June 17, 2025

Understanding Plate and Member Interaction in RISA

When modeling plate elements alongside member elements in RISA, users may notice that member forces—such as bending moments or axial loads—are lower than expected. This behavior is often the result of how loads are shared between plates and members based on their relative stiffness. If the distribution of forces doesn't align with your design expectations, it may be due to unintended interaction between these elements. Accurately capturing structural behavior requires not only the right modeling approach, but also a clear understanding of how plates and members interact within the analysis. In this article, we’ll explore common scenarios where this issue can arise, explain why it happens, and provide practical tips and workarounds to ensure your results match your design intent. Why Model Plate Elements in the First Place? Before diving into the interaction between elements, it’s worth asking: Do I need to model plate elements at all? In many cases, member design and stability can be accurately represented using simplified area or line loads—and plates might not be necessary. However, there are valid reasons to model plates, including: The model is unstable and needs plate elements for diaphragm action. Plates are part of the lateral load path. You want a…

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