Optimizing Non-Diaphragm CHS Connections: Predictive Models for Collapse Behavior under Biaxial Loading

Engineers today are increasingly focused on making steel connections, especially those involving circular hollow section (CHS) columns, more efficient and adaptable for complex structures like high-rise buildings, bridges, and other large-scale projects. CHS columns have become popular in modern construction due to their unique benefits. By distributing material evenly around a central axis, these columns offer a high strength-to-weight ratio, resist twisting forces well, and provide a clean, streamlined look. However, while CHS columns offer advantages, they also present unique challenges—especially when it comes to connecting them to beams under demanding load conditions. These connections, essential for transferring loads in moment-resisting frames, can face issues with plastic collapse, where the material deforms beyond repair. This challenge has motivated engineers to explore better ways to ensure these connections remain stable and resilient. Historically, engineers have strengthened these joints using diaphragm-type connections, which add stiffening plates or diaphragms for stability. While diaphragms effectively reduce deformation and improve load-bearing capacity, they also come with drawbacks. Adding diaphragms means higher material costs, more complicated fabrication, and limits on design flexibility, as these stiffeners affect the layout and aesthetics of the structure. In response, non-diaphragm connections—those without added stiffeners—have been proposed as an alternative. These offer simpler assembly and cost savings on materials, but they can be more susceptible to localized deformation, potentially making them less durable under complex, multi-directional loads. In a recent study published in the Journal of Constructional Steel Research, Assistant Professor Yoshiharu Sato from the University of Tokyo tackled this issue by studying how non-diaphragm connections between CHS columns and flange plates hold up under biaxial loads. Professor Sato wanted to better understand how these simplified connections behave under the real-world stresses they would encounter in large structures. As urban buildings grow taller and infrastructure becomes more intricate, it’s critical for engineers to have reliable data on designing these connections to handle heavy loads without needing additional stiffening.

The study’s main goal was to explore the plastic collapse mechanisms in these non-diaphragm connections. In engineering terms, plastic collapse refers to a point where the structure undergoes permanent deformation under load and can’t support additional weight without risking failure. Knowing where and how this collapse happens allows engineers to design connections that distribute weight more effectively and remain stable under intense stress. To analyze this, the author used finite element analysis (FEA), a method that lets them simulate how the CHS column and flange plate connection responds to biaxial loading. They closely examined how factors like material strength and connection shape influenced the point at which the connection would fail.

In this study, Assistant Professor Yoshiharu Sato set out to get a closer look at how connections between flange plates and CHS columns—without the usual internal supports known as diaphragms—hold up under complex loading conditions. Their goal was to understand at a detailed level where and how these connections start to fail under stress. They built sophisticated models using FEA, which allowed them to mimic the real forces these connections experience in structures like multi-story buildings. By setting up these simulations, the researchers could trace exactly where plastic collapse, or permanent deformation, began and see the steps that lead to structural failure. One of the most interesting findings from the simulations was that points of collapse tended to form right at the flange plates, especially where the plate joins the column. Because these connections lack the internal stiffeners typical of diaphragm setups, they tend to concentrate stress at certain areas rather than spreading it more evenly across the structure. As the load increased, these high-stress spots stretched beyond their elastic limits, meaning they deformed in a way that couldn’t spring back to the original shape. This insight was crucial, as it pointed to a specific vulnerability in non-diaphragm connections: while simpler and cheaper to build, these connections face greater risks of localized failure under high-stress conditions. To measure the load at which these connections would reach their breaking point, the author developed a set of equations based on what they observed in the FEA models. These equations aimed to predict the exact load levels that would cause collapse in different configurations of CHS-flange plate connections. Testing these equations across various scenarios, the team found that their predictions closely matched the FEA results. This validation suggests that these equations could become a useful tool for engineers, providing a way to predict collapse loads in non-diaphragm connections without the need for additional stiffening elements, helping balance structural safety and cost-efficiency.

Beyond just collapse load, the author also interested in how different material properties—like yield strength and hardening behaviors—affected the performance of these connections. He found that stronger materials offered some extra resilience against collapse but only up to a point. Once stress reached a certain threshold, even high-strength steels showed similar failure patterns. This finding emphasized that while stronger materials help, the overall design and geometry of the connection are equally crucial in handling stress. Simply using tougher materials doesn’t fully make up for the lack of internal supports in non-diaphragm designs, suggesting that thoughtful design adjustments will be necessary to improve load distribution and ensure long-term stability.

Professor Yoshiharu Sato’s study is significant because it could fundamentally change how engineers design non-diaphragm connections in CHS columns, particularly when these columns face complex, biaxial loads. As cities grow and infrastructure demands increase, the need for connections that are not only reliable and strong but also cost-effective has become essential. This study addresses a crucial gap in structural design by giving engineers valuable insights into how non-diaphragm CHS connections behave under real-life load conditions. Unlike traditional diaphragm-stiffened connections, these non-diaphragm connections are simpler and more economical to construct. By showing how these connections hold up in high-stress applications, the study opens new doors for using CHS columns more widely in load-bearing structures. One of the most practical outcomes of this research is the set of prediction equations it provides for determining collapse loads in non-diaphragm CHS connections. These equations give engineers a reliable way to estimate load capacities without needing to add extra stiffening elements, making it easier to design connections that balance simplicity with safety. This is especially useful in large-scale projects, like high-rise buildings, where reducing material and labor costs while maintaining safety is a top priority. The equations also make it easier for engineers to calculate load limits quickly, saving time and resources in ensuring these connections can handle expected loads. Another key takeaway from this study is that while material strength does improve resilience, the geometry and layout of the connection are just as important. This insight suggests that an effective design might not only depend on stronger materials but also on optimizing shapes and finding the best ways to distribute loads. By encouraging engineers to focus not only on materials but also on design configuration, the study promotes a more balanced approach that combines durability with efficiency. Looking to the future, this research could lay the foundation for updating building codes and standards for non-diaphragm CHS connections. Right now, these connections aren’t always fully covered in design guidelines because there hasn’t been enough reliable information on their load-bearing behavior. With this study’s validated models and data, it’s now possible to safely expand their use, giving engineers more flexibility and room for innovation in structural design. For complex projects where traditional connections may be too expensive or challenging to implement, non-diaphragm CHS connections could become a viable alternative, backed by strong data on their collapse points and load limits.