Singularities in Design of Reinforced Concrete Surfaces
Figure 01 - Exemplary Display of Singularity and Countermeasure
Figure 02 - Activating Averaged Internal Forces in RF-CONCRETE Surfaces
Figure 03 - Design Method in RF-CONCRETE Surfaces
Figure 04 - Convert Nodal Load to Surface Load
Technical Article
Singularities occur in a limited area due to the concentration of the stress-dependent result values. They are conditioned by the FEA methodology. In theory, the stiffness and/or the stress in an infinite size concentrate on an infinitesimal small area.
In reality, singularities or the resulting stress concentrations do not occur to the extent that they occur in the model. Basically, it is not reasonable to evaluate results in the area of singularity locations. However, it is quite reasonable to analyze and question singularity locations because singularity locations can indicate problems in the real model. A practical example in concrete design would be, for example, questioning a punching risk in the area of singularity locations.
In the concrete design in RFEM and RF-CONCRETE, singularities often result in non-designable situations.
Where can singularities occur?
- Point-shaped supports or load introduction
- Reentrant corners or corners of openings
- Stiffness jumps (jump in plate thickness, for example)
- Start and end of ribs
- Start and end of line supports or walls
Detection of singularities
In FEA, singularity locations can be identified by refining the mesh at a corresponding location in the model by means of an FE mesh refinement. If the stress-dependent result value increases in the considered area, but the area in which it acts decreases, it is very likely a singularity location.
Counteract singularities
In RFEM and the reinforced concrete design with RF-CONCRETE, you can counteract the singularities and the associated non-designable situations in different ways.
Smooth region
In RFEM, there are smoothing regions that you can use to either smooth the result peaks or set them to zero. The smooth region can be opened under "Results" in the menu bar. In the case of a smoothing, the underlying area has to be defined in terms of engineering. For the option "Set internal forces to zero", you can, for example, use the cross-section of a connected column as a range (see Figure 01).
Integrated surface
As an alternative to the smooth area with dimensions of the column cross-section, you can model surfaces and integrate them into the existing surface. These surfaces are then excluded in the design in RF-CONCRETE Surfaces (see Figure 01).
Figure 01 - Exemplary Display of Singularity and Countermeasure
The use of smoothed or zero internal forces must be activated in the RF CONCRETE Surfaces in the detail options (see Figure 02).
Figure 02 - Activating Averaged Internal Forces in RF-CONCRETE Surfaces
Both methods (smooth area and integrated surface) can be used for columns as well as for reentrant corners. In general, smooth regions are sufficient. However, the smooth regions do not have the desired effect for the nonlinear calculation because internal forces may be rearranged during the calculation and new singularity effects may occur.
Design method for washers
In a slab design, singularities can occur due to high axial forces, for example due to selective support. In addition, the design method can have an increasing influence on singularity effects or non-designable situations. For slabs, it is therefore recommended to deactivate the optimization of the design internal forces in RF-CONCRETE Surfaces (see Figure 03).
Figure 03 - Design Method in RF-CONCRETE Surfaces
Distributed load introduction
To avoid singularity effects, concentrated loads or line loads can be converted into surface loads. To find the Find function, for example, use the shortcut menu (see Figure 04).
Figure 04 - Convert Nodal Load to Surface Load
Rounding of reentrant corners
In the case of reentrant corners as well as corners at openings, it is possible to round a corner if necessary by using the "Round or Angled 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 regions.
Supports
The avoidance of singularities at nodal and line supports is described in this technical article.
Reference
Keywords
Stress peak Stress concentration Singularity
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Main Program
Structural engineering software for finite element analysis (FEA) of planar and spatial structural systems consisting of plates, walls, shells, members (beams), solids and contact elements
3,160.00 EUR
Add-on Module
Design of reinforced concrete members and surfaces (plates, walls, planar structures, shells)
720.00 EUR
