Imperfection cases describe the boundary conditions of imperfections. This allows you to specify whether geometric imperfections or equivalent loads should be considered. The specifications are regulated in the individual standards.
Base
The Base tab controls which type of imperfection is present. Imperfections can be considered, for example, in the form of equivalent loads, geometric pre-deformations, or story-wise inclinations.
In the Assignment tab, you can assign the imperfection case to specific load cases and combinations.
Settings
Various imperfection types are available for selection in the list.
Local imperfections only
The imperfections are represented by equivalent loads. The initial sways and precambers depend on axial forces. You can define the imperfections in the Local Imperfections categories for members and member sets.
Equivalent loads from load case
The imperfections are based on the loads of a specific load case. You can assign this load case in the Equivalent Loads from Load Case tab or create a new one.
Initial sway via table
With this imperfection type, imperfections can be represented story-wise. Define the parameters in the Initial Sway via Table tab.
Static deformation
The imperfections are based on the deformations of a load case or a load combination. Define the governing parameters in the Static Deformation tab.
Select in the 'Source type' list whether the displacements are based on a load case or a load combination.
Buckling mode
The imperfections are based on the mode shapes of a stability analysis. You can define the governing parameters in the Buckling Mode tab.
Select in the 'Source type' list whether the displacements are based on a load case or a load combination.
Dynamic mode shape
The imperfections are based on the mode shapes of a dynamic analysis. You can define the governing parameters in the Dynamic Mode Shape tab.
Select in the 'Source type' list whether the displacements are based on a load case or a load combination.
Group of imperfection cases
This imperfection type provides the option to combine the specifications from several imperfection cases. You can group the imperfection cases and scale them with factors in the Group of Imperfection Cases tab.
Options
The 'Consider for generated load combinations' check box is activated by default. This ensures that the imperfection case is considered for all load combinations that are not covered by a manual selection in the Assignment tab.
Example
In the 'Assignment' tab, the "Wind" load case is manually assigned. This automatically assigns all load combinations that use this load case as well. For load combinations without wind, however, the imperfection case is only considered if the 'Assign to all load combinations without assigned imperfection case' option is activated.
Equivalent Loads from Load Case
The Equivalent Loads are available as imperfection type (see the Selecting Imperfection Type image).
You can describe the imperfections in a load case by equivalent loads. Assign the load case category None to this load case so that it is not considered in the combinatorics, but only as an imperfection load case. In this way, you can define stabilizing loads independently of axial forces, for example - in contrast to the inclinations or "notional loads" that act as equivalent loads related to axial force.
Select the load case in the list or create a new load case with the
button.
Initial Sway via Table
The Initial Sway is available as imperfection type (see the Selecting Imperfection Type image).
Story Imperfections
You can describe the normative displacement of a building in a table. This way, you do not need to define imperfections for each individual column. The story areas of the table control how the nodes in the building sections are displaced.
Define the ordinates of the stories and assign the inclinations. With the
button, you can add rows. Ensure an ascending order of the ordinates. From the specifications, RFEM determines a geometric displacement and applies it to the FE nodes.
Options
If the imperfections should not be related to the global XYZ coordinate system, you can select or create a user-defined 'Coordinate system'. The 'Story direction' specifies the ordinate of the stories; thus, vertically oriented initial sways are also possible.
The 'Imperfection direction' specifies in which direction(s) inclinations are applied. With the default setting 'XY', the inclinations for the X and Y directions are specified in the table. If they only act in one direction, specify the corresponding axis in the list.
In the table, the 'Inclination coefficient as reciprocal of 1' is displayed. If you clear the check box, the values appear as decimal numbers.
Static Deformation
The Static Deformation is available as imperfection type (see the Selecting Imperfection Type image).
The 'Imperfection shape' is based on the deformations of a load case or a load combination. The geometric pre-deformations are scaled to a sag.
Imperfection shape from
Select the governing load case or load combination in the list. With the
button, you can create a new load case or load combination.
Imperfection sag
The 'Reference location' defines the location in the model whose displacement serves as the reference for the sag. This is usually the location with the maximum displacement. However, you can also select another node in the list or define it graphically with the
button.
If the imperfections should not be related to the global XYZ coordinate system, you can select or create a user-defined 'Coordinate system'. The 'Scaling direction' defines the orientation of the imperfections. With the 'S' and '-S' options, spatially pronounced imperfections are possible, such as those occurring in shell models.
Specify the 'Imperfection sag' that represents the maximum geometric displacement. The displacements of the FE nodes are scaled to this value. The deformation shape of the load case or load combination specified above forms the basis.
Buckling Mode
The parameters of the imperfection type Buckling Mode can be defined if the critical load is calculated for a load case or a load combination with the Structure Stability add-on.
The 'Imperfection shape' is based on the eigenvalues of a load case or a load combination. The mode shapes are scaled to a sag.
Imperfection shape from
Select the governing load case or load combination in the list. With the
button, you can create a new load case or load combination. Then define in the list the buckling mode on which the imperfections should be based.
Imperfection sag
The 'Reference location' defines the location in the model whose displacement serves as the reference for the sag. This is usually the location with the maximum displacement. However, you can also select another node in the list or define it graphically with the
button.
If the imperfections should not be related to the global XYZ coordinate system, you can select or create a user-defined 'Coordinate system'. The 'Scaling direction' defines the orientation of the imperfections. With the 'S' and '-S' options, spatially pronounced imperfections are possible, such as those occurring in shell models (see the Selecting Scaling Direction image).
Specify the 'Imperfection sag' that represents the maximum geometric displacement. The displacements of the FE nodes are scaled to this value. The mode shapes of the load case or load combination specified above form the basis.
Dynamic Mode Shape
The parameters of the imperfection type Dynamic Mode Shape can be defined if a Modal Analysis is performed for a load case or a load combination.
The 'Imperfection shape' is based on the dynamic eigenvalues of a load case or a load combination. The mode shapes are scaled to a sag.
Imperfection shape from
Select the governing load case or load combination in the list. With the
button, you can create a new load case or load combination. Then define in the list the mode shape on which the imperfections should be based. With the 'Automatic' option, you can have the governing mode shape determined by the program.
Imperfection sag
You can define the parameters as described in the Imperfection Sag section for a buckling mode.
Group of Imperfection Cases
The Group of Imperfection Cases is available as imperfection type (see the Selecting Imperfection Type image).
You can combine several imperfection cases in a group and thus consider them simultaneously, for example. This option is often useful for shell buckling to apply global imperfections and local buckling.
Define the imperfection cases to be combined. With the
button, you can add rows to the table.
You can apply a 'Factor' to each imperfection case. Use the 'Operator' to specify whether the imperfection cases act simultaneously ('and') or whether they are mutually exclusive ('or').
Assignment
In the Assignment tab, you can assign one or more load cases or load combinations to the imperfection case. Only then is the imperfection case considered in the calculation.
Assign
All load cases you have created are listed in the 'Assign' column. To transfer a load case to the 'Assigned Objects' list, you can use the following options:
- Double-click the load case.
- Select the load case. Then click the
button.
Assigned Objects
The 'Assigned Objects' column manages all load cases that you assign to the imperfection case. For example, if the imperfection case 'Wind in +X' should be considered with the load case 'Wind in +X', transfer this load case to the list as described above. If the Combination Wizard is active, all load combinations that use this load case are also assigned.
In this way, you can match imperfection cases with the effective directions of load cases: An imperfection case with inclinations in X, for example, acts for wind load in X, an imperfection case in Y for wind load in Y. For unassigned load combinations (those without wind loads), each imperfection case is considered separately. RFEM generates two load combinations each with identical superposition criteria but different imperfection cases.
