Get FEM CG

Engineering Tools — Get FEM CG (00324)

What this tool computes

Two things, in one Excel workbook:

  1. Per-element centroids of the selected elements in the coordinate system you choose, plus auxiliary columns (element shape, edge length, area, thickness, modulus, offset applied).
  2. Aggregate stiffness-weighted FEM CG in X / Y / Z from those elements, computed as the centroid weighted by per-element (Modulus × Area × Thickness).

The aggregate is a stiffness-weighted CG (elastic centroid), not a mass-weighted CG. For mixed-material structures this matters — see Weighting choice below.

Why per-element centroids alone aren't always enough

FEMAP has a per-element centroid query already (and List Element Data exposes it in a per-element table). That tells you where each element is. This tool answers a different question:

"If I treat the selected portion of the structure as a single solid object, where does its load-bearing centre sit?"

That's the structure-level CG, not the per-element centroid. It's the right number when you're laying out lifting points, balancing a panel about a pivot, or comparing two design candidates on load-path symmetry.

Weighting choice

The aggregate weights are w = E × A × t for each shell element — modulus times area times thickness. This is a stiffness-weighted average, which means it answers "where does the elastic centroid sit" rather than "where does the mass sit."

If your structure is single-material (all aluminium, all composite with the same layup, etc.), the two answers are identical up to a density constant. If you have mixed materials — aluminium face sheets with a titanium insert, for instance — the stiffness CG biases toward the titanium and the mass CG biases toward the aluminium volume. They can differ noticeably.

Pick the right weighting for the engineering question:

  • Stress / load-path / lifting-point layout: stiffness CG (this tool, as-is).
  • Inertia / dynamics / mass-balance: use FEMAP's own mass-property tools (e.g., Group Element Mass Compute) which use density × volume weights.

The README sheet in every output workbook restates this weighting choice explicitly, so anyone reading the file downstream knows what they're looking at.

Shell offset correction

PSHELL Z-offset is applied. For elements with a non-zero pr.zoffset, the tool computes the shell normal from the element's first three nodes, then adds z_offset × normal to the centroid before transforming into the chosen CSys.

This matters specifically for the "face-sheet-on-neutral-mesh" modelling pattern: a single mesh at the neutral plane, with each face-sheet property carrying a Z-offset to its physical location. The node-derived centroid puts every element at the neutral plane; the offset-corrected centroid puts each face sheet at its actual mid-thickness in space. For panel CG queries the difference can be the entire panel thickness.

For elements without a sensible offset (line elements, solids) the offset correction is a no-op.

How to use it

  1. Open from Custom Tools > Model Audit - Query > Get FEM CG.
  2. Click OK on the explanatory dialog.
  3. Pick ONE coordinate system for output (Cancel = Global Rectangular).
  4. Pick the elements to process.
  5. Excel opens with two sheets: FEM CG (data) and README (provenance + weighting note).

Output layout

  Element ID   CSys   X          Y          Z          Shape    Edge Len   Area      Thickness   Modulus    Offset
  ----------   ----   --------   --------   --------   ------   --------   -------   ---------   -------    ------
  1234         5      120.500    -45.220    8.500      Quad4    25.000     625.0     0.040       1.06E+07   0.020
  1235         5      120.500    -20.220    8.500      Quad4    25.000     625.0     0.040       1.06E+07   0.020
  ...

  [empty row]
  FEM CG (stiffness-weighted)   115.342    -22.481    8.500                                       sum(w)=...

The aggregate row sits two rows below the last element row, with a soft-yellow background. The sum(w) cell in the Modulus column shows the total accumulated stiffness weight, useful for sanity-checking that the selection had nonzero weights overall (zero-weight selections produce an "n/a" CG with a clear note).

CSys-type-aware axis labels: in a cylindrical CSys columns become R / T / Z; spherical becomes R / T / P.

Element-type support

  • Shells (Tri3 / Tri6 / Quad4 / Quad8): full support — centroid + offset correction + area + thickness + aggregate weight.
  • Line elements (Bar / Beam / Rod / etc.): centroid only. Area / thickness columns show 0 (or blank), aggregate weight contribution is 0. The centroid still appears in the per-element row so the CSys-transformed coords are available for engineering use.
  • Solids (Tet / Hex / Wedge): centroid only. Same treatment as line elements for the aggregate.

If your structure is mostly solid elements, this tool isn't the right pick — use Group Element Mass Compute instead, which handles volume-based mass + CG for solid geometry.

Cancel paths

  • Cancel on the main dialog: no Excel is created, nothing happens.
  • Cancel on the CSys picker: defaults to Global Rectangular (id 0) with a warning, continues.
  • Cancel or zero-selection on the element picker: tool exits with a "no elements selected" message, no Excel.

Related tools

  • List Element Data — per-element centroid columns (plus property, type, nodes, length, modulus, thickness, area, offset). Same per-element centroid calculation as the rows in this tool's output. When you don't need the aggregate, use that.
  • Group Element Mass Compute — group-based mass + CG + inertia, including solid elements, using density × volume weighting. The right pick for mass-CG questions on multi-material or solid-element structures.
  • Update Csy of Mass Prop — bulk-set the CSys reference on mass property records once you've identified the right reference frame.

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