A result combination (RC) superimposes the results of load cases according to specific combination criteria. The results of load cases, load combinations, and result combinations can be combined. Result combinations are primarily used for dynamic analyses.
The 'Result Combinations' tab is available if
- the Result combinations check box is activated in the 'Base' tab, or
- a Combination wizard is used to generate result combinations.
When activating the tab, the result combinations are created according to the combination rules of the standard. They are listed sorted by design situations. If you have deactivated the Combination wizard in the main tab Base , you have to create the result combinations manually (see section Defining Result Combination Manually).
The 'Name of the result combination' represents a short designation. It is based on the safety factors and numbers of the load cases and reflects the combination rules.
The 'To solve' check box controls whether the selected result combination is analyzed in the calculation.
Base
The Base tab manages the general data of the result combination selected in the 'List' on the left.
Categories
The Design situation describes the standard specifications according to which the load cases are combined (see chapter Design Situations).
The Combination type controls the criteria according to which the superposition of results is performed. If the Combination wizard was used, the entry 'General' appears here. For a user-defined result combination, there are several options available in the list.
- General: The combination criteria can be defined individually in the 'Assignment' tab.
- Envelope - Permanent: The load cases and combinations contained in the combination are always considered ('/p'), but exclude each other ('or'). The RC thus represents the envelope with the most unfavorable results of all components.
- Envelope - Transient: The load cases and combinations contained in the combination are only considered if they provide an unfavorable contribution to the result. They exclude each other ('or'). The RC represents the envelope from the results of the components. However, the signs can cause the results of components not to be included if they have no effect on the extreme values.
- Superposition: All load cases and combinations contained in the combination are always considered ('p') and added to each other ('+'). The superposition determines the sum of all results. There are no maximum or minimum values.
Options
By default, the results are combined additively: B = A1 + A2 + A3 + ... + An. If you select the SRSS Combination check box, a superposition is performed according to the square root of the sum of the squares method: B = √(A12 + A22 + A32 + ... + An2). This approach is relevant for a modal analysis. In the section SRSS Combination, you can then specify how the signs of the extreme values are handled.
The Generate sub-combinations of superposition type check box causes the components contained in the result combination to be generated as separate combinations. This allows you to see which combinations are used for the superposition of results and which criteria apply in each case.
SRSS Combination
This section is accessible if the 'SRSS Combination' check box was activated in the Options section. Here, you can specify how the modal responses are combined and how the signs are handled.
The standard form of the SRSS rule combines the results quadratically according to the square root of the sum of the squares method described above. As a result, the signs are lost. With the Use equivalent linear combination check box, the SRSS rule is applied as an equivalent linear combination. This allows results such as corresponding internal forces to be determined. These are usually significantly smaller than according to the standard form of the SRSS rule. The results according to the equivalent linear combination are consistent in themselves and the associated signs are correct with respect to the governing internal force.
Using the selection fields for Extreme value signs, you can control which extreme values of the load cases are included in the superposition and whether the signs should be maintained. In this way, the extreme values of the modal internal forces and deformations and the results associated with the leading component are determined with correct signs.
Assignment
In the Assignment tab, you can check which load cases or load combinations are included in the selected result combination and according to which criteria they are combined. Here, you can also define the criteria of a combination manually.
Assigned for RC
The load cases of the result combination are listed in the table. The values specified in the 'Factor' column are based on the partial safety and combination coefficients according to the standard. They are applied depending on the design situation and the action category.
The Action type describes how each load case is considered in the superposition:
- Permanent: The load is always effective.
- Imposed: The load is only considered if the results lead to an increase in internal forces and deformations.
The Operator expresses how the load cases are linked:
- +: The symbol marks an additive combination. The load case is considered independently of another load case as soon as the results have an unfavorable effect.
- or: The criterion describes an alternative combination. The results of specific load cases exclude each other, such as wind. Only the values of the load that provides the most unfavorable contribution are considered.
Auxiliary Combinations
If the Auxiliary combinations option is activated in the dialog Combination Wizard for result combinations, a result combination is generated separately for each action. The result combinations are then generated from these superordinate result combinations according to the action combinations.
Example
The model contains the following actions with the corresponding load cases:
| E1 | Permanent |
| E2 | Imposed loads |
| E3 | Snow |
| E4 | Wind |
According to EN 1990, three action combinations result for the ultimate limit state:
| AC1 | 1.35 * E1/p + 1.50 * E2/p + 0.75 * E3 + 0.90 * E4 |
| AC2 | 1.35 * E1/p + 1.05 * E2 + 1.50 * E3/p + 0.90 * E4 |
| AC3 | 1.35 * E1/p + 1.05 * E2 + 0.75 * E3 + 1.50 * E4/p |
Without auxiliary combinations, the load wizard generates the following result combinations:
With the 'Auxiliary combinations' option, the load cases are first combined according to actions and then superimposed.
Defining Result Combination Manually
Using the
button at the end of the 'List', you can add a result combination manually. The 'Assignment' tab is adapted accordingly. You will also find this scheme if you use the Combination wizard for load combinations and have activated the 'Result combinations' check box in the 'Base' tab.
All available load cases, load combinations, and result combinations are listed in the 'Assign' column. To transfer an entry to the 'Assigned for RC' list, you can use the following options:
- Double-click the entry.
- Select the entry or multiple entries while holding down the Ctrl key. Then click the
button.
The
and
buttons control which Operator is used when linking the entries:
- +: The load cases are added.
- OR: The load cases act alternatively.
To change the 'Factor' of a load case or a combination, activate the first column of the relevant row. Enter the correct coefficient or select it from the list.
Similarly, you can adjust the 'Action type' and the 'Operator': Activate the field and select the appropriate entry from the list.
The 'Syntax input' section allows you to define the result combination directly. Enter the load cases or combinations each with coefficient, action type symbol ('/p' or without), and operator ('+' or 'or'). Then assign the definition criterion using the
button.
Corresponding Combinations
In the Corresponding Combinations tab, you can define the "related" combinations that result from the same load cases for the individual design situations.
The principle is described in more detail in chapter Load Combinations.