Overview of diaphragm action in roof panels and slabs
Roof panels and slabs will both act as a diaphragms provided they have Include in diaphragm checked in their properties.
While Include in diaphragm is the only property used to determine diaphragm action in roof panels, for slab items the diaphragm properties are also determined by the solver model under consideration and the following properties:
- Diaphragm option slab item property
- Decomposition slab item property
- Mesh 2-way Slabs in 3D analysis level or slope property
How these and other choices affect the diaphragm model are described in the topics below.
Include in diaphragm
Roof panels:
The Include in diaphragm choice works as follows:
- On - a diaphragm is created within the roof panel using a mesh of semi-rigid 2D solver elements
- Off - no diaphragm is created
Slab items:
For slab items the Include in diaphragm choice is only available when the Diaphragm option is Semi-Rigid or Rigid. Individual slab items can then be included or excluded as follows:
- On - a diaphragm is created within the slab item which may take the form of a shell mesh, a semi-rigid mesh, or nodal constraints
- Off - no diaphragm is created
Note: Where 2 slab items share a boundary and one
item is included and one excluded, then the nodes along the shared boundary are
included in the diaphragm.
Note: When a slab item is excluded from the
diaphragm this has no effect on the mesh of shell 2D solver elements used in some of
the solver models for 2-way spanning slabs.
Diaphragm option
The choice of Diaphragm option is set in the slab item properties. The chosen option is applied to all slab items in the same slab.
The available options are:
- Rigid
- Semi-rigid*
- None
* See the following Decomposition section for further details of the solver models in which semi-rigid as opposed to shell 2D solver elements are adopted.
How your choice affects the 2D solver element types used depends on the method of decomposition selected.
Note: For roof panels there is no choice of
diaphragm option - it is always treated as semi-rigid.
Decomposition
While roof panels are always one-way spanning, for slab panels you can choose the Decomposition method in the slab item properties. The chosen option is applied to all slab items in the same slab.
Two options are available: Two-Way Spanning or One-Way Spanning - although for certain slab types the value is fixed as follows:
- Composite Slab - One-Way only
- Precast Slab - One-Way only
- Slab on Beams - Two-Way or One-Way
- Flat Slab - Two-Way only
- Steel Deck - One-Way or Two-Way
- Timber Deck - One-Way only
One-way spanning slabs are unmeshed unless the Diaphragm option is set to Semi-rigid, in which case they will be meshed with semi-rigid 2D solver elements.
Note: Semi-rigid 1-way spanning slabs cannot be
designed.
Two-way spanning slabs always adopt a mesh of shell 2D solver elements for the FE chasedown and FE Load Decomposition solver models. However for the 3D Analysis and Grillage chasedown solver models they are:
- unmeshed - if the Diaphragm option is set to None,
- meshed with semi-rigid 2D solver elements - if the Diaphragm option is set to Semi-rigid, or,
- meshed with shell 2D solver elements - if the Diaphragm option is set to Rigid.
Note: Semi-rigid 2-way spanning slabs can be
designed, but only using the results from the FE Chasedown, not the 3D Analysis or
Grillage Chasedown.
Mesh 2-way slabs in 3D analysis
Two-way spanning slabs are always meshed with shell 2D solver elements in FE Chasedown and Load decomposition solver models. When the Mesh 2-way Slabs in 3D Analysis property (specified in the Level, or Slope properties) is checked the same meshing parameters are then extended to the Grillage chasedown and 3D Analysis solver models.
Summary of diaphragm constraint and mesh type configurations
The configurations of mesh and nodal constraints applied to each solver model resulting from the different permutations of the Decomposition, Diaphragm option, and Mesh 2-way slabs in 3D analysis properties are recapped in the table below.
| Decom-position | Diaphragm Option | Mesh 2-way slabs in 3D Analysis | FE Load Decomposition & FE Chasedown Models | Grillage Chasedown & 3D Analysis Models |
| 1-way | None | Not Applicable | No mesh; no nodal constraints | No mesh; no nodal constraints |
| Semi-Rigid | Not Applicable | Semi-Rigid mesh; no nodal constraints | Semi-Rigid mesh; no nodal constraints | |
| Rigid | Not Applicable | No mesh; Nodal constraints | No mesh; Nodal constraints | |
| 2-way | None | Yes | Shell Mesh; no nodal constraints | Shell Mesh; no nodal constraints |
| No | Shell Mesh; no nodal constraints | No mesh; no nodal constraints | ||
| Semi-Rigid | Yes | Shell mesh; no nodal constraints | Semi-Rigid mesh; no nodal constraints | |
| No | Shell Mesh; no nodal constraints | Semi-Rigid mesh; no nodal constraints | ||
| Rigid | Yes | Shell Mesh; Nodal constraints | Shell Mesh; Nodal constraints | |
| No | Shell Mesh; Nodal constraints | No Mesh; Nodal constraints |
Diaphragm center of rigidity
Any given diaphragm (rigid or semi rigid) has a “center of rigidity” or bending stiffness based on the stiffness of the structure that supports it, (i.e. the column stacks, wall stacks and braced bays that connect to the diaphragm from below).
In any 2D or 3D Scene View, the center of rigidity for each diaphragm can be displayed by making an appropriate selection under Center of Mass & Rigidity in Scene Content. An “Analytical” or an “Approximate” approach can be selected, but note that the analytical approach is only suitable for "modal mass" and "base shear" combinations. The approximate approach is only suitable for loadcases and other combinations, but not "modal mass" and "base shear" combinations.
For further details of the two approaches, see: Center of mass and rigidity calculations.
Having selected your approach, you can then select the combination or loadcase for which the center of mass & rigidity calculations are required.
If you then hover the cursor over the resulting center of mass & rigidity arrows, their X and Y global coordinates are displayed in the tooltip.
Note: The center of rigidity that is displayed
for a diaphragm is only relevant to the area of the diaphragm and the
wall/column stacks/braced bays supporting that diaphragm - not the structure as
a whole.