What if one structural system could do it all – absorb seismic shocks, slash carbon emissions, accelerate building timelines, and cut down on future maintenance costs? Concrete-filled steel tubes (CFSTs) are the composite innovation that promises to merge performance with sustainability. With the recent publication of ISO 16521:2024, CFSTs now have a harmonized global standard, unlocking the potential to transform how we design and build infrastructure worldwide.

As cities expand and sustainability becomes non-negotiable, the construction industry is under pressure to build smarter, and find solutions that deliver both high performance and environmental responsibility. Steel-concrete composite and hybrid structures stand out as the most promising choice, blending the mechanical advantages of both materials while neutralizing their disadvantages. Their impressive resistance and ductility make them ideal for a wide range of infrastructure projects, particularly ambitious ones like high-rise buildings and large-scale bridges.

Developed by technical committee ISO/TC 71, the new ISO 16521 was published by its subcommittee SC 9 in September 2024. It marks a significant milestone for CFST technology, granting it formal recognition and providing a global framework for design, construction, inspection and maintenance.

In CFST systems, steel and concrete work in tandem to elevate performance, boosting resistance to complex loading and making them especially suitable in earthquake-prone areas. The steel tube acts as a shell, confining the concrete and enhancing its strength and ductility through triaxial compression, while the concrete improves the tube’s overall stability. This composite action offers excellent energy absorption and cuts the need for formwork, resulting in faster, cleaner construction and reduced emissions.

However, widespread adoption of CFST structures, particularly in less developed, budget-constrained regions, has faced persistent hurdles. High initial costs for materials and fabrication often discourage investment, while corrosion remains a key concern in aggressive environments. The absence of consistent research and standardized design guidelines has also created uncertainty, limiting confidence in the technology and its application.

To address these challenges, ISO 16521 sets out clear technical protocols that ensure consistency and reliability from concept to execution. It introduces innovative CFST hybrid forms that can significantly reduce material use and tackle steel tube corrosion through systematic protective design specifications.

As a globally recognized reference, the standard will accelerate adoption of CFSTs in both urban development and infrastructure projects. With standardization in place, stakeholders are better equipped to harness the full potential of this technology in modern construction, paving the way for a more sustainable and resilient built environment.

ISO 16521 gained global attention in June 2025, when Prof. Lin-Hai Han of Tsinghua University – Chair of the newly formed ISO/TC 71/SC 9 – was honoured with the ISO Excellence Award, becoming the first recipient recognized for work in concrete materials and structures.