The biggest challenge in the design of building structures when using finite element method (FEM) software is the representation of the object’s actual function.
The designer’s task is more difficult because they have to select the finite element analysis (FEA) mesh in a way that the results are not underestimated or overestimated. Peaks at critical points in the structure are also dangerous (e.g. above the supports in the column – slab superstructures).
Another challenge is mapping the connections at the intersections of the components, as well as determining the appropriate elasticity of such a connection. Often this issue is underestimated; however, in some cases it can cause disturbances in the cross-sectional forces.
In this article, I demonstrate how such problems can be solved using GRAITEC Advance Design software. In fact, it is very easy with this FEM structural analysis solution dedicated to structural engineers operating in a BIM environment.
- Creating a Link at Node
Imagine a situation where we have a simple structure consisting of several primary and secondary beams. These are continuous elements.
In FEA software, we can only specify the release of linear elements at their ends.
How then can we account for the hinged support on the central primary beam? How do we represent the actual working of the structure, where the secondary beam will not transmit a bending moment into the torsion of the primary beam?
The user could divide the secondary beams into two elements and add releases at their ends. However, this is not a good approach, as the static scheme will change. The beams will then operate as single-span beams.
We are interested in making the beams work as continuous, two-span beams.
Advance Design has a dedicated function for such cases: Create a link at node.
Once the function has been selected, the user must indicate where the node link will be created. This can be done by inputting a point or by selecting primary and secondary elements.
After inputting the node link, the user sees a ‘point’ at the intersection of the elements and then has to set relations to it.
An asymmetrical load is included in the structure shown to better illustrate the problem.
Now the “My bending moment” on the secondary beams looks like this:
Where the Node link has been imposed, the graph is ‘smooth’. Where it is not, there is a bending moment dodge.
Node links are also often used in roof trusses.
2. Column – Slab Structure
FEM software for civil engineering is characterized by the fact that the elements have no dimension. We imagine the cross-section of a linear element as follows:
In FEA, by contrast, such elements are treated as a single line (‘pin’). Therefore, they do not have a dimension.
If such a linear element is connected to a planar element, this will only occur at one node and not in relation to the entire column dimension.
This will result in a concentration of forces at one node and the occurrence of a characteristic peak at that node.
Advance Design has a solution to this problem. This is the DOF – Constraint function, which allows the column’s dimensions to be mapped and the peak to be eliminated.
Once this function has been selected, the user must indicate the primary node (the connection between the column and the slab), as well as the secondary nodes (the outline of the column’s dimension). It is a good idea to use the DXF underlay at this point.
The primary node is marked in red and the secondary node in green.
Advance Design has gone one step further. Now you can create such connections automatically!
Simply go to the software options and select the setting shown below.
DOF – Constraint will form automatically after the calculation.
Remember that the master-slave connection has its restraints and can also be used for other purposes.
Contact us today and learn how you can manage and operate projects better, safer and in a more cost-effective way with GRAITEC Advance Design.