Model stiffeners by beam elements.
It is preferable to model the stiffeners by beam elements. This way the model is more flexible for iteration purposes.
When using offset beams be aware that, where the beam ends, you introduce a moment in the structure, which is equal to the axial force in the beam times the offset.
The figure below shows some differences in result with and without offset beams. At the end of the stiffener the moment, due to the offset, gives an out of plane displacement. This results in a hump, as shown in the upper figure and the animation.
The model contains a box plated structure with T stiffeners at top and bottom panel. The plated structure is 10mm thick. The T stiffener has dimensions h x tweb ; w x tflange = 108.5x4.2; 100x5.5 (in mm). The beam is 1m x 1m x 8m and clamped at the left side. It is loaded by a force of 1000kN at the right side.
Dependent on how abrupt the offset is given, it will result in higher stresses in both, the beam and the plated structure. This could be far away from the truth.
(Please note that the legend was set to max 355MPa. The actual max stress is higher).
Since offset beams lead to larger stresses in the plates where the beam ends, it is preferable to work without offset beams when modelling stiffeners. When using the FEM Data Streamliner buckling tool, the offset will be corrected by the software. In such case be aware that the stiffness of the structure cross section slightly increases and the local stiffness of the stiffener reinforced plate is decreased.
If necessary the cross section stiffness needs to be adjusted by adjusting the stiffener cross section area. This leads to conservative results. The code check for buckling uses only the stresses in the plate elements (mid plane stresses) and assumes that this stress also occurs in the stiffener.
About the local stiffness: The buckling module will check the stiffener with the plate stresses. If an out of plane pressure is applied, the stiffener will also be checked for the resulting bending stresses.