Crossing the river sustainably with flax and organic binder

For thousands of years, flax has been used to make clothing, sacks, and durable ship ropes. This ancient, cultivated plant is now experiencing a "second bloom" and has the potential to become the building material of the future. As part of the EU-funded Interreg NWE project “Smart Circular Bridge”, two bridges were built using this renewable raw material. The first “Smart Circular Bridge” has been in operation in Almere in the Netherlands since 2022. The second bridge was inaugurated on February 7, 2025. It is located in Ulm and spans the river “Kleine Blau”, just steps away from the iconic Ulm Minster.

Researchers from the University of Stuttgart were also involved in the “Smart Circular Bridge” project. The architectural concept and design for the two bridges in Almere and Ulm were developed by Associate Professor Hanaa Dahy, Head of the research group Biobased Materials and Materials Cycles in Architecture (BioMat) at the University of Stuttgart’s Institute of Building Structures and Structural Design (ITKE), as well as Head of the BioMat@Copenhagen Research Center at Aalborg University in Denmark. "The Smart Circular Bridge project is an important milestone for sustainable architecture in Germany and internationally, as it demonstrates the potential of renewable raw materials for load-bearing structures on several levels - technically, structurally and aesthetically. New green infrastructure for the cities of the future and innovative architectural tectonics are emerging," says Dahy. Dahy and the BioMat team at the University of Stuttgart acted as architectural partners for the construction of the first Smart Circular Bridge in Almere and the Ulm bridge project.

"The inspiration for the bridge's design stemmed from the desire to make a bold statement about the future of green infrastructure. The sweeping curve represents a philosophical concept that connects the present with the future. It is a symbol of connection - especially between youth (as the future) and the past (embodied by Ulm Minster). I was particularly inspired by the rhythm and repetition of the cathedral's Gothic architecture, especially the vaults, which served as the initial impetus for the structural syntax of the natural fibers in the bridge's balustrade," says Dahy. She emphasizes, "The philosophy of 'Materials as a Design Tool' played a crucial role in the design. We view materials not merely as passive building elements but as active design tools that shape the form, function, and aesthetics of the structure."

Measuring nine meters in length and five meters in width, the bridge may initially seem unremarkable to passersby. However, a closer look reveals its design potential—an artistic railing with geometric patterns, intricately woven by robotic arms in a fully automated process. The ropes are made of flax fibers dipped in bio-based polyester binder, making them particularly stable and weatherproof. The railing posts are made of hardwood, recycled from a demolished bridge - a clear example of the sustainable circular economy.

At first glance, the bridge resembles concrete, but appearances can be deceiving. It consists of a high-performance composite material made from flax fibers and bio-based polyester binder and recycled PET foam. The combination of materials makes the bridge particularly light and so stable that it can compete with steel structures. The bridge performed so well in load tests and it can support not only pedestrians and cyclists but also vehicles!

Both bridges in the “Smart Circular Bridge” project will continue to undergo rigorous scientific monitoring. In Ulm and Almere, sensors continuously record data on load, deformation, temperature, and environmental influences in real time, ensuring comprehensive analysis. This data is analyzed using AI. In this way, research gathers valuable insights into materials and construction for future projects, while every person crossing the bridge contributes to this progress.

There's another striking aspect of the "Smart Circular Bridge" in Ulm: Stuttgart-based sound agency KLANGERFINDER has developed an interactive sound art concept that transforms sensor signals into a dynamic, playful dialogue with the bridge. Human movements and the effects of the environment are translated live into sounds and music - making it possible to experience the interaction between man, nature and the building.

The "Smart Circular Bridge" project
The “Smart Circular Bridge” in Ulm and the bridge in Almere, constructed in 2022, were created as part of the EU-funded Interreg NWE project “Smart Circular Bridge”. The project is backed by an interdisciplinary consortium of 15 partners led by Eindhoven University of Technology. The project is researching the potential of renewable raw materials as a building material in load-bearing structures, as the construction industry currently contributes a significant proportion of global CO₂ emissions. By developing innovative, high-performance materials from renewable resources, the initiative aims to advance the bio-based circular economy while achieving sustainable, long-term reductions in CO_₂ emissions.