Advancing Hybrid I-Girder Design: Enhancing Performance Through Refined Cross-Section Analysis and Optimized Standards

Hybrid steel I-girders are changing the game in modern structural engineering because they offer a balance in material efficiency and performance under bending loads. By placing high-strength steel in the flanges—where most of the stress from bending occurs—and using lower-strength steel in the web, engineers can save both weight and cost without compromising on strength. This approach fits perfectly with the growing push for sustainable and resource-efficient infrastructure, particularly in massive projects like bridge construction. Yet, for all their promise, hybrid I-girders are not fully understood, especially when it comes to how they behave under flexural loads. The design standards we currently rely on still have some gaps and inconsistencies that need addressing. One major challenge is the shortcomings in existing design codes like the American AASHTO specifications and the European Eurocode 3. These guidelines include some specific rules for hybrid girders, but they fall short when it comes to handling high-strength steels, like S690 used in compression flanges. For instance, the slenderness limits for what are classified as “compact sections” often fail to ensure the required ductility and rotation capacity, which are critical for plastic design. On top of that, there are inconsistencies between the two standards, such as differences in the web-to-flange yield strength ratios they allow. This lack of harmony in the codes causes confusion and makes it harder to adopt hybrid I-girders effectively. Another sticking point is the limited experimental data available on how hybrid I-girders actually perform under uniform bending. Although some past studies have looked into local buckling and moment resistance, many of them rely heavily on numerical simulations rather than real-world physical testing. This leaves a gap when it comes to validating the structural performance of these girders with hard data.

To this account, a new research study, published in Thin-Walled Structures, investigated these challenges. Led by Dr. Shuxian Chen and Professor Tak-Ming Chan from The University of Hong Kong, and Associate Professor Jun-Zhi Liu from Beijing Normal University, investigated how hybrid I-girders behave under uniform bending and proposes practical improvements to current design methods. The research team tested girders made with S690-grade steel flanges and webs of varying strengths—Q690, Q460, and Q355. To replicate real-world conditions, they used a four-point bending test, which ensured uniform bending moments across the girders. They closely monitored the moment when the flanges first started to yield and compared it with the point of failure or local buckling. While the strong flanges held up under greater loads, the lower-strength webs played a significant role in determining how the girder would fail and how ductile it could be. Alongside the physical tests, the authors built finite element models using ABAQUS software to simulate girder behavior which carefully tuned to match the experimental data, incorporating realistic factors like geometric imperfections and residual stress. The authors reported impressive accuracy of these models and allowed the team to predict ultimate moment resistance and buckling behavior with a high degree of confidence. With this tool in hand, they ran simulations on 544 different girder configurations, tweaking parameters like flange thickness, web thickness, and plate slenderness ratios. They found that while high-strength flanges boosted bending resistance, high slenderness in either the flange or the web could cause early local buckling, which limited how much the girders could rotate before failure.

One surprising discovery came when they examined girders classified under Eurocode 3 as Class 1 sections, which are supposed to have enough ductility for plastic design. Even though these girders technically met the slenderness requirements, they often fell short of the necessary rotational capacity. The issue was especially obvious in girders with S690-grade flanges, where the limited strain-hardening ability of the steel made matters worse. On the flip side, AASHTO’s stricter rules for flange designs still allowed some overly lenient cases. In response, the team suggested tightening Eurocode 3’s limits by introducing a reduction factor to ensure S690 girders would meet ductility standards. The researchers also looked into the effects of residual stresses which come from thermal cutting and welding during fabrication. These stresses had a noticeable impact on buckling behavior, especially in girders with slender webs. Interestingly, they found that tensile residual stresses at the flange tips—common with thermal-cut edges—could slow down buckling and lead to smoother strength reductions. This showed how the manufacturing process itself could influence performance, reinforcing the need to consider residual stresses when designing hybrid girders. Their findings also revealed weaknesses in existing design methods. For instance, By adjusting the DSM to better reflect flange-web interaction of hybrid I-girders, the team developed a revised formula that proved far more accurate when tested against both their experiments and simulations. Moreover, the team studied the Continuous Strength Method (CSM), which takes material strain-hardening into account. The multi-linear material models they used allowed them to simulate how the girders transitioned from elastic behavior to strain hardening. Through a rigorous analytical analysis, the CSM design expressions for the cross-section resistance of hybrid I-girders were derived.

In conclusion, this new study brings a fresh perspective to the design and performance of hybrid I-girders, offering some exciting advancements in structural engineering. By taking a closer look at how the flanges and webs—made from different grades of steel—work together, the researchers uncovered some major gaps in current design codes like AASHTO and Eurocode 3. They did not just stop there, though. They came up with practical fixes to address these issues, which is a big step forward, especially as infrastructure projects increasingly demand materials that are both efficient and safe. One of the most important takeaways from the research is how it improves width-to-thickness ratios for high-strength girders. The existing design rules for Class 1 sections, it turns out, do not always ensure the flexibility and rotation capacity needed for plastic design. The team introduced a reduction factor that makes sure hybrid girders using S690-grade steel flanges have the right ductility. This is crucial for big structures like bridges that deal with heavy or repeated loads—where flexibility can mean the difference between success and failure. The authors also introduced new tools for engineers: DSM and CSM which make it easier to predict how hybrid girders will perform under bending, while also factoring in the complex relationship between flanges and webs. They cut down on the guesswork and improve accuracy, making designs both safer and more cost-effective. These approaches could soon replace the outdated calculations that have been in use for years. Another fascinating discovery involved the impact of residual stresses, which are created during manufacturing processes like welding or cutting. The researchers found that these stresses, particularly from thermal cutting, can affect how the girders handle buckling. For example, tensile stresses at the flange tips can actually delay buckling, helping the structure stay stronger for longer. This finding shows how important it is to consider how girders are made, not just what they are made of. The study also calls attention to some of the limits of using high-strength steels like S690. While these materials are incredibly strong, they do not always perform well in designs that require significant deformation and with fine-tuning slenderness limits and improving material models, the researchers successfully offer a practical guide to use these materials without risking failure.