Limit Values for Stability Analysis in RF-/STEEL EC3

Technical Article

As of the program version X.11, the filter options of small compression forces or moments for stability analysis in RF‑/STEEL EC3 have been revised. The revision of these filter options in the “Stability” tab of the “Details” dialog box allows you to work in the module transparently since they are now independent of the design.

The filter options should be used to permit for design in the case of design problems due to a non‑regulated stability analysis of certain cross‑sections and load situations and also to neglect the problematic internal forces by using freely definable limits. The limit values can be defined regardless of the default setting in the dialog box shown in Figure 01.

Figure 01 - Defining "Limit Values for Stability Analysis"

Application and Example

In the following, two simple examples are provided to explain the application. There are two single‑span beams with a length of five meters each. Both are made of the structural steel used S235. The left beam is the angle section L 100 x 100 x 10, the right one is the channel section UPE 300. The load is shown in Figure 02.

Figure 02 - System and Loading

If you try to perform the design directly in RF-/STEEL EC3, using the German National Annex [2] to Eurocode 3, both beams are considered as non-designable.

Figure 03 - Result of Unsuccessful Design Attempt

For both the L‑section and the channel section, no interaction according to 6.3.3 of EN 1993 [1] is possible since the cross‑sections must be double symmetric. Similarly, the design according to 6.3.4 of EN 1993 is also not possible since only I‑sections are permitted here. It would be possible to perform the design according to the second‑order analysis in RF‑/STEEL Warping Torsion. However, the small bending moment on the L‑section and the small compression axial force on the channel section are to be neglected in this example.

In this case, the problem is that by neglecting the axial force on the channel section, for example with a limit of 10%, the axial force on the L‑section would be excluded, which must be used on the L‑section for the buckling design. The other way around, by neglecting the bending moment on the L‑section, the bending moment on the channel section may also be neglected. In this example, the correct procedure is to create two module cases and design the members separately.

Thus, two cases are considered in RF‑/STEEL EC3:

  • Design of the L‑section and neglecting the bending moment
  • Design of the channel section and neglecting the axial force

For Case 1, the limit values for neglecting the moment are increased to 0.100. The buckling design is now performed.

Figure 04 - Limit Values for Case 1 and Successful Buckling Design

For Case 2, the limit values for neglecting the axial force are increased to 0.100. The lateral-torsional buckling design is now performed.

Figure 05 - Limit Values for Case 2 and Successful Lateral-Torsional Buckling Design

Reference

[1]   Eurocode 3: Design of steel structures - Part 1‑1: General rules and rules for buildings; EN 1993‑1‑1:2010‑12
[2]  National Annex - Nationally determined parameters - Eurocode 3: Design of steel structures - Part 1‑1: General rules and rules for buildings; DIN EN 1993‑1‑1/NA:2015‑08

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