For the design of the structure against earthquakes, you can superimpose the results of the spectral analysis in a seismic result combination with the results of other load cases.
The results of the spectrum analysis are generated in two steps, the parameters of which are stored in the spectrum analysis settings:
- The modal answers are combined according to the selected 'combination rule for periodic answers' (see picture Select combination rule for periodic replies ).
- The direction-dependent results are combined according to the specification for the 'combination of directional components' (see picture Select combination rule for directional components ).
In order to use the results of the spectral analysis for the design of the model against earthquakes, they must be further combined on the basis of an exceptional design situation. This combination is specified, 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" |
Generate design combination
The Combination wizard supports you in the task of assigning one or more design combinations for the seismic analysis create. With these result combinations, you can apply the results of the envelope or certain partial results.
Enveloping
In the 'Base' tab of the 'Load Cases and Combinations' dialog box, activate the Combination Wizard (if unchecked) and Result Combinations (see chapter # extbookmark manual|result combinations|Basis # of the RFEM manual).
Create a ULS (STR/GEO) - earthquake type design situation (1). Assign a combination wizard that creates result combinations (2). You can also use the
button to define this assistant in the 'Design situations' tab.
With these specifications, the program generates one or more result combinations with the maximum seismic loads that you can use for the design of the model. In these combinations, the envelope from the spectral analysis is taken into account.
Subresults
In the 'Base' tab of the 'Load Cases and Combinations' dialog box, activate the Partial Results option for the result combinations.
The available partial results depend on which of the spectral analysis settings has been assigned to the response spectrum load case and for which excitation directions a response spectrum has been 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 partial results are available in the 'Result combinations' tab.
In this way, you can control which partial result is included in the earthquake result combination.
If you combine the directional components according to the scaled sum, these subtotals are available for selection:
