How tall can we build from CLT?
CLT as ecological, light-weight natural material becomes more and more popular as material for tall buildings. Tall buildings from CLT tends to become standardized solution for commercial and residential buildings.
But how tall can we actually build from CLT?
This directly depends on physical and mechanical characteristics of CLT and timber.
CLT is a light-weight material and this characteristic allows to build high and light structure. This is great advantage because of lower seismic impact on building and for reducing total loads to the foundations, but on the other hand, this can be disadvantage because lighter and taller buildings are more sensitive to wind loads.
Slabs of tall buildings can be designed as simple CLT slabs or as hybrid systems with concrete topping or stiffening ribs. Concrete toppings are good way to increase the stiffness of diaphragms and complete tower like it is used for last floors of the second tallest wooden building in the world – 81 m tall Mjøstårnet.
What is important when building tall with mass timber?
Distribution of walls is directly connected with lateral stability of tower. Stiffness of building is the most important and governing in designing towers of CLT and timber. CLT have good performance to be used in shear walls as stiffening elements. Creating massive cores and placing walls in shape of I, C or box will significantly increase stiffness of the building. In systems with less walls it is common to use GL diagonals as bracing in façade or inside of the building, but joints of huge diagonals, beams and posts is impossible to build without using complicated steel brackets.
Ductility of tall wood structures comes from ductility of connections and this is the weakness of timber buildings. These connections need to be designed carefully and tested in order to ensure appropriate behavior.
Alternate solution for lateral stability of tall CLT buildings are massive concrete cores. But this also reflects on building CO2 footprint because use of concrete and steel makes bigger impact on CO2 emissions.
Great example of only timber use in high rise buildings is the Mjøstårnet (81 m) and TREET (53 m) buildings in Norway that have lateral stability systems with CLT core and diagonal GL beams on façade. Buildings like HoHo Tower Vienna, Austria tallest wooden building in the world (84 m), UBC Brock Commons (53 m) in Canada have their lateral stability secured with concrete cores.
Today, we have 84 m high timber building, we have buildings of similar heights under construction across Europe, USA, Japan… We have announced over 100 m timber buildings in near future and this seems to be approximate limits of timber. With intensive research in field of connections, timber strengthening and new materials, we are sure we can expect even taller buildings.