Volumes [FEA]

An FE Volume is a three-dimensional solid element spanning eight nodes (a hexahedron, or "brick"). It is used where the full three-dimensional stress state matters and neither beam nor shell idealization is adequate — anchorage zones, thick pier caps, footings, deep bearing seats, and local regions around a discontinuity.

A Volume contributes to the stiffness matrix, carries mass through its material density, and generates load through self-weight. Its nodes have translational degrees of freedom only; a solid element has no rotational stiffness, which is the single most important consequence of choosing it.

The Volumes node is currently disabled in the FEA tree, so this object is not reachable from the object tree in the running product even though the element type is fully supported by the solver. Solid models are still built through meshing and import paths.

Only the 8-node hexahedron is available. There is no tetrahedron, wedge, or higher-order solid, so a region that cannot be swept into a reasonably regular brick mesh is usually better idealized with shells. Solid elements also do not round-trip through SAP2000 import/export.

Geometry

Node 1 … Node 8: The eight corner joints of the hexahedron.

Engineering influence. Node order must follow the standard brick convention — the four nodes of one face, then the four of the opposite face in the same rotational sense. An inconsistent order produces an inverted or self-intersecting element with a negative Jacobian, which the solver cannot integrate meaningfully; the usual symptom is a failed solve or grossly wrong local stresses.

Element shape matters more for solids than for any other element type. Accuracy is best when the brick is close to a cube; it degrades as elements become elongated, skewed or warped. An 8-node brick also suffers from shear locking in bending: a single layer of bricks through the depth of a member in flexure is far too stiff, so several elements through the thickness are needed wherever bending dominates.

Because solid nodes carry no rotational stiffness, connecting a beam or a shell directly to a solid transfers force but not moment — the beam effectively becomes pinned where it meets the solid. This is the most common modelling error with solids and it is silent: the model solves, and the member simply sheds the moment the designer expected it to carry. Where moment must transfer, the connection has to be made over a patch of several solid nodes, or through a constraint that couples the rotation.

Material: The material supplying modulus, Poisson's ratio and density. A volume with no material has no stiffness and no self-weight.

Engineering influence. Poisson's ratio is more consequential in a solid than in a beam or shell, because a fully three-dimensional element develops confining stress: a value approaching 0.5 (incompressible, as for some rubbers) causes volumetric locking, in which the element becomes artificially stiff and stresses become unreliable. For concrete and steel the usual values cause no difficulty.

Volume (readonly) / Weight (readonly): Derived values. Comparing total weight against a hand calculation is the quickest confirmation that the mesh covers the intended region and the density is right.

These two columns are derived and cannot be edited, but they are not flagged read-only, so typing into them appears to work and the value is discarded on redraw.

Global Offset

Node 1–8 Offset X / Y / Z: Rigid offsets in global directions between each node and the element's actual corner. Zero (the default) places the corner at the node. Offsets are used far less often on solids than on lines or shells, and a non-zero offset distorts the element's shape, which degrades integration accuracy.

Local Offset

Node 1–8 Offset X / Y / Z: The same offsets expressed in the element's local axes.

Do not enter the same physical offset in both tabs; they add.

End Releases

Node 1–8 Tx / Ty / Tz / Rx / Ry / Rz: The connection condition at each corner, in global axes.

Sign convention — the same as node supports and line end releases:

Entering -1 intending to release a corner produces a fixed connection. Use 0 to release.

Value	Meaning
−1	Rigid / fully connected (the default)
0	Released — free in that direction
positive	Semi-rigid spring of that stiffness

Engineering influence. Releasing a translation at a solid corner disconnects that node from the load path in that direction and normally leaves a hole in the mesh; it is rarely what you want. The rotational columns are of little practical use here, because a solid element has no rotational stiffness at its nodes to release in the first place.

Overrides

Material Modulus of Elasticity: An absolute override of the material's modulus for this element.

Material Poisson's Ratio: An absolute override of the material's Poisson's ratio for this element.

Engineering influence. These substitute properties for an individual solid without editing the shared material, which is how a locally softened region — cracked concrete around an anchorage, a degraded zone, or a deliberately reduced-stiffness block — is represented. Reducing the modulus softens that element and sheds force into the surrounding solids.

Poisson's ratio deserves particular care here: a solid element develops confining stress, and a value approaching 0.5 causes volumetric locking, making the element artificially stiff and its stresses unreliable. Overriding toward that range is more consequential in a solid than it would be in a beam or shell.

Groups

Group: The FE Group used for construction-stage activation and selection. A volume whose group is inactive in a stage does not exist in that stage.

Casting Day: The day the element is cast, used by time-dependent analysis to compute creep, shrinkage and modulus development. Concrete loaded young creeps substantially more than mature concrete, so an incorrect casting day misstates long-term deformation and the redistribution of force between stages. It is inert unless the analysis includes time-dependent effects.

Verification

• Check Volume and Weight against a hand calculation of the region being modelled.
• Confirm no element reports an inverted or negative Jacobian; if the solve fails on a solid region, node ordering is the first thing to check.
• Refine the mesh and confirm stresses converge. Solid stress results are more mesh-sensitive than beam or shell results, and a single run tells you little on its own.
• Check that at least two or three elements span the thickness anywhere bending is significant, and compare against a shell or beam idealization of the same region — a solid model far stiffer than the equivalent beam usually indicates shear locking from too few elements through the depth.
• Where a beam or shell frames into the solid, check whether moment is actually transferring. If the member reports near-zero moment at that face, the connection is behaving as a pin.
• Compare a simple block under uniform compression against σ = P/A and δ = PL/AE.

Common mistakes

• Connecting a beam or shell to a single solid node and expecting moment continuity. Solid nodes have no rotational stiffness, so the connection is effectively pinned and the moment silently disappears.
• Incorrect node ordering, producing an inverted element.
• One element through the thickness in a bending region, giving a grossly overstiff result through shear locking.
• Highly distorted elements — elongated, skewed or warped bricks — which degrade accuracy without any warning.
• Using solids where shells would do. A solid model of a slab needs far more elements and produces results that are harder to interpret than the plate forces a shell reports directly.
• A Poisson's ratio close to 0.5, causing volumetric locking.
• Expecting solids to survive a SAP2000 round-trip, which they do not.