ProtaStructure : Analysis Warning : Diaphragm Nodes not in the same Plane

Analysis Warning : Diaphragm Nodes not in the same Plane

If a warning appears during analysis, indicating that nodes constrained by a diaphragm do not lie in the same plane, it should not be ignored :

Warning: [DIAPHRAGMS] Diaphragm Nodes not in the same Plane. Nodes in different planes are constrained to move together, undesirable effects may accrue…

The warning message is triggered when various inter-connected slab diaphragms are not created in the same plane due to the following situation :  

  1. An inclined slab connected to a horizontal slab
  2. An inclined slab connected to another inclined slab
  3. Combination of the above

If there is only 1 inclined slab, a single inclined diaphragms can be created and are catered for correctly. It is also possible to have multiple differently inclined slab diaphragms as long as they do not interconnect. However, it is not possible to cater for differently inclined and interconnected diaphragms. In such a situation a single diaphragm constraint is formed that constrains all nodes within it equally & the diaphragm warning message is triggered.

Idea
You should review the diaphragm assumption in the Display  Analytical Model and consider excluding any slabs from the diaphragm creation, which can rectify the problem. In some situations, you may choose to ignore this warning if the result is deemed acceptable after due verification.

Example Model -  Inclined slab connected to horizontal slab

In this example model, 2 horizontal slabs are connected to an inclined slab.


If either the Slabs to Define Rigid Diaphragm or Single Rigid Diaphragm at Each Floor Level option is used, a warning message is displayed during the analysis indicating that the nodes constrained by the diaphragm do not all lie in the same plane.

Complete the analysis and go to Display Analytical Model to further investigate the analytical model.  Turn on “Diaphragm” in Element Tab to display the extent of the diaphragm and the nodes constrained by the diaphragm.  A single diaphragm for all the horizontal and inclined slabs are created as shown below.


On first viewing the above may not appear unreasonable. The center of diaphragm is where all the grey lines (rigid arm) join - it is located in a diaphragm node above all the slabs, not in the same plane of any of the slabs. 

Because the rigid arms that constrain all the nodes to move together are not co‐planar there are incorrect side effects as demonstrated by the axial load diagram below.


All the horizontal beams and inclined beams are subjected to axial loads even under gravity G load case.  In normal practice, we would expect the horizontal beams to experience no axial force as they should be constrained by a horizontal diaphragm; while axial force in the inclined beams are acceptable. 

To rectify the situation, you may choose to exclude the slanting slab from the rigid diaphragm by checking the option "Slab Does Not Contribute to Floor Diaphragm" in slab properties (as shown below).


Run analysis and similarly verify the results in  Display Analytical Model.

In this case, 2 discrete & separate diaphragm are formed for the horizontal slabs and no axial force exist in the horizontal beams.  As expected, there is  axial force in the inclined beams as they are not constrained by any diaphragm.  Overall, this is a more acceptable result when compared to previous assumption.


Notes
If the angle of inclination of the slanting slab in very small, the adverse impact due to a default diaphragm creation may be minimal.  In this case, based on your engineering judgement, you may choose to ignore the diaphragm warning to allow for all the beams to be contrained by a single diaphragm.

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