🧱 Which element
The element follows the shape of the part and what it has to carry. A part with one dimension much longer than the other two is a beam, one with a dimension much smaller than the other two is a shell, and a part with neither is a solid. A connection with a stiffness of its own, between two parts, is a spring link. Most models mix several kinds.
At a glance
| Element | Made from | Takes | Carries | Read with |
|---|---|---|---|---|
| ForceBeamColumn | a curve | a beam section | axial force, two shears, torsion, two moments | Beam Forces, Beam Stresses, Utilisation, Node Forces |
| Beam With Hinges | a curve | a beam section, a Hinge Release per end | the same, with released ends | the same |
| Truss | a straight line | a beam section, for its area and material | axial force only | Beam Forces (N), Node Forces |
| ASD Shell | quad and triangle mesh faces | a Plate Fiber Section | membrane forces, bending, transverse shear | Shell Forces, Shell Section |
| SSP Brick | an 8-vertex mesh | an nD material | the full 3D stress state | Brick Stresses |
| Ten Node Tetrahedron | a 4-vertex mesh | an nD material | the full 3D stress state | Brick Stresses |
| Spring Link | a line between two existing nodes | a uniaxial material per direction, read as a stiffness | what each direction is given | no result component yet |
Beams: ForceBeamColumn, Beam With Hinges, Truss
The three are types of one switcher, ForceBeamColumn (Alpaca4d), picked in its Element Type menu.
ForceBeamColumn is the element for any line-like member: beams, columns, braces. It is force-based: the section response is integrated along the member, at five Newton–Cotes points, rather than a shape being assumed for the curvature. The sections Alpaca4d gives a beam are elastic, section Elastic: a fibre section cannot yet be assigned to one.
Beam With Hinges is the same element with a hinge zone at each end. A released degree of freedom becomes a hinge section with that stiffness scaled by 10⁻⁶. It is how a member end is pinned, or released about one axis. Because the release is a very soft section rather than a true free end, it is approximate, and its accuracy depends on the hinge length, which is why that length is clamped.
Truss is a bar that carries axial force only, in tension and compression, or in one of the two: a cable or a bracing rod that goes slack when pushed, a strut that lets go when pulled. It is always corotational, so it holds for large displacements too. A line load on it goes to its two end nodes, half each, as it has no bending to carry the load with. A cable needs a Prestress: straight and slack, it has no stiffness across its length. A tension-only or compression-only bar needs a non-linear analysis to let go: Newton, in several steps.
Beam forces are read at the integration points of each element, and Gravity Load puts a beam's weight on its end nodes. Both are reasons to split a beam into five or more elements per span. A member that can buckle on its own needs splitting too, which Linear Buckling does with Divisions.
Shells: ASD Shell
For slabs, walls, plates, roofs and tanks: anything thin compared with its other two dimensions, where membrane action and bending both matter. Each mesh face becomes one element: ASDShellQ4 for a quad, ASDShellT3 for a triangle, and the two may be mixed in one mesh. Is Corotational turns on the corotational formulation, for large displacements and rotations. Set Local X Axis whenever you will read the forces: see Axes and signs.
Solids: SSP Brick and Ten Node Tetrahedron
For bodies that are truly volumetric, where a full 3D stress state matters: foundations, massive walls, blocks, regions of stress concentration.
- SSP Brick is a hexahedron with one integration point and a stabilisation term, which keeps it free of the shear locking a fully integrated 8-node brick suffers in bending, and cheap. Use it when the volume meshes into hexahedra.
- Ten Node Tetrahedron is the quadratic tetrahedron: its strain varies linearly through it, so it can bend. Use it for volumes that only mesh into tetrahedra, which is most geometry not drawn for bricks. A cantilever 10 × 1 × 1 cut into 60 tetrahedra deflects 0.4018 with it, against beam theory's 0.4024, and 0.098 with four node tetrahedra. It needs an OpenSees newer than 3.8.0, and its supports have to hold the edge nodes as well as the corners.
Connections: Spring Link
A spring link joins two nodes that are apart, with one uniaxial spring per local direction and nothing in the directions left out: a bearing, a bolt group, the interlayer of laminated glass. It has no mass and makes no nodes, so both ends have to land on nodes the model already has. When the join has no give at all, a Rigid Link is exact; when a node is held against the ground, that is a Support.
Mixing elements
Assemble joins elements wherever their geometry meets within its Tolerance. A node gets 6 degrees of freedom when a beam or a shell touches it, and 3 when only solids do. So a brick or a tetrahedron sharing a node with a beam passes force to it but no moment, and a spring link on a solid's node has only its three translations.
How closely each element reproduces the textbook answers is shown in the benchmarks: beams, shells and bricks.
See also
- Elements — every element component
- Sections — what a beam or a shell is made of
- Materials — uniaxial or nD
- How a model is built — where the elements go
- Axes and signs — the local axes of beams and shells