How to model a Transfer Beam supporting discontinuous columns and walls

How to model a Transfer Beam supporting discontinuous columns and walls

Before proceeding with the modelling of the transfer beam, it is important to understand how the physical model translate to an analytical model when building analysis is performed.

  • The physical model is what you see on the modelling screen where all members are rendered in 3D according to the actual sizes
  • The analytical model is a wire-frame model that is created by solver which is actually what is analyzed finally.
  • The analytical model can be viewed by clicking Analytical Model in Analysis tab. 
Do not use FE Floor Analysis Chasedown for the analysis and design of transfer beam as it is beyond the scope of FE Floor Analysis. Always run Building Analysis. 
For more explanation refer :  Modelling, Analysis & Design Flowchart

Physical Model Versus Analytical Model

In ProtaStructure, we can visualize all members as single line wire-frame created along the axis or axes intersection:

  • A column will be a single line wire frame at the intersection of 2 axes.


               Physical Model                                                Analytical Model

 

  • A beam is a single wire-frame line along the insertion axis. Hence for the column and beam members to connect properly, they need to meet at the exact axes’ intersection.


      Physical Model                                            Analytical Model


Transfer Beam Supporting Discontinuous Column

When modelling a discontinuous column supported by a transfer beam, the column should use the same longitudinal axis as the beam to be supported.  In the example below :


  1. The transfer beam is modeled using axis 1
  2. The upper column is inserted using intersection of axis 1 & C
  3. The lower column is inserted using intersection of axis 2 & D
When viewing the plan view transfer storey, it's useful to switch on "Display Transfer Columns and Walls on Plan" via Visual Interrogation to show the discontinuous columns above the transfer storey. As shown above, the discontinuous columns are shown in a different color (blue) on the plan view of ST01.
After the analysis, the Model and Analysis Results Display clearly shows only the upper column is supported as it inserted using the same Axis 1 as the transfer beam.


To solve this problem, there are 2 alternative : 
1.  Mark the column as "Transfer Member" in Column Properties & rigid links will be automatically created during the analysis to join the bottom of the column to the transfer beam.
2.  Manually create a "dummy" beam to join the bottom of the column to the transfer beam (if auto-rigid link does not work)

Transfer Columns with auto Rigid Links

Discontinuous columns & walls must be marked as “Transfer Member” in the Properties > Geometry tab, reasons being : 
  1. The design of beams supporting these transfer members will be correct. 
  2. Discontinuity message will not be issued for marked transfer members during the analysis. 
  3. Additionally, if transfer columns are inserted on different gridline from that used by transfer beam, the analysis will automatically created during the analysis to join the bottom of the column to the transfer beam. 
In example model below, transfer column S1 is using the same insertion axis 1 as the beam.  Transfer column S2 is inserted using a different offset axis 2.  
A comparison is done with analysis done with "Transfer Member" unchecked and checked (as shown below). 


After the analysis, the Analytical model clearly shows the lower column S2 is now supported by the transfer beam via a rigid beam.  
The rigid beam is rendered as dark blue line.  The eccentric forces of the offset column will be considered, e.g. there will be additional torsion on the transfer beam due to this offset.

Support Offset Transfer Column with Dummy Beam

There maybe rare situation where auto rigid is not created despite having "Transfer Member" checked in column properties. 
In this case, a dummy beam may be created joining the beam insertion axis 1 and the transfer column insertion axis 2 (as shown below) :  


  1. Click Beam element icon
  2. Connect the dummy beam from the transfer beam axis to insertion axis of the discontinuous column
  3. The dummy beams may be of any size and will be considered as rigid links to connect the transfer beam and the eccentric discontinuous column.

After the analysis, recheck the Analytical Model, which shows the lower column is now supported by the transfer beam (via a rigid beam) :

The rigid beam is rendered as dark blue line.  The dummy beam has been converted to a rigid beam automatically is it is fully embedded inside the transfer beam. The eccentric forces of the offset column will be considered, e.g. there will be additional torsion on the transfer beam due to this offset.

Transfer Beam Supporting Discontinuous Wall

A wall is a single plane along the longitudinal insertion axis. Therefore, for a discontinuous wall to be supported by the beam, both the beam and wall members must share the same longitudinal axis. 

For transfer wall, these settings must be selected correctly in Wall Properties > Geometry tab : 
1.  "Transfer Member" must be checked (similar to transfer column)
2.  "FE Shell Model" must be selected for wall analysis type (instead of Mid-Pier)
We will use the sample model below to demonstrate the difference in "FE Shell Model" & "Mid-Pier" wall analysis type. 



                                                          Physical Model                                  

 

                       

  1. As shown in the Analytical Model above, the Mid-Pier assumption will be unrealistic or unreasonable as the transfer wall will exert a point load in the middle of the wall.  This inaccuracy will worsen with longer transfer wall. 
  2. The FE Shell model will give an accurate & realistic result as the transfer wall loading on beam approximate that of a uniform load; as each shell will exert a separate load on the transfer beam. 
Do not use Wall Model Type as "Mid-Pier wall" for transfer wall. This is because Mid-Pier will transfer the load as a single point load on the transfer beam, which is inaccurate. 

Supporting Offset Discontinuous Wall with Dummy Beams

If the transfer wall is not using the same longitudinal insertion axis as the transfer beam below, it will not be supported, even if "Transfer Member" is checked in wall properties (unlike similar situation for transfer column). 
In such cases, dummy beams must be manually created to connect both ends of the wall to the transfer beam insertion axis.  The steps are : 

  • Create ghost axes for the dummy beams.
  • Connect the dummy beam from the transfer beam axis to the axis of the transfer wall.
  • Ensure that appropriate intersection points are available for the creation of the wall and beam elements.
  • The dummy beams may be of any size and will be considered as rigid links to induce the appropriate forces onto the transfer beam.


As shown in the example model above, the dummy beams will be converted to rigid link during the analysis and will result in :

  1. The load transferred from the discontinuous walls to the transfer beam
  2. Induced torsional moment to the transfer beam
For further details on transfer wall settings please read this article :  Define discontinuous / transfer walls and columns

Moment of Transfer Beam Supporting Wall



When meshed shearwall are supported by transfer beam, the moment can be displayed in the Analytical Model view as per above snapshot.
This is due to the nature of meshed walls which analyzed as an isotropic elastic plate. This means that the plate can be span horizontally and develop in plane axial forces.
Therefore, the load path in the wall acts as "strut and tie" manner (Truss analogy) as well as in flexure (spanning horizontally) by distributing load directly to supports at its ends and none to the bottom of the beam practically.



In the design, transfer wall will not be specially designed and detailed as "deep beam".  All shear walls will always be designed and detailed as normal shear wall, irrespective of size, height, length and continuity.
We recommend you read this design guide to understand more about 3D effects of building with transfer beam supporting discontinuous column : 3D Effects of Building with Transfer Beam Supporting Discontinuous Column  


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