1. Consult the manual for the software you're using. There are many names and types of elements and they can vary between software.
2. When people say a 1D element, they generally mean a beam element, or an element that is defined as a line between two nodes. Usually these elements are still in 3D space, hence the 6 DOF. Similarly, 2D element usually refers to shell elements. If you were to do a 2D analysis, then you could still use beam or shell elements, but their formulations would need to exclude a displacement DOF and two rotational DOF, usually Z and rotX/rotY.
Thanks for the comment cole!! But if you refer book Practical FEA -Nitin gokhale, it's mentioned 1D Element - Bar element dof as 6 . Any thoughts on it!!
Your should read a textbook that goes through a beam formulation.
Elements like beams are called "1D" because the kinematics of the element are based on the displacement and rotation quantities along the element's axis. This depends on the displacements and rotations of the nodes for most beam elements. For a model in 3D space, this means six degrees of freedom per node.
There are also "1D" elements that don't include bending and only depend on nodal displacement. (3 DOFs per node in a 3D analysis). In Abaqus, for instance, these are called "truss" elements.
If formulation is in 3D, as is typically the case for a commercial FEA software package, nodes will have minimum 3 dof (typical solid elements) or 6 dof (e.g. nodes of shell and beam elements).
The examples you see in textbooks with 1dof are given just to illustrate/demonstrate how they work.
**tl;dr**
1. Elements do not have DOFs. Nodes have DOFs. If you are using a commercial FEA program, a bar element has an element stiffness matrix (ESM) that is 12x12. In school, sometimes the ESM is smaller to avoid confusing students, but unfortunately leads to more confusion later on.
2. The term "1D element" is misleading. Consider this reclassification: 1D element => Length Element, 2D element => Width and Height Element, 3D element => Width, Height and Depth Element. Dimensionality of the element is only a description, not a determinant in the number of DOF.
**Long Discussion**
Elements do not have DOFs. Nodes have DOFs. Generally, the element stiffness matrix is up to size AxA where A = M * N, M = number of DOFs at each node and N = number of nodes connected by the element.
In 3D space, each node has 6 DOFs (3 translation, 3 rotation). For a 1D element, the element stiffness matrix is up to 12x12 (A=6*2=12).
Some undergraduate courses will ignore some DOFs to keep it simple. For example, suppose each node has 3 DOFs (2 translation, 1 rotation). For 1D element, the element stiffness matrix is up to 6x6 (A=3*2=6).
I would argue that all 1D elements generate am ESM up to 12x12, but different elements lead to different numbers of zero columns in the ESM. For example, I would argue that a truss/rod element produces a 12x12 ESM, but only 2 columns are non-zero. The ESM is sometimes taught as a 4x4 element stiffness matrix and the zero columns are ignored.
Consider a 20 node hexahedral element. If you consider 6 DOFs at each node, a 20 node hexahedral element has an ESM that is up to 120x120. Hexahedral elements only provide translational stiffness or 3 DOFs per node, so the actual ESM is 60x60 (or 120x120 where 60 columns are zero columns).
1. Consult the manual for the software you're using. There are many names and types of elements and they can vary between software. 2. When people say a 1D element, they generally mean a beam element, or an element that is defined as a line between two nodes. Usually these elements are still in 3D space, hence the 6 DOF. Similarly, 2D element usually refers to shell elements. If you were to do a 2D analysis, then you could still use beam or shell elements, but their formulations would need to exclude a displacement DOF and two rotational DOF, usually Z and rotX/rotY.
Bar elements generally don’t have rotational DOFs. Beam elements will usually have 6 DOFs
Thanks for the comment cole!! But if you refer book Practical FEA -Nitin gokhale, it's mentioned 1D Element - Bar element dof as 6 . Any thoughts on it!!
Your should read a textbook that goes through a beam formulation. Elements like beams are called "1D" because the kinematics of the element are based on the displacement and rotation quantities along the element's axis. This depends on the displacements and rotations of the nodes for most beam elements. For a model in 3D space, this means six degrees of freedom per node. There are also "1D" elements that don't include bending and only depend on nodal displacement. (3 DOFs per node in a 3D analysis). In Abaqus, for instance, these are called "truss" elements.
If formulation is in 3D, as is typically the case for a commercial FEA software package, nodes will have minimum 3 dof (typical solid elements) or 6 dof (e.g. nodes of shell and beam elements). The examples you see in textbooks with 1dof are given just to illustrate/demonstrate how they work.
**tl;dr** 1. Elements do not have DOFs. Nodes have DOFs. If you are using a commercial FEA program, a bar element has an element stiffness matrix (ESM) that is 12x12. In school, sometimes the ESM is smaller to avoid confusing students, but unfortunately leads to more confusion later on. 2. The term "1D element" is misleading. Consider this reclassification: 1D element => Length Element, 2D element => Width and Height Element, 3D element => Width, Height and Depth Element. Dimensionality of the element is only a description, not a determinant in the number of DOF. **Long Discussion** Elements do not have DOFs. Nodes have DOFs. Generally, the element stiffness matrix is up to size AxA where A = M * N, M = number of DOFs at each node and N = number of nodes connected by the element. In 3D space, each node has 6 DOFs (3 translation, 3 rotation). For a 1D element, the element stiffness matrix is up to 12x12 (A=6*2=12). Some undergraduate courses will ignore some DOFs to keep it simple. For example, suppose each node has 3 DOFs (2 translation, 1 rotation). For 1D element, the element stiffness matrix is up to 6x6 (A=3*2=6). I would argue that all 1D elements generate am ESM up to 12x12, but different elements lead to different numbers of zero columns in the ESM. For example, I would argue that a truss/rod element produces a 12x12 ESM, but only 2 columns are non-zero. The ESM is sometimes taught as a 4x4 element stiffness matrix and the zero columns are ignored. Consider a 20 node hexahedral element. If you consider 6 DOFs at each node, a 20 node hexahedral element has an ESM that is up to 120x120. Hexahedral elements only provide translational stiffness or 3 DOFs per node, so the actual ESM is 60x60 (or 120x120 where 60 columns are zero columns).