Defining load combinations

Last modified by Fredrik Lagerström on 2022/01/04 16:46

  • All loads are regarded as unfavorable by default
  • Loadcases and associated load combinations are only applicable to buildings by default


Loadcases are extended with information about type classification, see Figure 1.


Figure 1: Loadcases dialog

The classification type will affect the ψ-values that are used in the load combinations. If a type is changed after the creation of a load combination, the associated ψ-values will automatically be updated, see "Load combinations".

Calculation settings


Figure 2: Calculation settings

In the "Calculation settings" tab the user specifies applicable consequence class, see Figure 2 (red square). For those annexes where the location of the building has an impact, an additional input field is presented, see Figure 2 (blue square).

The information that will be shown, if the mouse pointer hovers over the info icon, is adopted to current annex i.e. it will alter depending on the chosen annex.

If any of these inputs are altered after the load combinations has been created, the load combinations will automatically be updated, see "Load combinations".

Load combinations


Figure 3: Load combinations


Figure 4: Automatic generation of load combinations

warning.pngNo load combination is automatically generated unless the user choose to activate automatic generation of load combinations based on dead load at the initial creation of the beam, see Figure 4. These initially generated load combinations will thus only contain dead load, see Figure 3.

There are four areas of special interest in the load combination tab, see Figure 3;

  1. Edit box where the generated load combination is presented in plain text format. It is possible for the user to edit the load combination by altering this text string.
  2. Loadcase list. All loadcases that the user has activated/defined in the Loadcases dialog, see Figure 1, are listed here and available for usage in one or several load combination
  3. The creation of a load combination is based on the chosen combination of actions which is set here. Available/relevant combination of actions are dependent on chosen limit state and type, see Figure 5. "-" indicates a user-defined combination of action where the user has to specify the appropriate coefficients.
    Figure 5: Available combination of actions
  4. If the "Original text" check-box is ticked all contributing coefficients in the chosen active load combination will be showed. If it's unticked the text string will be compressed, see Figure 6.
    Figure 6: Original text action

Create a Load combination

The creation of a load combination is a straight forward click maneuver;

  1. Begin by deciding what kind of limit state and type that should be used, see Figure 7.
    Figure 7: "Limit state" selector
  2. Choose applicable predefined combination of action depending on current annex (or "-" if a user-defined load combination is to be created), see Figure 8.
    Figure 8: "Combination of action" selector
  3. A loadcase is added to the load combination simply by clicking on it in the Loadcase list (area 2), see Figure 3. Chosen loadcase will be added to the text string in area 1, see Figure 3 with corresponding coefficients based on the current annex, calculation settings and chosen combination of action.

    The added loadcase will change color to red in the Loadcase list to indicate that it's a part of the current load combination, see Figure 9.

    Figure 9: Adding a loadcase

    When including variable loadcases the adding order is of importance i.e. the first variable loadcase that is added will by default be the main loadcase. This is indicated in the Loadcase list by a "x" behind the loadcase, see Figure 10 and 11.

    Figure 10: Adding main variable loadcase

    Figure 11: Adding variable loadcase
  4. To remove a loadcase from the load combination just click on the loadcase that should be removed in the Loadcase list and it will be removed from the text string in area 1, see Figure 3, and also change color to black i.e. unused, see Figure 12.

    Figure 12: Removal of a loadcase

    When a main variable loadcase is removed the program will automatically choose the next variable loadcase in the text string as main.

    If the user choose to edit the text string in area 1, see Figure 3, the "Combination of action" in area 3, see Figure 3, will automatically switch to "-" i.e. user-defined.
  5. Don't forget to press the "Change" button when the definition of the load combination is done.

PRE-Stress uses a feature called dependency to model the effects of preceding load combinations, such as transfer losses and remaining displacement due to creep, shrinkage and cracking. A given load combination can be set to depend on previous load combination. Dependency on an ultimate limit state (ULS) load combination cannot be set (and is not a meaningful concept), whereas dependency on a serviceability limit state (SLS) load combination is always possible.  Thus, a ULS can depend on an SLS, but never the reverse. Load combinations are calculated in the order given in the load combination list, meaning a load combination can only depend on earlier load combinations. Chains of dependency (depending on a load combination which in turn depends on a load combination, and so on) are permitted and indeed common.

For the current load combination, start values such as remaining deformation after release, cracking etc., are inherited from the load combination it is dependant on.

If your list of load combinations includes a serviceability limit state (SLS) load combination of the long term type, you may define specific parameters such as creep coefficient and shrinkages by clicking the Long Term parameters tab in the project tree handler.