Example 12: Construction of interaction diagram for a built-up steel section

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Data for Cross Section Analysis & Design application

In this example we will construct the interaction diagram of the built-up section created in Example 11.

Interaction diagram data
Case 1: Moment about Y axis vs. Axial force
Case 2: Moment about Y axis vs. Moment about Z axis for axial force values -750 kN (compression) and 900 kN (tension)
Solution with Cross Section Analysis & Design

Opening a file from disk

First of all we click on the File menu and select Open in order to open the file we created in Example 11.

Unlock the model

Afterwards, if the model is locked, we click on Edit -> Unlock model, in order to modify the geometry of the cross section.

Unlocking the model to enable editing

In the popup window, we choose Yes to unlock the model.

Unlock model confirmation

Review of Analysis Parameters

In order to review the Analysis Parameters, we click on the Analysis -> Analysis Parameters menu item. The reinforced concrete related parameters are hidden, since no concrete regulation has been selected. We can select a material from the list on the left and modify the existing preferences.

Analysis parameters to be used

We do not change anything and click OK.

Definition of load cases

We will use the advanced interaction analysis in this example. The other option for interaction analysis (as per selected R/C code) is available only when a reinforced concrete code has been specified.

The load cases can be selected by clicking on the Analysis -> Interaction (advanced) -> Load cases menu item.

Case 1

We click on the Add new load case button button to add a new load case. We change the name to “Case 1” and select “Axial force - Moment about y” option. The assigned Analysis Parameters set should be “ULS”.

Definition of a new load case for advanced interaction analysis in terms of Axial force vs. Moment about Y

Case 2

We click on the Add new load case button button to add the next load case and change the name to “Case 2”. We make sure that the option “Moment about y – Moment about z” is selected and the Analysis Parameters set is “ULS”. Then we click on “Options” to specify the axial force levels for the interaction diagram.

Specifying a compressive axial force of -750 kN for the interaction load case 2

On the above form, we select “Alternate Axial Force” in the “Variations” box on the left. Next, we enter at the top right corner the values -750 and click on “Add” button. We repeat this step for the value 900.

Two axial force levels for the interaction analysis have been specified

In this way we request the interaction diagram to be produced for both specified axial force levels. We click OK to close the form.

The 2 load cases are now defined.

Load cases for advanced interaction analysis have been defined

Carry out the analysis

We just click Analysis -> Interaction (advanced) -> Analyze, to perform the analysis.


The requested interaction diagrams can be obtained by clicking click Analysis -> Interaction (advanced) -> Show results.

Interaction diagram for Case 1:

We choose “Case 1” from the list at the top right corner of the form. The interaction diagram for Moment about Y vs. Axial force is shown.

Interaction curve in terms of Axial force vs. Moment about Y axis for load case 1

Interaction diagram for Case 2:

By choosing “Case 2”, the corresponding interaction diagrams in terms of Moment about Y axis vs. Moment about Z are shown.

Interaction diagrams for each predefined axial force value in load case 2

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