I have a Validation > Analysis > First-order linear > “Non-linear elements changed to linear”” warning. Why do I get this and how do I resolve it?

Question

I have a Validation > Analysis > First-order linear > “Non-linear elements changed to linear” warning. Why do I get this and how do I resolve it?

Answer

“Non-linear elements changed to linear”, indicates that elements have been defined in the model as non-linear i.e. able to withstand either tension or compression only, (which requires non-linear analysis) but a linear analysis has been undertaken. Linear analysis cannot cater for non-linear behaviour, and so non-linear elements are converted to linear elements to allow a solution. It is likely the analysis results would be incorrect for this analysis.

Using the Design ribbon

The Design ribbon tab > Options > Analysis page controls the analysis to be performed when you undertake a Design command.

In the screenshot below there are no non-linear elements defined within the model so a first-order linear analysis is performed and the message does not appear.



There is a note within the Design ribbon tab > Options > Analysis page, that advises that should a non-linear element exist then a non-linear analysis will be run during the design. The screenshot below highlights this note and also shows that following a Design command both a First-order linear (with the warning message), and a non-linear analysis is performed when a non-linear element is found in the model.



Tekla Structural Designer as part of its design routine, will always run a linear analysis, this is used for a number of operations including the calculation of equivalent horizontal forces (EHF’s). Since a linear analysis cannot deal with tension or compression only elements, then the warning is raised - and for this type of analysis the element is set as linear i.e. able to take both compression and tension. The warning is displayed against this analysis type to bring this to your attention. The non-linear elements in the model have however, been identified so a non-linear analysis is ALSO carried out on the model and reported just below the warning. When designing the elements in the model the non-linear results are used. You can therefore ignore the warning in this situation. Note. When creating brace patterns such as cross flats it is important to ensure that you use the brace patterns (X, K, V, A) available.

Using the Analyse ribbon

When running analysis from the Analyse ribbon tab, then you control the type of analysis you perform. In this case you would need to run the appropriate non-linear analysis from the ribbon tab in order to consider the non-linear elements.



Running a linear analysis would result in the warning and hence the elements are converted to linear elements and able to take both tension and compression.

If you opted to run Analyse All, this has the same effect as running a Design (Statics) command, albeit without running the final phase code checking (design) phase. It follows the Design ribbon tab > Options > Analysis page so would run both a linear and non-linear analysis if non-linear elements were encountered.

Note, when reviewing Results via the Results view regime, ensure you are reviewing the correct analysis results by setting the Analysis Result type to display results for.