Should I choose the purlins as a part of the analysis system?

If the purlins are included in the analysis system, the stiffness of the purlins will be included in the analysis and will appear as a rod element. It will carry the biaxial bending-axial force and shear force depending on the actions.

If the purlins are not included in the analysis system, their stiffness is not included in the analysis and they do not appear as a rod element. However, they transfer the load that comes from the sheetings to the main beam as a concentrated load. The design of purlins is made according to biaxial bending and shear force.

When the purlins are not included in the analysis system, it is necessary to define the sag-rods which are stability elements that pass through the ends of the roof braces and continue throughout the structure in order for the roof to work as a whole structural system. These elements will both reduce the lateral buckling length of the beam and operate the roof as a whole structural system. 

To add or remove purlins from the analysis system:

1. Click the "steel purlin" command from the menu.

2. Click on the "select analysis part" command from the opened toolbar.

3. When the command is clicked, the purlins included in the analysis model will be selected on the 3D perspective screen.

4. When the purlins to be removed from the analysis model are clicked and deselected, the purlin will be removed from the analysis system and does not appear as a rod element in the analysis.

5. By clicking on the purlins it is possible to include these in the analysis model.

6. By clicking on the sag-rods it is possible to include these in the analysis model and the command is closed.

7. The wall girts or side rails defined to the steel columns with the steel purlin command are not included in the analysis system.