Floating Column or Hanging Columns
The floating column is a vertical member which rest on a beam but doesn’t transfer the load directly to the foundation. The floating column acts as a point load on the beam and this beam transfers the load to the columns below it. The column may start off on the first or second or any other intermediate floor while resting on a beam. Usually columns rest on the foundation to transfer load from slabs and beams. But the floating column rests on the beam.
This means that the beam which supports the column acts as a foundation. That beam is called as a transfer beam. This is widely used in high storied buildings for both commercial and residential purpose. This helps to alter the plan of the top floors to our convenience. The transfer beam which supports the floating column, transfers the loads up to foundation. Hence this has to be designed with more reinforcement.
Floating Column in Buildings
Now a days multi-storey buildings constructed for the purpose of residential, commercial, industrial etc., with an open ground storey has become a common feature. For the sake of parking, the ground storey is kept free without any constructions, except for the columns which transfer the building weight to the ground. For a hotel or commercial building, where the lower floors contain banquet halls, conference rooms, lobbies, show rooms or parking areas, large interrupted space is required for the movement of people or vehicles. The columns which are closely spaced in the upper floors are not advisable in the lower floors. So to avoid this problem, floating column concept has come into existence.
In urban areas, multi storey buildings are constructed by providing floating columns at the ground floor for the various purposes which are stated above. These floating column buildings are considered safe under gravity loads and hence are designed only for those loads. But these buildings are not designed for earthquake loads and therefore, these buildings may be unsafe in seismic prone areas. When the floating columns are employed in buildings in seismic prone areas, the entire earthquake of the system is shared by the column or the shear walls without considering any contribution from the floating columns.
Floating Column & Earthquake
Though floating columns have to be discouraged, there are many projects in which they are adopted, especially above the ground floor, where transfer girders are employed, so that more open space is available in the Ground Floor. In the earthquake zones, the transfer girders which are employed have to be designed and detailed properly with care. If there are no lateral loads, the design and detailing is not difficult.
Concept of floating column mainly comprises of disrupting flow of transfer of EQ force.
- Floating columns are to be designed as a normal compression member.
- But while designing transfer beam, it is designed as beam carrying all that load of column as a single point load.
- But is to be kept in mind that EQ force developed must be brought down along the shortest path i.e. load is distributed among two intermediate column supporting that beam.
Floating column is supported by high shear capacity beams/deep beams. At some places we can’t avoid floating columns. Hence, it is wise to revise code provisions for deep beams.