This article describes special considerations for solid elements and rotational degrees of freedom at their joints.
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Basic behavior, only translational degrees of freedom are activated
Per CSI Analysis Reference Manual, Chapter "The Solid Element", Section "Degrees of Freedom":
The solid element activates three translational degrees of freedom at each of its connected joints. Rotational degrees of freedom are not activated. This element contributes stiffness to all of these translational degrees of freedom.
Situations where rotational degrees of freedom may get activated
When solid elements coexist in the same model with other types of elements that are connected to the joints of solid elements, rotational degrees of freedom may be also activated at the joints of the solid element. This is due to the presence of the other elements that activate rotational degrees of freedom. When no element connected to such solid joint provides rotational stiffness, the .LOG will contain warning messages about zero rotational stiffness. These warning messages can be handled as follows:
- If no moments or no rotational masses are applied at joints, then these warning messages can be ignored, because no stiffness is required to resist zero moments.
- Soft rotational springs can be assigned to solid joints to provide rotational stiffness and make the warning messages about zero rotational stiffness go away.
- If the entire model contains only solid elements, then only the global UX, UY and UZ DOFs can be made available for the analysis in the solver options. This will globally deactivate rotational DOFs.
Some scenarios when zero rotational stiffness is reported in the .LOG file for solid elements:
- Mass is assigned to the joints of the solid elements. Even if the mass assignment contains only translational masses and all rotational masses are set to zero, rotational DOFs are activated and zero rotational stiffness is still reported.
- Connecting frames elements to joints of solid elements essentially provides moment releases to the joint of the frame where it is connected to the solid (because the solid element provides no rotational stiffness). When the frame element has other releases assigned to it, this may results in the zero rotational stiffness reported in the .LOG file.
Edge constraints
The implementation of edge constraint activates three rotational DOFs at the joints of the solid element. Since solid joints do not provide rotational stiffness, internal rotational springs are generated at affected joints to provide rotational stiffness such that local numerical instabilities do not occur. These springs are displayed using red symbols when reviewing the analysis model in the GUI.
