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).
The dynamic analysis in RFEM 6 and RSTAB 9 is divided into several add-ons. The Modal Analysis add-on is a prerequisite for all other dynamic add-ons, since it performs the natural vibration analysis for member, surface, and solid models.
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).
With the RF-/TIMBER Pro add-on module, you can perform the vibration design known from DIN 1052 for the design according to EN 1995-1-1. In this design, the deflection under permanent and quasi-permanent action at the ideal one‑span beam may not exceed the limit value (6 mm according to DIN 1052). If you consider the relation between the natural frequency and the deflection for a hinged single-span beam subjected to a constant distributed load, the 6 mm limit value results in a minimum natural frequency of about 7.2 Hz.
Structures react differently to wind action depending on stiffness, mass, and damping. A basic distinction is made between buildings that are prone to vibration and those that are not.
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.
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.
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 vibration design of cross‑laminated timber plates often governs for wide-span ceilings. The advantage of timber as a lighter material compared to concrete is turned into a disadvantage here, since a high mass is advantageous for a low natural frequency.
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.
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.
The new RF‑/DYNAM Pro - Natural Vibrations module has been available since RFEM version 5.04.xx and RSTAB version 8.04.xx were released. Masses can now be imported directly from load cases and load combinations.