The most regularly used shapes for columns in building projects are rectangular shape, circular shape and square shape.
In most of the projects, rectangular column shape is preferred over circular columns. Some times square columns are also preferred over circular columns. Seismic resistance point of view, circular column may be better than rectangular columns. Even then circular columns are not much preferred.
It is important for the young engineers to understand the practical reasons. This helps to have the right co-ordination at the right time and helps in avoiding abortive works during the scheme development stage. Deciding the shape of the column is as important as deciding the size and orientation and position of columns and beams.
This blog is the result of a question asked by a student in my ETABS learning community. I have a dedicated community for all who sign up for my Structural design courses for structural discussions. I keep discussing questions and answers in this community.
The question was ‘’ Why circular column is not used in the construction even though it undergoes less buckling and have less stress concentration and have the same moment of inertia in all directions?’’
This is in fact a good question. Most students focus on analysis aspects or the ETABS software aspects more. Here this question has an excellent practical relevance. Many times, this practical knowledge will allow you to take decisions while scheme development and allows you to reduce the amount of co-ordination time and rework. Practical understanding is as important as structural and software skillset.
I will answer this structural related question in this blog. Before that let me note the advantages of circular columns. The advantages of a circular column are mentioned in the question itself. However, let me point out the most significant advantages.
Main Advantage of circular column
There are some structural advantages for circular columns. Many a time, this is over ruled by other architectural and functional requirement. These points are discussed below.
Seismic Resistance
Circular column is suitable in higher seismic zones where high compressive strength and higher ductility are needed in all directions. We don’t know the direction of ground movement during earth quake. In fact, earth quake is ground movement or acceleration. Though we model seismic loads in only orthogonal directions (X and Y) in ETABS, it can occur in any direction. It is therefore, good to have seismic resistance and ductility in all direction for a better seismic performance.
Ductility is one of the key virtues for seismic resistance. Circular column can be confined more easily than rectangular columns. Confinement ensure more ductility and you can read more about the ductility criteria in IS 1893: 2016. Confinement of a column is more related to end zone shear reinforcement and it is more easily achievable in circular columns.
No stress concentration
As mentioned in the question itself, since there are no sharp corners, there will be no stress concentration in circular members.
Many times, structural advantages alone will not dive the selection of the shape of a structural member.
Reasons that decide the shape of a column
The shape of a column like any building, is decided based on many factors like architectural shape of the building, functional use of the building, structural requirement of the building and so on.
Architectural Reason
Aesthetics is one of the main reasons that decides the shape of a column. For that matter, circular shape for a column is aesthetically pleasing and hence it is used many times. However, the usage is limited in colleges, verandas and institutional buildings. In colleges and corridors and where ever there are double heights or more, circular columns are elegant. It looks aesthetically pleasing. Indian Parliament building is an example where the shape of the building itself suits the column to be circular.
One main reason for not preferring circular column is also an aesthetic reason. Many times, the walls in a building are 200mm and hence most prefer to have the column tucked inside the walls. So, if one dimension is 200, the area needed to resist the forces can be achieved by a larger length for the column. Circular columns will not be feasible in this case when you want to keep the column with in the 200mm wall thickness. In many cases, strength point of view, the column size needs to be more than 200mm. Even then square or rectangular column itself is preferred over circular columns.
Other reasons for the lack of preference for circular columns are mentioned in the subsequent points below.
Difficulty and cost of Formworks
It is easy to do the formwork of a rectangular or square section than circle. Achieving circular shape that do not have corners and edges need more workmanship as well as better formwork materials. Many times, you may need to use new plywood or aluminium formwork. It is more difficult to achieve concrete finishing. Even plastering a curved surface to desirable quality is challenging than that of a plane surface. This will result in additional time and costs. Most medium scale projects are on tight budgets and clients will be more concerned about the cost increase aspects.
It is also more difficult to control the pressure of the concrete in circular formwork. Especially if the column heights are more, it will be challenging to ensure stability of the formwork against the concrete pressure. More quality control and formwork supporting arrangements will have to be adopted at site. The workmanship needed is more for ensuring all of these aspects and hence the cost will be much higher that that of a rectangular column construction.
Functional Requirement
In many buildings like offices, columns are used as display walls for pictures and images and notice boards and signages. Rectangular surfaces provide plane surface where you can post notices and directives. I am not saying this will become a stand-alone reason for the shape selection of the column. Also, if the circular column is large enough, even that can be used for the same purpose.
In basements used as parking areas too, the rectangular or square columns provides a plane surface for notices, warning boards and signages for the movement of traffic. You can see these in basements of shopping malls and other commercial establishments.
Structural Reasons
Though we discussed that, seismic point of view, circular columns are needed, some times we will have the need of more stiffness in one direction. Think of a slender building with less base width and a larger length. We may prefer to have more stiffness in the slender direction. If we adopt a circular column, say you need 450dia. But then providing an equivalent area as a rectangle will be 200x800 which may give more stiffness if 800 is oriented towards the slender direction. This helps in reducing deflection and drift better than a circular column.
Another structural reason that decides the shape is the aspect ratios needed and the minimum member sizes needed for columns as per IS 456 and IS1893. This reason decides between square and rectangular shapes than circular though.
Summary
The reasons mentioned above gives you clarity on why rectangular columns are generally preferred. Its not a single reason that will be the decider but the overall project requirement. The preference also differs depending on the project requirement and the aesthetic requirement. There can also be situations where some columns that are of aesthetic significance and gelling well with architecture being circular and others rectangular. There are also some situations in some types of buildings where lower-level exposed columns are circular and upper levels rectangular. It can also be that lower-level columns are square and upper circular. These can result in undesirable weaknesses in story levels and should be avoided preferably.
Though we cannot have a rigid rule, most of the times it is the ease of construction and the economy that drives the shape of the column.
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