12.16: Numerical investigation of the local buckling behaviour of high strength steel circular hollow sections

The introduction of high‐strength steel (HSS) hollow sections with yield strengths of f y =690 MPa and beyond in the construction sector is currently made more difficult by a lack of knowledge on the specific local and local+global buckling behaviour of slender HSS cylindrical hollow‐sections. With rising yield strength, buckling phenomena become more relevant. Eurocode 3 design provisions classify CHS as “slender \ class 4” at a diameter to thickness ratio of D\t=90·∊ 2 , with ∊ 2 =235\f y . For HSS with f y =690 MPa and beyond, the majority of cold‐formed and many hot‐finished CHS will be class 4 and should be designed accordingly. However, no straightforward rules for the design of class 4 HSS CHS are found in EC3. This paper discusses initial numerical studies on the specific local buckling behavior of HSS CHS sections. The study represents the initial steps in the recently initiated RFCS research project “HOLLOSSTAB”, during which new design rules for HSS hollow sections are developed on the basis of an “Overall Interaction Concept” (OIC). This concept – similarly to the Direct Strength Method (DSM) used in North America for the design of cold‐formed steel open cross‐sections – makes use of the results of (numerical) linear buckling analyses (LBA) for the whole member to determine the slenderness and consequently an “overall” buckling reduction factor. The paper discusses how this approach fits into the general framework of buckling design checks for cylindrical structures, discusses existing rules and their implications for HSS CHS, shows numerical results and introduces initial design proposals based on the OIC approach.