The Rise of CLT

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Cross-laminated timber (CLT) is growing in popularity, with worldwide production of CLT expected to reach 1 million m³ by 2015 (Schickerhoffer). The ambition of timber engineers around the world is growing in equal measure as we push the limits of the material, by building ever taller structures.

In 2008 the Stadhaus, in London, became the tallest residential timber building in the world, reaching a lofty 9 storeys. It was a game changer; modern timber structures had never been built this tall before. Five years passed before the Stadthaus lost its title of tallest residential timber structure to the Forte building in Melbourne; a residential timber building one storey higher than its predecessor, standing at 32.2m.

The race continues to build the next world beater, but going higher poses technical challenges, not least ensuring the stability of the building. The uplift forces at the base of a CLT building, due to the wind on the structure increase in proportion to the square of the building height. This rapid increase in loading means that the next generation of timber buildings (10-20 storeys in height) will require the development of new connection details to prevent, the floor plates from crushing between the walls and the building lifting off the ground.

Uplift Force at Ground Floor vs Number of Storeys
Uplift Force at Ground Floor vs Number of Storeys

Recently at Ramboll, we have been involved in the design of, what will become, the tallest residential timber building in the UK. Naturally this has caused us to consider some solutions to overcoming these problems. Here is a selection of ideas that we have been considering:


Notched floors and walls

This solution uses castellated CLT floor slabs to provide direct load transfer between walls. Nail plates are positioned at the castellations to transfer uplift forces between the CLT walls.


3D View: Continuous CLT Wall and Notched CLT Floor with Nail Plates
3D View: Continuous Wall and Notched Floor with Nail Plates


Hardwood Dowels

Hardwood dowels within the CLT floor slabs prevent crushing of the floor by loading timber in the strongest and stiffest grain direction.

3D View: Hardwood Dowels Prevent Crushing of CLT Floor Plates
3D View: Hardwood Dowels Prevent Crushing of CLT Floor Plates


Bolt-through Connection

This bolt-though connection uses proprietary high capacity uplift connections bolted through the CLT slab to transfer forces between the walls.

Section: Bolt-Through Connection
Section: Bolt-Through Connection