How to load submodel from a hyperstatic structure in IDEA StatiCa Detail (2D)

Your linear global model says you need more reinforcement over the support — but the nonlinear submodel only carries 78% of the applied load. Here's the physics behind that gap. 👇 In this session I take a continuous two-span beam (fixed hinge in the middle, sliding hinges at the ends, 68 kN/m uniform load) and show what happens when you feed LINEAR global internal forces into a nonlinear submodel in IDEA StatiCa 2D Detail. From the linear analysis: 🔹 Peak shear force: 414 kN 🔹 Field bending moment: 454 kN·m 🔹 Bending moment above the middle support: 808 kN·m (negative) 🔹 Support reaction: 828 kN Apply those linear forces to the submodel and 2D Detail transfers only 78% of the load — failing on the reinforcement bars, with the concrete in compression close behind. The full nonlinear model, on the other hand, carries everything. Why? ➡️ Redistribution. As the section above the support cracks, its stiffness (E·I) drops, the negative moment sheds toward the spans, and the system finds a new equilibrium. Using a redistribution tool I built for this, dropping the mid-zone stiffness (K ≈ 0.3, i.e. ~70% reduction) brings the reaction down to ~792 kN and the support moment down to 561 kN·m — about a 30% reduction. Re-run the submodel with 561 kN·m and the equilibrium shear, and it passes — with trimmed-end moments around 216 kN·m in tension, just like the complete model. KEY TAKEAWAYS 1️⃣ Always preserve static equilibrium — internal forces AND applied loads must be carried to the trimmed ends (same rule as for determinate structures). 2️⃣ Linear models OVERESTIMATE the negative moment over the middle support → submodels loaded this way are overloaded and push you toward unnecessarily heavy reinforcement there. 3️⃣ The redistribution is driven by stiffness loss from cracking and depends on the section stiffness and reinforcement ratio. 4️⃣ The flip side is dangerous: linear models can UNDERESTIMATE the span moment, since the diagram shifts down toward the field — a real design risk if ignored. 5️⃣ Linear global forces don't represent the real (cracked) state of a hyperstatic structure. RECOMMENDATIONS ✅ For hyperstatic structures, analyze the whole model when you can. ✅ If the model is too large, place the boundary conditions and trimmed ends far from the zone where stiffness reduces — so the redistribution isn't distorted. This is the follow-up to my session on submodels for statically determinate structures — watch that one first if you haven't. ⏭️ Next up: how to apply prestressing in submodels. 📩 I'm a certified IDEA StatiCa trainer — for training or consultation, reach out here or on LinkedIn. CHAPTERS 00:00 Intro 00:18 Linear global model — continuous two-span beam 02:47 Nonlinear model in IDEA StatiCa 2D Detail 04:00 Loading the submodel with linear internal forces 05:46 The result: only 78% of the load transfers 07:13 Linear vs nonlinear reactions & redistribution 09:52 Reducing stiffness (E·I) and shifting the diagram 11:38 The redistribution tool in action 14:36 Applying redistributed forces — submodel passes 16:05 Key takeaways 18:22 Recommendations for hyperstatic structures 18:57 Certified training & contact 19:35 Coming next: prestressing in submodels #StructuralEngineering #IDEAStatiCa #CBFEM #Eurocode #ReinforcedConcrete #FEA #MomentRedistribution #StructuralAnalysis