Top 7 FAQ about the plastic shell model in FEM-Design

Earlier this year, we introduced plastic analysis as a material non-linearity module in FEM-Design 23. By using the plastic shell model in FEM-Design to compute 3D shells of reinforced concrete and structural steel, you get an accurate and realistic structural analysis that offers material savings and increased safety—an outcome that’s challenging to achieve with manual calculations for complex structures.

To help our users better understand what the plastic shell model can do, we’ve compiled a list of the top 7 questions we’ve received. See our answers below. If you need further assistance, don’t hesitate to contact us.

The plastic shell model in FEM-Design: your questions answered (FAQ)

1. When can I apply this model?
This model is applicable for analysing structural elements made from structural steel and reinforced concrete walls and plates. It can be applied to single elements like walls or complex 3D models like shafts, element wall systems, and cast-in-situ structures, as well as all kinds of steel structures and details made out of steel plates.

2. Can it handle out-of plane loading?
Yes, the model can handle out-of-plane loading, making it suitable for assessing the structural integrity of elements subjected to complex loading, including bending loads. Out-of-plane and in-plane combined. For reinforced concrete, the out of plane shear effects are modeled with elastic model but non-linear effect of cracking to shear stiffness is taken into account.

3. Does it capture plastic deformation capacity?
Yes, the model includes failure model. This means that in every load step in the analysis, the failure criterion of rebars, bi-axial failure of concrete, and strain of structural steel are evaluated. If failure is found in finite element, failure is activated and the element loses its load bearing capacity. All this means a very safe analysis, and the user can be sure that failure has not occurred at any step of the analysis. This cannot be accomplished by afterward strain checks in complex load paths changes of concrete structures. Failure models can be adjusted or completely turned off to make the analysis more robust.

4. Can I check a slab with only one layer of reinforcement?
Yes, the model allows for the design of slabs with varying reinforcement configurations, from single to multiple layers. Reinforcements can be single bars or meshes, and anchoring is also taken into account. So, any complex layer arrangement of rebars is allowed. In fact, the model can also analyse slabs without any reinforcement in some FE elements or whole elements without rebars, with some restrictions to the applied material model.

5. Does it handle cracks?
Yes, our model effectively and robustly simulates average cracking effects. The model is the so-called smeared reinforcement model. The model simulates the position and changing direction of cracks to assess their impact on structural behaviour. However, the model does not have discrete cracks, so it does not directly give crack widths due to the analysis. The benefit of this model is that it is very robust and not sensitive to FE mesh, and it also suits big structures. The focus is on accurately predicting where and how cracks might occur and develop and on getting the correct behaviour of a structure. Afterwards, crack widths can be calculated based on section forces with concrete check in SLS.

6. Is this model verified?
Yes, the model has been extensively verified through hand calculations, experimental data, and additional benchmarks. Detailed verification information and examples are available in the plastic shell manual.

7. Is this method supported by Eurocode?
Yes, the model complies with Eurocode standards for plastic and non-linear analysis, ensuring it meets the rigorous requirements for structural safety and performance.