floating coloumn.pptx

INFORMATION ABOUT FLOATING COLOUMN

1. FLOATING COLOUMN

2. Floating coloumn 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.

3. 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.

4. 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.

5. Floating Column & Earthquake The lateral forces due to earthquake need to be transferred to the foundation through clear force transfer path. This force transfer path is disrupted in case of floating columns. These floating columns attract a lot of seismic force, which is unfavorable in the high seismic zone. The floating columns act well when only vertical forces are considered. But they are highly undesirable for lateral forces such as earthquake. It is highly discouraged to have floating columns in high seismic zone regions. But even then, we see a lot of buildings be it residential, commercial or industrial using floating columns in their construction. And the only reason being the flexibility to alter the plan above or below to suit the client requirement. This is where our knowledge and art of structural engineering comes into play. So , in this situation, it is the job of structural engineers like us to ensure that such buildings are not only analyzed properly but the detailing of such buildings is also done properly. Detailing of the steel becomes a crucial part of floating column construction.

6. Need of Floating Column Now a days multi- storey building construction for residential, industrial or commercial purpose has become a common feature. These multi- storey building need ample of parking or open spaces below. In multi- storey residential building to accommodate for the number of parking places and the turning radius, some of the columns from the floors above create a problem. In these cases, these columns are designed as floating columns . Even in commercial building there might be a need for conference hall or banquet hall on the lower floors. For these purposes we prefer to have a clear open space rather than having columns in between. This is where floating columns come into the picture. Floating columns gives the liberty to alter the floor plans above.

7. How are Floating Columns Incorporated in the structure? Like in any structure, the load from the floors above is transferred to the column. The entire load is then transferred to the beam on which the floating column is going to rest. The floating column is designed as regular column. The beam on which it rests is designed as a beam carrying all the load of the column as a single point load . This beam referred to as girder beam or transfer beam usually has a big cross section with heavy steel. This girder beam is also subjected to torsion. The design and detailing of this girder beam is very crucial in the construction of floating columns.

8. Load Transfer in Floating Columns and Non-floating Columns The load transfer is directly done by the non-floating column where it is safely transferred to the foundation. In case of floating column, the load is taken by the below beam. The column is arranged as a point load over the beam. The load is equally distributed to the beam . Advantages and Disadvantages of Floating Column The floating column is undesirable in building construction. Some of the reasons are: 1.Increased storey  displacement in buildings 2.Attract seismic forces extensively 3.There is no continuity with the above and below floors making it vulnerable Still floating column is used due to following advantages 1.The plan can be varied on each floor 2.Construction of Soft- storeys 3.Architectural Importance

9. Why People or Architects take Floating Columns? In modern times the buildings are becoming complex particularly the mix use ones. There are different uses on different floors and hence to follow it structural grid becomes difficult as columns on any floor would become a hindrance. Even in residential buildings when there is a parking on ground floor or lower stories or huge cantilevers are taken to exploit ambiguities in Local bye-laws for gaining more FSI, the lower floors need column-free spaces for easy movement of vehicles; while on upper floors which are more in no, the columns have been designed based on room layout. They are also frequently used when there are shops on ground floor and residence on upper floors. Rather than finding an architectural solution one easily take recourse to floating columns and remove columns on lower stories, which is “a dangerous proposal”. Those architects who are not able to find design solutions in their design or those who do not respect the structural engineering and force the structural engineer to alter the uniform vertical alignment of columns and to provide floating columns. What has happened during last few years is that; that someone took a risk and the building survived and hence architects and structural engineers thought that this could be done. Hence now this practice has reached a stage where it is followed by many in violation of all norms. People do not understand that the building has survived in Gravity loads only. But they forget that how it would behave in Earthquake or wind i.e. when subjected to horizontal loads.

10. They also forget that earthquake occurs once in a life of building i.e. once in 50 or 100 years and at such times it might prove absolutely fatal. In our opinion taking floating columns is not a sound engineering practice.

11. Floating Column Applications The most important application of floating column is for the construction of soft storey in the ground floor to facilitate free space for parking or entrance corridors. This free space will provide parking option for residential, industrial and commercial buildings. In banquet halls, lobbies, conference room, large interrupted column free space is required for free movement of people and vehicles. This must avoid closely spaced columns in the area. A wrong practice that has come into force is to solve such problems by providing floating column. Floating columns are constructed to obtain different grid arrangement in different floors.

12. The floating column is designed as a normal compression member over the beam or slab. The beam below the floating column is designed as transfer beam, where the beam carries the load of the floating column as a point load. The beam that is employed to support the floating column is designed with high shear capacity or as deep beams. These provisions are not stated in Indian codes. The system is so designed that the stub column is not the part of lateral load resisting system (Earthquake and wind loads). The floating column is analyzed in software by providing pin joints at the ends.

13. Plan of G+2 building

14. FLOOR NAME COLOUMN NO. SIZES WIDTH DEPTH GROUND FLOOR C1,C4,C28,C23 400mm 230mm C6,C7 450mm 230 FIRST FLOOR C1,C4,C28,C23 240mm 230mm C2,C12,C16,C18,C28, C17,C13,C21 270mqm 230mm C5,C6,C7,C10 350mm 230mm SECOND FLOOR C1,C4,C28,C23 220mm 230mm C2,C3,C12,C14,C16, C18,C20,C24,C28,C27,C26,C21,C19,C17,C13 240mm 230mm C5,C6,C7,C10,C11,C22,C23 310mm 230mm SIZES OF COLOUMN:-

15. REINFORCEMENT OF COLOUMN:- FLOOR NAME COLOUMN NO. COLOUMN BAR TRANSVERSE REINFORCEMENT No. of bars Dia of bars Dia of bar distance GROUND FLOOR C1,C4,C28,C23 10 14 8 190 c/c C6,C7 10 14 8 170 c/c FIRST FLOOR C1,C4,C28,C23 10 12 6 190 c/c C2,C12,C16,C18,C28, C17,C13,C21 10 12 6 190 c/c C5,C6,C7,C10 10 12 6 180 c/c SECOND FLOOR C1,C4,C28,C23 8 12 6 190 c/c C2,C3,C12,C14,C16, C18,C20,C24,C28,C27,C26,C21,C19,C17,C13 8 12 6 185 c/c C5,C6,C7,C10,C11,C22,C23 8 12 6 195 c/c

16. FLOOR BEAM NO. SIZE REINFORCEMENT d b GROUND FLOOR B1,B2,B3,B4,B5 300 230 Provide 10mm dia. 2 legged stirrups at 100mm c/c B6,B7,B8,B9,B10 320 230 Provide 10mm dia. 2 legged stirrups at 95mm c/c FIRST FLOOR B1,B2,B3,B4,B5 320 230 Provide 10mm dia. 2 legged stirrups at 110mm c/c B6,B7,B8,B9,B10 300 230 Provide 10mm dia. 2 legged 100stirrups at mm c/c SECOND FLOOR B1,B2,B3,B4,B5 300 230 Provide 10mm dia. 2 legged stirrups at 110mm c/c B6,B7,B8,B9,B10 310 230 Provide 10mm dia. 2 legged stirrups at 105mm c/c BEAM REINFORCEMENT:-

17. FOOTING NO. SIZE Development length(mm) REINFORCEMENT D(mm) B(mm) C1,C2,C6,C5 450 310 1385 Provide 16mm dia. Bars at 105mm c/c on both side C3,C4 430 300 1240 Provide 16mm dia. Bars at 150mm c/c on both side Design of footing:-

18. Are Floating Columns Advisable? The floating column construction, a dangerous practice adopted by many structural designers is found undesirable due to the following reasons . They are: 1.Floating column is constructed over the beam and hence it won’t be having any form of continuity with the structural elements below. 2.Floating columns can make every floor different, with different grid system. This will affect vertical symmetry of the building. This will affect the building performance during earthquake forces. 3.This forms one of the critical elements in the building, which will attract large seismic forces in seismic prone regions or during earthquakes. 4.Using floating columns increases the story displacement and story drift under the action of lateral force. 5.The base shear created at the building support during earthquake activity is high. 6.The floating column tends to increase the base shear and story displacement of buildings under earthquake action. This is not a desirable performance. Hence, past performance of floating column in seismic regions make them unsuitable in future construction. They are limited to take up gravity loads alone. 7.During Bhuj Earthquake (Jan 26, 2001) many multi storied buildings collapsed like pack of cards within seconds, as there were presence of soft stories, floating columns and mass irregularities .

19. As per the 6 th  revised edition of IS: 1893 – 2016 (Part 1) – Clause 7.1, it is not recommended to have floating columns in building construction.  FLOATING COLUMNS AS PER THIS CODE ARE NOW PROHIBITED .  Hence floating columns not being a sound engineering practice do not recommend it, especially for high-rise structures .