Element Registry¶

The element registry (ops_elements.py) maps OpenSees element type names to their finite element topology. This is used throughout ops-post to determine how to build meshes, compute Gauss point positions, and extrapolate results to nodes.

API¶

Lookup¶

from ops_post.ops_elements import lookup, lookup_or_guess

# Exact match
info = lookup("ASDShellQ4")  # returns ElemInfo or None

# Match with fallback to connectivity guess
info = lookup_or_guess("ASDShellQ4", num_conn_cols=4)

ElemInfo¶

Each entry in the registry is an ElemInfo object:

Attribute	Type	Description
`family`	`ElemFamily`	Element family enum
`num_nodes`	`int`	Number of element nodes
`num_gp`	`int`	Number of Gauss points
`gp_coords`	`ndarray (n_gp, 2)`	GP natural coordinates
`shape_fn`	`callable`	Shape functions: `(xi, eta) -> (n_nodes,)`
`extrap_fn`	`callable`	Returns extrapolation matrix: `() -> (n_nodes, n_gp)`
`face_nodes`	`list[int]`	Node indices for the display face

Convenience properties:

Property	True for
`is_surface`	Shells, 2D quads/tris
`is_beam`	Beams, trusses
`is_solid`	Bricks, tetrahedra
`is_point`	Zero-length, links

ElemFamily enum¶

from ops_post.ops_elements import ElemFamily

ElemFamily.SHELL_QUAD   # 4- or 9-node shell
ElemFamily.SHELL_TRI    # 3-node shell triangle
ElemFamily.QUAD_2D      # 2D continuum quad
ElemFamily.TRI_2D       # 2D continuum triangle
ElemFamily.BRICK        # 8- or 20-node brick
ElemFamily.TET          # 4- or 10-node tetrahedron
ElemFamily.BEAM         # 2-node beam/column
ElemFamily.TRUSS        # 2-node truss
ElemFamily.LINK         # 2-node link
ElemFamily.ZERO_LENGTH  # 2-node zero-length

Shape functions¶

from ops_post.ops_elements import shape_q4, shape_t3, shape_q9

N = shape_q4(xi, eta)   # (4,) at (xi, eta) in [-1,1]^2
N = shape_t3(xi, eta)   # (3,) at (xi, eta) in area coordinates
N = shape_q9(xi, eta)   # (9,) at (xi, eta) in [-1,1]^2

Gauss point coordinates¶

Pre-defined arrays of natural coordinates:

Constant	Points	Domain
`GP_QUAD_1x1`	1	[-1,1]^2
`GP_QUAD_2x2`	4	[-1,1]^2 at +/-1/sqrt(3)
`GP_QUAD_3x3`	9	[-1,1]^2 at +/-sqrt(3/5), 0
`GP_TRI_1`	1	Triangle centroid
`GP_TRI_3`	3	Triangle 3-point rule
`GP_BRICK_2x2x2`	8	[-1,1]^3
`GP_BRICK_3x3x3`	27	[-1,1]^3
`GP_TET_1`	1	Tetrahedron centroid
`GP_TET_4`	4	Tetrahedron 4-point rule

Extrapolation matrices¶

Map Gauss point values to corner node values:

from ops_post.ops_elements import (
    extrapolation_matrix_q4,      # () -> (4, 4)
    extrapolation_matrix_t3_3gp,  # () -> (3, 3)
    extrapolation_matrix_t3_1gp,  # () -> (3, 1)
)

Usage:

E = extrapolation_matrix_q4()
nodal_values = gp_values @ E.T  # (n_elems, 4)