Rigid Diaphragms

A rigid diaphragm constrains selected nodes to move as one rigid body in the diaphragm plane. It is intended for frame models where a floor deck or wall panel is assumed rigid in-plane, but is not modelled with shell elements.

When to Use a Rigid Diaphragm

Use a rigid diaphragm when the physical floor or wall can reasonably be idealised as in-plane rigid relative to the lateral framing stiffness:

• Frame-only building floors— floor joints share lateral translation and rotation about the vertical axis.
• Rigid wall panels represented by framing only— selected wall joints share in-plane motion without introducing a shell mesh.
• Seismic load distribution— a floor force and accidental torsion applied through a diaphragm master can redistribute to the vertical elements by their stiffness.

Creating a Diaphragm

1. Select one or more structural nodes in the model view.
2. Right-click and choose Add Rigid Diaphragm, or open Constraints in the properties panel and click Add Rigid Diaphragm.
3. Choose the diaphragm normal axis, DOF mode, slave nodes and master-node method.
4. Resolve any red validation messages, then click Add diaphragm.

The add command is available for a node-only selection when at least one selected node is not part of an enabled diaphragm. If all selected nodes already belong to enabled diaphragms, select a participating node and edit its existing diaphragm from Constraints. A disabled diaphragm is retained in the model but does not reserve its nodes for this add-command check.

For a new diaphragm, the opening selection determines the starting mode:

• Three or more selected nodes — starts in Pick manually with those nodes preselected.
• Fewer than three selected nodes— starts in the horizontal-floor Auto mode, centred on the selected elevation. The selected nodes remain ready if you change to manual selection.

Axis and Degrees of Freedom

The Axis (normal) setting defines the diaphragm plane. In the default In-plane (3 DOF) mode, only the two translations in that plane and the rotation about the normal axis are constrained.

The six-character code is ordered DX DY DZ RX RY RZ. In this constraint display, Fmeans that the slave DOF follows the master's rigid-body motion and R means it remains independent. This is separate from the node-support restraint code.

A horizontal floor diaphragm remains supported when the building grid or footprint is rotated in plan: a level floor is still in the global XZ plane with normal axis Y. The alignment limitation applies to wall and inclined diaphragms. In-plane wall diaphragms must lie in the global YZ or XY plane; a vertical wall rotated in plan is not supported in In-plane mode.

DOF Mode

• In-plane (3 DOF)— appropriate for a floor or wall that is rigid in its own plane but should remain flexible or unrestrained out-of-plane.
• Full rigid (6 DOF) — uses FFFFFF and ties all translations and rotations. Use it only when the selected region should be rigid in all directions, not merely diaphragm-rigid.

In-plane mode is formulated on a global axis-aligned plane. The dialog blocks an in-plane diaphragm whose selected nodes would require projection onto that plane, including a wall rotated in plan or an inclined diaphragm. For a genuinely rigid inclined or non-planar body, full-rigid mode represents the three-dimensional rigid motion, but it also removes out-of-plane flexibility.

Slave Nodes

A diaphragm must contain at least three slave nodes. The master does not count towards that minimum. If you use an existing selected node as the master, select at least four nodes in total so that three slaves remain. Choose nodes that span the diaphragm surface rather than a collinear set.

• Auto — all nodes at Y elevation is available only for a horizontal floor with normal axis Y. It collects eligible nodes within the displayed elevation tolerance; the default tolerance is 50 mm and is displayed in the active length units.
• Pick manually lets you select nodes from the canvas or enter IDs in the picker. Manual selection is required for wall diaphragms with normal axis X or Z.

Phantom diaphragm masters and generated plate-mesh nodes cannot be slaves. The picker also prevents a node that is already a slave of another enabled diaphragm from being added to a new one. Pick real framing joints and plate boundary joints instead of interior mesh nodes.

Master Node

• Phantom master at centroid— the recommended dialog option. It creates an elementless node at the geometric centroid of the selected slave nodes and updates that position if the slave set changes. The node is owned by the diaphragm and is deleted with it.
• Use existing slave node— promotes one selected structural node to master and leaves the remaining selected nodes as slaves. Use this when loads or restraints must be applied directly through an existing physical joint.

A phantom master is read-only in the node properties panel: do not attach members, supports, settlements or manual node loads to it. The seismic load wizard creates and manages its own floor masters when it needs to apply storey forces at a mass centre.

Restraints on the Master

A support on the master is well-defined and accepted by the dialog, but the kinematic tie means every pinned master DOF that is also tied by the diaphragm propagates to every slave on that DOF. With the master fixed in a tied translation, all reaction in that direction collects at the master node instead of redistributing through the other vertical lines of bracing. If you intended the floor to redistribute lateral load, use the phantom master option or pick a master node that is not restrained on the tied DOFs. Restraints on the master's out-of-plane DOFs (the ones marked R in the per-DOF code) are not propagated and act only on the master node itself.

The dialog's per-DOF preview reflects this. A tied DOF whose master is also pinned on that DOF is shown as F⏚ (grounded) and an explanatory note lists the affected DOFs and master node ID. Untied DOFs and ordinary master-to-slave ties are unchanged.

Validation Rules

The dialog checks the constraint while you author it. Existing or imported issues are also surfaced in Model Health.

• Repeated slave membership is blocked. A node cannot be a slave of two enabled diaphragms because the ties overlap and over-constrain it.
• Chained constraints warn. Reusing masters across diaphragms, or combining diaphragm nodes with node-link constraints, can create a fragile constraint graph and should be reviewed.
• Supports on tied slave DOFs are blocked. For example, a Y-normal in-plane floor cannot use a slave fixed in DX, DZ or RY. The rigid tie and the support would fight on the same DOF and the result is ill-posed. Apply the intended diaphragm-level restraint at the master, or keep the supported node outside the diaphragm. Supports on the master are allowed but propagate through the tie as described under Restraints on the Master above.
• Prescribed displacement on a slave is blocked in the dialog. A settlement on an existing constrained slave is ignored by the solve and reported by Model Health; apply settlement at the appropriate master or support node instead.
• Unsupported in-plane geometry is blocked. Horizontal floors remain valid regardless of building plan rotation, but a wall diaphragm rotated in plan or an inclined diaphragm cannot be saved in In-plane mode. Select a supported global plane, use plates, or choose Full rigid only for a body intended to be rigid in all directions.

Loads and Mass

A rigid diaphragm enforces the selected rigid-body kinematics; it does not represent finite slab stiffness or add gravity load or mass by itself. Loads on ordinary structural slave nodes remain part of the analysis and are transferred through the constrained system and the connected framing.

Editing, Display and Deletion

• Select a master or slave node and open the diaphragm under Constraints to edit it or toggle Enabled. Selecting a phantom master shows its owning diaphragm and provides the same open and enable/disable controls.
• Disabled diaphragms remain in the project but are not sent to the solver and are not drawn as active diaphragm ties.
• Enabled diaphragms are displayed with dashed coloured tie lines for pairs of nodes that are not already joined by a two-node member. Colour varies with master elevation.
• Delete a diaphragm from its edit dialog. Deleting a phantom master also removes its owning diaphragm.
• If node deletion leaves fewer than three slave nodes, the diaphragm is retained but automatically disabled so that it can be repaired or removed.

Verification

The implementation is covered by automated tests for rigid-body in-plane kinematics, out-of-plane freedom in in-plane mode, modal behavior, phantom master handling and validation of conflicting diaphragm constraints.