Imperfection cases describe the boundary conditions of the imperfections. This allows you to specify whether geometric imperfections or equivalent loads are to 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-by-story 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
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 imperfection via table
This imperfection type allows you to represent imperfections story by story. Define the parameters in the Initial Sway Imperfection via Table tab.
Structural deformation
The imperfections are based on the deformations of a load case or a load combination. Define the governing parameters in the Structural Deformation tab.
In the 'Source type' list, specify 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. In the 'Source type' list, specify whether the displacements are based on a load case or a load combination. The governing imperfection shapes can also be determined automatically from the buckling modes for all assigned load cases and load combinations.
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. Only one option is available in the 'Source type' list because the displacements in modal analyses can only be considered for load cases.
Group of imperfection cases
This imperfection type offers the possibility of combining the specifications from several imperfection cases. You can group the imperfection cases in the Group of Imperfection Cases tab and scale them with factors.
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 load case "Wind" is assigned manually. This automatically also assigns all load combinations that use this load case. 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 an imperfection type (see image Selecting the Imperfection Type).
You can describe the imperfections in a load case using 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 stabilization loads independently of axial forces – in contrast to the inclinations or "notional loads" that act as equivalent loads related to axial forces.
Select the load case from the list or create a new load case using the
button.
Initial sway imperfection via table
The initial sway imperfection is available as an imperfection type (see image Selecting the Imperfection Type).
Story imperfections
You can describe the normative displacement of a building in a table. This means you do not need to define imperfections for each individual column. The story ranges in the table control how the nodes in the building sections are displaced.
Define the ordinates of the stories and assign the inclinations. Use the
button to add rows. Make sure the ordinates are in ascending order. From the specifications, RSTAB determines a geometric displacement and applies it to the nodes.
Options
If the imperfections are not to 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; this also allows vertically oriented initial sway imperfections.
The 'Imperfection direction' specifies the direction(s) in which inclinations are applied. With the default setting 'XY', the table specifies the inclinations for the X and Y directions. If they act in only one direction, specify the corresponding axis in the list.
In the table, the 'Inclination coefficient as reciprocal of 1' is displayed. If you deactivate the check box, the values are displayed as decimal numbers.
Structural deformation
The Structural Deformation is available as an imperfection type (see image Selecting the Imperfection Type).
The 'imperfection shape' is based on the deformations of a load case or a load combination. The geometric pre-deformations are scaled to a camber.
Imperfection shape from
Select the governing load case or load combination from the list. Use the
button to create a new load case or load combination.
Imperfection camber
The 'Reference location' defines the location in the model whose displacement serves as the reference for the camber. This is usually the location with the greatest displacement. However, you can also select another node from the list or define it graphically using the
button.
If the imperfections are not to be related to the global XYZ coordinate system, you can select or create a user-defined 'Coordinate system'. The 'Scaling direction' specifies the orientation of the imperfections. With the options 'S' and '-S', spatially pronounced imperfections are possible, such as those occurring in spherical models.
Specify the 'Imperfection camber', which 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 Buckling Mode imperfection type can be defined when the critical load is calculated for a load case or load combination using the Structural Stability add-on. The concept of the tab is aligned with the Source type.
Buckling mode from load case/load combination
The 'imperfection shape' is based on the eigenvalues of a load case or a load combination. The mode shapes are scaled to a camber.
Imperfection shape from
Select the governing load case or load combination from the list. Use the
button to create a new load case or load combination. Then define the buckling mode on which the imperfections are to be based in the list.
Imperfection camber
The Reference location defines the location in the model whose displacement serves as the reference for the camber. This is usually the location with the greatest displacement. However, you can also select another node from the list or define it graphically using the
button.
If the imperfections are not to be related to the global XYZ coordinate system, you can select or create a user-defined Coordinate system. The Scaling direction specifies the orientation of the imperfections. With the options 'S' and '-S', spatially pronounced imperfections are possible, such as those occurring in spherical models (see image Selecting the Scaling Direction).
Specify the Imperfection camber, which 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.
Automatic imperfection
In the automatic determination of imperfections, the imperfection shape is derived from the respective load-dependent mode shape of the model and scaled with the defined imperfection amplitude.
Settings for automatic determination
Define the Number of shapes to be examined. Please note the following: For each load case and each load combination, so-called "sub-load cases" or "sub-load combinations" are created in which the imperfection shape is considered. This creates two sub-load cases or sub-combinations per mode shape because each mode shape is scaled once in the positive direction and once in the negative direction. These sub-load cases and combinations can then be analyzed like normal load cases or load combinations.
Example: You have assigned automatic imperfections to LC1 and specify two shapes to be examined. This creates the sub-load combinations LC1.1 to LC1.4 – that is, four sub-load combinations.
The Definition type allows you to control the input for scaling the mode shapes. Two options are currently available for selection in the list:
- Relative: You can specify the maximum imperfection camber directly in the 'Parameter' section (see image Defining Parameters for Automatic Determination of Imperfections). This imperfection camber depends on the virtual effective length of the object with the maximum amplitude within the mode shape.
- EN 1993-1-1: The imperfection camber is determined according to the normative specifications of EN 1993-1-1 Table 5.1. You can adjust the imperfection parameters in the further sections.
Parameter
Specify whether the cross-section design is to be performed 'Elastically' or 'Plastically'. The imperfection cambers to be used depend on this decision.
Precamber for
The buckling curve of the cross-sections is generally known. For 'Members without buckling curve' (with unknown buckling curve), you can specify the value manually. The same applies to 'Surfaces and solids', which do not have a buckling curve according to EN 1993-1-1.
Dynamic mode shape
The parameters of the Dynamic Mode Shape imperfection type can be defined when 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 camber.
Imperfection shape from
Select the governing load case or load combination from the list. Use the
button to create a new load case or load combination. Then define the mode shape on which the imperfections are to be based in the list. With the 'Automatic' option, you can have the governing mode shape determined by the program.
Imperfection camber
You can define the parameters as described in the Imperfection Camber section for a buckling mode.
Group of imperfection cases
The Group of Imperfection Cases is available as an imperfection type (see image Selecting the Imperfection Type).
You can combine several imperfection cases in a group and thus, for example, consider them simultaneously. This option is useful in special analyses to apply global imperfections and local buckling.
Define the imperfection cases to be combined. Use the
button to 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
The 'Assign' column lists all load cases you have created. 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. If, for example, the imperfection case 'Wind in +X' is to 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, this also assigns all load combinations that use this load case.
In this way, you can align 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. RSTAB creates two load combinations each with identical superposition criteria but different imperfection cases.
