Development and testing of strong-middle steel sections with enhanced buckling behaviour

This study experimentally investigates the compressive behaviour of steel hollow sections with a specified strength heterogeneity along their length achieved through localized Induction Heat (IH) treatments. By converting the ferrite microstructure of S355 circular hollow sections (CHS) into coarse or fine martensite through IH, material yield strength can increase from 1.5 up to 2.4 times, respectively, in the area of IH application. The proposed strength-heterogenous steel sections, named Strong-Middle Sections (SMSs), are designed with higher material strength at mid-length and lower strength at the ends. As a result, SMSs can sustain higher compressive loads than conventional steel sections (CSSs) owing to their increased flexural resistance at mid-length. To validate the concept, column buckling tests were conducted to examine the effect of slenderness ratio, middle-to-end material strength ratio and the length of the strong mid-section in the proposed SMS design. Based on test data, SMSs achieved approximately up to 40 % higher buckling loads and up to 120 % greater post-buckling strength than the corresponding CSSs, displaying either a symmetrical or a novel asymmetrical buckling mode. Digital Image Correlation (DIC) measurements revealed a more unifrom distribution of strain demands along the length of the SMSs, indicating that larger portions of the steel section participated in plastic deformation. The novel deformation mechanism of SMS led to smoother post-buckling transitions and less abrupt drop of post-peak strength. Although approximate, EN 1993–1–1 well estimates the yielding strength of SMS by shifting the location of the first-fibre yielding from the mid-length of the section to the end regions of the section. However, this approach appears to underestimate the maximum compression load achieved by SMSs, which was found to exceed the axial plastic resistance of the section at low slenderness ratios.