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 it.

How to create new one

In the dialog for a" Non-linear combinations" it is possible to define 'Bow imperfection' or 'Global imperfection'. Every possibility has several other types of imperfections with specific settings, which must be fill in. According these settings imperfections are taken into account in " Nonlinear calculation" for nonlinear combination with this setting.

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	"Simple inclination"	The imperfection is expressed in the form of a simple inclination. The inclination may be defined in millimetres per a metre of height of the structure. It means that only horizontal inclination in the global X and Y direction may be specified. The inclination is linearly proportion to the height of the building. This option is applicable mainly for high-rise buildings. It has no or minimal influence on horizontal structures.
	"Inclination functions"	The initial imperfection is defined by a function (or curve). The user inputs the curve by means of height-to-imperfection diagram. This option is applicable mainly for high-rise buildings. It has no or minimal influence on horizontal structures.
	"Deformation from load case"	This option requires two-step calculation. First, a calculation for a required load case must be performed. The deformation due to this load case is then used as the initial imperfection for further calculation.
	Buckling shape	This option requires two-step calculation. First, a stability calculation must be performed. The calculated buckling shape is then used as the initial imperfection for further calculation. Regardless of the approach, the initial imperfection can be defined for a non-linear combination only. It is one of the parameters that the user may define in the Non-linear combination manager.

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.