To perform seismic resistance design of a building, you can superimpose the results of the spectral analysis with the results of other load cases in a seismic result combination.
The spectral analysis results are created in two steps, and the parameters are stored in the Spectral Analysis Settings:
- The modal responses are combined according to the selected "Combination rule for periodic responses" (see the image Selecting Combination Rule for Periodic Responses ).
- The directional results are combined according to the specification of the "Combination of Directional Components" (see the image Selecting Combination Rule for Directional Components ).
In order to use the spectral analysis results for the design of a seismic model, it is necessary to further combine them on the basis of an accidental design situation. This combination is defined, for example, in EN 1990 [1], Section 6.4.3.4, as follows:
| Gk,j | Characteristic value of the permanent action j |
| P | Governing representative value of prestress |
| AEd | Design value of a seismic load |
| Qk,i | Characteristic value of a non-governing variable action i (a secondary action) |
| "+" | "to combine" |
| ∑ | "common effect of" |
Generating Design Combination
The Combination Wizard supports you in the task of creating one or more design combinations for a seismic analysis. You can apply the results of the envelope or specific subresults to these result combinations.
Envelope
In the "Main" tab of the "Load Cases and Combinations" dialog box, activate the Combination Wizard (if not selected) and Result Combinations (see the chapter Base of the RFEM manual).
Create a design situation of the ULS (STR/GEO) – Seismic type (1). Assign the combination wizard that creates Result Combinations (2). Click the
button to define the same wizard in the "Design Situations" tab.
Using these specifications, the program generates one or more result combinations with the maximum seismic loads that you can use to design the model. The envelope from the spectral analysis is considered in these combinations.
Subresults
In the "Main" tab of the "Load Cases and Combinations" dialog box, select the Subresults option for Result Combinations.
The available subresults depend on which of the spectral analysis settings is assigned to the response spectrum load case and for which excitation directions the response spectrum is specified. For example, if you save the results of all selected mode shapes and combine the directional components according to the SRSS rule, the following subresults are available for selection in the "Result Combinations" tab.
This way, you can control which subresult is included in the seismic result combination.
If you combine the directional components according to the scaled sum, the following subresults are available for selection:
