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When do you use a polyline to model rafters?

A polyline is an option in SCIA Engineer that allows to model a continuous 1D element constituted by multiple bars and treat it as one object.

How to create a polyline

A polyline element can be modelled using the command to create a 1D element and then selecting the option polyline in the command menu.

Polyline Command

An alternative is using the command “Polyline Edit” in the “Modify” menu and then selecting the option “Join curves into polyline”

Polyline Command

In this case, it is important to check that additional data haven’t been lost.

 

Differences between the model of rafters as a polyline or as separate elements

The polyline could be useful in a model because it allows the user to have less element in the results menu. From the other side, any data on nodes (releases, nodal supports, etc) can only be added to the beginning and end of the polyline. 

What is more interesting to compare is instead the buckling factors of the rafters when they are modelled as separate elements or as polylines.

In the first case, two symmetric rafters are modelled as separate elements and then as a polyline.

Symmetric Structure

The buckling length ly is the same for the two rafters and it is 13,93 m. 

System lengths and buckling settings symmetric rafter

Then the same rafters are modelled as a polyline.

System lengths and buckling settings symmetric polyline

The buckling length ly is 14,12 m so, in this case, the results are similar enough. 

 

The second case shown, it is one in which two rafters of different length are modelled as separate elements.

Asymmetric Structure

The buckling length ly is different for the two rafters and it is respectively 6,17 m and 10,51 m. 

System lengths and buckling settings asymmetric rafter System lengths and buckling settings asymmetric rafter

Then the same rafters are modelled as a polyline. 

System lengths and buckling settings asymmetric polyline

The buckling length ly is 14,25 m so, in this case, the results differ between the two configurations.