RF-/STEEL EC3 - Online Manual Version 5/8

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RF-/STEEL EC3 - Online Manual Version 5/8

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2.7 Nodal Supports - Sets of Members

This window is displayed if at least one set of members is selected for design in the 1.1 General Data window and the stability analysis is performed by the general method according to [1] clause 6.3.4 (default setting).

If the Equivalent Member Method for sets of members is selected in the Details dialog box (see Figure 3.2), Window 1.7 is not displayed. In this case, lateral intermediate supports can be defined in Window 1.4 by using division points.

Figure 2.27 Window 1.7 Nodal Supports - Set of Members

The current table manages the boundary conditions of the set of members that is selected on the left in the navigator!

The supports defined in RFEM or RSTAB (for example, in Z for a continuous beam) are not relevant in this window: The distributions of moments and shear forces for the determination of the amplification factor are automatically imported from RFEM/RSTAB. Here, you define the support conditions affecting the stability failure (buckling, lateral-torsional buckling).

Supports on the start and end nodes of the set of members are preset. Any other supports, for example due to connected members, must be added manually. Use the  button to select nodes graphically in the RFEM/RSTAB work window.

According to [1] clause 6.3.4 (1), it is possible to design singly symmetric cross-sections that are stressed solely in their principal plane. For this analysis method, it is necessary to know the amplification factor αcr,op of the entire set of members. In order to determine this factor, the program creates a planar framework with four degrees of freedom for each node.

The orientation of the axes in the set of members is important for the nodal support definition. The program checks the position of the nodes and internally defines the axes of the nodal supports for Window 1.7 according to Figure 2.28 to Figure 2.31. The [Local Coordinate System] button below the model graphic can help you with the orientation: Use it to display the set of members in a partial view where the axes are clearly visible. You can find an example in the Knowledge Base on our website.

Figure 2.28 Auxiliary coordinate system for nodal supports - straight set of members

If all members of a set of members rest on a straight line, as shown in Figure 2.28, the local coordinate system of the first member in the set of members corresponds to the equivalent coordinate system of the entire set of members.

Figure 2.29 Auxiliary coordinate system for nodal supports - set of members in vertical plane

If the members of a set of members do not rest on a straight line, they still have to be located in the same plane. In Figure 2.29, the members rest in a vertical plane. In this case, the X'-axis is horizontal and oriented in the direction of the plane. The Y'-axis is horizontal as well and defined perpendicular to the X'-axis. The Z'-axis is oriented perpendicular downwards.

Figure 2.30 Auxiliary coordinate system for nodal supports - set of members in horizontal plane

If the members of a buckled set of members rest in a horizontal plane, the X'-axis is defined parallel to the X-axis of the global coordinate system. Thus, the Y'-axis is oriented in the opposite direction to the global Z-axis, and the Z'-axis is directed parallel to the global Y-axis.

Figure 2.31 Auxiliary coordinate system for nodal supports - set of members in inclined plane

Figure 2.31 shows the general case of a buckled set of members: The members do not rest on a straight line, but in an inclined plane. The definition of the X'-axis results from the intersection line between the inclined and the horizontal plane. Thus, the Y'-axis is perpendicular to the X'-axis and in vertical position to the inclined plane. The Z'-axis is defined perpendicular to the X'-axis and Y'-axis.

The buttons below the graphic have the following functions:

Table 2.4 Buttons for cross-section graphic

Shows model or system sketch

Shows members as 3D rendering or wire-frame model

Shows current set of members or entire model

Displays irrelevant members of model as transparent or opaque

Shows set of members with local coordinate system or entire model

Shows view in direction of X-axis

Shows view in opposite direction of Y-axis

Shows view in direction of Z-axis

Sets isometric view

With the [Edit warp stiffener] button it is possible to determine the constant of a warp spring by the program.

Figure 2.32 Dialog box Edit Warp Spring

The following warp stiffening types are available in the Edit Warp Spring dialog box:

  • End plate
  • Channel section
  • Angle
  • Connecting column
  • Cantilevered portion

Materials and cross-sections can be selected by using the lists and [Library] buttons. With the  button you can select them also graphically in the RFEM/RSTAB model.

Based on the parameters, RF-/STEEL EC3 determines the Resulting warp spring Cω which can then be imported with [OK] in Window 1.7.

Warping analysis with seven degrees of freedom

To analyze sets of members according to the second-order analysis for flexural-torsional buckling with warping torsion, select the corresponding check box in the Warping Torsion tab of the Details dialog box (see Figure 3.9). The table titles of Window 1.7 will be adjusted accordingly.

The warping analysis requires a license of the RF-/STEEL Warping Torsion module extension.

Figure 2.33 Window 1.7 Nodal Supports - Set of Members for warping analysis with seven degrees of freedom

Here, you define the support conditions of the set of members singled out from the system, which are available on the nodes of the members involved. Nodal supports defined in RFEM or RSTAB are preset, as well as supports on both ends of the set of members.

Lateral supports of the set of members must be added in the form of additional supports. In this way, you can represent, for example, a purlin's effect given in the spatial model of RFEM or RSTAB. If this support is missing in the model of the singled out set of members, instabilities are possible.

With the button, the supported nodes can be selected graphically in the RFEM/RSTAB work window.

In columns B to N, you specify the support conditions of the selected nodes. To activate or deactivate the supports and restraints for the corresponding degrees of freedom, click in the check boxes. Alternatively, you can enter the constants of the translational and rotational springs manually.

The Support rotation and Eccentricity parameters allow for modeling support conditions close to reality.

You can find an example describing the warping torsion analysis of a tapered single-span beam in an article of our Knowledge Base on our website.

Literatur
[1] Eurocode 3: Design of steel structures - Part 1‑1: General rules and rules for buildings; EN 1993‑1‑1:2010‑12