To evaluate whether it is also necessary to consider the second-order analysis in a dynamic calculation, the sensitivity coefficient of interstory drift θ is provided in EN 1998‑1, Sections 2.2.2 and 4.4.2.2. It can be calculated and analyzed using RFEM 6 and RSTAB 9. The coefficient θ is calculated as follows:$$\mathrm\theta\;=\;\frac{\displaystyle{\mathrm P}_\mathrm{tot}\;\cdot\;{\mathrm d}_\mathrm r}{{\mathrm V}_\mathrm{tot}\;\cdot\;\mathrm h}\;$$
For the ultimate limit state design, EN 1998‑1, Sections 2.2.2 and 4.4.2.2 require a calculation considering the second‑order theory (P‑Δ effect). This effect may be neglected only if the interstory drift sensitivity coefficient θ is less than 0.1.
Both the determination of natural vibrations and the response spectrum analysis are always performed on a linear system. If nonlinearities exist in the system, they are linearized and thus not taken into account. They are caused by, for example, tension members, nonlinear supports, or nonlinear hinges. This article shows how you can handle them in a dynamic analysis.
The “Modal Analysis” add-on in RFEM 6 allows you to perform modal analysis of structural systems, thus determining natural vibration values such as natural frequencies, mode shapes, modal masses, and effective modal mass factors. These results can be used for vibration design, as well as for further dynamic analyses (for example, loading by a response spectrum).
Modal analysis is the starting point for the dynamic analysis of structural systems. You can use it to determine natural vibration values such as natural frequencies, mode shapes, modal masses, and effective modal mass factors. This outcome can be used for vibration design, and it can be used for further dynamic analyses (for example, loading by a response spectrum).
Seismic Analysis in RFEM 6 is possible using the modal analysis and the response spectrum analysis add-ons. As a matter of fact, the general concept of the earthquake analysis in RFEM 6 is based on the creation of a load case for the modal analysis and the response spectrum analysis, respectively. The standard groups for these analyses are set in the Standards II tab of the model’s Base Data.
An FE mesh quality display is available in RFEM as a tool for structural analyses of two-dimensional components. It leads to the execution of an internal check of the generated finite elements for defined criteria.
The RF-STABILITY add-on module determines any critical load factors, effective lengths, and eigenvectors of RFEM models. Stability analyses can be carried out by various eigenvalue methods, the advantages of which depend on the structural system as well as computer configurations.
The SHAPE‑THIN and SHAPE‑MASSIVE cross-section programs are suitable for determining the cross-section properties of common thin-walled or thick-walled sections. These cross-section properties are also available for further analyses in RSTAB and RFEM.
In EN 1993-1-1, the General Method was introduced as a design format for stability analyses that can be applied to planar systems with arbitrary boundary conditions and variable structural height. The design checks can be performed for loading in the main load-bearing plane and simultaneous compression. The stability cases of lateral-torsional buckling and flexural buckling are analyzed from the main supporting plane; that is, about the weak component axis. Therefore, the issue often arises as to how to design, in this context, flexural buckling in the main load-bearing plane.
In RFEM, orthotropic plastic analyses using the Tsai‑Wu plasticity criterion have been possible for quite some time now. The hardening modulus Ep,x or Ep,y can be used to consider the hardening of the material during the iterative calculation.
In this article, representations of a blast scenario of a remote detonation performed in RF-DYNAM Pro - Forced Vibrations are shown, and the effects are compared in the linear time history analysis.
The steady state for periodically excited structures can be determined by means of the modal analysis in the DYNAM Pro - Forced Vibrations add-on module. This is an advantage if only the structure's steady state is of interest. Instead of a complete solution of the equation of motion, only a special solution is displayed.
With the RF-STABILITY and RSBUCK add-on modules for RFEM and RSTAB, it is possible to perform eigenvalue analyses for member structures in order to determine the effective length factors. The effective length coefficients can then be used for the stability design.
Both the determination of natural vibrations and the response spectrum analysis are always performed on a linear system. If nonlinearities exist in the system, they are linearized and thus not taken into account. Straight tension members are very often used in practice. This article will show how you can display them approximately correctly in a dynamic analysis.
In order to consider inaccuracies regarding the position of masses in a response spectrum analysis, standards for seismic design specify rules that have to be applied in both the simplified and multi-modal response spectrum analyses. These rules describe the following general procedure: The story mass must be shifted by a certain eccentricity, which results in a torsional moment.
The input windows in RF-/STEEL EC3 distinguish between the flexural and lateral-torsional buckling analyses. In the following text, an example will show the parameters for lateral-torsional buckling.
Different methods are available for calculating the deformation in the cracked state. RFEM provides an analytical method according to DIN EN 1992-1-1 7.4.3 and a physical-nonlinear analysis. Both methods have different features and can be more or less suitable depending on the circumstances. This article will give an overview of the two calculation methods.
With RF-FOUNDATION Pro, it is possible to determine the settlements of single foundations and resulting spring stiffnesses of the nodal supports. These spring stiffnesses can be exported into the RFEM model and used for further analyses.
The product range of Dlubal Software contains various modules for design of steel and timber connections. The RF-/JOINTS Steel – Column Base add-on module allows you to analyse footings of hinged or restrained steel column bases. The fastener selection, foundation geometry, and material quality are crucial for the cost-effective and safe design of the column base.
The Time Course Monitor displays the results of a time history analysis from RF‑/DYNAM Pro – Forced Vibrations. The graphic can be adjusted in the settings. This can be reached by right-clicking in the shortcut menu. For example, you can activate or deactivate the grid in the graphic. Those changes are overtaken into the printout report when you print the graphic.
The steady state for periodically excited structures can be determined by means of the modal analysis in the DYNAM Pro – Forced Vibrations add-on module. This is an advantage if only the structure's steady state is of interest. Instead of a complete solution of the equation of motion, only a special solution is displayed.
With RF-DYNAM Pro – Forced Vibrations, you can perform a time history analysis. For example, you can analyze an explosion acting on a nearby building structure. In "Dynamik der Baukonstruktionen" by Christian Petersen, formulas for time diagrams and load distribution are described to specify an explosion. The image shows the input of such an explosion load. Free variable loads are available in RFEM that enable flexible load distributions.
Not only do RF-/STEEL EC3 and RF-/TIMBER Pro perform cross-section designs and stability analyses, they allow you to perform serviceability limit state designs. For this, it is possible to relate the deformation to the undeformed initial system or to shifted members ends.
To simulate an excitation that varies over time and changes its position, you can combine several loading time diagrams in RF‑/DYNAM Pro - Forced Vibrations.
In RF-/DYNAM Pro - Forced Vibrations, you can define time diagrams directly as functions in an edit field. The parameter t is reserved for the time steps, but apart from that, all parameters as defined in the "Edit Parameter" dialog can be used in RF‑/DYNAM Pro.
In RF-/DYNAM Pro - Forced Vibrations, you combine static load cases with time diagrams to define the type of excitation of your structure. This way, you can use not only nodal loads, but also use line, surface, free, or generated loads in the time history analysis.
RF-/DYNAM Pro - Forced Vibrations provides the option of a time course monitor. During the evaluation process, you can compare several graphs directly in the program. In addition, you can transfer the figures to the printout report or export them directly to Excel as a value table.
In RF-/DYNAM Pro - Natural Vibrations, you can import axial forces and stiffness modifications from any Load Case (LC) or Load Combination (CO). You can modify material, cross‑section, member, and surface properties and activate these modifications in the LC/CO calculation parameters.
In RF-/DYNAM Pro - Natural Vibrations, it is possible to transfer complete load cases/load combinations as masses. To do this, you can simply save the load case or the load combination to be considered as a mass case in the add‑on module.