Circular RC Section Check - EC2 & TCVN 5574

What it checks

This tool verifies a circular reinforced concrete section to Eurocode 2 or TCVN 5574: the axial-moment interaction diagram, shear, slenderness and second-order effects, and the serviceability checks for crack width and deflection. It is the right tool for a circular column, a bored pile or a circular pier.

A circular section is harder to analyse than a rectangular one because the compression zone is a circular segment rather than a rectangle, and the reinforcement is distributed around the perimeter rather than concentrated in layers. The tool integrates across the chord rather than using a simplified equivalent rectangle for the ULS case.

Why the N-M interaction diagram matters more here

For a circular column with bars distributed around the perimeter, the moment capacity depends on axial load in a way that is not intuitive: adding axial compression increases the moment capacity up to the balance point, then reduces it. A column checked only at its maximum axial load can be unsafe at a lower axial load with the same moment - the load case that governs is often not the one with the biggest numbers.

The interaction diagram makes this visible. The tool plots the full curve rather than checking a single point, so you can see where your load cases sit relative to the balance point and whether a lightly loaded case is actually the critical one.

Formula reference

Design concrete strength

$f_{cd} = \dfrac{\alpha_{cc}\,f_{ck}}{\gamma_C}$

Design steel strength

$f_{yd} = \dfrac{f_{yk}}{\gamma_S}$

Circular segment area

$A_{seg} = R^{2}\left(\theta - \sin\theta\cos\theta\right)$

Squash load

$N_{Rd} = \eta f_{cd} A_c + A_s f_{yd}$

Equivalent width for shear

$b_w = D$

Slenderness ratio

$\lambda = \dfrac{l_0}{i},\quad i = \dfrac{D}{4}$

Shear without links

$V_{Rd,c} = \dfrac{0.18}{\gamma_C}k(100\rho_l f_{ck})^{1/3} b_w d$

Assumptions and limits

Reinforcement is assumed evenly distributed around the perimeter at a constant cover, which is the normal arrangement for a circular column or pile. A section with bars bunched on one face behaves differently and the interaction diagram would not be symmetric as assumed here.

Shear uses an equivalent rectangular width equal to the diameter, which is the standard simplification. Confinement from circular links - which genuinely enhances the concrete in a well-detailed column - is not taken as an increase in capacity, so results are conservative for a heavily confined section.

The tool checks a section. Buckling of a slender column is addressed only through the slenderness and second-order provisions; a full frame stability analysis, torsion, punching, fatigue, fire and seismic detailing are outside the scope.

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