Advancing Sustainable Infrastructure through High Strength Steel:

The study of composite columns, especially concrete-filled steel tubes (CFSTs), is gaining significant traction in modern structural engineering. CFSTs combine the advantages of steel and concrete into single robust structural elements, and by this provide enhanced load-bearing capacities, increased rigidities, and improved fire resistance which made CFSTs increasingly popular for large-scale infrastructure like high-rise buildings and bridges. But as construction demands grow, and the need for even stronger, more efficient structures becomes apparent, engineers are now looking to incorporate high-strength materials such as S690 steel and C75 concrete. These materials surpass the properties of traditional materials, such as S355 steel and C60 concrete, typically covered by existing building codes. High-strength materials like S690 steel are still relatively new in civil engineering, and they haven’t been fully integrated into design codes like the European standard EN 1994-1-1, which primarily addresses traditional S235 to S460 steel. High-strength steel can deliver remarkable benefits in terms of material efficiency and structural adequacy, but its behavior, especially in composite structures, is not yet fully understood and quantified. This lack of knowledge creates a challenge: while high-strength steel and concrete could elevate the performance of CFST columns, engineers currently lack proven guidelines for safe and effective use of these materials in various structural applications and under different load conditions. A particular challenge with CFSTs made from high-strength materials is understanding how they behave when subjected to compressive loads. Stocky CFST columns, which are widely used in practice, exhibit significant pressure between the steel tubes and the concrete cores during compression. This confining pressure can significantly boost the load-bearing capacities of the columns, but accurately measuring and predicting it is complex. Traditional models tend to make simplified assumptions about how the materials behave, often overlooking the unique stress-strain responses and the resulting deformation characteristics that both the high-strength steel and the concrete exhibit under the influence of one another.

Addressing these challenges, a new study led by Professor Kwok-Fai Chung and Professor Xu-Hong Zhou, along with Peng-Fei Men and Ho-Cheung Ho at The Hong Kong Polytechnic University and the Chongqing University, investigated the behaviour of stocky CFST columns constructed with S690 steel tubes filled with high-strength concrete. Published in Engineering Structures, the research shed light on load resistances, deformations, and confinement effects within these columns, deepening understanding to their structural performance and paving the way for effective use of high-strength materials in modern construction.

The researchers tested 21 stocky CFST columns which had different cross-sectional dimensions and material grades, carefully devised to represent real-world conditions seen in high-demand structural settings. Their goal was to uncover how the S690 high-strength steel and the C60/75 high-strength concrete worked together to resist applied loads under the presence of confining pressure. The experiments took place in a controlled environment where each column was subjected to steadily increasing axial compression. This setup allowed the researchers to carefully monitor how the columns responded as the applied loads increased. As they observed each of these columns during testing, they collected key data on applied load and local deformation for subsequent analyses on stress interaction between the steel tube and the concrete core. One of their main findings was that the high-strength steel tubes played a significant role in confining the concrete, adding strength and ductility to the columns overall, despite of a reduced ductility in the S690 steel when compared with that of the traditional steel.  This confining effect was especially clear in the thicker steel tubes, which held up higher loads with less deformations, revealing the real benefits of using the S690 steel in these types of CFST columns. To understand how these columns deform during testing, the team placed strain gauges on the surfaces of the steel tubes to measure precisely how local strains were distributed as the applied loads increased. They noticed that under large applied loads, the steel tubes were able to restrain the concrete cores effectively, preventing them from expanding too much outward, and thus, allowing the columns to hold up under even larger loads. This behaviour illustrated how the S690 steel maximize the confinement effect in these types of CFST columns despite of its reduced ductility.

An unexpected outcome was that the high-strength CFST columns didn’t entirely align with predictions from existing design codes, which were developed based on traditional steel grades. The S690 steel tubes, in particular, supported significantly larger loads than existing design codes would suggest, hinting that the contribution of the S690 steel might be undervalued in today’s design codes. These results suggest a need to rethink and possibly update the design rules to better match the actual behavior of these CFST columns when high-strength materials are used. The team also tested different column sizes and steel tube thicknesses to see how these factors impacted the columns’ performance. They found that the thicker steel tubes were remarkably strong, holding their shapes even under advanced compression, while the thinner tubes showed more visible deformations. This suggests that thicker steel tubes provide a larger confinement effect, contributing to greater enhancement in strength overall. These results imply that using thicker steel tubes could significantly boost load-bearing capacities. To investigate more into how confinement affects structural resistance, the authors looked into how these columns behaved as they approached their load limits. They observed that the steel tubes often started to show small signs of buckling as the applied loads increased. It should be noted that these deformations helped distribute stresses more evenly between the steel tubes and the concrete cores, allowing the columns to handle larger applied loads before ultimately failing. At extremely large loads, however, the steel tubes eventually reached their load limits, leading to localized buckling and signaling the beginning of structural failure. This balance among mobilization of material properties, effective confinement, and load-sharing illustrated that while the high-strength steel tubes greatly increase the load carrying capacities of these columns, there is a cap on such an increase, because of failure of the concrete cores.

In conclusion, the new study by Professor Kwok-Fai Chung and colleagues stands out for its potential to change how we approach designing and using composite columns in structural engineering. By investigating CFSTs made with high-strength S690 steel and C60/75 concrete, the study opens up new possibilities for using advanced materials with various strength and ductility to be heavily loaded members in buildings and bridges.  In time, this study could drive updates to design codes, giving engineers more confidence to use high-strength materials that maximize both structural safety and efficiency. It is believed that one of the most impactful implications of this study is its potential to shape stronger and more efficient infrastructure, especially in urban areas where limited spaces and growing demands require innovative solutions. The study demonstrates that CFSTs made of high-strength steel and concrete can meet these demands better than traditional materials. Thanks to the exceptional strength and confinement of the S690 steel tubes, these CFST columns can carry large loads with small deformations, making them highly effective to carry loads in buildings and bridges, probably even in extreme events like earthquakes. Beyond immediate construction applications, this study also supports a global trend toward sustainable building practice by showcasing materials that achieve high performance while potentially using less. By maximizing the load carrying capacity of each of these columns, high-strength materials could help cut down on the amount of steel and concrete needed, reducing both material costs and environmental impact. This focus on efficiency is especially relevant as the construction industry faces pressure to balance load carrying capacities with environmental considerations. In practical terms, the study offers structural engineers key data they need to confidently consider high-strength CFSTs for projects requiring high structural resistances with low environmental impact. By focusing on mobilization of material properties, effective confinement, and load-sharing, the study gives a more refined understanding of CFST performance which will help engineers make optimized design decisions, ensure structural safety, and enhance sustainability over a structure’s lifespan.  “We should develop new knowledge and skills to build stronger structures with fewer materials, and to offer structural solutions with improved structural efficiency and performance using high strength S690 steel” said Professor Kwok-Fai Chung.