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Analysis Wizard : Response Spectrum

The basic response cases are set up in the data. This page confirms which output is required. If no modal results are present the user is warned: “There are no modal results present. This analysis will terminate”.

Definition​

Modal Analysis Task

If more than one set of modal dynamic results exist GsSpec needs to know which set of results are to be used. In this situation the task selection is enabled with a list of the available analysis tasks.

Separate or CQC3

Normally a response spectrum analysis considers each direction separately, and some rule if used to combine the effects from different directions. a CQC3 analysis considers all three directions concurrently.

Combination Method

This determines how the modes are to be combined. The combination methods offered are

  • Absolute sum
  • Complete quadratic combination (CQC)
  • Modified Rosenbluth
  • Square root sum of squares (SRSS)

The CQC combination method requires damping value which depends on the particular analysis options.

The combination of basic responses allows for an SRSS combination across the different directions.

Axis

In general the calculation is carried out in global axes, however a different axis can be specified provided that the z axis is aligned with the global z axis.

Source

This allows for this analysis to be included as a source for combining results.

Direction, Spectrum and Modes

Each of the x, y and z directions can be selected for inclusion in the analysis. The spectra that have been defined are offered as options or the user may enter a spectrum number directly. The user may select particular modes to be included in the response, although in most cases the default of “all” will be most appropriate.

Rigorous or Fast

This option is only relevant for This gives a choice between a rigorous calculation only applies for combination method which couple the responses in different modes (CQC and modified Rosenbluth). A rigorous calculation calculates the coupling between all modes. A fast calculation ignores the coupling between modes where the coupling coefficient is less than 1%.

Note​

There are two combination in response spectrum analysis. The part of the calculation is for the response in each mode. These modal responses need to be combined to get the response to excitation in that direction. This can use any of the combination methods noted above but is generally CQC. Once the response in the individual direction has been calculated these need to be combined to get overall responses. The second combination is to combine the results for different directions. The simplest is a linear combination where there is a full contribution from one direction and reduce combinations from the others. This can be handled just like any other combination case. The other option is the SRSS option and this takes the responses calculated in the first step and then combines these using SRSS.

Analysis wizard: Response spectrum details​

Unlike most other analysis options derived results from a response spectrum analysis cannot be derived from the basic response spectrum results. Instead the derived modal results must be calculated and then these combined.

Definition​

Storey drifts

Storey drifts can be specified at various locations. A storey drift is identified by a name, a location in the global xy plane and a list of storeys that are included. By specifying storeys it is possible to carry out drift calculations where storey levels may be different in different parts of the structure.

Relative nodal displacements

In a response spectrum analysis relative displacements cannot be calculated from the difference in the response displacements at the nodes but need to be calculated from the relative modal displacements. The relative nodal displacement is specified by a name, a pair of nodes and an axis for the output.

Analysis wizard: Response spectrum modes​

A pseudo response spectrum analysis is based on mode shapes arrived at by static analysis. In order to combine these for a response spectrum analysis the modal information must be supplied.

Definition​

Pseudo Dynamic Details

The dynamic details must be supplied for each pseudo mode to be considered. These are the frequency, model mass and effective mass. Only the pseudo modes for which values are not specified will be included in the analysis