Staircases: wood before concrete

In large timber construction projects, builders often concrete staircases when not much is visible on the construction site except the basement. Carpenters then assemble the prefabricated wooden components.

Timbatec has reversed this sequence: The timber construction is erected, then the liquid concrete is poured in - or we do without concrete altogether and design the staircases from wood. The reversal of the work steps is the logical consequence of today's working methods, because timber constructors calculate with smaller tolerances than concrete constructors. The wooden elements are prefabricated with millimeter precision. This accuracy is not possible when concreting on the construction site. That is why we plan the concreting of the lift shaft as the last piece of the puzzle.

Staircase core in wood construction
Ceiling connection staircase

2-D and 3-D representation of the subsequently concreted staircase core: The number of necessary connectors and the thickness of the concrete wall can be reduced thanks to this method.

Less steel and concrete needed

The advantages of the new approach are obvious: significantly fewer steel profiles and screws are needed, the connection from the timber structure to the concrete core is simpler, and the construction time is shorter. The erection and dismantling of the scaffolding and formwork for the concrete core is no longer necessary and the quantity of concrete used can be reduced. This is achieved with the same performance: the subsequently poured concrete is positively and non-positively connected to the wooden structure. The stairway cores serve as static bracing for the buildings and ensure escape routes in the event of fire.

Staircases erected in advance

Timbatec developed the efficient and resource-saving method with the subsequently concreted staircase cores in 2019 for Haus Krokodil in Winterthur, a timber building with 248 apartments. Since then, it has been applied to a large number of multi-storey timber buildings.

The 42 millimeter thick 3-layer panels were clad with 3x15 millimeter gypsum fiberboard on the inside of the room for fire protection. On the other side, the board serves as lost formwork for the concrete. Until the concrete has cured, the stairwell is temporarily reinforced.

Wooden staircases and elevator shafts

Staircases stairwells and elevator shafts can also be constructed without concrete. This also applies to safety stairwells, for example in hospitals or high-rise buildings.

Buildings with a total height of more than 30 meters are considered high-rise buildings in Switzerland. By law, such buildings must be built with safety stairwells. This means that all vertical escape routes must be specially protected against the ingress of smoke and fire. If access for the fire department is impossible in the event of a fire, buildings under 30 meters high must also be planned with a safety stairwell. The same applies to hospitals.

Fire safety regulations 2015

The 2015 fire safety regulations are an important milestone in the development of modern timber construction. Since the 2015 fire protection regulations came into force, buildings of all use categories may be constructed in wood. Even safety staircases in buildings up to the high-rise limit may be realized in wood since then. These are buildings up to 30 meters high and correspond to about 9-10 floors. The same applies to elevator shafts.

With the current Swiss Fire Protection Regulations BSV 2015 of the Association of Cantonal Fire Insurers VKF, wooden structures can be erected in all building categories and uses. In the definition of fire resistance, a construction with combustible components is treated the same as non-combustible components. Link to BSV 2015.

Until now, such stairwells and escape routes were always built in concrete. The fact that safety stairwells are now possible in timber construction is demonstrated by the apartment building on Albulastrasse in Zurich: Here, the stairwell was constructed with cross-laminated timber and enclosed with 18-millimeter-thick gypsum fiberboard - encapsulated, as experts call it.

Wood staircase core

Two minutes' walk from Zurich Altstetten station is an apartment building with over 30 apartments on seven floors. Since the tight space conditions do not allow access for the fire department everywhere, Timbatec's engineers planned a safety stairwell - made of wood.

Bachelor thesis on wooden lift shafts

The wood-based materials manufacturer Olwo AG and the elevator manufacturer Emch AG collaborated to develop an elevator shaft made of glulam. Timbatec supported both companies in the product development. Among other things, a planning manual with a construction and detail catalog was created as part of Florian Fritschi's bachelor thesis.

Timber constructors thus have the possibility to produce an elevator shaft module from cross laminated timber at short notice. Link to the bachelor thesis.

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