Resilience of Concrete-Filled Steel Tubular Columns Under Repeated Lateral Impacts

When you think about the strength of buildings, bridges, or offshore platforms, the first thing that comes to mind is usually how well they stand up to everyday use. But what happens when they get an impact? Not just once, but over and over again? That is a big question in structural engineering, especially as cities grow, traffic increases, and infrastructure faces more wear and tear from vehicles, ships, and even natural forces like waves and earthquakes. One type of structure that has been getting a lot of attention for its ability to handle impact is the concrete-filled steel tube (CFST). It is exactly what it sounds like—a hollow steel tube filled with concrete, combining the best of both materials. The steel gives it flexibility and tensile strength, while the concrete provides high compressive strength. Together, they create a structure that is tough, durable, and good at absorbing energy from impacts. Engineers already know CFSTs perform well when they take a single hit but what about when they get hit multiple times in a row? That is where things get tricky, and surprisingly, there is still a lot that is not fully understood. The challenge is that damage does not always happen all at once. Sometimes, a structure holds up fine after the first impact, but with each additional hit, it weakens in ways that are hard to predict. Cracks might start to form, the steel may buckle, and the overall stability of the structure can gradually decrease. This slow breakdown is not well-documented, making it difficult for engineers to know exactly when a CFST column will reach its limit or if it will eventually settle into a stable state—something called pseudo-shakedown. To tackle these unknowns, new research paper published in Engineering Structures Journal and conducted by Associate Professor Shan Gao and Jie Yang from Harbin Institute of Technology alongside Professor Shao-bo Kang from Chongqing University and Fangyi Li from China Civil Engineering Construction Corporation and Xiaona Shi from ZhongTu Dadi International Architectural Design Co., LTD investigated the behavior of square CFST columns under repeated lateral impacts. They studied how much damage occurred and whether the structures could stabilize over time.

The research team set up nine square CFST specimens, each with different steel tube thicknesses and impact heights, to see how they would react under continuous lateral impacts. To make the tests as realistic as possible, they placed each specimen on a simple support system, just like what you would find in real-world structures such as bridge piers or protective barriers. A drop-weight testing machine delivered repeated hits, while high-speed cameras and force sensors captured every detail of the impact process. Right away, the first few impacts made a difference. Specimens with thinner steel tubes showed noticeable deformation, with local buckling appearing near the point of impact. As the impacts continued, an interesting pattern started to emerge. The damage did not keep increasing at the same rate. Instead, in many cases, the deformation began to slow down. This indicated something known as pseudo-shakedown, a state where the material gradually adjusts to repeated loading and stops accumulating significant new damage. This effect was especially noticeable in specimens with thicker steel tubes. Their outer steel shell was more effective at absorbing and spreading out the energy from each impact. In contrast, the thinner tubes struggled to handle the repeated stress, eventually leading to cracks and complete structural failure. One of the biggest takeaways was how impact forces changed over time. The first hit always produced the highest force. However, as the structure absorbed more and more impacts, its stiffness decreased, and the peak impact force dropped anywhere from 15 to 41 percent by the final impact. This drop was much more severe in thinner specimens, where the steel lost its ability to resist further impacts. The thicker tubes, on the other hand, maintained a much higher level of resistance, allowing them to last significantly longer.

The authors also found that the strength of the concrete inside the steel tube had far less impact than expected. The steel shell was doing most of the work, absorbing the energy and controlling deformation through plasticity and strain hardening. This challenges the long-standing assumption that increasing the concrete’s strength will significantly improve impact resistance. The researchers also discovered that where the impact happens makes a big difference. When the force was applied near the center of the span, the structure bent the most and was at the highest risk of failure. But when impacts happened closer to the supports, the structure held up much better, showing much less deformation and a higher chance of reaching pseudo-shakedown. To push their findings further, the team developed a detailed computer model in ABAQUS to simulate the impact scenarios. Their model matched the experimental results very closely, confirming that CFST deformation could be predicted with a high degree of accuracy. Using this model, they tested additional scenarios, such as increasing impact velocity, changing boundary conditions, and modifying section dimensions. One key discovery was that fixing both ends of the CFST column instead of simply supporting them significantly improved impact resistance. Stronger boundary conditions reduced overall deformation and increased the number of impacts needed to reach pseudo-shakedown. Perhaps the most valuable outcome of this study was the development of predictive formulas. These formulas provide engineers with a practical way to estimate how many impacts a CFST column can endure before stabilizing and how much deformation it will experience in the process. By using these models, engineers can design structures that are better prepared for repeated impacts without having to rely on costly trial-and-error testing. This research does more than just expand scientific understanding—it provides real-world solutions for making structures stronger, safer, and more durable.

In conclusion, the research work of Professor Shao-bo Kang and colleagues expected to significant implications for how engineers design and assess structures that need to handle repeated impacts because it showed that repeated impacts can gradually wear down a structure, either leading to a stable deformation state (pseudo-shakedown) or eventual failure, depending on the material properties and design choices. Moreover, it successfully demonstrated how much steel tube thickness matters when it comes to impact resistance and that thicker steel tubes handle repeated impacts much better, as they slow down damage accumulation and allow the structure to settle into a stable state rather than breaking down over time. This is especially valuable for designing things like bridge piers, offshore platforms, and protective barriers, which are constantly exposed to low-speed impacts from vehicles, waves, or floating debris. Engineers now have clear evidence that prioritizing steel thickness over concrete strength makes a huge difference in CFST performance, since the outer steel shell is what absorbs most of the impact energy. Additionally, the authors successfully showed the ability to predict pseudo-shakedown behavior and they developed predictive formulas to estimate how many impacts a CFST column can take before reaching a steady deformation state. This is a huge time and money saver since it allows engineers to make data-driven design decisions without having to rely on expensive, large-scale physical testing. Furthermore, the new study reported how impact location affects structural performance. When a force is applied at mid-span, the column bends significantly more, making it much more likely to fail. On the other hand, impacts closer to the supports result in much less deformation and a greater chance of reaching pseudo-shakedown. Therefore, engineers should focus reinforcements on the most vulnerable areas, instead of distributing them evenly across a structure which will result in smarter material usage and lower costs.  On a larger scale, this study highlights the need to update structural codes and design guidelines to better account for repeated impact loads. Right now, many industry standards still rely on static load assumptions or single-impact testing, which do not capture the full picture of long-term damage accumulation. The new findings provide a much-needed update to engineering best practices and helps ensure that future structures are designed with real-world conditions in mind.