In reality, singularities, or the resulting stress concentrations, do not occur to the same extent as they do in the model. In principle, evaluating results in the vicinity of singular points is not meaningful. However, investigating and questioning singular points is quite useful, as singular points can indicate problems in the real model. A practical example in concrete design would be questioning the risk of punching shear in the vicinity of singular points.
In concrete design in RFEM and RF-BETON, singularities often result in design failures.
Where can singularities occur?
- Point supports or load application
- Re-entrant corners or corners of openings
- Changes in stiffness (for example, a step in slab thickness)
- Start and end of ribs
- Start and end of line supports or walls
Identifying singularities
In FEM, singular points can be identified by refining the mesh at a corresponding location in the model using FE mesh refinement. If the stress-dependent result value in the area under consideration increases, but the area in which it acts decreases, then it is highly likely to be a singular point.
Counteracting singularities
In RFEM and reinforced concrete design with RF-BETON, singularities and the associated design failures can be counteracted in various ways.
Smoothing area
In RFEM, smoothing areas are available, which can be used to either smooth result peaks or set them to zero. The smoothing area can be accessed under "Results" in the menu bar. For smoothing, the underlying area must be defined by engineering judgment. With the option "Set internal forces to zero", for example, the cross-section of an adjacent column can be used as the basis for the area (see Figure 01).
Integrated area
As an alternative to a smoothing area based on the dimensions of the column cross-section, areas can be modeled and integrated into the existing area. These areas are then excluded from design in RF-BETON Surfaces (see Figure 01).
The use of the smoothed or zeroed internal forces must be activated in RF BETON Surfaces in the detailed options (see Figure 02).
The two methods described (smoothing area and integrated area) can be applied to both columns and re-entrant corners. In general, smoothing areas are sufficient. However, for nonlinear analysis, smoothing areas do not have the desired effect, as internal forces are redistributed during the calculation and renewed singularity effects can occur.
Design method for walls
In wall design, singularities can occur due to high normal forces, for example as a result of point supports. In addition, the design method can have an amplifying effect on singularity effects or design failures. For walls, it is therefore recommended to deactivate the optimization of design internal forces in RF-BETON Surfaces (see Figure 03).
Distributed load application
To avoid singularity effects, point loads or line loads can be converted into area loads. The function can be accessed, for example, via the context menu (see Figure 04).
Rounding off re-entrant corners
For both re-entrant corners and corners at openings, a corner can, if necessary, be rounded off using the "Round or chamfer corner" function. The function can be accessed via "Tools" in the menu bar. In general, however, many singularity effects can be sufficiently counteracted by smoothing areas.
Supports
The avoidance of singularities at node and line supports is explained in this technical article: