Engineering Structures 28 (2006) 1223–1235 www.elsevier.com/locate/engstruct Variable amplitude cyclic pure bending tests to determine fully ductile section slenderness limits for cold-formed CHS Mohamed Elchalakani a,∗,1 , Xiao-Ling Zhao b , Raphael Grzebieta b a School of Architectural, Civil and Mechanical Engineering, Victoria University, Victoria 8001, Australia b Department of Civil Engineering, Monash University, Victoria 3800, Australia Received 22 August 2005; received in revised form 18 October 2005; accepted 19 October 2005 Available online 24 April 2006 Abstract This paper describes an experimental investigation of the cyclic inelastic flexural behaviour of cold-formed circular hollow section (CHS) beams. Controlled-rotation, symmetrical cyclic bending tests were performed using a variable amplitude loading history on different sizes of compact CHS with diameter-to-thickness ( D/t ) ratios ranging from 20 to 40. The CHS beams exhibited stable hysteresis behaviour up to local buckling and then showed considerable degradation in strength and ductility depending on the D/t ratio. Seismic capacity parameters are presented including strength, stiffness, hysteresis loops and modes of failure for each specimen. Peak moments obtained in the cyclic tests were compared with those obtained in monotonic and cyclic tests published previously and also with design moments predicted using a number of steel specifications. New section slenderness limits suitable for the design and construction of seismic resisting structural systems were determined. A comparison is made between these seismic slenderness limits and the limits available in the design codes. The effect of the number of cycles on ductility and energy absorption is examined. c  2006 Published by Elsevier Ltd Keywords: Beams; Slenderness limits; Cold-formed steel; Circular tubes; Cyclic loading 1. Introduction The study of plastic properties of tubular structures under cyclic loading is important being a significant loading condition during a seismic event. A basic assumption made in regards to plastic design of a tubular framed structure is that the flexure member shall have a sufficient plastic deformation (Point A in Fig. 1) capacity while maintaining the plastic moment M p during the course of deformation. Without such rotation capacity at the member level, the collapse mechanism of the framed structure cannot develop. The need for substantial rotation capacity is even greater (point B in Fig. 1) under seismic (cyclic) loads because it is expected that not only will such a yield mechanism form but also that the structure will deform beyond the point of initial collapse and cycle back and forth under severe earthquake shaking. Section ductility ∗ Corresponding author. E-mail address: mohamed.elchalakani@vu.edu.au (M. Elchalakani). 1 Former Ph.D. student, Department of Civil Engineering, Monash University. requirements that allow a tubular frame structure built using CHS to deflect up to point A or B in Fig. 1 were examined in the past by the authors in a series of papers [17,18,20–22]. 2. Section slenderness and loading history 2.1. Section slenderness Although steel material with nominal yield stress up to 450 MPa usually exhibits high ductility, the plastic deformation capacity is often limited by section profile instability. Instability in RHS (rectangular hollow sections) and SHS (square hollow sections) flexural members induces flange local buckling, web local buckling and lateral torsional buckling [28,34,35,41]. In CHS, however, it induces only local plate (shell) buckling. For a given grade of structural steel, the tendency of local plate buckling is a function of the diameter-to thickness ratio D/ t and the amount of moment gradient. AS 4100 [6] and NZS 3404 [32] define the section slenderness for CHS as λ s =  D t  ·  σ y 250  . (1) 0141-0296/$ - see front matter c  2006 Published by Elsevier Ltd doi:10.1016/j.engstruct.2005.10.022