Column Outline Modelling for Flat Slab and Transfer Slab Model

Column Outline Modelling for Flat Slab and Transfer Slab Model

For flat slab or transfer slab models, the slab will always be meshed to the perimeter outline of the column. 

There are 2 options how the analytical outline is formed, which is available under Building Analysis dialog > Analysis Options > Settings tab :  

1.  Use Finite Stiffness Rigid Links
The rigid links can deflect depending on the sustained loadings.  This option may reduce the spike in forces at column edges, resulting in lesser steel requirements. 
This option is suitable for lightly or moderately loaded flat slab model. 
This is the only option for ProtaStructure 2025 and prior versions. 

2. Use Body Constraint Model 
Body Constraints will interconnect a pair of nodes as a body, as if they are connected with an infinitely stiff link. Hence, it cannot deflect as compared to Rigid Link option. This option should be used for heavily loaded flat slab or transfer slab model. 
This option is only introduced in ProtaStructure 2026. 

Detail investigation with Transfer Slab Model 

We will investigate the above  2 options using a simple example model of transfer slab supported by columns and shear walls as below. 


The transfer slab is supported by columns in front and shear walls at the back. It is heavily loaded as it is supporting discontinuous walls with high manually input gravity loads at the top.  

Firstly, we run the analysis with Rigid Links assumption (Option 1).  After the analysis, we can check the analytical assumption and results by accessing the  Analytical View as shown below :  
  1. The rigids links in dark blue are auto-created from centroid of the column wire-frame to the perimeter outline of the column
  2. For shearwall, rigid links  not created to the perimeter outline of the wall. 
The below shows the analytical model displacement & animation enabled with the discontinuous transfer walls is hidden.

If the animation is slow, right-click & save the image into our computer for viewing.
  1. Notice that the rigid links are deflecting downwards from the column frame to the perimeter of the column. 
  2. As a result, the connected slab will also deflect downwards - causing differential deflection between the column node and slab nodes.
  3. This does not happen to the shear walls supporting the slab - as the slab is connected directly to the wall element.   
  4. The net effect is that the transfer slab is deflecting downwards unexpectedly in a significant amount at the column position .
  5. This will result in unreasonable forces in the slab and connected columns and walls, as deflection is a direct reflection of internal member forces
  6. We can conclude that Rigid Links is not a sound assumption for this model. 

2. Use Body Constraint Model 

Let us re-run the analysis using Body Constraint assumption.  Similarly, we can check the analytical assumption and results by accessing the  Analytical View as shown below : 


  1. The body constraint in grey are auto-created from centroid of the column wire-frame to the perimeter outline of the column
  2. For shearwall, body constraint is not created to the perimeter outline of the wall. 
The below shows the analytical model displacement & animation enabled with the discontinuous transfer walls is hidden.
If the animation is slow, right-click & save the image into our computer for viewing.
  1. The body constraint model forces the column frame and outline perimeter column to deflect the same amount as if it is a single entity
  2. This means there is no differential deflection between the column frame and the connected slab mesh
  3. The entire deflection of the transfer slab is expected and reasonable, so will be the forces generated as deflection is a reflection of internal member forces
  4. We can conclude Body Constraint is the more reliable and sound assumption. 

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