Minimum Size of Columns

//Minimum Size of Columns

Minimum Size of Columns


Jagvir Goyal

Collapse of a four storeyed building in Solan District few days back causing death of 14 persons including 13 army men sent shockwaves across the region. In the present times, when the structural engineering and concrete technology are at most advanced stage, such a collapse of building speaks of poor and ill-designed construction still going on without check with sole aim of earning money and playing with the life of the users.

As per news reports, a preliminary cause of collapse of building has been assessed as inadequate size of its columns. Let’s have a look at some important aspects related to design, construction and safety of columns. These may be basics for a civil engineer but important for a common man to know:

RCC Framed structures: Building columns constitute part of RCC frames used these days to construct multi-storeyed buildings. Use of RCC frames is essential from earthquake resistance point of view also. Building columns should only be in RCC as no other material except steel serves so well. Steel columns or composite columns are rarely used in particular areas and circumstances only.

Size of Columns: Size of columns in a building should be decided through proper structural design of the building and not on the basis of thumb rules. Neither the size followed by an adjacent building should be adopted. A 9 inch x 9 inch size column is too small and may be at the maximum sufficient for a single storeyed building only. In a building, the floors transfer all their weight and live load to beams which further transfer it to columns. Columns transfer the whole load of the building to the ground. Therefore, these must be designed professionally. Even for a single storeyed building, size of RCC column shouldn’t be less than 9 inch x 12 inch.

Future additions: It is common practice to add more storeys to an existing building to reap more floor area and profits. The beams and columns are examined at a glance, declared ok and new floor is added. This is most dangerous practice and reported to have been done in case of Solan building also. Simplest way for it is to design the columns by keeping future addition of storeys in view. The structural engineer will consider the load of future addition also and design the columns accordingly.

Steel reinforcement: Diameter of steel bars provided in RCC columns plays vital role in determining their load carrying capacity. In no case, vertical bars of diameter less than 12 mm should be used. Use of 16 mm or 20 mm diameter or even 25 mm diameter is still preferable depending upon the load on the column. These vertical bars need to be further tied and kept in designed place by using lateral ties of 8 mm or 10 mm diameter at regular vertical intervals. IS Codes also prescribe minimum area of steel reinforcement to be provided in the column section area. This criteria should also be fulfilled.

Adequate cover: It is very important to provide adequate concrete cover to steel bars provided in a column. This concrete cover keeps the steel well hidden inside it and saves it from rusting and corrosion. Rusting of steel bars, if exposed, may ultimately lead to collapse of the column over the years. The clear cover of concrete beyond the outer edge of lateral ties holding the vertical bars in position shouldn’t be less than 25 mm all around the column throughout the height of column. If cover is not uniformly provided, steel shall be attacked at the point where concrete cover is minimum.

Horizontal ties or beams: Continuous height of columns without any horizontal RCC beams at regular vertical intervals increases the slenderness ratio of the columns, resulting in reduction of their load carrying capacity. It should be ensured to keep unsupported height of columns as minimum possible. Slenderness ratio shall depend on the unsupported height and dimension of column. Lesser it is, better is the strength of column.

Concrete grade: Columns should be built by using properly designed and controlled concrete and not by using the uncontrolled mixes of 1 : 2 : 4 or 1 : 1 ½ : 3 etc. M15 grade of concrete shouldn’t be used in columns. Minimum grade of concrete for columns should be M20. Higher grades are preferred.

Adequate footing: It is very important to provide adequately sized and well-designed RCC footing for columns to safely transfer all the load from them to the ground without settlement of the footing. Further, the footing should be provided at adequate depth below ground level and rested on firm soil. The structural engineer will design how the vertical steel bars of the column enter the footing to create a fully integrated joint between the footing and the column and will also check the joint of column and footing for punching shear and other factors to make it safe against cracks.

Cut entry of water: It should be carefully ensured that there is no such source of water leakage around the building foundation from where the water enters the footing of the columns and weakens or erodes the soil underneath them. This is a very common phenomenon. Water keeps entering a building’s foundation area from a leaking pipe or rain water flow or any other source and weakens its foundation leading to cracks in the building or its collapse.

Structural appraisal: Irrespective of whether a law exists in this regard or not, the owner of a building should ensure its structural safety first by engaging a structural design engineer to design all its RCC components, getting its supervision done from a civil engineer for proper quality of materials, proper workmanship and execution of work as per structural drawings and later by getting its structural appraisal done from a qualified engineer who’ll certify the building as safe or not.