A frame truss is loaded by a uniform load of 17.5 kN/m (on the top flange). You can specify the load application point in Details ("top flange, destabilizing effect" by default).
The boundary conditions of the separated substructure are defined in Window 1.6 of the RF‑/STEEL EC3 add‑on module. You can then start the calculation.
In this model, the critical load factor αcr of 2.376 results (see the image). You can display the mode shape graphically in a new window for checking and evaluation purposes. The governing stability problem comprises the lateral-torsional buckling of the frame truss.
In the second post, the mode shape and the critical load factor will be calculated on a member model using the RF‑/FE‑LTB add‑on module.
In the third and final post, the mode shapes and the critical load factors determined previously will be evaluated using an FE model (surface elements) and the RF‑STABILITY add‑on module.