Sztywność początkowa stanowi nachylenie krzywej obciążenie-przemieszczenie dla połączenia, wskazujące na odporność połączenia na odkształcenia pod wpływem niewielkich obciążeń. This parameter is essential for understanding force distribution within a structure, its behavior under service loads, and the stability of the overall system. Hence, initial stiffness analysis in steel joints is one of the crucial topics in structural engineering, primarily for understanding the behavior of connections under load.
Many design standards, including Eurocode 3 (EN 1993-1-8) and AISC, outline guidelines for analyzing the initial stiffness of steel joints. Structural analysis tools frequently incorporate options for joint stiffness modeling as well. Dlubal’s programs RFEM and RSTAB are among the leading solutions offering comprehensive joint stiffness analysis features, enabling users to model connections and specify joint properties that integrate seamlessly with FEA. This capability supports accurate stiffness estimation and enhances the realism of structural models.
In this context, the following article clarifies how initial stiffness analysis can be done in Dlubal’s Steel Joints add-on using the end plate connection of a continuous column and beam (as shown in Image 1) as an example. The focus is on calculating rotational stiffness for moments about the y-axis. The article demonstrates how to initiate and conduct the analysis in the software, followed by a short discussion of the underlying concept.
Initial Stiffness Analysis in Steel Joints Add-on
The stiffness analysis is offered as an additional analysis feature in the Steel Joints add-on. You can set up the analysis configuration here, or, if a configuration has been created previously, you can select it from the drop-down menu (Image 2). Note that existing configurations can also be modified at this stage. The available design parameters for configuration are shown in Image 3.
The initial stiffness analysis can be conducted for selected members with respect to the internal forces N, My, and Mz. Within the Members tab of the add-on’s input dialog, you can select the desired internal forces using checkboxes, with the ability to choose multiple forces at once (Image 4). The stiffness analysis can then be carried out for these internal forces with both positive and negative values.
Concept Behind
The Steel Joints add-on, as outlined above, allows for initial stiffness analysis in just a few straightforward steps, providing results efficiently and quickly. For those interested in the methodology behind it, the approach focuses on analyzing the deformation of the selected beam in the joint while rigidly supporting the other beams. It is worth noting that, alongside the surface model, a corresponding member model is also utilized (Image 5). In both models, the member ends are fully supported; however, the member model uses analogous members to those in the surface submodel and features rigid connections between them. This setup compensates for deformation effects, effectively accounting for the beam's own deflection within the joint.
