Imperfection cases describe the framework conditions of the imperfections. This allows you to specify whether geometric imperfections or equivalent loads are to be taken into account. The specifications are governed by the individual standards.
Base
The Base tab controls which type of imperfection is present. Imperfections can be taken into account, for example, in the form of equivalent loads, geometric pre-deformations, or story-related inclinations.
In the Assignment tab, you can assign the imperfection case to specific load cases and combinations.
Settings
The list provides various imperfection types to choose from.
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 it anew.
Initial Sway Imperfection via Table
With this imperfection type, imperfections can be represented 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 load combination. Define the governing parameters in the Structural Deformation tab.
In the 'Source type' list, select 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, select whether the displacements are based on a load case or a load combination. However, 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, since displacements in modal analyses can only be taken into account for load cases.
Group of Imperfection Cases
This imperfection type offers the possibility to combine 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 taken into account 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 manually assigned. As a result, all load combinations that use this load case are automatically assigned as well. For load combinations without wind, on the other hand, the imperfection case is only taken into account if the 'Assign to all load combinations without an assigned imperfection case' option is activated.
Equivalent Loads from Load Case
The equivalent loads are available as an imperfection type (see image Selecting imperfection type).
You can describe the imperfections in a load case by means of equivalent loads. Assign the load case category None to this load case so that it is not taken into account in the combinatorics, but only as an imperfection load case. In this way, you can, for example, define stabilization loads independently of axial forces – in contrast to inclinations or "notional loads," which act as equivalent loads in relation to axial forces.
Select the load case in 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 imperfection type).
Story Imperfections
You can describe the normative displacement of a building in a table. This means you do not have 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. You can add rows using the
button. 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 makes vertically oriented initial sway imperfections possible.
The 'imperfection direction' determines the direction(s) in which inclinations are applied. With the default setting 'XY', the inclinations for the X and Y directions must be specified in the table. If they act in only one direction, define the corresponding axis in the list.
The table shows the 'inclination coefficient as reciprocal of 1'. If you deactivate the check box, the values appear as decimal numbers.
Structural Deformation
The structural deformation is available as an imperfection type (see image Selecting imperfection type).
The 'imperfection shape' is based on the deformations of a load case or 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. Using 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. As a rule, this is the location with the largest displacement. However, you can also select another node in 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' determines the orientation of the imperfections. With the 'S' and '-S' options, spatially pronounced imperfections are possible, such as those occurring in spherical models, for example.
Specify the 'imperfection sag', 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 if the critical load is calculated for a load case or load combination using the Structural Stability add-on. The concept of the tab is matched to the source type.
Buckling Mode from Load Case/Load Combination
The 'imperfection shape' is based on the eigenvalues of a load case or load combination. The mode shapes are scaled to a sag.
Imperfection Shape from
Select the governing load case or load combination in the list. Using the
button, you can create a new load case or load combination. Then define in the list the buckling mode on which the imperfections are to be based.
Imperfection Sag
The reference location defines the location in the model whose displacement serves as the reference for the sag. As a rule, this is the location with the largest displacement. However, you can also select another node in 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 determines the orientation of the imperfections. With the 'S' and '-S' options, spatially pronounced imperfections are possible, such as those occurring in spherical models, for example (see image Selecting scaling direction).
Specify the imperfection sag, 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 respectively present 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 analyzed. 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 taken into account. Two sub-load cases or sub-combinations are created per mode shape, since each mode shape is scaled once in the positive and once in the negative direction. These sub-load cases and sub-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 analyzed. As a result, the sub-load combinations LC1.1 to LC1.4 are created – that is, four sub-load combinations.
The definition type allows you to control the input for scaling the mode shapes. Currently, two options are available in the list:
- Relative: You can specify the maximum imperfection sag directly in the 'Parameters' section (see image Defining parameters for automatic determination of imperfections). This imperfection sag depends on the virtual buckling length of the object with the maximum amplitude within the mode shape.
- EN 1993-1-1: The imperfection sag is determined according to the normative specifications of EN 1993-1-1 Tab. 5.1. In the further sections, you can adjust the imperfection parameters.
Parameters
Specify whether the cross-section design should be 'elastic' or 'plastic'. The imperfection sags 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' that 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 if a modal analysis is performed for a load case or load combination.
The 'imperfection shape' is based on the dynamic eigenvalues of a load case or load combination. The mode shapes are scaled to a sag.
Imperfection Shape from
Select the governing load case or load combination in the list. Using the
button, you can create a new load case or load combination. Then define in the list the mode shape on which the imperfections are to 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 an imperfection type (see image Selecting imperfection type).
You can combine several imperfection cases into one group and thus take them into account simultaneously, for example. This option is useful for special analyses in order to apply global imperfections and local buckling.
Define the imperfection cases that are to be combined. You can add rows in the table using the
button.
You can apply a 'factor' to each imperfection case. Use the 'operator' to specify whether the imperfection cases act simultaneously ('and') or whether they exclude each other ('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 taken into account in the calculation.
Assign
The 'Assign' column lists all load cases that 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. For example, if the imperfection case 'Wind in +X' is to be taken into account 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: For example, an imperfection case with inclinations in X applies to wind load in X, and an imperfection case in Y applies to wind load in Y. For unassigned load combinations (those without wind loads), each imperfection case is taken into account separately. RSTAB creates two load combinations each with identical superposition criteria but different imperfection cases.
