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The most common causes of unstable models are failing member nonlinearities such as tension members.
The following model of a frame whose columns are simply supported at the base point and which has moment releases at the column head serves as an example of this.
However, if it is loaded vertically, both tension members fail and the structural system becomes unstable, which causes a calculation error. You can avoid such an error by selecting the exceptional handling of failing members under "Calculate" → "Calculation Parameters" → "Global Calculation Parameters".
Model Used in
Steel Frame Structure
| Number of Nodes | 4 |
| Number of Members | 5 |
| Number of Load Cases | 3 |
| Total Weight | 0.668 tons |
| Dimensions (Metric) | 5.500 x 0.500 x 5.250 m |
| Dimensions (Imperial) | 18.04 x 1.64 x 17.22 feet |
| Program Version | 8.04.00 |
You can download this structural model to use it for training purposes or for your projects. However, we do not assume any guarantee or liability for the accuracy or completeness of the model.
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The most common causes of unstable models are failing member nonlinearities such as tension members. As the simplest example, there is a frame with supports on the column footing and moment hinges on the column head. This unstable system is stabilized by a cross bracing of tension members. In the case of load combinations with horizontal loads, the system remains stable. However, if it is loaded vertically, both tension members fail and the system becomes unstable, which causes a calculation error. You can avoid such an error by selecting the exceptional handling of failing members under "Calculate" → "Calculation Parameters" → "Global Calculation Parameters".
If the calculation of a member model according to the second-order analysis is terminated with an error message, this instability is often caused by failed tension members: As soon as compressive forces appear in a tension member during a calculation step, this member is no longer considered in the following iterations. Thus, the model can become unstable.
Plate girder is an economical choice for long spans construction. I-section steel plate girder typically has a deep web to maximize its shear capacity and flange separation, yet thin web to minimize the self-weight. Due to its large height-to-thickness (h/tw) ratio, transverse stiffeners may be required to stiffen the slender web.
Understanding steel connection rigidity is crucial in structural design. Often, connections are treated as strictly pinned or rigid, but this can lead to uneconomical or even dangerous designs. Explore how Dlubal Software's RFEM and Steel Joints add-on help verify connection stiffness and moment resistance, ensuring safer and more economical designs.
- Numerous component types, such as base and end plates, web angles, fin plates, gusset plates, stiffeners, tapers, or ribs for easy input of typical connection situations
- Universally applicable basic components (such as plates, welds, bolts, auxiliary planes) for modeling complex connection situations
- Graphical display of the connection geometry with dynamic updating during the input
- Wide range of cross-section shapes: I-sections, U-sections, angles, T-sections, hollow sections, built-up cross-sections and thin-walled sections
- Library in the Dlubal Center with a large number of program-side template connections, including user-defined templates
- Automatic adaptation of the connection geometry based on the relative arrangement of the components to each other – even in case of subsequent editing of the structural components
In the ultimate configuration of the steel joint design, you have the option to modify the limit plastic strain for welds.
The "Base Plate" component allows you to design base plate connections with cast-in anchors. In this case, plates, welds, anchorages, and steel-concrete interaction are analyzed.
In the "Edit Section" dialog box, you can display the buckling shapes of the Finite Strip Method (FSM) as a 3D graphic.
A member that is obviously supported insufficiently does not lead to the instability of the entire structure. Why?
My model is unstable. What could be the reason?
What is the component-based finite element method (CBFEM)?
Is it possible to perform stability analysis according to EN 1993‑1‑1 in RSECTION 1?
Why is it not possible to select the plate I want to cut/edit in the Plate Editor/Plate Cut?
Is it possible to consider the eccentricity of unsymmetrical line welds for the stress analysis in RFEM 6?
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