Questo articolo ti mostrerà un esempio pratico di come eseguire un'analisi di instabilità flesso-torsionale per il capannone in acciaio mostrato nell'immagine 1. You also can see this article as a continuation of the article titled Determinazione dei coefficienti di carico critici utilizzando l'add-on Stabilità della struttura in RFEM 6 e RSTAB 9 , which shows how to use the Structure Stability add-on to determine critical load factors and the corresponding stability modes for this 3D model.
Tuttavia, il suddetto articolo ha osservato che la considerazione della torsione di ingobbamento nell'analisi di stabilità della struttura è necessaria a causa dei risultati dell'analisi statica (cioè, i carichi applicati danno luogo a momenti flettenti My, quindi un problema di instabilità da torsione laterale del principale è prevista anche la trave).
As a result, the following text will show you how to use the Torsion Warping (7 DOF) add-on in combination with the Structure Stability add-on to consider cross-section warping as an additional degree of freedom when performing the stability analysis. It is important to know that the calculation is on the entire model, so the stiffness of the adjacent members or defined support conditions is considered automatically.
Both the Torsional Warping and Structure Stability add-ons can be found in the Base Data, as shown in Image 2. You must know that in RFEM 6 and the Torsional Warping add-on, warping is associated with members only; therefore, no global support conditions have to be defined for it.
Furthermore, the warping at the member ends is assumed to be unobstructed by default; hence, you should use Member Transverse Stiffeners to define warp springs at the member ends and consider the warping stiffness or warping stress. You can read more about this in Knowledge Base article Verifica di instabilità flesso-torsionale con il nuovo add-on Torsione di ingobbamento (7 gradi di libertà) per RFEM 6/RSTAB 9 .
Since the warping is assumed to be unhindered at the member end, there is no transfer of the momentum between adjacent members. In other words, all members are considered individually for the warping calculation (that is, individual members can warp freely at their ends). To transfer the warping between individual members that are connected, you can define a set of members. In this regard, the main beams of the steel hall of interest are defined as separate sets of members, as shown in Image 3.
At this point, a parallel can be made between the load application point when calculating members with 6 degrees of freedom, and calculating members with 7 degrees of freedom. Namely, if other objects are connected to a member to be calculated with 6 degrees of freedom, shear forces from other components are introduced at the shear center.
When calculating members with 7 degrees of freedom, however, the connection point is considered to be in the center of gravity (that is, the centroid of the cross-section), and the defined member loads are also applied there. To address this issue in the analysis, you can define member eccentricities or use rigid members to define such connections. In this regard, the application point of the load in this example will be defined with the eccentricity shown in Images 4 and 5.
Next, the stability analysis settings can be defined in the same way as explained in Knowledge Base article Determinazione dei coefficienti di carico critici utilizzando l'add-on Stabilità della struttura in RFEM 6 e RSTAB 9 . Namely, you can select the analysis method and consider the other options shown in Image 6.
As discussed in the aforementioned article, the stability analysis can be considered in terms of load cases, load combinations, and design situations. In this example, the stability analysis is considered in terms of the ultimate design situation, as shown in Images 7 and 8. Hence, you can calculate this design situation and obtain results in the same way as discussed in the aforementioned article.
Infine, è possibile eseguire il calcolo e ottenere i risultati dell'analisi di stabilità tenendo conto della torsione di ingobbamento della sezione trasversale. In the Overview of the Static Analysis table, you can see the most critical load factor of all the load combinations (Image 9), together with the governing load combination with which the critical load factor is associated. This way, you can open the stability analysis results for the specific load combination and display the associated mode shape.
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