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.

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.


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.

    • Related Articles

    • Diaphragm Modelling Example

      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 ...

    • How to model slanting / inclined beam

      When a beam is modeled, it is by default created horizontally at the top of the active storey in view. The elevation of the beam can be changed using the del z boxes for each end of the beam in the Beam Properties > 3D tab. Method 1 : Input delZ ...

    • Building Analysis Overview

      Having modeled your structure and defined the parameters/loading to be applied, you are now ready to generate the member design forces. These are determined by performing a 3D analysis of the whole building. The pre and post analysis stages, along ...

    • Analytical Model

      Physical Model vs Analytical Model The 3D model you built is a physical model with actual member size rendered visually. The analysis converts the physical model into the analytical model (as shown below). The analytical model is essentially a ...

    • Model Options

      Options to be used in analysis can be specified in “Model Options” page of “Building Analysis” form. “Model Options” comprises of four tab pages namely, Model, Shear Wall Model, Slab Model, and Settings. Any modifications made in this page can be ...