Axial load carrying capacity of short RCC columns per IS 456:2000 Clause 39.3. Formula Pu = 0.4·fck·Ac + 0.67·fy·Asc applies only to short columns where effective length divided by least lateral dimension does not exceed 12. Tables below cover M20 + M25 concrete grades with Fe415 + Fe500 reinforcement steel — the four combinations that account for over 90% of Indian RCC building columns. Each table gives ultimate (factored) load carrying capacity in kN at standard steel percentages from 0.8% to 4.0% across rectangular column sizes from 230×230 mm up to 600×600 mm. For slender columns where the slenderness ratio exceeds 12, additional moment is induced due to lateral deflection and the capacity must be reduced per IS 456 Clause 39.7 — those reduction factors are not in this table. The minimum reinforcement percentage is 0.8% per IS 456 Cl. 26.5.3.1 to ensure ductile behaviour; the maximum is 6% (overall) or 4% in laps. Most economical design typically falls in the 1.0%-2.0% steel range — beyond 2.5%, marginal capacity gain per kg of steel diminishes.
Formula (IS 456 Clause 39.3)
Pu = 0.4 × fck × Ac + 0.67 × fy × Asc
Where Pu = ultimate axial load (kN), fck = characteristic compressive strength of concrete (MPa), Ac = area of concrete = Ag − Asc, fy = yield strength of steel (MPa), Asc = area of steel reinforcement
Design Notes
• Values for short columns only — effective length divided by least lateral dimension must not exceed 12. Beyond this slenderness ratio, columns are 'slender' and develop additional moments due to lateral deflection (P-delta effect) that the capacity formula does not account for.
• For slender columns, apply additional moment reduction factor per IS 456 Clause 39.7 — typically reduces capacity by 10-30% depending on slenderness ratio. For very slender columns (ratio > 60), a full second-order analysis is required.
• Minimum 4 bars for rectangular / square columns (one in each corner). Minimum 6 bars for circular columns equally spaced around the perimeter. Bundled bars (2-bar or 3-bar bundles) are permitted but each bundle is treated as a single bar for tie spacing.
• Minimum bar diameter is 12 mm (8 mm permitted only in tied non-load-bearing situations). For seismic detailing per IS 13920 in Zones III/IV/V, use diameter ≥ 12 mm and ensure all bars are within distance of 150 mm from the nearest stirrup corner.
• Maximum spacing of lateral ties / stirrups is the least of: (a) least lateral dimension of column, (b) 16 times the smallest longitudinal bar diameter, (c) 300 mm. Tie diameter is the larger of: (a) one-fourth of the longitudinal bar diameter, (b) 6 mm. Closer spacing is required in confining zones near beam-column joints per IS 13920.
• Factor of safety of approximately 1.5 is applied — to get working (service) load capacity, divide Pu values in the tables by 1.5. The 0.4·fck·Ac term applies partial safety factor of 1.5 for concrete; the 0.67·fy·Asc term applies partial safety factor of 1.15 for steel. The 0.4 + 0.67 coefficients are NOT the same as direct stress fractions — they are derived from limit-state ultimate strain assumptions per IS 456 Cl. 39.3.
• Tables assume the column is axially loaded only (no moment). For columns subjected to moment (most real-life columns), the capacity reduces and an interaction diagram per IS 456 Cl. 39.5 must be used. Most peripheral building columns develop biaxial moment from beam framing — the interaction analysis is mandatory for these.
• Concrete cover to longitudinal reinforcement is typically 40 mm for moderate exposure (IS 456 Table 16). The Ac (area of concrete) in the formula = Gross area Ag minus Asc (area of steel) — for accurate small-section calculations, the cover does not change Ag but affects the gross-to-effective area ratio.
• Steel percentages above 4% should be detailed carefully — laps create high local steel ratios that can exceed the 6% absolute maximum at lap zones. Avoid laps at locations of high stress (top of column, beam-column joints).