Initial imperfections

Definition, application

The initial deformation curve may be used in a " Non-linear combinations" to define the shape of the structure at the beginning of the analysis. Thus a state of initial imperfection in shape can be easily modelled.

Two types of imperfection are available:

- Global imperfections, also known as sway imperfections

- Local imperfections, also known as bow imperfections

Global imperfections

Global imperfections are taken into account in the analysis by altering the initial geometry of the finite element mesh, before starting the analysis. The coordinates of each mesh node are modified according to settings defined by the user.

The imperfection may be defined manually, by some function that depends on the initial position of each mesh node, or based on the deformed shape of the structure, for instance from the linear analysis of a load case, of from a buckling shape obtained from a stability analysis.

Local imperfections

Local imperfections are taken into account using the method of notional loads, automatically applied in the most unfavourable direction for each considered load combination. A detailed description is available in the chapter related to bow imperfections.

This article describes how bow imperfections are taken into account in the nonlinear analysis in SCIA Engineer.

Where to find it

In project data dialog in part 'Functionality' it is necessary to switch ON 'Initial imperfection'. After that in dialog for a " Non-linear combinations" it is possible to define the local bow imperfection, or to assign a global imperfection entity from the library of Global imperfections.

How to create new one

In the dialog for a" Non-linear combinations" it is possible to directly define 'Bow imperfection' or to assign a 'Global imperfection' from the library.

According these settings, the imperfections are taken into account in " Nonlinear calculation" for nonlinear combination.

Possibilities

Imperfections	Specification	Description
"Bow imperfections - Theoretical background"	"Inclination and curvature of beams"	The programme automatically determines which direction of curvature is critical and uses that direction for calculation. The curvature is taken into account in all beams in the structure regardless of their spatial orientation.
	"Buckling settings that invalidate 2nd order results"	The buckling settings allows the user to define local imperfections (bow imperfections) which are taken into account during a nonlinear analysis
Global imperfection	Item from the Global imperfection library	The selected library imperfection is considered for the nonlinear combination.

The calculated and displayed deformation is ALWAYS measured from the imperfect model, i.e. it does not represent the total deformation from the ideal shape, but the overall deformation from the imperfect shape.

Calculations taking account of any type of imperfection are non-linear calculations and as that they are sensitive to the size of finite elements. Or to be precise, they are sensitive to the number of finite elements per a member. The user MUST remember that a division giving just one finite element per a beam is NOT sufficient and may give completely misleading results.