Non-linear combinations

Definition, application

Non-linear combinations are similar to standard combinations and are used for non-linear calculations. A nonlinear combination MUST be defined if a nonlinear calculation is supposed to be carried out. Without a nonlinear combination defined, the program is NOT CAPABLE of running the nonlinear calculation of any kind.

Where to find in SCIA Engineer

All non-linear combinations are stored in Load cases, Combinations / Nonlinear combinations in main tree. It is possible go there in menu Tree / Load cases, Combinations / Nonlinear combinations too.

How to create a new one

In 'Nonlinear combinations' dialog there are possibilities 'New' or 'New from combination'. For modification it is possible to use buttons 'Insert', 'Edit' and 'Delete'. 'New' starts dialog 'Nonlinear combinations - NCX' where it is possible to define new one and add some LCs to it .

'New from combination' allows to create nonlinear combination from standard linear and envelope combinations saved in 'Combination' dialog. After click on this button 'Make selection' dialog will appear.

In dialog there are several properties:

Type of combination

Linear / Envelope

According this choice available combinations are displayed in left window. it means one nonlinear combination will be created from one selected linear combination, several nonlinear combinations will be created from one envelope combination because it contain several linear ones.

Note: 'Envelope' choice means all code depended combinations like ULS, SLS, Characteristic., Seismic...

Used filter

All / All dangerous / Most dangerous

All = is set as default value. It means envelope combination will be decomposed on the background on linear ones and from every linear combination one nonlinear combination will be created. Approximate number of generated nonlinear combinations is displayed in right bottom corner of dialog.

If linear analysis is successfully passed and results are available, other two possibilities are available in combobox.

All dangerous = nonlinear combinations will be generated only from dangerous combinations from the selected envelope combination. Dangerous combination is a combination which causes a stress extreme on FE in the envelope. Detection of dangerous combinations is based on stress evaluation after a linear analysis. The number of generated combinations will be lower.

Most dangerous = it is possible to chose how many most frequent dangerous combination from envelope combination will be transformed to nonlinear ones.

Note: See schema below the table.

Number of dangerous

Number of most frequent dangerous combination from envelope combination which will be transformed to nonlinear ones.
Create class From all generated nonlinear combinations the result class will be automatically created. So this result class from nonlinear combinations generated from envelope combination will behave for nonlinear analysis as envelope combination for linear analysis.

Explanation of theoretical background on an example :

Descriptions for five tables in five columns :

Model of structure covered by 10 FEs ---> Envelope combination contains 12 linear ones --->Analysis of extreme stress for every FE ---> Calculated frequency ---> Sorted as most dangerous

Initial imperfection

The non-linear combination may be defined with a specific kind of initial imperfection in shape of the modelled structure. There are several ways to define the initial imperfection. In general, the imperfections are divided into two groups:

Bow imperfection

None

No initial imperfection is imposed.

Simple curvature

f

Curvature of one 1D member

1/f

Radius of curvature

According to buckling data

The imperfection is derived from the buckling data. For standard sections the buckling curves are determined according to the appropriate national standard (e.g. for EC3 Table 5.5.3 is used ). For other sections, the buckling curves are taken from the user input made in cross-section definition.

Global imperfection

None

No initial imperfection is imposed.

Selection of an entity from library

Since SCIA version 22.1., the global imperfections are treated as a standalone entity, which has its own library and items from this library (global imperfections) might be assigned to nonlinear combinations.

Initial global geometrical imperfections are in more detail described in this chapter .

Nonlinear composite analysis

Stage for composite analysis model

This setting is available in case the composite analysis model is enabled in the project. It defines the reference stage for the analysis of the structure in case composite members are present in the model. For more details, see Nonlinear & Stability analysis of composite structures.