This article presents the basic concepts in structural dynamics and their role in the seismic design of structures. Great emphasis is given to explaining the technical aspects in an understandable way, so that readers without deep technical knowledge can gain an insight into the subject.
This article will show you the Building Model add-on, which has been enhanced with one important advantage: calculating the center of mass and center of rigidity.
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).
Given that realistic determination of the soil conditions significantly influences the quality of the structural analysis of buildings, the Geotechnical Analysis add-on is offered in RFEM 6 to determine the soil body to be analyzed.
The way to provide data obtained from field tests in the add-on and use the properties from soil samples to determine the soil massifs of interest was discussed in Knowledge Base article “Creation of the Soil Body from Soil Samples in RFEM 6”. This article, on the other hand, will discuss the procedure to calculate settlements and soil pressures for a reinforced concrete building.
The stand-alone program RSECTION is at your disposal for determining section properties and performing stress analysis for thin-walled and massive cross-sections. The program can be connected to both RFEM and RSTAB so that sections from RSECTION are also available in the RFEM and RSTAB library. Likewise, internal forces from RFEM and RSTAB can be imported into RSECTION.
You can use the stand-alone program RSECTION to determine the section properties for any thin-walled and massive cross-sections, as well as to perform a stress analysis. The previous Knowledge Base article titled "Graphical/Tabular Creation of User-defined Cross-sections in RSECTION 1" discussed the basis of defining cross-sections in the program. This article, on the other hand, is a summary of how to determine the section properties and perform a stress analysis.
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).
RSECTION 1 is a stand-alone program for determining section properties for both thin-walled and massive cross-sections, as well as for performing a stress analysis. In addition, the program can be connected to both RFEM and RSTAB: sections from RSECTION are available in the RFEM/RSTAB libraries, and internal forces from RFEM/RSTAB can be imported into RSECTION.
The add-on modules for designing structural member components according to national, European, and international standards also show design results in addition to numerical output in tables graphically, as diagrams displayed on the framework.
The number of National Annexes for Eurocode 2 with regard to the design of reinforced concrete cross-sections has been extended since SHAPE-MASSIVE 6.54. Therefore, the following NAs of EN 1992-1-1:2004 + AC:2010 are available:
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.
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.
RF-/DYNAM Pro - Equivalent Loads allows you to determine the loads due to equivalent seismic loads according to the multi‑modal response spectrum method. In the example shown here, this was done for a multi‑mass oscillator.
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.
In a multi-modal response spectrum analysis, it is important to determine a sufficient number of eigenvalues of the structure and to consider their dynamic responses. Regulations such as EN 1998‑1 [1] and other international standards require activation of 90% of the structural mass. This means: to determine so many eigenvalues that the sum of the effective modal mass factor is greater 0.9.
In theory, an ideal gas consists of freely moving mass particles without extension in a volume space. In this space, each particle moves at a speed in one direction. The collision of one particle with another particle or the volume limitations leads to a deflection and a change in the speed of the particles.
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.
As of the version 5.06.1103, masses of nodes, lines, members, and surfaces can be neglected in RF‑DYNAM Pro. The setting to activate this feature can be found in the Details dialog box; the neglected masses are valid for all defined mass cases.
There are two ways of adding cross‑sections that are not included in the extensive cross‑section library: 1. You can create the cross‑section in the cross‑section programs SHAPE‑THIN or SHAPE‑MASSIVE and import it to RFEM/RSTAB. 2. If the cross‑section properties are provided by the manufacturer, you can add it to the RFEM/RSTAB cross‑section library using the option "New User‑Defined Cross‑Section".
In the DYNAM Pro add‑on module for RSTAB, you can now neglect masses that may have a negative effect on the equivalent mass factor when calculating eigenvalues. To do this, you can disable the masses under [Details]. These include primarily mass points located in the support of the structures.
If a slender component (member) is to be connected to a massive component (solid), it is necessary to pay attention to the correct connection of both elements.
For unprotected I‑sections, the standard provides the correction factor ksh according to Equation 4.26a in Section 4.2.5.1 (2) to consider the shadowing effect. The term [Am/V]b is used there. This section factor includes Am, which represents the box enclosing the cross‑section (Index b = boxed). In the case of a three-sided fire exposure (a girder with a massive ceiling), the flange surface not exposed to fire is not taken into account when determining [Am/V]b.
With the latest version of RF‑/DYNAM Pro, you can exclude the mode shapes from the seismic design. In most standards, there are provisions to exclude the effective modal mass factors that are too small. The "Mode Shapes" tab offers the option to specify the factor and to disregard the mode shapes automatically.
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.
User-defined views are a very useful tool for effective modeling, as the previously selected and adapted objects appear directly with a click of the mouse. These objects can also be used easily to create informative and clearly arranged result graphics. With just a few clicks, you can create all result graphics at once using the mass print function.