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5.4 Action Combinations
When you open this dialog tab or Table 2.4, actions are automatically superimposed according to the combination rules and identified as so-called "action combinations". This overview is sorted by actions. It corresponds to the way actions are described in the standards. Now you can define the action combinations that will eventually be worth considering for the generation of load or result combinations.
An action combination includes all ways how load cases can be combined in the action. It should therefore not be confused with a load or result combination that represents only a single variant of these possibilities.
The combinations generated from actions are numbered consecutively. An action combination includes all the possibilities how the load cases contained in the action can be considered. These possibilities depend on the action category and the combination expressions.
In the bottom left corner of the Existing Action Combinations section in the Edit Load Cases and Combinations dialog box, it is possible to filter generated combinations by design situation or relevance.
RFEM automatically assigns short descriptions that are based on the safety factors and symbols of the actions, and that express combination rules. You can change these descriptions, if necessary.
Click the dialog button to jump to the previous dialog tab where the combination expression is selected that the current action combination was created with.
The check box allows you to control if the selected action combination is considered for creating load or result combinations. In this way, it is possible to exclude action combinations from the generation or reactivate them.
If RFEM creates an action combination twice because of special constellations, one of them is automatically deactivated.
The design situation of the current action combination is indicated once more so that you can check data. Use the [Info] button to look at the combination rule of the design situation. A dialog box with explanations opens (see Figure 5.13).
The columns provide information about actions with the corresponding partial safety factors and combination coefficients.
If an action is assumed to be Leading in the combination, it is marked accordingly in the dialog box. In this case, it is inserted as the action Qk,1 in Equation 5.1 to Equation 5.7 (see Chapter 5.3 Combination Expressions).
The values shown in the Factor table column are based on coefficients that depend on the selected standard. For EN 1990, they are the partial safety factors γ, the combination factors ψ, the reduction factors ξ, and, if applicable, the reliability factors KFI of each action, which result from the design situation and action category.
Use the [Settings] button to check and, in case of a user-defined standard, adjust the partial safety factors and combination coefficients. The factors are organized in several tabs of the Coefficients dialog box. The first tab, Partial Safety Coefficients, is shown in Figure 12.27. The Combination Coefficients tab manages the factors ψ and ξ.
The Actions in Action Combination dialog section lists the Load Cases contained in the actions, with all possibilities how they can be taken into account in the action. These possibilities depend on the action type and the defined action (simultaneous or alternative). It is assumed that all assigned load cases are always used together for the action types "Permanent loads" and "Prestress", unless the relation is defined as "Alternative". In case of variable, extraordinary, and seismic actions, assigned load cases can be superimposed in all relevant combinations.
This dialog section or table column is filled during the generation that starts automatically when closing the dialog tab or table. The entries provide a short overview of the numbers and quantity of generated load or result combinations.
In the example shown on the left, a total of 47 load combinations are generated for the design situation ULS. For the action combination AC12 (penultimate row), the four load combinations CO40 to CO43 occur with the following background:
The first action A1 has been categorized as the "Permanent loads" action category and is provided with the factor γ = 1.35 in the generated load combinations. The contained load cases 1 and 2 occur together in all load combinations.
As the second action A2, we have the "Snow" action category, included in the load combinations with the factor γ * ψ = 1.50 * 0.50 = 0.75.
The third action A3 doubles the number of generated load combinations, because the "Wind" category is available with the two alternatively acting load cases 4 and 5. This action is multiplied with the factor γ ⋅ ψ = 1.50 ⋅ 0.60 = 0.90 in the load combinations.
The fourth action A4 is classified as the "Imposed load category B" action type and provided with the factor γ = 1.50 in all four load combinations. This action is a leading action.
In addition, we have to take the two imperfection load cases 7 and 8 into account, which are coupled with the directions of both wind load cases. We want to create load combinations, once with imperfections and once without.
With these specifications, RFEM forms the following load combinations for AC12:
1.35LC1 + 1.35LC2 + 0.75LC3 + 0.9LC4 + 1.5LC6
1.35LC1 + 1.35LC2 + 0.75LC3 + 0.9LC4 + 1.5LC6 + LC7
1.35LC1 + 1.35LC2 + 0.75LC3 + 0.9LC5 + 1.5LC6
1.35LC1 + 1.35LC2 + 0.75LC3 + 0.9LC5 + 1.5LC6 + LC8
Click the dialog button to jump to the Load Combinations dialog tab where the first combination created from the current action combination is selected.
Enter a user-defined note or select an entry from the list.