Monotonic and cyclic behaviour of beams and beam-columns using cold-formed hollow steel sections with moderate heat-treatment

Abstract

In construction structural steel field, Hollow Structural Sections (HSS) have always been considered a viable solution to achieve both architectural and structural needs. In particular, HSS members due to their good compression and flexural properties as well as high torsional stiffness, can provide undeniable advantages in Moment Resisting Frame (MRF) to withstand seismic action. However, the use of HSS in earthquake-resistant applications has been limited owing to a possible lack of ductility, due to the presence of cold-formed regions, and unstable behaviour under large cyclic deformations. For these reason, over last decade, the attention has been moved to concrete filled tubes (CFT), leaving out the potential benefits of empty tubular sections, such as reduced weight, less time construction and costs, especially in low-to-mid-rise structures. Collection of past studies highlighted that HSS members adopted in seismic applications are mainly manufactured as cold-formed steel sections. However, these sections can be subsequently heat–treated by providing potential improvements in terms of homogeneity and ductility. Yet, the influence of heat-treatment levels on hysteretic behaviour is not well investigated. Therefore, this research aims to (i) investigate key parameters affecting the structural performance of cold-formed HSS with heat-treatments, and to (ii) evaluate the deformation capacity of hollow steel members under flexural and combined axial and flexural loading through experimental and numerical assessment. Firstly, tensile and cyclic material tests have been conducted to evaluate the influence of material properties on structural behaviour of hollow members, i.e. the material homogeneity along the section and the cycling hardening. Then, monotonic, and cyclic quasi-static tests of HSS beam and beam-columns (full scale) were carried out to assess the parameters influencing the deformation capacity. The key parameters are the axial load level and the web and flange slenderness. Lastly, an extensive parametric study has been conducted on a wide range of square and rectangular hollow sections to compare the findings with the requirements specified in the current seismic codes. The investigation has been carried out through detailed finite-element models that are first validated with their experimental counterparts. In particular, the rotation capacity has been chosen as an index to define the ductility in the static case, that follows the same approach of Eurocode 3 and AISC 360-16. Whereas, for the seismic case, plastic rotation has been chosen as the index to define ductility as per AISC 341-16. The findings suggest reviewing the cross-section classification limits under non-seismic and seismic conditions, with due consideration of the axial load level, the web-flange interaction, and the improved material properties of cold-formed HSS members with moderate heat-treatments.