Question
What is Drilling DOF? How does it affect the force distribution in the structure?
Answer
Plate Elements have the following DOF, as shown in the figure below. The typical plate elements have three translation and two rotational DOF at a node. The plate element does not have rotational stiffness. To account for the in-plane rotational stiffness, the Drilling DOF is used.
The deficiencies of this element type are the zero energy mode (hourglass deformation) and that the node rotations are not true rotations. The boundary conditions of these elements have a high influence on the results. The rotations should be fixed at the boundaries because the node rotations are not true rotations.
To further clarify, let us consider the simple example shown in the image below. Let’s say we have a slab (modeled using a plate element without the drilling DOF) supported by a column (modeled using a beam element) and subjected to a lateral load, which causes rotation about the vertical axis of the plate element. Since plate elements do not have rotational stiffness about their vertical axis, this will result in singularity errors in the analysis.
Now, if we consider the plate element with drilling DOF supported by a column (modelled using beam element) and apply lateral load. for this case, no singularity errors are observed in the analysis.
Now, if we compare the torsion moment of the column for both the case.
- Without drilling DOF: No torsional moment is generated since plate element does not have the rotational stiffness,
- With drilling DOF: Torsional moments are accounted with Drilling DOF