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Analysis wizard: Modal dynamic

For both a modal dynamic and modal dynamic p-delta analysis the user is brought to this page.

Definition​

Mode calculation strategy​

Specify which one of the three mode calculation strategies to use. The options are as follows:

  • Specify number of modes - With this option, modes are calculated sequentially starting from mode 1 (or what is specified in the start mode in advanced analysis settings dialog) and returns the exact number of modes specified by the user. The following items need to be defined for this option:

    • Number of Modes - Specify the number of modes required from the eigenanalysis.

  • Specify frequency range - With this option, modes are calculated sequentially starting from the first mode that is within the frequency range up until either the upperbound frequency or maximum number of specified modes is exceeded. The following items need to be defined for this option:

    • Frequency range - Specify the lowerbound and uppberound of the frequency range of interest.
    • Limit maximum number of modes to - The maximum number of modes to calculate. When the analysis commences GSA needs to allocate space for result sets. This sets a maximum number of analysis cases that can be populated by the solver. If analysis cases are not required they will remain empty.

  • Specify target mass participation ratio - With this option, the solver will calculate the relevant modes until either the target mass participation ratios is met or until the maximum number of modes is reached. The following items need to be defined for this option:

    • Target participation - The mass particiaption ratios in global X, Y, and Z directions.
    • Limit maximum number of modes to - The maximum number of modes to calculate. When the analysis commences GSA needs to allocate space for result sets. This sets a maximum number of analysis cases that can be populated by the MASIL solver. If analysis cases are not required they will remain empty.
    • Skip modes with low mass participation - If this option is checked off, the MASIL algorithm will be used to return only the significant modes which when aggregated give the target participation (some modes with low participation may be skipped). If this option is not checked off, then GSA will return all modes starting from mode 1 until either the target participation is met or the maximum number of modes is exceeded.

Mass Option​

This specifies how the mass of the elements is to be included in a dynamic analysis, the following three options are available:

  • Lumped mass at nodes - in this option, the total element mass is assigned to the nodes of the element and form the element mass matrix, the proportion of the total mass at each node depends on the length (1D element), area (2D element) or volume (3D element) covered by the nodes for 1D, 2D and 3D elements respectively, e.g. the two nodes of a beam element will each have half of the total mass of the element, the 4 nodes of a rectangular Quad4 element will each have 1/4 of the total mass of the element. The element mass matrix from this option is a diagonal matrix, this option should be used in most cases unless local vibration modes of the elements are interested.
  • Calculate mass from the element shape function - in this option, the element mass matrix, called consistent mass matrix, will be calculated using the same shape function for element stiffness matrix calculation. The principle of this method is that the work done by the inertia forces of the distributed mass within the element is the same as the work done by the inertia forces of the calculated nodal masses are equal. The element mass matrix from this option is not diagonal. This option is normally used if element local vibration modes are interested
  • Ignore element mass except for nodal mass - if this option is selected, the mass of all elements except explicitly defined nodal masses will be ignored in the dynamic analysis. The nodal masses will be always considered in modal dynamic analysis

A scale factor can be applied to modify the element masses. This is typically used to increase the mass to allow for connections.

Additional Mass Derived from Loads​

In addition to the mass of the elements, extra mass can be inferred from loads (Case definition) applied in a particular Direction, with a Scale factor if required. This is used where “dead loads” have been applied to the structure which represent mass in the real structure. Only externally applied forces are included; the effect of gravity (i.e. self weight) is taken into account by the mass option.

P-delta Case​

This option is only present if a “modal dynamic p-delta” analysis has been requested.

This is the basis on which the geometric stiffness is calculated. This can either be a load description (e.g. 1.2L1 + 1.2L2) or an analysis or combination case (e.g. A3 or C4). When a load description is specified, GSA first analyses the structure under this load and generates a geometric stiffness from the resulting forces/moments. When an analysis or combination case is specified the geometric stiffness is constructed from the results in this analysis or combination case. Combination cases can only be simple combination cases (i.e. of the form a.Aa+b.Ab+...a.A_a + b.A_b + ... )

Calculate modal (material) damping​

If a damping value has been specified in materials used by elements in the analysis, this will calculate the resulting damping effect due to the various materials. Note that if only a single material is used in the model then the modal damping will be equal to the material damping.

Advanced options​

For advanced options on dynamic analysis see