The Behaviour of Cold-Formed Steel Built-up Structural Members

Cold-formed steel sections offer many benefits to construction, such as a high strength-to-weight ratio, an ease of handling, transportation and stacking, and important sustainability credentials. For these reasons their range of application has rapidly expanded from being mainly used as secondary members in steel structures to an increasing use as primary members. This trend in construction is exerting an increased demand on cold-formed steel structural members in terms of the span length and the load carrying capacity they need to provide. A common and practical solution to address these new demands consists of creating built-up sections by connecting two or more individual sections together using fasteners or spot welds. However, a lack of understanding of the way these sections behave and a gap in specific design provisions has prevented the exploitation of the real potential which these types of sections can offer. This research aims to develop an improved understanding of the behaviour, stability and capacity of built-up cold-formed steel members in compression and bending, paying special attention to the various interactions resulting from cross-sectional instabilities, buckling of the individual components in between connector points and global buckling of the built-up member, as well as the role played by the connector spacing in these interactions. To this end, a series of experiments on built-up beams and columns was carried out. A total of 20 stub column tests were completed with four different built-up geometries, each constructed from four individual components assembled with either bolts or self-drilling screws at varying spacings. The columns were tested between fixed end conditions and were designed to exclude global instabilities of the built-up specimens. In addition, 24 long column tests with almost identical built-up cross-sectional geometries, assembled with the same types of connectors, were also conducted. The columns were compressed between pin-ended boundary conditions and the load was applied with eccentricities of L/1000 or L/1500. Each built-up geometry was tested with three different connector spacings, and this time the columns were designed to exhibit global buckling of the whole column in addition to cross-sectional buckling of the components and possible buckling of the components in between connector points. A series of 12 beam tests was also carried out for two different cross-sectional geometries, constructed from multiple channel sections and connected with bolts at varying spacing. The built-up beams were tested in four-point bending, with lateral restraint provided at the locations where the concentrated loads were applied in order to avoid global instability. All tests on columns and beams showed that the different components of the built-up geometry mutually restrained each other while buckling, relative to their individually preferred buckled shapes, and that while the connector spacing may significantly affect the amount of restraint they exert on each other, its effect on the ultimate capacity is considerably less. The material properties of all tested specimens were determined by means of coupon tests taken from the corners and flat portions of the constituent sections, while detailed measurements of the geometric imperfections of each specimen were carried out using a laser displacement sensor mounted on a specially designed measuring rig. In addition, the mechanical behaviour of the connectors used to assemble the built-up specimens was determined by means of single lap shear tests. Detailed finite element models were created of the built-up beams and columns, which included the material non-linearity obtained from the tensile coupons, the geometric imperfections recorded on the actual specimens and the connector behaviour obtained from the single lap shear tests. The models were first validated against the data gathered from the experimental programmes and were further used in parametric studies, in which the sensitivity of the ultimate capacity to contact between the components and to the connector spacing was investigated. The numerical studies revealed that the effects of both contact between the components and the connector spacing on the ultimate capacity was most pronounced when the connector spacing was shorter than the natural local buckle half-wave length of the components. However, this range of connector spacings may prove impractical in construction due to the large amount of labour it requires.