Plastic behaviour of polygonal hollow sections in bending

Steel poles made from polygonal sections are an economic alternative to circular hollow sections for use as lighting towers or transmission line pylons. For transmission lines especially, the intention behind the design of the poles is to make them more compact in order to avoid land usage and achieve an inconspicuous appearance. If the pole diameter is reduced, the slenderness of the sections needs to be reduced as well in order to achieve the desired resistance, but increasing the wall thickness disproportionately reduces the cost-effectiveness of the structure. The European design rules for overhead electrical lines [1] do not allow plastic behaviour of stocky sections to be used, which reduces the advantages of compact sections.
In modern standards for steel structures, such as Eurocode 3 (EC3) [2], activating plastic reserves is generally acknowledged as state of the art. According to EC3-1-1, the slenderness of the section determines whether the cross-section is allocated to class 1 or 2, where plastic design is allowed, class 3, where only elastic behaviour is used, or class 4, where local buckling is assumed. When changing from class 2 to class 3 sections, EC3-1-1 prescribes a sudden drop in resistance, which has long been recognized as unjustified. Research has been done to overcome this mismatch for circular hollow sections [3].
In this paper the transition between the plastic bending moment capacity and the limit state of buckling is investigated in order to identify the ultimate load of polygonal sections. The 16-sided polygon investigated can be treated as a collection of plate strips using the plate buckling design rules of EC3-1-5 for class 4 cross-sections. Detailed examinations illustrate the inconsistency of EC3 when it comes to the bending capacity of compact sections even before the start of stability problems.
Numerical investigations have been performed on the basis of experimental data gained from full-scale bending tests, along with imperfection measurements via laser scanning. These results suggest that the bending moment capacity can be increased beyond the calculated elastic capacity for more compact sections, but might be overestimated when calculating the full plastic capacity according to EC3-1-1.