FAQ 002403 EN

Helpful Questions & Answers

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In RF-CONCRETE Surfaces, I get non-designable situations at some locations. Note 5 is displayed: "Allowable compressive stress of concrete in strut direction exceeded." 


RF-CONCRETE Surfaces displays failed design at some locations in Window 2.1 because the concrete compression stress is too high there (see Figure 01). In this window, you can also use the [Info] button to check the design details for a result row selected in the table above. Based on the values shown in a new window, it is obvious that the membrane force resistance is smaller than the design membrane force in the strut direction (see Figure 02).

As a result, the program displays a corresponding message "not designable" on all finite elements with too great concrete compression stresses.

The theoretical background to this design can be found in the manual of RF‑CONCRETE Surfaces, among others. You can download it using the link under Reference or open it in the module by pressing the [F1] key.

In order to prevent non-designable situations, you can follow this procedure:

  • The failed design may be caused by a singularities.
  • If there is no pure slab, the optimization of internal forces may be the cause. This should be deactivated in the case of shells and slabs (Figure 03).
  • It may be helpful to add a third reinforcement direction (Figure 04).
  • The concrete quality can be increased.
  • The surface thickness can be increased.


note 5 concrete strut non-designable failed design concrete compression stress


[1]   Eurocode 2: Design of concrete structures
[2]   Manual RF-CONCRETE Surfaces. (2017). Tiefenbach: Dlubal Software.

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  • Updated 5 April 2021

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