In reality, singularities or the resulting stress concentrations do not occur to the extent that they occur in the model. In principle, result evaluation in the vicinity of singularity locations is not meaningful. However, examining and questioning singularity locations is quite useful, as singularity locations can indicate problems in the real model. A practical example in concrete design would be, for example, questioning the risk of punching shear in the vicinity of singularity locations.
In concrete design in RFEM, singularities often result in not designable situations.
Where can singularities occur?
- Point supports or point load introduction
- Re-entrant corners or at the 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
Recognizing singularities
In FEM, singularity locations can be identified by refining the mesh at the relevant location in the model using FE mesh densification. If the stress-dependent result value in the considered area increases, while the area over which it acts decreases, it is with very high probability a singularity location.
Counteracting singularities
In RFEM 6, singularities and the associated not designable situations can be counteracted in various ways.
Surface Result Adjustments
In RFEM 6, surface result adjustments are available, which can be used to smooth result peaks. The results in the specified area are displayed as average values instead of the actual distribution. The "Surface Result Adjustments" can be accessed under "Special Objects" in the navigator. For a surface result adjustment, the underlying area must be defined using engineering judgment.
Integrated Surface
As an alternative to surface result adjustments with dimensions corresponding to the column cross-section, surfaces can be modeled and integrated into the existing surface. These surfaces must then be excluded from design.
Distributed Load Application
To avoid singularity effects, point loads or line loads can be converted into surface loads. A possible approach using "Free Rectangular Loads" is shown below. In the "Edit Global Parameters" menu, t stands for the thickness of the concrete slab, F is the nodal load, and phi is the load distribution angle
The two methods listed (surface result adjustments and integrated surface) can be applied to both columns and re-entrant corners. In general, surface result adjustments are sufficient. However, for nonlinear analysis, surface result adjustments do not have the desired effect, since internal forces may redistribute during the calculation and new singularity effects may arise.
Design Method for 2D Members
In the design of 2D members, singularities can occur due to high normal forces, for example as a result of point support. In addition, the design method can have an amplifying effect on singularity effects or not designable situations. For 2D members, it is therefore recommended to deactivate the optimization of design internal forces (in the navigator) under: "Concrete Design", "Design Configurations", "Surfaces".
Avoid Nodal Loads and Line Loads
To avoid singularity effects, it is recommended to use surface loads instead of nodal or line loads.
Rounding Re-entrant Corners
Both in the case of re-entrant corners and corners at openings, a corner can be rounded or chamfered if required using the "Round or Chamfer Corner" function. Both functions can be accessed via "Tools" "Modify Lines" in the menu bar. In general, however, many singularity effects can be sufficiently counteracted by smoothing regions.
Supports
The avoidance of singularities at nodal and line supports is explained in this technical article: