Interactive Buckling and Design Evaluation of Circular Hollow Sections - Assessment of Design Guidelines and Buckling Mode Interaction Through Numerical Analysis and Parametric Study of Buckling Resistance of Circular Hollow Steel Sections

Abstract

This master’s thesis investigates the behaviour of circular hollow section (CHS) in steel under axial loading. Particular emphasis is placed on how the interaction between global and local buckling modes affects member strength. The objective was to complement and compare how current design guidelines assess these sections against the obtained results. To fulfil the aim, the work began with a literature review to deepen understanding of the subject. This was followed by the development and validation of an FE-model capable of replicating buckling behaviour observed in previous studies. A subsequent parametric study was conducted, covering geometries and parameters relevant for a comprehensive perspective on CHS buckling behaviour. The parametric study results, compared with design guidelines, show that current methods create issues for CHS classified as CSC 4. These are treated as shell structures, although their behaviour resembles columns, leading to unsafe and inefficient design. To improve accuracy, an effective area method and a local buckling reduction factor from the British steel design standard (BS5950-1) and a study by Toffolon and Taras (2017) were investigated. These methods aligned more closely with results and provided safer, more efficient CSC 4 designs. In addition, the interaction between global and local buckling was studied. In columns showing interactive failure, ultimate load was reduced by 8 % compared to columns with only local modes, despite nearly identical geometry. Two parameters were identified for determining the transition from interactive to fully local buckling: L′ which is defined as L′ = q l2t r3 and Ncr,global Ncr,local . TThis master’s thesis investigates the behaviour of circular hollow section (CHS) in steel under axial loading. Particular emphasis is placed on how the interaction between global and local buckling modes affects member strength. The objective was to complement and compare how current design guidelines assess these sections against the obtained results. To fulfil the aim, the work began with a literature review to deepen understanding of the subject. This was followed by the development and validation of an FE-model capable of replicating buckling behaviour observed in previous studies. A subsequent parametric study was conducted, covering geometries and parameters relevant for a comprehensive perspective on CHS buckling behaviour. The parametric study results, compared with design guidelines, show that current methods create issues for CHS classified as CSC 4. These are treated as shell structures, although their behaviour resembles columns, leading to unsafe and inefficient design. To improve accuracy, an effective area method and a local buckling reduction factor from the British steel design standard (BS5950-1) and a study by Toffolon and Taras (2017) were investigated. These methods aligned more closely with results and provided safer, more efficient CSC 4 designs. In addition, the interaction between global and local buckling was studied. In columns showing interactive failure, ultimate load was reduced by 8 % compared to columns with only local modes, despite nearly identical geometry. Two parameters were identified for determining the transition from interactive to fully local buckling: L′ which is defined as L′ = q l2t r3 and Ncr,global Ncr,local . The major conclusions were: (i) current guidelines result in unsafe and inefficient design for CHS in CSC 4, and an effective area method, similar to the one established in the British standard, eliminate unsafe design but show conservative results for the reduction factor stated; and (ii) interactive buckling reduces buckling resistance of CHS of up to 8 % and thusly should be considered in design.