Appreciating wooden skyscrapers on Skyscraper Appreciation Day

Today (10 August) is Skyscraper Appreciation Day, so let’s appreciate skyscrapers! Not just any skyscraper, but skyscrapers made from an innovative material… Wood!

Building with wood
Wood in itself, of course, is not a new building material. Pretty much since people started building their own houses, wood has been used as a construction material in parts of the world where it was readily available. However, one major drawback of wood, at least as a construction material, is that it easily catches fire. In times of urbanisation when most buildings in a city were made from wood, fires were a major threat as they easily could do a lot of damage (as for example during the Great Fire of London in 1666, destroying most of the city).

Stone, in that regard, is a much safer building material, and nowadays, less flammable materials like steel and concrete are favoured for construction. Especially the use of steel made it possible to build high-rise buildings and later skyscrapers.

Wood in high-rise buildings
Wood is historically not often used for high-rising buildings, because on its own, its strength has limits. However, innovations in the production of wooden materials in the past 100 years have managed to increase its strength. Examples of these innovations are glue-laminated timber (glulam) and laminated veneer lumber.

The most popular wood material that is used in the construction of high-rise buildings is cross-laminated timber (CLT). CLT was developed in the mid 90s in Austria and is now widely used in construction. It is made by gluing layers of dimension lumber together in a large cross laminated sandwich, in intervals of layers from 3, 5, 7, and up to 9 thick. Thanks to its layered structure, CLT panels have immense strength in two directions.

Advantages of CLT
Using CLT has many advantages. First of all, unlike ‘normal’ wood, it does not burn easily. Instead, it behaves more like concrete. When it does catch fire, it wants to put itself out. It is safer even than steel, which can melt at high temperatures. A thick plank of wood will char on the outside, sealing the wood inside from damage.

In addition to being just as strong as steel, wood has the advantage of being lightweight as well, and because CLT panels are pre-fabricated, they are can be installed easily and quickly.

Perhaps the most important advantage of CLT is its low environmental footprint. Concrete and steel require enormous amounts of CO2 to produce and transport. Wood, on the other hand, generates much less CO2, even engineered wood like CLT. Even after a tree is felled, the wood still absorbs CO2, unlike many other construction materials.

In addition, wood is a much better insulator than concrete or steel, which means that it costs less energy to heat and cool a building.

However, while wood is a renewable material, it takes many years before a tree is fully grown. So is CLT as sustainable as claimed?

The short answer is yes. CLT is made from lumber from trees harvested from sustainably managed forests, and often trees that are killed by mountain pine beetles. If this wood is left to decay, it emits carbon into the environment. In addition, the US CLT Handbook states that in the US, less than 2 per cent of the standing tree inventory is harvested each year, while the net tree growth is close to 3 per cent.

Wooden skyscrapers
There are many wooden high-rise buildings already constructed or planned that use CLT, such as HoHo Wien in Vienna, Baobab in Paris, Canopia in Bordeaux, Dalston Lane in London, T3 in Minneapolis, and Patch22 in Amsterdam.

When the term skyscraper was originally coined in the 1880s, a building had to have 10 to 20 storeys, but now the average skyscraper has 40 to 50 floors and is over 100 metres tall. Canopia, for example, would fit the term like a glove, as the building is planned to be 50 metres tall. Each wooden skyscraper seems to become a little higher, pushing the limits of building with wood. Let’s just hope that in this case, it’s not the bigger they are, the harder they fall…

Photos: Ema Peter / C.F. Møller / SHoP Architects / HoHo Wien / Patch22