Comparative assessment of EN 1994-1-1, AISC 360 and JGJ 138 for multi-profile composite mega-columns, with a parametric extrapolation towards ultra-high-performance concrete

Abstract

Mega-columns that embed several discrete hot-rolled profiles in a single reinforced-concrete section, sometimes called isolated steel reinforced concrete (ISRC), have increasingly been adopted in place of welded steel caissons in super-tall lower storeys, but their design falls in a regulatory gap. The simplified method of EN 1994-1-1 covers only single-profile sections, AISC 360-22 admits multiple encased shapes provided they are interconnected but caps the cylinder strength at 69 MPa, and JGJ 138-2016 is confined to concrete grades up to about C80. This study examines how far those provisions can be extrapolated, using a closed-form code-based assessment of a nine-specimen database that combines the six monotonic Phase-1 columns of Bogdan et al. with three axial multi-profile columns of Zhao et al. A single plastic-stress-distribution engine builds the four-point interaction polygon identically under each code, so the only difference is the concrete treatment. All predictions are computed at the characteristic (measured) material level, with partial safety factors set to unity, so that the comparison isolates the cross-sectional mechanics and does not represent factored design capacities. EN 1994-1-1 and AISC 360-22 both apply the 0.85 effective-stress block and coincide, at a mean test-to-prediction ratio λ = 1.12 (CoV 13.4%) against 1.02 for JGJ 138-2016 (CoV 12.2%), whose near-unity α₁ makes it least conservative. The predictions are generally conservative across the eccentric range and for the Zhao cage, the single exception being E10-2 under JGJ 138-2016 (λ = 0.977), but they overpredict Bogdan's two pure-axial specimens at f’_c ≈ 55-61 MPa by 11–14% (EN/AISC) and 19–23% (JGJ), reflecting the non-attainment of the full plastic state at peak–the steel still at or below yield and the cover spalling prematurely under full-area loading. In the parametric sweep at e/h = 12.5%, the JGJ margin over the coincident EN/AISC prediction holds within 7–10% up to 180 MPa. Extending these rules to UHPC will require a calibrated effective concrete stress, a minimum steel-fibre dosage, and explicit treatment of eccentricities up to e/h ≈ 15%.