1. Classification of Columns
Columns are categorized based on several structural and geometric parameters:
Rectangular / Square Columns: Most common in residential and commercial framed buildings because they easily align with brick masonry walls and simplify formwork (shuttering) fabrication.
Circular Columns: Frequently used for aesthetic architectural features, porch supports, stilt parking levels, and bridge piers. They offer uniform resistance to lateral loads from all directions.
L-Shaped / T-Shaped (Corner/Edge) Columns: Speciality columns used at building corners or interior junctions to eliminate projecting column corners inside rooms, improving interior space utilization.
B. Based on Slenderness Ratio (Height-to-Lateral Dimension)
Short Columns: When the effective length relative to its lateral dimension is low, the column fails primarily by material crushing under direct compressive stress.
Slender (Long) Columns: Columns with a high height-to-width ratio. They are prone to buckling (lateral bending) under heavy axial loads and require specialized structural design reduction factors.
C. Based on Reinforcement Type
Tied Columns: Main longitudinal bars are held together by individual closed lateral ties (stirrups) spaced at regular intervals. This is the standard choice for most building frames.
Spiral Columns: Longitudinal bars are enclosed within a continuous helical (spiral) steel wire. Spirals provide lateral restraint that significantly enhances ductility and load capacity before failure.
2. Key Design and Reinforcement Principles
Proper steel detailing ensures columns can handle combined axial loads and bending moments (eccentric loading caused by beams).
Longitudinal Reinforcement: As per IS codes, the minimum area of steel reinforcement (Asc) in a column must be at least 0.8% and can go up to 6% of the gross cross-sectional area of the concrete (though 1% to 3% is typically practical to allow proper concrete flow around congested bars). The minimum number of bars is 4 for rectangular columns and 6 for circular columns.
Transverse Reinforcement (Ties / Stirrups): Stirrups prevent the longitudinal bars from buckling outward and shear failure. Their diameter and spacing are determined by the main bar size and calculated shear forces.
Clear Cover: Columns require a minimum clear cover of 40mm (increasing to 50mm if exposed to severe weathering or soil contact) to protect the steel from moisture and carbonation.
3. Step-by-Step Construction Workflow for Columns
Executing a column correctly on site requires strict adherence to vertical alignment, mix quality, and pouring techniques.
Step 1: Setting Out and Starter Bars (Dowel Bars)
Verify column centerlines using grid lines and reference pillars established during the foundation layout.
Ensure column starter bars (dowels) projecting from the footing or lower floor slab match the size, count, and position specified in the structural drawing.
Check that lap lengths (typically 42ϕ to 50ϕ depending on steel grade and bar diameter) are maintained correctly above the slab level.
Step 2: Reinforcement Cage Fabrication and Placement
Fabricate the column steel cage on the ground or directly over starter bars, ensuring longitudinal bars are tied securely with closed lateral ties or ring stirrups at exact design intervals (closer spacing is required near beam-column junctions for seismic confinement).
Attach durable polymer or concrete cover blocks on all four sides to maintain the mandatory 40mm clear cover.
Step 3: Formwork (Shuttering) Erection
Use rigid marine plywood panels or steel shuttering plates supported by timber or adjustable steel props (cup-lock system).
Check column verticality using a plumb bob or spirit level on multiple adjacent faces. Secure the formwork tightly with column clamps, yokes, or tie-rods to prevent bulging or shifting under wet concrete pressure.
Provide a small temporary cleaning pocket or window at the base of the shuttering to clear out any construction debris, tied wire scraps, or sawdust before pouring.
Step 4: Concreting and Compaction
Use structural grade concrete matching the design specifications (minimum M20 or M25 for framed structures).
Pouring Height Rule: Avoid dropping wet concrete from heights greater than 1.5 to 2 meters to prevent aggregate segregation. For tall columns, use a concrete chute or pump pipe.
Vibration: Consolidate the concrete thoroughly using a needle vibrator inserted systematically. Ensure the vibrator does not hit the formwork or displace the steel cage.
Step 5: Stripping Formwork and Curing
Formwork can typically be stripped safely after 24 to 48 hours, depending on ambient temperature and concrete setting speed.
Immediately initiate rigorous water curing by wrapping the column in Hessian cloth/jute bags or plastic sheets and keeping them moist continuously for at least 7 to 14 days.
