Editing Earthquake Load Dialog

Opens when Edit Load is selected in a load case (RSA) under Earthquake Load group. Response spectrum analysis (RSA) is a linear analysis method, which converts a problem in dynamics into an equivalent problem in statics. It considers the contribution from each natural mode of vibrations of structure, and calculates the likely maximum elastic seismic response of the structure. RSA provides insight into the dynamic behavior by measuring acceleration, velocity, or displacement as a function of structural period for a given time history and level of damping. It is practical to envelope response spectra such that a smooth curve represents the peak response for each realization of structural period. A description of input follows:

Setting	Description
Code	Specify the design code to generate design response spectrum. The option of Custom allows users to manually input the data of the design response spectrum under the table of Special Data and Curve.
Combination Method	Combine the responses from each mode into a total response. SRSS: Square Root of Summation of Squares method. ABS: Absolute sum. This method is very conservative and represents the worst case combination. CQC: Complete Quadratic Combination method. This method is recommended for closely spaced modes instead of SRSS. TEN: Ten Percent Method of combining closely spaced modes. NRC Reg. Guide 1.92 (Rev. 1.2.2, 1976). CSM: Closely Spaced Method as per IS:1893 (Part 1)-2002 procedures. GRP: Closely Spaced Modes Grouping Method. NRC Reg. Guide 1.92 (Rev. 1.2.1, 1976).
Site location and ground motion coefficients	When AASHTO option is selected, it allows to specify the site zip code, the corresponding ground acceleration coefficients (PGA, Ss, and S1) will be derived from the national ground motion database. If "Custom" option is selected ground acceleration coefficients should be entered manually.
Spectrum Table	Long Period (TL): to specify the range of the Period in the design response spectrum Site Class: a site shall be classified as A through F according to the site class definition per LRFD Art. 3.10.3.1. For AASHTO code option, the site factors (Fpga, Fa, and Fv) are automatically calculated based on the site class and the ground acceleration coefficients.
Application Direction and Factor	Specify the global direction(s), in which the design response spectrum is to be applied. The response spectrum may be applied in one, or more directions simultaneously. Directions not applied will use zero in the related factors. Two methods to produce response spectrum results are offered. The Dominant and Sign options may be used in the input file to produce signed output. STAAD solver can assign a mathematical sign (positive or negative) to the modal results by one of two means. The Dominant and Sign options may be used in the input file to produce signed output. If more than one direction is applied, the mode shape number is needed in order to use the sign. If only one direction is used and mode number is zero, application will automatically find the mode shape for which the mass participation is the highest and use the sign from that mode to assign to the response (displacement/reaction/girder force). In the SIGN method, sum of responses in each mode with same sign of the response are accumulated and then compared to see which one is overweighing the other one to use the sign of that. For instacen, while superimposing modal responses of girder shear, if sum of all the shears in all modes having positive sign is more than the same with negative sign, final (modal) supoerimposed the shear will have positve value. On the contrary, Dominant mode either uses user input of the mode number to get the sign of the resposse in that mode to assgin to it or looks in the mass participation table under the same direction as the application of the spectrum, and selects the mode that has maximum paticiapation. In the latter method as the application is looking along one direction only, application of the spectrum in this method is limited to only one direction only.
Missing mass	Use Missing Mass method to include the static effect of the masses not represented in the modes. The spectral acceleration can be entered as a factor of acceleration due ot gravity ("g") or the frequency (ZPA) beyond which the structure could be treated as vibrating in static mode. By Default ZPA is considered as 33 Hz. The spectral acceleration corresponding to 33 Hz is used from the provided spectral curve if none of the input is provided. A static mode is created with base acceleration same as static acceleration and the results of this mode are combined with other selected modes. Missing mass does not include the effect of masses lumped at the supports unless the support is a stiff spring.
Structural Damping	This is to be used for specifying the damping ratio which will be applied to all modes. The default value is 0.05.