In a typical building lateral resistance is provide at a few discrete points and it is assumed that applied lateral loads will be distributed to the lateral load resisting systems via floor diaphragm action. Within ProtaStructure diaphragm modelling is achieved using diaphragm constraints.

A diaphragm constraint will maintain exact relative positioning of all nodes that it constrains, i.e. the distance between any two nodes constrained by a diaphragm will never change, therefore no axial load will develop in any member that lies in the plane of a diaphragm between any two constrained nodes.

When running the building analysis the Model Options > Slab Model tab gives 3 options :

• Slabs to define rigid diaphragms (Default Setting)
  
• Single rigid diaphragm at each floor level
  
• No rigid diaphragm floor levels.
  

The differences between these options can be demonstrated with the simple model below.

Slabs to define rigid diaphragms (Default Setting)

If the building is analysed with this setting then ProtaStructure will find any discrete areas of interconnecting slabs and set up discrete diaphragms as appropriate. Separate notional loads are calculated and applied to each diaphragm area. These applied notional loads and the resulting sways can all be examined in the Display Analytical Model.

The easiest way to see that 2 discrete diaphragms have been created is to turn on “Diaphragms” & look at exaggerated view of “Displacements” in the Display Analytical Model.

For each of the two diaphragm areas a separate centre of mass location is determined and the notional load is applied at that position. The view above shows the values for the Fy case in this example. Note that the mass of the walls increases the applied notional load in the left hand diaphragm area. Clearly the right hand area is moving independently to the left hand area which is better restrained by the walls as opposed to frame action.

Single rigid diaphragm at each floor level

If the building is analysed with this setting then ProtaStructure will find apply a single diaphragm constraint to every node at any given level. The existence of slabs is completely ignored/irrelevant. A single notional load is calculated and applied at the overall centre of mass as shown below.

Once again, the easiest way to verify the single diaphragm has been created is to turn on “Diaphragms” & look at exaggerated view of “Displacements” in the Display Analytical Model.

This time we can see the entire level translating and rotating as a unit, since the centre of mass and hence the applied notional load is very eccentric to the core walls, the dominant effect in this example is one of rotation.

No rigid diaphragm floor levels

If you have defined slabs but for some reason you do not wish a diaphragm effect to be considered you can completely eliminate diaphragm constraints using this option. In this case notional loads are applied separately at every node in the floor level, these applied notional loads and the resulting sways can all be examined in the Display Analytical Model.

Once again, the easiest way to verify no diaphragm has been created is to turn on “Diaphragms” & look at exaggerated view of “Displacements” in the Display Analytical Model.

The frame that is restrained by the wall hardly moves at all, other frames move to differing degrees. Note that the frames which do not include walls have the same stiffness and so the differing deflections relate to differing notional loads.

Excluding Specific Slabs from Diaphragms

In the example above there was no slab defined in the area linking region between the middle grids. What if there was a slab in this area as shown below?

If the linking slab was substantial you might consider that it maintains the diaphragm action between the two areas by using default option Slabs to Define Rigid Diaphragms.

However, as the link becomes more slender then at some point you will decide it cannot maintain diaphragm action between the two areas.

In this case you can edit the slab properties and exclude it from diaphragm by checking "Slab Does Not Contribute to Floor Diaphragm" as shown above.

In this case you can still use the default option Slabs to Define Rigid Diaphragms and the resulting behavior & deflections will be the same as 2 separate & discrete diaphragms being formed (as shown below)