Between square and circle : A study on the behaviour of polygonal steel profiles under compression

Abstract: The present study explores the structural properties of regular convex polygonal cross section (RCPS). Such profiles are commonly used to construct masts and towers, e.g.  transmission towers, light poles etc. The aim is to provide a better understanding of how the RCPS profiles behave on the local level under meridional compression.For that matter, the study is limited to short specimens to avoid interaction with flexural buckling. Depending on the geometric and material characteristics, local buckling of an RCPS can either be contained within the width of individual facets, with the polygon edges acting as simple supports, or overcome the edge barriers of a facet extending to its neighbouring ones (spillover), which is best described as a type of shell buckling behaviour. Distinguishing between these two cases is the main task of this study.The investigation is performed by means of numerical simulations and laboratory experiments, in both cases paying due attention to the influence of initial geometric imperfections. Four different imperfection patterns are identified by observing the collapse mechanisms occurred on experiments.Through Finite Element Modelling (FEM), the influence of the four imperfection types as well as of the yield strength are studied over a wide range of RCPS models with varying number of vertices and local slenderness. Based on the minimum FEM obtained resistances, the RCPSs are categorised into two groups, one where a plate-like (facet contained) buckling is more critical than the spillover and vice versa.The suitability of the Eurocode parts corresponding to plated and shell structures (EN~1993-1-5 and EN~1993-1-6 respectively) is evaluated for the design of RCPS on the basis of the two identified regions. The comparison showed that the two codes in conjunction can cover the design of RCPS. A method expressed as a simple empirical formula is suggested for categorising RCPS and choosing the respective Eurocode part.The results of the numerical simulations are verified through an experimental matrix consisting of nine High Strength Steel (HSS) specimens. The specimens are of 16, 20 and 24 vertices and range from class 1 to class 4. Precise measurements of the geometric initial imperfections are taken by 3D scanning and digital processing of the specimens.Experimental results from the present study and from literature come in good agreement with the conclusions from the numerical simulations, validating the proposed model.