Research on dynamic response law and impact performance of steel pipe concrete rock-socketed pile under horizontal impact load

In the world of heavy infrastructure, some of the most critical structures are the deep, hidden roots that hold up bridges and docks. These are often piles driven deep into the earth or rock, designed to bear immense weight. Among the most robust of these are concrete-filled steel tubes, where a thick steel pipe is filled with solid concrete. The steel acts as a protective shell, while the concrete provides the bulk and strength. Engineers rely on these composite columns because they are strong, flexible, and relatively easy to build. However, these structures face a unique and violent threat: accidental collisions. When a ship drifts off course or a barge is pushed by a strong current, it can slam into a dock or bridge pier with tremendous force. Unlike the slow, steady weight of a building, this is a sudden, horizontal blow that sends shockwaves through the material. Understanding how these deep, rock-socketed piles react to such a hit is vital for safety, yet the complex dance between the steel shell and the concrete core during a split-second impact has remained somewhat of a mystery.

To solve this puzzle, a team of researchers from China Merchants Chongqing Communications Technology Research & Design and Chongqing Jiaotong University turned to the power of computer simulation. They built a detailed digital model of a large-diameter steel pipe concrete pile, anchored firmly into a block of simulated rock. This virtual pile was not just a static image; it was a dynamic system where the steel, the concrete, and the reinforcing bars inside were all programmed to react to stress, strain, and the rapid changes of a collision. They subjected this digital model to a series of horizontal impacts, mimicking a ship striking the pile at different speeds. By watching how the forces traveled through the structure in their computer, they could see exactly what happened inside the pile at every fraction of a second, something that is incredibly difficult to measure in a real-world crash.

The researchers discovered that when the pile is hit, it does not simply bend and stop. Instead, the entire structure begins to vibrate, shaking back and forth like a plucked guitar string, but with a rapid, dampening rhythm that fades away over time. This vibration happens in two distinct phases. First, there is a sudden, sharp spike in force as the impact energy hits the pile, causing the internal forces to surge. Then, the structure enters a phase of gradual decay, where the shaking slows down and the forces settle. Inside the pile, the steel tube and the concrete core do not move in perfect unison. While they vibrate together, the way they handle the internal stress is surprisingly different. The researchers found that the concrete core carries the vast majority of the heavy lifting. At any given moment during the impact, the shear force—the sliding force that tries to cut the pile in half—and the bending moment—the force trying to snap it like a twig—are significantly larger in the concrete than in the steel shell. Specifically, the steel tube contributes approximately 26% of the shear force and about 26% of the bending moment compared to the concrete core, meaning the concrete bears the overwhelming majority of the load.

Perhaps the most counterintuitive finding was how the steel and concrete interact. One might assume the steel shell helps share the load, acting as a partner to the concrete. While the steel does contribute a measurable portion of the internal forces, the simulations showed that the steel tube does not act as a simple load-sharing partner in the way one might expect. In fact, the presence of the steel tube often results in the concrete core experiencing even greater internal forces than it would alone. The two materials seem to work against each other in terms of force distribution; as the shear force in the concrete rises, the force in the steel often moves in the opposite direction. This means the steel tube acts primarily as a constraint that alters how the concrete bears the brunt of the impact, rather than simply sharing the burden equally. The pattern of these forces along the length of the pile also revealed a specific shape. The forces in the concrete core form a shape that is narrow at the top and wide at the bottom, resembling a gourd, with the most intense stress concentrated near the point where the pile enters the rock. The steel tube, conversely, shows a different pattern, with its forces spreading out in a shape that is wide at the bottom and narrow at the top, like an upside-down trumpet.

The speed of the impact proved to be a critical factor. When the researchers increased the velocity of the simulated collision, the forces inside the pile grew significantly. However, the relationship between the speed of the hit and the resulting damage was not a simple straight line. They also looked at how the "constraint coefficient" affected the outcome. This is a measure of how tightly the steel shell squeezes the concrete, determined by the thickness of the steel and the strength of the concrete. The study revealed that changing the thickness of the steel or the strength of the concrete changes how the pile responds, but not always in a predictable way. For instance, making the steel thicker increases the constraint, which can sometimes lead to a decrease in the maximum bending force the concrete experiences, but only up to a certain point. Beyond a specific threshold, the relationship flips. Similarly, if the concrete is weaker, the steel shell has to work harder, but the overall behavior of the pile changes in complex ways depending on the speed of the impact.

Ultimately, the research provides a clear map of how these deep foundations behave under the extreme stress of a ship collision. The simulations confirmed that the concrete core is the primary defender, absorbing the majority of the impact energy and internal forces, while the steel shell plays a more complex role of confinement that influences the distribution of forces rather than simply sharing the load. The study established that the maximum sideways movement of the pile and the maximum bending force in the steel tube follow a consistent, predictable relationship based on the speed of the impact and the tightness of the steel constraint. These findings offer engineers a clearer understanding of the hidden mechanics at play when a dock or bridge is struck, allowing for designs that are not just strong, but specifically tuned to survive the violent, sudden jolts of the real world. By knowing exactly how the forces flow and where the structure is most vulnerable, engineers can build safer, more resilient infrastructure that stands firm even when the unexpected happens.