Transfer beam is a horizontal structure that transfers heavy gravity loads from the above structural elements to the below structures. These are beam elements that are mainly employed in the design of high-rise building structures. They are designed to take up load more than what a normal beam is designed for.
|Fig.1. 500 North Michigan Avenue|
The transfer beam can be explained by studying a real example of the 25 storey office building, 500 North Michigan Avenue in Chicago. The transfer beam provided at the ground floor is designed at a depth of 1.7m. These are supported by column spanning at a clear distance of 3.5m. If you check the fig.2, the ground floor front view is shown, that gives the idea about the transfer beam.
|Fig.2. 500 North Michigan Avenue - Ground Floor - Transfer Beam|
As shown in the figure-2 above, the transfer beam is supported by two columns and it takes the single column point load above.
The figure-3 above shows a 178.6m high rise building which is a tubular structure that have closely spaced columns. The beams as shown are perforated as provisions for windows. The structure also posses a heavy transfer beam that have a depth of 4.2m. It is supported by columns placed at a clear span of 7.5m to take heavy column loads from above.
Load Action in Transfer Beam
Transfer beams predominantly transfer the loads to the below structure by shearing action. Unlike normal beams, where the gravity loads are transferred through flexural action. Transfer beams are built continuously and monolithically with the spanning supports. It provides a load path when there comes a redundancy.
Also Read: What is Flexural Member?
|Load Transfer Through Transfer Beam|
We don't have to bother about deflection in case of normal beams. In case of normal beams, satisfying the deflection condition as per the code will just be enough. But in case of transfer beam, a deflection in transfer beam make the floor above to deflect. Hence, it is differently designed as it is subjected more non-linear stresses that are caused by the large concentrated point loads above it. Hence, the region of load action is discontinuous. So, we cannot employ linear elastic theory.
The load path in the figure-3 shows the need for transfer beam. If all the columns were aligned top to bottom, there was no need for transferring medium. But when a shift in column location as shown in figure-3 is required, a transfer structure is necessary to lead the load transfer.
Design of Transfer Beam
A transfer beam is more subjected to shear. The design of transfer beam is either done by deep beam method or strut & tie model, based on the assumptions chosen.
Th transfer beam is subjected to non-linear stresses that demands for ductile design. The shear failure predominant in transfer beam structure results in sudden or brittle failure. Hence, these structures demand ductile designing.
The requirement of transfer beam is a critical decision as it can be subjected to large critical stresses. But the need for free column spaces in the ground floor demand the need for transfer beam.
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