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DYNAM Basic 8.xx
Analysis of Natural Vibrations
First, you define the geometry of the structure in RSTAB.
By taking into account the additional or net masses, DYNAM can represent the mass distribution within the structure appropriately. Furthermore, it is possible to import nodal or member loads defined in RSTAB, which are acting in the Z-direction, as equivalent additional nodal or member masses to the module. You can take advantage of the following two options to emphasize either computational accuracy or speed of calculation:
- The consistent mass matrix allows for the representation of a distributed mass.
- The diagonal mass matrix represents the structure's masses as concentrated in its structure nodes.
The module's calculation algorithm allows for an explicit member division by means of which the program can calculate more natural frequencies without dividing the members by additional structure nodes.
The mass matrix can include either the mass from self-weight and/or the additional member and node masses. DYNAM determines the mass from self-weight automatically by using the material data specified in RSTAB.
In case axial force values from a previous structural analysis are already available, they can be transferred automatically from the RSTAB calculation to the corresponding DYNAM input table. In addition to the determination of up to 10000 eigenvalues for any type of structure, DYNAM allows for the following additional calculations.
The structure of the mass matrix can be influenced in several ways. The consistent mass matrix allows for the representation of a mass distribution. A diagonal structure of the mass matrix represents the structure's masses concentrated in its structure nodes.
In addition to the determination of eigenvalues, DYNAM provides the following additional calculations:
Scaled Node Deformations
By default, DYNAM scales the normal modes of the structure to the value 1 and lists them member by member. If you are interested in the displacement of the individual structure nodes, you can use this option to display the displacement in an additional output table.
Depending on the support conditions and the structural model, structure nodes may freely oscillate. Due to its mass inertia, any additional mass assigned will respond with a corresponding dynamic reaction which can be taken as a quasi-static mass. This quasi-static load equals the additional mass applied to the non-moving structure and is usually unequal to the static mass.
The substitute masses can be listed as absolute sums or as sum of the factors. This allows you to see which natural frequency is dynamically relevant in the respective directions and how many natural frequencies are needed for the dynamic analysis.
Substitute masses are mainly used for slender and high structures. Such a structure is entirely reduced to a single-mass oscillator using energy considerations. Its mass is determined for each natural frequency. While the program calculates the kinetically equivalent masses, further values are calculated. The modal masses result from the structure's mass and its normal modes. The participation factors result from the same values and show how much a certain location of the structure is affected, concerning its dynamic behavior, by the respective mass.
Subsequent to the calculation, DYNAM displays the results in clearly arranged output tables.
The structure's mode shapes can be represented graphically in the RSTAB work window. All input and results tables as well as any kind of graphic with mode shapes can be integrated into the global RSTAB printout report.
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