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Column Interaction Diagram Calculator

Outer ring

Bars distributed around the perimeter — one per corner, remainder split between faces. Additional rings are inset progressively inward at real clear-spacing offsets.

b=400h=400d=342d'=59c=408T25As=3927 mm²NEd=500kN
N-M Interaction Diagram
01k2k3k4k-1k0100200300Bal(500, 150)M (kN·m)N (kN)
Key Interaction Points
PointN (kN)M (kN·m)
Pure Compression4224.00.0
Balanced Point1155.7310.3
Pure Bending0.0249.6
Pure Tension-1707.40.0
Design Point (Pu, Mu)500.0150.0
Design basis
  • National Annex: none applied as a whole — the EN recommended values are used, except where noted below. There is no National Annex selector; check the values against the annex for your project.
  • αcc = 0.85 in fcd = αcc·fck/γc (the UK National Annex value; EN 1992-1-1 §3.1.6(1) recommends 1.0).
  • Pure compression N0 = fcd·(Ac − As) + As·min(fyd, Es·εc2) with εc2 = 0.002 (EN 1992-1-1 §3.1.7, §6.1(5)): bars displace concrete and the steel stress is capped at 400 MPa for B500; sections fully in compression pivot about εc2 at 3h/7 (Fig. 6.1).
  • γc = 1.5 and γs = 1.15 (EN 1992-1-1 Table 2.1N, persistent and transient), editable.
  • Short-column check only — no slenderness or second-order moment magnification is applied; use RC Column Design for slender columns.
  • Uniaxial bending about one axis only.
  • Circular sections are assumed spiral-tied and rectangular sections tied (governs the ACI φ transition floor and the axial capacity cap factor) — verify this matches your actual detailing.
  • Checks a single user-supplied load point (Pu, Mu) against the section’s interaction envelope — this tool doesn’t generate load combinations for you.
  • Verify all provisions against the governing code edition and applicable National Annex before use.
16 independent benchmarks — view validation
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Column Interaction Diagram v1.1Validated against 16 benchmark cases →
Interaction CheckPASS
Utilization
52.2%
Utilization52.2%
Calculation
EC2 §6.1Section properties
Ac = b×h, As = Σ(area of each bar)
Ac = 160000 mm², As = Σ over 8 bars
As = 3927 mm²
EC2 §3.1.7, §6.1(5)Maximum axial capacity (pure compression, uniform strain εc2 = 0.002)
N0 = fcd·(Ac − As) + As·min(fyd, Es·εc2)
N0 = 17.00×(160000 − 3927) + 3927×min(434.8, 400)
N0 = 4224.0 kN
EC2 §6.1N-M interaction check
Design point must lie within interaction curve
(NEd, MEd) = (500.0, 150.00) kN, kN·m
Utilization = 52.2% — ✓ PASS