At the 2024 constructsteel conference, the University of Luxembourg received the Innovation of the Year Award for its development of steel-to-timber shear connectors—a key enabler for demountable, reusable steel-timber composite flooring systems. These innovative connectors support the transition to circular construction by facilitating disassembly and reassembly without compromising structural performance.
We spoke with Professor Christoph Odenbreit to learn more about the research and its implications for the future of sustainable construction.
How did the principles of the circular economy shape the objectives of this research?
The starting point was the European Green Deal and the pressing need to reduce waste production, natural resources consumption and finally embodied carbon in the construction sector. While recycling has been a traditional strategy, true circularity requires us to think new ways in terms of disassembly and reuse. This is a challenge, especially in concrete and also in steel-composite structures, where unresolvable connections make reusability impossible.
Our aim was to develop systems that enable reuse without degrading performance. We began by developing adaptable connections that enable beam reuse. By standardizing beam lengths basing on a grid pattern and making them 500 mm shorter than the grid dimensions, we developed a push-in and draw-out extension to adjust the beam for application in a new building. Next, we designed connectors for steel-concrete prefabricated elements that allow disassembly from either the top or the bottom. This led us to rethink how steel and timber could be joined in a similar way that allows for easy separation, preserves the integrity of the materials, and facilitates standardisation for future reuse-applications.
What technical challenges did you resolve by designing steel-to-timber shear connectors that are both high-performing and demountable?
The main challenge lies in the material differences: steel is strong and ductile, timber is anisotropic and susceptible to crushing and brittle failure in tension. Standard bolted or screwed connectors, while technically removable, often damage the timber during disassembly, and hence limiting reuse.
To address this, we developed three novel connector types which feature a steel “shear connection device” embedded in the timber and use high-strength bolts preloaded with direct tension indicators (DTIs). These devices maximise the contact area between the steel shear connector and the timber, and to minimise crushing, enable at the same time full bolt preload without damaging the timber. The connectors are entirely mechanical—no adhesives—which supports disassembly and component reuse.
Could you briefly explain how these connectors performed under testing?
We carried out a series of tests to check how strong and reusable the connectors are. In a set of pushout tests, we looked at how well the connectors held the steel and timber together when shear force was applied. The connectors performed very well, staying firmly in place with high loads and only minimal movement and no damage only at excessive high loads.
Then we tested entire beams built with the connectors, applying vertical loads on the beam to see how these steel-timber composite beams would perform. The beams remained stable and flexible, bending as expected without breaking. Most importantly, after testing, all timber and connection components were still in good condition and could be reused in future construction.
How do these systems translate into practical applications on construction sites?
These connectors can readily be used in flooring systems that combine steel beams with timber panels. The timber panels rest on downstanding I-profile steel beam and are designed so they can be taken apart and reused. Small gaps between adjacent panels are filled with mortar and lined with small steel—or plastic elements— to make them easy to separate later without damage.
Most of the preparation—like cutting the timber, drilling holes, and installing the connectors—is done in a factory. This helps ensure everything fits perfectly and reduces the chances of mistakes on site. Once at the construction site, the pieces are simply assembled and the bolts tightened. This method works well with modern, automated building techniques and helps speed up construction while lowering costs.
What distinguishes this system from other hybrid flooring solutions?
This system offers a combination of excellent composite performance and at the same time the possibility for demountability and reuse. Compared to conventional steel-concrete composite floors, it is lighter, enabling larger spans compared to timber construction, reduced foundation loads. Unlike other composite floors that rely on permanent fasteners, our solution maintains high mechanical performance without sacrificing reuse.
Moreover, the system bases on a grid pattern and is standardised and modular, which supports futureproofing. It is not only technically robust but also adaptable to future regulations and design-for-disassembly strategies.
Looking ahead, how do you see this innovation contributing to the wider adoption of circular construction?
We see this as a step toward a new mindset in structural engineering—where design for disassembly and reusability is not a compromise, but an integral part of the initial design process. The pavilion “Petite Maison” of the European Cultural Capital that applied our design and principles highlighted circular economy benefits in steel construction. All connections were circular, and, as mentioned above, slightly shorter beams with push-in connectors for beam to column connections were used—the system works and allows maximum reusability. Our immediate goal is to scale up through further industrial collaboration, adapt the system to further products of engineered timber, standardise and integrate it into regulatory frameworks.
As industry and regulation evolve to prioritise environmental performance, we believe innovations like these connectors will play a vital role. They not only reduce material waste, consumption of natural resources and emissions but also support more flexible, adaptable, and very resource-efficient buildings.