Overview of diaphragm action in roof panels and slabs

Tekla Structural Designer
2021
Tekla Structural Designer

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

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
Esta informação foi útil?
Anterior
Seguinte