Second-Order Analysis
Glossary Term
The calculation according to the second-order analysis (also: P‑delta) detects the deformation effects due to loading, which affect the distribution of internal forces. Axial forces in the structural system usually lead to an increase of bending moments: in the case of pressure-loaded members, additional moments arise and thus overlinear effects of loads and moments occur. On the other hand, tension forces have a positive effect.
In RFEM and RSTAB, it is possible to calculate load cases and load combinations according to the various methods of analysis (see Figure 01). However, result combinations superimpose the results of the calculated load cases and combinations, so that the method of analysis can only be controlled indirectly.
The calculation according to the second-order analysis examines stability problems, for example buckling. In the RF‑/STEEL EC3 add‑on module, you can also perform the stability analysis considering lateral-torsional (torsional-flexural) buckling. The module extension RF‑/STEEL Warping Torsion allows you to perform the analysis according to the second‑order theory with 7 degrees of freedom (warping torsion).
In the case of steel structures, the second-order analysis must be used if the compression force in a member exceeds 10% of the elastic critical buckling load [4]. For concrete structures, certain limiting slendernesses are governing. Timber structures are usually calculated according to the linear static analysis.
Chapter 7.2.1.1 of the RFEM [1] or RSTAB manual [2] contains further information about the methods of analysis.
In the attached example, a load is analysed according to the linear static analysis and the second-order analysis. A vertical load with a small horizontal load acts on the column head (see Figure 02). The axial force has no influence on the moment distribution My in LC 1. However, the additional moment according to the second-order analysis is clearly recognisable in LC 2. If the load is further increased (or the cross-section is reduced), the program will display a message about the instability.
Keywords
Second-order analysis Deformed system P-delta Load combination Stability Critical load
Bibliography
Downloads
Links
Stability Analysis of a Purlin With I-Section Without Lateral and Torsional Restraint
This technical article deals with the stability analysis of a roof purlin, which is connected without stiffeners by means of a bolt connection on the lower flange to have a minimum manufacturing effort.
![Vertical Soil Stress under Corner Point of Uniform Rectangle Load [2]](/-/media/Images/website/img/000001-010000/009601-009700/009675.png?la=en&mlid=AEBCAC28C25A4A2F93F419D5DA4345C3&hash=C2A6383AD9BEC4A6A8EB68C3F997CE680E35D3AC)
New
Increased Calculation Performance by Reducing the Nodal Degrees of Freedom
The number of degrees of freedom in a node is no longer a global calculation parameter in RFEM (6 degrees of freedom for each mesh node in 3D models, 7 degrees of freedom for the warping torsion analysis). Thus, each node is generally considered with a different number of degrees of freedom, which leads to a variable number of equations in the calculation.
This modification speeds up the calculation, especially for models where a significant reduction of the system could be achieved (e.g. trusses and membrane structures).
- To which axes refer the support rotations and support eccentricities in RF‑/STEEL EC3 Warping Torsion?
- What does the load application point in RF-/STEEL EC3 Warping Torsion refer to?
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What are the options in RFEM or RSTAB for determining the ideal elastic critical moment for any cross-sections and systems/loads?
Is it also possible to design flat steel (brackets, flat steel stringers of staircases)? - Are warping stresses in structural components subjected to torsion considered in RFEM/RSTAB and the STEEL or RF‑STEEL Members add-on modules?
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I have calculated a frame according to the second-order analysis. The testing engineer says that the frame is not stable because the torsional buckling check has failed.
What do I have to be aware of in the case of the second-order analysis? Do I have to calculate the system as 3D, even if it is a 2D frame?
What modules do I need for a lateral-torsional buckling analysis when calculating according to the second-order analysis?
- For a member with an unsymmetric cross-section, the load plane runs through the centre of gravity of the cross-section and not through the shear centre. Therefore, torsion resistance must occur. Why is there MT = 0 ?
- After installing version 8.25, the profiles in RSTAB appear different to me than originally - much more darker. Is there anything that can be done about it?
- How can I use the COM interface to B. delete all nodes or members in a model?
- Why does the vertical position of a result member in my model have no influence on the bending moment My of the member?
- Is it possible to keep the results from RWIND Simulation when deleting the FE mesh if I only adjust materials or support conditions?
Associated Products
Main Program
Structural engineering software for finite element analysis (FEA) of planar and spatial structural systems consisting of plates, walls, shells, members (beams), solids and contact elements
3,540.00 USD
Main Program
The structural engineering software for design of frame, beam and truss structures, performing linear and nonlinear calculations of internal forces, deformations, and support reactions
2,550.00 USD
Module Extension for RF-STEEL EC3 and RF-STEEL AISC
Warping torsion analysis according to the second-order theory with 7 degrees of freedom
850.00 USD
Module Extension for STEEL EC3 and RF-STEEL AISC
Warping torsion analysis according to the second-order theory with 7 degrees of freedom
850.00 USD
