November 2, 2016

Basic Code Terms for Seismic Behavior and Why We Use Them

Seismic Forces are generated automatically using seismic parameters R, SD1, and SDS in the Seismic Load Generator when using the Director to go from RISAFloor to RISA-3D or when inserting Seismic Loads on a rigid diaphragm in RISA-3D. And Seismic Load Combinations can be generated using the seismic parameters ρ, Ωo, and SDS and applied to the structure. These seismic parameters are used to approximate the inelastic response of the structure subjected to seismic design lateral forces. Figure 1 shows a simplified example of what some of the seismic parameters represent when looking at force versus drift for a lateral system.


Figure 1. Definition of Seismic Parameters

R is the Response Modification Factor as defined in Table 12.14-1 of ASCE 7-10. It provides a reduction for the design seismic force based on the ductility of the system. The program offers a single user-input R value in each Global direction.


R is used in the calculation of the Seismic Response Coefficient, Cs, which determines the Seismic Base Shear, V, and these values get reported in the Seismic Generation Detail Results.


Deflection amplification factor, Cd, is a factor to approximate the inelastic drift due to seismic forces. This factor is not considered at this time for the drift calculations calculated in RISA-3D. The user must consider the effects of Cd outside of RISA.

Omega, Ωo, is an overstrength factor as defined in Table 12.14-1 of ASCE 7-10. This factor accounts for the inherent strength in the design equations, the material used, and the lateral system specified.

Rho, ρ, is the redundancy factor as specified in Section 12.3.4 of ASCE 7-10. The redundancy factor is used to provide for redundancy in each horizontal direction within the system.

In RISA-3D, Omega and Rho are user specified values that are defined in the Global Model Settings.


In seismic design some members may be required to be designed to an overstrength load. In addition, the lateral system subjected to seismic forces may be required to have some redundancy. In RISA-3D, this is done by creating load combinations with the Omega overstrength and Rho redundancy factors applied to the earthquake loads. This can be done using the Seismic Load Combination Generator.


SDS represents the 5% damped spectral response Design acceleration for short period response as specified in ASCE 7 Section 11.4.4. SDS can only be used if Dead Load is selected. It is used for the vertical portion of seismic load and is typically multiplied by 0.2*DL in ASCE 7 Section 12.4 load combinations.

The Seismic Load Combinations with these parameters are shown below as an example.


To learn more about how to apply the seismic parameters in RISA, go to and type seismic in the search window.

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