Many young civil engineering students pose me this question '' what is a floating column'' during structural design seminars and etabs training or staad pro training. I thought I will write a very quick article on this subject. I will also discuss some generic points about floating columns and columns in general in this article.I have added some provisions of IS 13920, the ductile detailing code that will help you know the special attention needed for ductility.
What is a column?
Of course a column is a vertical compressive member that transfers the load from the beams to the footings.Some times there might be tensional forces in a vertical member fully or partially especially when it is a long shear wall.Some times there will be suspenders to hang a wall or a stair in duplex homes which will be having only tensional forces in it.
If you compare a beam and a column, have you ever thought about its difference? Both are RCC members and one is horizontal and one is vertical? Apart from this the main difference is that the beams predominantly will have one direction bending alone. The axial loads in a beam will be generally less and there will be only predominantly one direction bending. In coulumns the axial load will be higher and there can be bending in either direction. Column design is interactive in nature.
As per IS 13920, ductility clauses for columns is applicable if the stress in the member is more than 0.08fck.This can be considered as a rule for differentiating a beam and column in a major large structures.This is by keeping in mind that some times,beams especially coupler beams and the beams closer to shear walls also can have large axial forces and can be treated as columns in design and detailing if the stress is more than 0.08fck.
Where do you need them
Location of columns needs a lot of carefull consideration. Column position and numbers needed, depends on the loads,spans,architectural requirement and on the seismic demand and so on. I have other articles specifically written for this. The only point to make here is that, it is decided based on careful analysis which some times requires iterations depending on the size and complication of the project.The orientation of column also plays a huge role in deciding the total stiffness and thereby deflection control of the building.
This structural blog am writing is focusing beginners and hence the idea is to convey that careful scheming is very important for better performance of a building.Dont think that any orientation of column is always ok.
What should be the size of the column
Sizing of column also needs careful consideration as the location. However there are certain minimum size requirement as per IS 456 aswell as IS 13920 which is the ductile detailing code which is mandatory to adopt in seismic zones 3 and above.
See another blog on the topic - MINIMUM MEMBER SIZES This blog gives you more insights on what are the minimum member sizes requirement.
What is a floating column
A column that is floating on a beam or a transfer slab is a floating column.That means a floating column is not starting from the ground from a footing. This may be due to functional/architectural reasons.
An example may be a drive way in the basement that cannot have a column.But the upper levels needs because of the architectural scheme of the building.The scheme of the building will decide all these. However these floating columns are making the structure very irregular and this needs special attention as per IS 1893 and IS 13920
Is there a special attention needed if there is a Floating column
Many beginners dont even know that there is some thing special needed when there is a floating column.Some engineers know that there is some thing to be done but most understand that the beam needs more attention than the column as the gravity load is taken by the transfer beam.Yes this is correct to an extent.Earlier seismic code IS 1893 2002 allowed us to design the beams for a larger safety factor.The floating column also need special attention.The most important aspect is that the structure shall not have soft storey due to the stiffness variation.Then we should ensure that the floating columns are not part of the lateral load resisting mechanism.This means that you need special attention when you model in any structural analysis software. You need to ensure that the floating columns is not generating a frame action and thereby resist lateral forces. It shall not be considered to resist any lateral forces and the lateral forces shall be passed on to other structural members. This can be done by correct releases in model and correct detailing at site to ensure what we envisaged in analysis model takes place in site. How we do it and the details will be covered in our courses.
Also follow this civilera blog for future articles that will cover the special attention that is needed.
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Some basic attention for irregular structures can be seen in the link IRREGULAR BUILDING
Summary
Many students think that learning ETABS modelling or taking a structural design training using ETABS is all what is needed for being an independent engineer. In reality, an etabs training is only a part of the learning curve. It is the basics and sound understanding of a scheme development that is far more important than a mere staad pro training or revit training or any design training for that matter.A careful attention of code provision on these important points like floating columns,why,how and when to use member releases etc is of primary importance before starting any work.