CLT has three layers and a 5-ply CLT has 5 layers. CLT has a substantial load-bearing capacity when used in wall panels and floor slabs and it shrinks less than sawn wood used in light-frame construction”.
He remarks that “one of the advantages of CLT over sawn lumber is that it does not lose its strength during a fire. CLT develops a char layer on the outside that protects the inner portions of the load-bearing CLT wood member. It is also more resilient in a wider range of environments because it does not experience the same magnitude of shrinkage that sawn lumber does. In tall buildings, CLT is being used for both the floor and roof slabs, and the wall panels; the CLT wall panels support the gravity loads from the floors and roof slabs. They are also strong enough to be used to resist the lateral or horizontal forces from wind and earthquakes”.
On the other hand, continues Professor Abi Aghayere, “the fact that wood structures also have a comparatively smaller carbon footprint compared to concrete or steel structures is a benefit that could have been a factor in the International Code Council consideration. Some estimates put the carbon footprint of a typical wood building at approximately 25 to 40 percent of the carbon footprint of steel or concrete structures. And since CLT is made from sawn lumber pieces, which do not necessarily have to be of the highest quality or highest stress grade, using it doesn’t drive up the demand for old-growth (large) timber. There has been some research on using low grade sawn lumber for CLT members. So CLT might allow for the use of a wider variety of wood species and stress grades”.
Growing demand for timber from the building industry
As wood becomes trendy again in the building sector, demand for environmentally-friendly timber is expected to grow by as much as 15% over the next decade.Cross Laminated Timber (CLT) Market – Global Industry Analysis, Size, Share, Growth, Trends, and Forecast 2017 – 2025 forecasts remarkable growth for timber used from residential units to government buildings.
As ProPopulus Chairman Pedro Garnica states, “the role of the wood industry in the pursuing of a more sustainable built-environment is essential. We can provide substitutes to non-renewable materials not only for the building industry but in a series of businesses”. But, he points out, “there is a paradox that we must solve: although studies show that the demand for timber will increase in the near future, as far as populiculture is concerned, the lack of a common European regulation for our activity is causing poplar plantations in Europe to decrease alarmingly. Poplar is one of the most efficient trees in terms of sustainability since one hectare of poplar captures 11 tonnes of CO2 every year and it also has a fast-growing cycle”. He calls for a clear, common European rule that paves the road to growers and sustainably managed plantations in the EU in order to satisfy the future demand for wood products.