A column is supposed to be a vertical member starting from foundation level and transferring the load to the ground. The term floating column is also a vertical element which ends (due to architectural design/ site situation) at its lower level (termination Level) rests on a beam which is a horizontal member. The beams in turn transfer the load to other columns below it. Such columns where the load was considered as point load. Theoretically such structures can be analyzed and designed. In practice, the true columns below the termination level [usually the stilt level] are not constructed with care and more liable to failure.
Hypothetically, there is no need for such floating columns – the spans of all beams need not be nearly the same and some spans can be larger than others. This way, the columns supporting beams with larger spans would be designed and constructed with greater care.
For Floating columns, the Transfer Girder and columns supporting Transfer Girder needs special attention. If load factor needs to be augmented (for Transfer Girder and its columns) to have additional safety of structure, shall be adopted. In the given system, floating columns need NOT be treated to carry any Earth Quake forces. Therefore entire Earth Quake of the system is shared by the columns/shear walls without considering any contribution from Float columns. However in design and details of Float columns, minimum 25% Earth Quake must be catered in addition to full gravity forces.
This way the overall system as some breathing safety during Earth Quake. However, Floating columns are competent enough to carry gravity loading but Transfer Girder must be of adequate dimensions (Stiffness) with very minimal deflection.
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.
