Bulk Data Entry Define the initial geometric imperfection at the beginning of a nonlinear analysis.


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IMPERF ID TYPE              

The imperfection can be specified either based on an existing H3D result file (TYPE=H3DRES) or defined explicitly on each grid (TYPE=GRID).

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  G X Y Z          


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IMPERF 5 H3DRES              
  1 1 1 0.1          
  1 2 2 0.05 11        
  2 101 LAST 0.02 12        
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
IMPERF 5 GRID              
  11 0.1 1.0            
  12 0.2   3.0          
  13 0.3 2.0 6.0          


Field Contents SI Unit Example
ID Set identification number.

No default (Integer > 0)

TYPE Imperfection type. 1
The H3D result file should be provided through ASSIGN, H3DRES Subcase Entry.
The imperfection on each grid can be defined directly. The fields X/Y/Z are the imperfection in the grid coordinate system.

No default (Integer > 0)

H3DRESID H3D results file identification number that is specified with ASSIGN,H3DRES.

No default (Integer > 0)

SUBID Subcase identification number.

No default (Integer > 0)

NRES Result number (the buckling mode number or the increment number from the displacement result).
Last increment of a subcase.

No default (Integer ≥ 0)

FACT Scale factor.

Default = 1.o (Real ≥ 0.0)

GSET (Optional) Grid set identification number, on which the imperfection is applied.

No default (Integer > 0)

G Grid identification number.

No default (Integer > 0)

X, Y, Z Imperfection at one grid (in grid coordinate system).

No default (Real)



  1. Either the buckling modes or the displacement results can be treated as imperfection. The H3DRESID continuation lines can be repeated as many times as required. Each continuation line represents a particular imperfection shape. Multiple imperfection shapes are combined by means of linear superposition. H3DRESID is associated with the H3D result file ID. SUBID can be either the subcase ID from linear buckling analysis, or any subcases containing displacement result. The field NRES is the number of the result, which can be either the number of buckling modes for the linear buckling analysis, or the increment number containing displacement result. GSET is the grid set ID, which is optional for each imperfection shape.
  2. In post-buckling analysis with arc-length method, imperfection is sometimes recommended to solve the instability problem, which can practically transform the instable bifurcation problem into a stable limit-point problem.
  3. Imperfection can currently only be applied in nonlinear analysis. IMPERF=ID is specified in the Subcase Information section. The IMPERF Bulk Data Entry is added to define the imperfection shape.
  4. From the point of view of a finite element model, the imperfection will change the translational coordinates of the grids. The imperfection shape can either come from an existing H3D result file, or be defined explicitly on each grid. For TYPE=H3DRES, the H3D result file should contain at least either the buckling modes or the displacement results. The buckling modes and displacement results can be combined in one IMPERF card at the same time. Optionally, for each continuation line a grid set ID can be specified to limit to a particular region of the model where the imperfection is to be applied.
  5. For the same model, different imperfections can be defined for multiple nonlinear subcases. However, the imperfection is only considered in the solver calculation, and the final H3D result file still contains the mesh of the original structure. Usually the magnitude of the imperfection is very tiny, otherwise, it is recommended to create a new model.
  6. In case of subcase continuation, the imperfection can only be specified in the first subcase. All continue subcases following this first subcase are all using it as the reference configuration.
  7. Imperfection is supported for nonlinear static and nonlinear direct transient for both small and large displacement analyses.