LAUSR

Abstract For circular steel sections, axial slenderness limits are typically defined in terms of the diameter-to-thickness ratio and indicate whether the member will first yield or locally buckle under axial compression. The common opinion is that filling a hollow structural section with concrete would enhance its resistance against local buckling by restraining its inward deformations. On the basis of this concept, the axial slenderness limits stipulated by most international design standards are notably higher for filled sections when compared to their hollow counterparts. Although there is extensive evidence in support of this approach for box sections, the available data for circular sections is limited and, in some cases, inconsistent. While some studies and most international design standards suggest that filled circular sections should have a notably higher axial slenderness limit compared to hollow circular sections, a number of studies argue that the concrete infill is in fact not very effective in increasing the local buckling strength of circular sections. Such studies are however very limited in the literature and often incomplete. Consequently, a comprehensive numerical study with over 140 analyses is conducted herein considering different material types, a wide range of slenderness values, and five initial imperfection modelling techniques to clarify the role of concrete infill in the local buckling behaviour of circular sections. Results of the study strongly suggest that the concrete infill has limited effect on enhancing the local buckling strength of circular tubes. The enhancement is found to be much lower than that implied in international design standards. The roots for this discrepancy are identified and recommendations are proposed in light of the findings of the study. The results suggest that the axial slenderness limit for hollow circular tubes can be significantly relaxed to bring it closer to that of filled circular tubes.