Seismic Analysis Parameters (Norwegian Code NS-EN 1998-1:2004/NA:2008)

Parameters of a structure seismic analysis depend on a seismic code used during calculations of a structure influenced by seismic impact. Select the seismic code NS-EN 1998-1:2004/NA:2008 in the New Case Definition dialog or click Parameters in the Analysis Types dialog.

To complete the seismic analysis according to the rules given in this code, define the following parameters.

• Value of the a_g40Hz coefficient - it denotes ground acceleration as specified on the seismic zone maps in NA.3.2.1. This is the basis for calculating the reference acceleration value a_gR:

  a_gR = 0.8 × a_g40Hz.

• Ground type (A, B, C, D, E), as specified in the table NA.3.1. It specifies parameters of the ground type influence that describe the response spectrum (S, Tb, Tc, Td) according to the tables NA 3.3 and NA 3.4.

  If the Envelope option is selected, the Parameters button becomes accessible; click it to open the Spectrum Envelope dialog.

• Importance class (I, II, III, IV), as specified in table NA.4(902). The building importance class specifies the importance factor γ (according to the table NA.4(901)); it is necessary to calculate the design acceleration a_g:

  a_g= γ × a_gR.

• Spectrum (Design or Elastic), as specified in the main code EN 1998 point 3.2.2.
• Direction (horizontal, vertical).
• Value of the Behavior factor q. According to the main code EN 1998 point 3.2.2.5, this is a reduction factor of the design response spectrum in relation to the elastic response spectrum. You can edit its value after selecting the Design spectrum option.
  Note: The behavior factor q reduces the design spectrum; this way it also decreases seismic loads and the resulting internal forces. However, as specified in EN 1998 point 3.2.2.5, the displacement based on the design response spectrum is multiplied by the behavior factor q. Thus, usually such displacement is not reduced in relation to the elastic response spectrum.

Click Eccentricity Definition to open the Definition of mass eccentricities dialog where you can specify how to account for the eccentricity of a mass in the seismic analysis

Below in the dialog, the Direction definition, Filters, and Residual Mode buttons are located.

The details concerning this structure analysis method can be found in the mentioned code.