I’m trying to model a composite slab in FEM Design, but it seems the option is not available, or I may be missing something.
One approach I’m considering is to model the composite section in Grasshopper, extract the equivalent stiffness properties, specifically the membrane stiffness matrix, flexural stiffness matrix, and shear stiffness matrix, and then define a fictitious shell in FEM Design using those stiffness values.
Alternatively, if there is a direct way to define a composite slab in GH and then transfer it in FEM Design and run the analysis without manual stiffness input, that would obviously be preferable.
I’m looking for ideas or references if anyone has already tackled this workflow and can share their experience/ idea how to proceed with and the modelling.
Hi Ravi,
I think you’ve already been in contact with my colleague in the support and selected a modelling approach, but since the question was posted here as well, this summary may be useful for others exploring similar workflows.
There are several possible workflows for modelling a composite slab in FEM‑Design, depending on how much detail is required and which parts of the behaviour you want to capture. Since the composite slab object is not available as a native element, the options generally fall into two categories: modelling the components explicitly in 3D, or deriving equivalent stiffness properties and applying them to a shell element.
1. Using Section Editor Section Editor is suitable for calculating cross‑sectional properties for both concrete and steel geometries. If the contribution of the profiled steel sheeting needs to be included with higher accuracy, the steel profile can be modelled directly in Section Editor. The resulting section properties may then be used either to define a custom section or to support stiffness factor calculations for a standard shell.
2. Full 3D modelling of concrete and steel
Another option is to model the concrete slab and the steel sheeting as separate physical parts in a 3D Structure model. These can then be connected using a Surface Connection. This allows the composite action to be represented through the interaction definition, while maintaining explicit geometry for both materials.
3. Equivalent stiffness and fictitious shell
If a simplified model is sufficient, the stiffness results from the Section Editor can be converted into an equivalent orthotropic shell. These values can be assigned directly to a Fictitious Shell element, representing the composite slab with a single object. Or to simply approximate the composite action by adjusting the Stiffness modifier factors of a regular shell. This approach is useful when the primary focus is global stiffness, dynamic behaviour, or footfall response, and when full geometric detail is unnecessary.
4. Grasshopper workflow
It is also possible to set up a more parametric workflow if needed. Some users generate slab geometry or stiffness parameters in Grasshopper and then send simplified shell definitions to FEM-Design through the API. The API supports assigning stiffness modification factors for membrane, bending, and shear behavior, which already covers many practical use cases. More advanced approaches may be possible depending on the required level of detail, but the best method depends on the specific project setup. We are happy to discuss suitable workflows if you want to explore this direction further.
