RSA parameters

Response-spectrum analysis (RSA) is a linear elastic static - (pseudo)dynamic -statistical analysis method which provides the peak values of response quantities, such as forces and deformations, of a structure under seismic excitation, as described in here.

In RSA users are asked to provide as input the response spectrum and the seismic loading combination(s) for which the RSA will output the results. This spectrum is employed for both the two horizontal (EX, EY) and the vertical (EZ) seismic directions.

The parameters that need to be defined in Response Spectrum Analysis are:

Loading combinations
In the loading combination module different response spectrum factors between horizontal and vertical directions may be defined. The modal combination rule (ABSSUM, SRSS, CQC) should be specified, as well as which modes are to be combined, in terms of accumulation of effective modal mass. User may define a minimum cumulative mass percentage and the program selects the appropriate number of modes that mobilise the largest amount of modal mass, until the target cumulative percentage is reached for every seismic direction.

For each loading case (G, Q, and ±E), users are asked to define the factors for the static gravity or live loading (f_G+Q) and the factors of the seismic loading (f_E). Seismic loading directions may be combined linearly (E = ±EX±EY±EZ) with different factors per direction (f_EX, f_EY, f_EZ) or by the SRSS rule (E = ± ). It is noted that the gravity loads have an explicitly defined algebraic sign, while for the seismic loadings both signs for every direction are taken into account. Consequently, the results of RSA loading combinations in terms of any response quantity are presented as envelopes.

Note: Code-defined ready-to-use loading combinations can be defined with the Add Standard Combinations button. The combinations consist of the gravity+live loads, plus 100% of the prescribed seismic forces in one direction and 30% of the prescribed forces in the perpendicular directions, one combination for every seismic direction. Further, a combination of the gravity+live loads plus 100% of the seismic forces is also provided.

Spectral Data
The response spectrum may be defined directly by the user or may be calculated from a given accelerogram.

• Given Accelerogram. The user introduces an acceleration time-history and defines the desired level of viscous damping to automatically create the spectrum. The resulting response spectrum, as opposed to the accelerogram, is shown to the user. The latter however, can be visualised through the Accelerogram button.
• User Defined Spectrum. The pairs of period and response acceleration values can be directly introduced by the user in an input table. This option is usually employed to introduce code-defined spectra and it is noted that, as in all other SeismoStruct modules, the list of values may be pasted from any other Windows application, as an alternative to direct typing.