Column Design for Buildings: Sizing, Steel and Ties Step by Step

A column is the last stop in the vertical load path. Slab loads travel to beams, beams deliver their end reactions to columns, and columns carry the accumulated load down to the foundation. Reinforced concrete columns combine concrete in compression with longitudinal steel that also resists compression, and lateral ties that hold the bars in place and confine the core. This article covers the column design sequence used for a low rise building: collecting the axial load, sizing the cross section from the capacity equation, choosing the longitudinal steel, and fixing the lateral tie diameter and pitch. The capacity logic parallels column buckling and connection design in steel frames, where the same axial force has to be carried with different material behaviour.

The Column Capacity Equation

The design axial load capacity of a short axially loaded column is Pu = 0.4 fck Ac + 0.67 fy Asc, where fck is the concrete strength, fy is the steel yield strength, Ac is the net concrete area and Asc is the area of longitudinal steel. The factors 0.4 and 0.67 convert characteristic strengths to design values and account for the different strain behaviour of concrete and steel at failure. The equation applies to columns that are short and loaded close to the centroid, where the bending moment is small enough to ignore. Real columns also receive small moments from the beam connections, and IS 456 permits a simplified design when that moment stays small; beyond it, the column is designed for combined axial load and bending with interaction curves. In a low rise building with a regular grid, the axial case usually sets the size.

Gross Area and Steel Area

Ag is the gross area of the column, and the net concrete area is Ac = Ag – Asc. When the steel percentage is expressed as a fraction of the gross area, the equation can be solved directly for Ag before any bar is selected.

Design Constants

Typical values for a low rise building are fck = 20 N/mm² and fy = 415 N/mm². With 2% steel, Asc = 0.02 Ag and Ac = 0.98 Ag, which lets the capacity equation collapse to a single term in Ag. The same constants are used for the slab and beam in the same building, which keeps the concrete supply and the bar bending schedule simple.

Column positions are fixed early in the architectural design and building envelope design process, because the grid has to line up with walls, partitions and the slab spans above.

Collecting the Axial Load

Load Sources

• Slab contribution: half the load from each adjacent panel
• Beam end reactions from the beams framing into the column
• Self weight of the column below the floor being designed
• Live load from the floors and roof above

Each source is taken from the previous design steps. The slab contribution comes from the slab reactions, and the beam end reactions come from the beam shear calculation. The column self weight is added once the trial size is known, which means one short iteration for tall columns. For multi storey buildings the live load on lower columns can be reduced because the chance that every floor above is fully loaded at the same time drops as the tributary area grows, though the reduction rarely changes a 230 mm minimum size.

Worked Load Take Down

1. Take half of each adjacent slab panel load: 121.95 kN.
2. Add the beam end reaction along the x direction: 13.54 kN.
3. Add the beam end reaction along the y direction: 111.94 kN.
4. Sum the components: 121.95 + 13.54 + 111.94 = 247.435 kN.

The beam end reactions come from the beam design: each beam delivers w l / 2 at its support. The same load take down procedure appears in building foundation design procedures, where the column load becomes the input for the footing size calculation.

Sizing the Column Cross Section

Solving for the Gross Area

Substituting into the capacity equation: 247.435 x 10³ = (0.4 x 20 x 0.98 Ag) + (0.67 x 415 x 0.02 Ag). The terms combine to 13.401 Ag, so Ag = 18,496 mm² and the side of a square column is the square root, about 136 mm.

Practical Minimum Size

Code practice and construction practicality set a minimum column size of 230 mm for buildings, so the calculated 136 mm is rounded up to 230 mm. The minimum also guards against slenderness effects and leaves room for the bars and ties with adequate cover. Columns smaller than 230 mm are hard to concrete properly because the aggregate cannot flow around the cage. Slenderness is judged by the ratio of the effective length to the least lateral dimension of the column. When that ratio stays below 12 the column is short and the axial equation applies directly; keeping the section at 230 mm normally keeps low rise columns out of the slender range.

The section can be confirmed quickly in beam design and analysis using SAP2000 or similar software, where the axial force from the frame analysis is compared with the section capacity before drawings are issued.

Longitudinal Reinforcement

Steel Percentage

A common starting assumption is 2% steel. The required area is Asc = 0.02 Ag = 0.02 x 230² = 1058 mm². The percentage stays low for a stocky column because the concrete carries most of the load, and the code minimum of 0.8% still applies when the load is small.

Bar Selection

A 20 mm bar has an area of 314.16 mm². Four bars give 1256.6 mm², which covers the required 1058 mm². Provide 4 Nos. 20 mm diameter bars, one in each corner of the column.

Bar Area Comparison

The three options all exceed the required area. Four 20 mm bars are the usual choice because the bars sit in the corners where the ties can grip them, and the cover stays uniform on all four faces. Round columns follow the same logic with the bars spaced around the circumference. Longitudinal bars are lapped at every floor level. The lap length follows the development length rules, and laps are staggered so that not all bars are spliced at the same section. The ties are spaced closer through the lap zone to hold the spliced bars together.

The envelope that wraps the column grid also matters for energy performance. Nested building envelopes for energy performance show how multi layer walls keep the interior warm while the structural frame stays exposed, and the column positions influence where insulation and cladding can run continuously.

Lateral Ties: Diameter and Pitch

Tie Diameter

The diameter of the internal ties is the larger of two values: one quarter of the diameter of the largest longitudinal bar, and 6 mm. With 20 mm bars, one quarter is 5 mm, so the rule gives 6 mm. The example provides 8 mm ties for extra robustness during handling.

Pitch of the Ties

The pitch is the smaller of three values: the least internal dimension of the column, 16 times the diameter of the smallest longitudinal bar, and 300 mm. For the 230 mm column with 20 mm bars, the least internal dimension is 230 mm, 16 x 20 = 320 mm, and the absolute maximum is 300 mm.

Pitch Limits

The smallest of the three values is 230 mm, so the column is detailed with 8 mm ties at 230 mm centre to centre. The ties are closed loops bent around the corner bars, and the ends are anchored with 135 degree hooks. The spacing is reduced near the beam column joint and at the bar laps, where the confinement demand is highest.

Ties are the part of the column that keeps the cage together during concreting. Column wraps and column design in cladding systems cover the finished member, but the structural ties inside decide whether the column stays stable under load and whether the bars stay in position while the concrete is placed.

The column design closes with a 230 x 230 mm section, 4 Nos. 20 mm bars, and 8 mm ties at 230 mm spacing. Every column in the building repeats the same sequence: collect the loads, solve the capacity equation, round the section up to a practical size, and detail the ties. The full sequence of slab, beam and column work comes together when constructing a concrete building, and the same step by step routine is followed from the foundation to the roof.