Reinforced concrete floor slabs account for about 40 percent of buildings' embodied carbon emissions. However, until now, there has been no practical alternative for complex, multi-story buildings. Using a construction demonstrator, researchers at the University of Stuttgart have now shown for the first time how floor slabs can be built sustainably from an innovative timber-concrete composite system.
The demonstrator, which was tested in June as part of the Future Cleantech Festival in Remscheid, showcases "UniversalTimberSlab" – the first point-supported timber-concrete composite flat slab for two-way long spans. “With the UniversalTimberSlab, we aim to make multi-story timber-concrete composite construction significantly more flexible, performative, and adaptable. The demonstrator is the result of several years of research at the University of Stuttgart and is based on innovative digital design and manufacturing methods,” says Professor Achim Menges, director of the Institute for Computational Design and Construction (ICD) and spokesperson for the Cluster of Excellence “Integrative Computational Design and Construction for Transformative Architecture” (IntCDC) .
A timber-concrete composite flat slab for multi-story mixed-use buildings
“UniversalTimberSlab” is as slender and performative as a reinforced concrete slab, but significantly more sustainable. This system largely relies on wood, a renewable resource, material-efficient construction, and circular design.
Compared to conventional post-and-beam timber floor slab systems, which require deep beams for larger two-way spans, the compact “UniversalTimberSlab” reduces the structural height by 30 to 70 centimeters – a significant cost advantage: With the same floor-to-ceiling height, a building can accommodate more stories, and the facade area is reduced by up to 20 percent.
“UniversalTimberSlab” also sets entirely new standards for design possibilities in timber construction. The structural system is based on point supports, meaning that the building loads are transferred through columns that can be spaced relatively far apart. Load-bearing partition walls are not required; walls can be arranged freely as needed. In mixed-use buildings, this allows for flexible layouts tailored to retail, dining, offices, and residential spaces. In addition, floor plans can be easily reconfigured at a later date, if the building is to be repurposed. The novel floor slab could thus help promote the further spread of timber construction in inner-city areas.
The game-changers: new slab segmentation, digital design methods
In timber construction, larger slabs are divided into individual prefabricated elements. While these elements and their supports previously had to be arranged along rigid grids for technical reasons, the “UniversalTimberSlab” system now makes it possible to also build regular and irregular floor slab geometries. The principle is based on a new, patent-pending segmentation method. Glued-laminated timber segments with straight laminations that are easy to prefabricate ensure cost efficiency. The fibers in the floor slab are arranged to follow the force flow to optimize the system's load-bearing capacity.
The project also developed new digital design methods for timber buildings: During the planning phase, AI-powered tools generate detailed models for numerous floor plan and building variations and assist with design optimization. For example, if a planner moves a column, the impact of that action on costs, slab height, and sustainability can be immediately shown.
Milestone: successful demonstrator in Remscheid
“What was particularly exciting for us was that the first load test wasn’t conducted in the lab, but in front of the approximately 200 visitors at the Future Cleantech Festival in Remscheid. This made it possible to verify the load-bearing capacity required for construction under real-world conditions,” says Professor Jan Knippers, director of the Institute of Building Structures and Structural Design (ITKE ).
For the structural test, the team built a 9 x 5 meters full-scale demonstrator of the floor slab with a two-way span of 8 x 4 meters in just three months. At just 36 cm, it was as thin as a reinforced concrete slab of equivalent load and span, yet required about two-thirds less reinforced concrete.
As a preliminary step, the researchers performed computer-aided calculations – so-called FEM simulations – to investigate how the floor slab structure deforms and vibrates under load. In the structural test, the demonstrator was loaded with a total of 20 metric tons in addition to its own weight. This made it possible to test the load-bearing capacity for a heavily used office building with finish loads of 1.4 kN/m² and a live load of 3.0 kN/m². The measurements confirmed the simulations, with a maximum deformation of only 12 mm. With a natural frequency of over 8 Hz – that is, eight oscillations per second – the floor slab met the requirements for multi-story buildings in terms of deflection and vibration.
Prospects for rapid technology transfer: first pilot project in the planning stages
With the successful feasibility test, the researchers have reached a crucial milestone on the path to the widespread use of the “UniversalTimberSlab” in multi-story wood construction. "The system builds on the existing manufacturing expertise of the wood construction industry. This means it has the potential to be rapidly deployed in larger construction projects once further development work is successfully completed,” says Hans Jakob Wagner, group leader at the ICD and head of the EIC Pathfinder project “UniversalTimberSlab.”
The first pilot building is already in the works: The town of Oberkochen in the German state of Baden-Württemberg is planning the “Zukunftsforum,” a three-story, 1,400 m² building constructed using the innovative “UniversalTimberSlab” method. The building is intended to house exhibitions, workspaces, makerspaces, workshops, and laboratories.
EIC Pathfinder project: "UniversalTimberSlab"
The “UniversalTimberSlab” floor slab is being developed as part of an EIC Pathfinder project funded by the European Innovation Council under Horizon Europe. „UniversalTimberSlab” is part of the Cluster of Excellence “Integrative Computational Design and Construction for Transformative Architecture” (IntCDC) . In addition to the ICD, the Institute of Building Structures and Structural Design (ITKE ), the Institute for Acoustics and Building Physics (IABP) , and the Institute of Construction Materials (IWB) together with the Materials Testing Institute (MPA) at the University of Stuttgart are involved, as is BUILT CoLAB in Porto. The floor slab segments for the construction demonstrator were manufactured by the industry partner HASSLACHER Group and installed by Dünschede Holzbau in just two days.