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The performance of structures under severe seismic action is ensured by properly calibrated ductility, stiffness, and strength. The "plastic" members of ductile mild carbon steel (S235 to S355) dissipate the seismic energy, acting as "structural fuses", while the "elastic" members must be provided with adequate overstrength, to have the capacity to carry the stresses corresponding to yielded and strain hardened plastic hinges. High strength steels, such as S460 to S690, might be used for these elastic members, providing them with the necessary overstrength, without enhancing the stiffness, as that could unbalance the structural system. Dual structural systems, as moment resisting frames (MRFs) combined with eccentrically braced frames (EBFs), can be equipped with replaceable mild carbon steel dissipative links acting as seismic fuses. The MRFs, possibly fabricated from high strength steel, remain elastic, being able to recover the initial position. The key problem in such systems is the design and calibration of the fuses, i.e. the replaceable bolted links, which should be carefully qualified by tests. The present paper reviews past research on bolted replaceable links and discusses the experimental qualification requirements. [ABSTRACT FROM AUTHOR]

The paper presents the outcome of a finite-element numerical investigation program developed and carried out in connection to a set of experimental investigations performed on welded beam-to-column joints, with columns realized as concrete-filled tubes (CFT) with rectangular hollow sections (RHS). The current study was aimed to calibrate a set of numerical models, to conduct a parametric study, and to validate the proposed design procedure. Joint models were calibrated, and additional parameters were investigated numerically, such as: influence of the concrete core, influence of the axial force in the column, response of one-way and two-way joints. It was shown that (1) the influence of concrete core was proved to be low for the single sided RBS or CP joint assemblies, but significant corresponding to joints with strengthened beam, (2) the response of the joint assembly was not affected by the presence of the axial compression force in the column, and (3) the connection solution with external diaphragms was characterized by an adequate response considering one-way and two-way joints. The current study evidenced a good conception and design of the reduced beam section (RBS) and cover plate (CP) joints. In particular, the study proved the feasibility of using higher steel grades in columns and joint components, and the use of an external diaphragm for the transfer the loads from the beam to the side walls of the rectangular hollow section (RHS) steel tube. [ABSTRACT FROM AUTHOR]

The paper presents the outcomes of an experimental program developed and carried out with the aim of characterizing the behavior of two types of moment-resisting joints in multistory frames of concrete-filled high-strength steel rectangular hollow section (RHS) columns and mild carbon steel beams. The paper describes the design approach and displays the specific detailing for the two joint typologies: with reduced beam section (RBS) and with cover plates (CP). In spite of the various past and recent studies which were focused on the evaluation of full-strength welded beam-to-concrete-filled-tube (CFT) column joints, particular design procedures are missing at the European level. The objectives of the current research were to (1) development a design procedure for welded beam-to-CF-RHS joints, (2) experimental validation of the of the design procedure, and (3) assessment of the seismic performance. The monotonic and cyclic tests evidenced a good conception and design of the RBS and CP joints. In particular, the current study proved the feasibility of using higher steel grades in columns and joint components, and the use of an external diaphragm for the transfer the loads from the beam to the side walls of the RHS steel tube. The seismic performance of the joints was proved adequate, the joints sustaining rotation capacities larger than the 40 mrad at the significant damage performance level. [ABSTRACT FROM AUTHOR]

The difference between local and distortional buckling of thin-walled cold-formed steel sections with stiffened elements, of members in both compression and bending are taken into consideration applying relevant design formulas available in the codes. However, for unstiffened elements, or for those unstiffened enough, the difference between distortional and local buckling modes, in some cases, and particularly for U-, Z- and L-sections, might be unclear. When these sections are edge stiffened, in the case of lipped flanges, it is not always easy to identify which of the two are dominant, while in the case of interaction with an overall buckling mode, the situation might be even more difficult. Local buckling modes involve the buckling of cross-sectional walls, while distortional buckling, also known as "stiffener buckling" or "local-torsional buckling", is characterised by the rotation of the flange at the flange/web junction in members with edge stiffened elements. Both types of buckling are also known as "sectional" buckling modes. For interactive local/distortional buckling with the overall one, the correct evaluation of the "short" member capacity might play a significant role when checking the design capacity of a slender thin-walled member prone to interactive sectional-overall buckling mode. In the case of a stiffened flange, the effectiveness of the stiffeners can make the difference between local and distortion buckling. A boundary between local and distortional bucking of a given open section can be established to be considered in the design. To correctly apply the Ayrton–Perry formula, in order to get the ultimate capacity of a thin-walled cold-formed steel slender member, prone to interactive local or distortional buckling with the overall one, the strength of the short member has to be identified. This paper summarises the study of this problem, including the main results and conclusions, emphasising that it is difficult to separate the local and distortional modes in the case of cold-formed steel open sections used in practice, and there is some interaction between these two modes. • Local/distortional and overall interactive buckling of thin-walled cold-formed steel columns. • Effectiveness of edge stiffeners in the interactive buckling of open section. • Interactive buckling through the Erosion of Critical Bifurcation Load (ECBL) method. • Relevant results and graphic comparison related to the performance of the approach. • Design approach to consider the interactive local–distortion failures of members. [ABSTRACT FROM AUTHOR]

Part II of the paper presents the results of the experimental program into the post-failure behaviour of thin-walled cold-formed steel lipped channel members subjected to eccentric compression about the minor axis. The tested samples are subjected to compression with a wide range of positive and negative eccentricities. Thus, the evolution of plastic mechanisms and the use of them to characterize the ultimate strength is examined, both by experimental tests and by FE simulations. Comparative diagrams of equilibrium paths obtained experimentally and from FE simulations together with post-ultimate curves obtained via yield line analysis are shown based on developed mechanisms models. Some conclusions are drawn regarding the applicability of the plastic mechanisms developed to determine the load-carrying capacity of the members under investigation. Finally, further research perspectives are presented. • Experimental tests on thin-walled cold-formed steel members subjected to minor axis eccentric compression. • Ultimate and post-ultimate behaviour members under eccentric compression. • Experimental and numerical results and graphic comparison. • Validation of the theoretical models for plastic mechanisms using experimental tests. • Concluding remarks on the developed plastic mechanisms. [ABSTRACT FROM AUTHOR]

The lightweight steel structural systems such as trusses or built-up beams, consisting of thin gage steel elements, are highly efficient and have the advantages in ease of handling and construction. The major concern relates to the joining technique between each part of the element. Commonly, the connection between thin-walled elements is performed by self-drilling screws but the quantity of time and manpower necessary for a large number of connections, demands an improved solution. Conventional welding technology is not appropriate for connecting thin sheets, such those of 0.4 mm–1.0 mm, to thicker ones of 1.0 mm–3.5 mm. The electric resistance spot or seam welding are widely used in the automotive industry to connect thin-to-thin metal materials. These technologies can be applied in case of thin-walled cold-formed construction, too. The paper presents a study on a new technical solution for lightweight cold-formed steel beams made of corrugated sheet web and built-up steel profiles for the flanges, connected by spot welding. Starting from the investigations on simple lap joint specimens tested under tensile-shear loading for various thicknesses combinations, the research concerns also metallographic aspects and investigates the response of the full-scale beams subjected to bending. Based on the obtained results, the study intends to demonstrate the feasibility and advantages of this solution compared to self-drilling screw solution. Particularly, besides the structural performance, the spot-welding solution increase significantly the work productivity and enables for automated fabrication in order to develop the mass production of these types of beams. • Built-up cold-formed steel beams of trapezoidal corrugated web connected using spot welding. • Validation test on two different versions of technical solution. • Detailed calibration tests of connections on the purpose of further numerical modelling. • Relevant results and graphic comparison related to the performance of tested specimens. • Concluding remarks on the feasibility of technical solution and further developments. [ABSTRACT FROM AUTHOR]