Multi-storey timber construction Up to the high-rise building level

H8 in Bad Aibling
H8 in Bad Aibling | Photo (detail): Schankula Architekten

Multi-storey buildings are not usually associated with wood, far more with materials such as steel, glass and concrete. Timber, however, should not be immediately ruled out as an alternative material for buildings with several storeys.

Quite the contrary: in future, topics such as the saving of resources or the reduction of CO2 emissions are going to increase the demand for sustainable materials of a renewable source in all fields of construction. Apart from being a feasible construction method as such, multi-storey timber construction can also produce buildings of high and lasting durability. This is made evident by existing historic structures of timber design, such as timber-frame houses, for example, or traditional log houses.

Fire safety in timber construction

While in many countries today buildings that exceed the high-rise building limit of 22 metres use timber for the supporting structure and shell, in Germany such projects have a host of obstacles to face. These are set out in statutory provisions and first and foremost concern fire safety. Building Code regulations are the responsibility of the individual federal states or Laender in Germany and are therefore not uniform. The Master Building Code (Musterbauordnung) prepared by the Conference of Building Ministers is supposed to serve as the basis for harmonising basic requirements. Its implementation in the building regulations of the 16 Laender, however, varies significantly. Speaking in simple terms and in compliance with the Master Building -Code it can be said that in Germany timber can be used for load-bearing elements in buildings that have up to five storeys.

Pilot projects in multi-storey timber construction

For buildings that have more than five storeys the supporting structure has to be of non-combustible construction materials. This practically rules out the use of wood. It is, however, possible, in individual cases, to obtain approval for buildings that deviate from these prescriptive requirements if appropriate enhanced fire safety measures are established. This could include such facilities as sprinkler systems, for instance, the encapsulating of combustible materials or the provision of additional escape routes. While observing the described statutory provisions, over the past years a number of pilot projects with multi-storey timber structures were implemented in Germany. Initially, the highest project was the seven-storey residential block “e3” that was erected on a vacant plot in Berlin.

Eight storeys in Bad Aibling

Just recently, the construction of an eight-storey timber building “H8” was completed in the Bavarian town of Bad Aibling, in the county of Rosenheim. This project was to demonstrate that timber is also a suitable basic construction material for high-rise buildings and can replace conventional materials. The project was supported by the German Environmental Foundation and it is part of the project “From Military Wasteland to Zero Emission City” funded by the Federal Ministry of Economics and Technology.

Prefabricated wall elements

The supporting walls are made of solid wood cross-sections of between 8 by 8 cm and 16 by 16 cm that stand close together on a threshold and are joined at the top by means of a plate (horizontal end profile). With fire protection provided by two layers of gypsum fibreboard these walls are used both as an outside and inside wall. For the outside wall the elements were brought to the site fully assembled with mineral wool insulation, timber cladding and inserted windows. Partial areas of the façade are plastered, such as the ground floor and the stairway.

Floors of cross-laminated timber boards

For the floors 20 cm thick, cross-laminated timber boards have been used that are covered with a grit layer and have a plasterboard panelling underneath. In the living areas this panelling is omitted to expose the wood surface. The stairway is of a concrete design for fire safety reasons. It was also brought to the site as prefabricated elements. The access to the apartments via an external corridor prevents the development of smoke in the event of fire. The balconies are made of steel due to the direct exposure to weather conditions.

Flexible floor plans

Besides the building’s suitability for residential purposes, the floor plan concept has been designed to permit different types of office use as well. Due to there only being a few load-bearing walls, the use of the available space can be varied to suit individual requirements and adjust to future developments. All units have barrier-free access. The building has a decentralised air conditioning system with heat recovery, and boasts a very low heating requirement of approximately 18 kWh/m²a.

The timber structure was erected in 16 working days, one storey every two days. During this process some 600 cubic metres of timber were installed. This means that approximately 600 tons of CO2, that were absorbed from the earth’s atmosphere during the growing period of the trees, is now stored in the building as carbon.

High-rise buildings for the future

The feasible limit does not stop at eight storeys. In the London district of Shoreditch a residential tower measuring almost 30 metres was built in 2008 – eight of the storeys are designed in solid wood and are built on a plinth of reinforced concrete. Another building, the LifeCycle Tower, will accommodate up to 20 storeys and is the work of an international team of architects, engineers and building contractors who are participating in a research project. As a first pilot project with just eight storeys initially, the tower is set for completion in the spring of 2012 in Dornbirn, Austria, close to the Bavarian border. In the long term, Germany will also not be able to stop timber construction exceeding the existing high-rise building limit.