A column is a vertical structural member that carries the loads of the entire structure above it — slabs, beams and walls — down to the foundation. Columns work primarily in compression, and they are the most safety-critical elements in a framed building: a column failure is sudden and can collapse the structure, unlike a beam which usually deflects and warns first.
Because of this, columns are reinforcement-dense and cast in richer concrete grades (M20 or higher). They contain main vertical bars tied together with lateral ties or stirrups that prevent the bars from buckling. Residential columns commonly start at 230 × 230 mm and increase with load and number of floors.
Steel content in columns is high — around 150 kg per cubic metre as a planning estimate, more than slabs or footings. Estimating a column means its concrete volume (width × depth × height × number of columns) plus reinforcement. The right column size is always a structural-design decision based on the load coming down from above.