Issue 48

V. M. G. Gomes et alii, Frattura ed Integrità Strutturale, 48 (2019) 304-317; DOI: 10.3221/IGF-ESIS.48.30 305 I NTRODUCTION ver the years the advances in investigation of manufacturing processes of cold-formed steels and the development of new technologies able to follow customers’ requirements have allowed the used of cold-formed steel sections in various industry sectors. Due to the manufacturing process of cold-formed steel profiles, it is possible to obtain sections with mechanical characteristics optimized for the structural design with significant weight reduction. Cold-formed steel sections have been applied in energy (e.g. electricity) distribution industry, telecommunications transmission towers and automotive industry (e.g. frames for cars, buses, trains, etc). Moreover, cold- formed steel sections are also used in rack structures applied in logistics warehouses where “storage and retrieval” (S/R) machines carry weighty goods at high speed over these structures in “7days-24 hours” economy. For this reason, loading conditions on this type of rack structures and their auxiliary components are not anymore quasi-static but dynamic, and cold-formed steel structural details may be subjected to load cycles in the order of 0.5 million/year. Currently, the EN 1993-1-3 standard only covers static design of structural solutions made of cold-formed thin-walled profiles. Because of this reason, FASTCOLD European project aims at providing rules for fatigue design of cold-formed steel members, with special focus on logistics industry. Cold-formed steel members are characterized by thin-walled sections and they are very often supplied with zinc coating. The connections between those members, mostly bolted connections, could be one of the critical locations that need to be conveniently addressed by research. Those connections are characterized by thin adjoining plates and different surface coatings could be used (e.g. zinc coating). Distinct preloads could be applied in practice since industrial practice, for example in rack structures, does not use controlled preloaded bolted joints. Instead, many times snug tight bolted joints are used. Fully threaded bolts (small shank lengths), associated to relative low preloads and low friction, leads to joint slipping and consequent inner hole surface indentation and damage. Therefore, research on bolted joints behaviour considering the previous specificities have been identified as a need to improve the existing rules for both static and fatigue behaviours. In this paper, the monotonic/static behaviours will be addressed. Many researchers have been studying bolted connections in order to develop novel solutions or to improve existing solutions. In general, bolted joints provide many advantages in relation to other joining solutions, such as the good fatigue capacity, especially when using high-preloaded bolts, energy saving, usage of less skilled labour and adaptations. In addition to reasons mentioned, this type of connections is less dangerous for workers, so there is no risk of fire during erection works. However, bolted connections may have some disadvantages such as torque loss due excessive vibrations, which is a risk for structural integrity, the demanding positioning of the holes in the plates in the manufacturing processes and a greater control to selection the cross-section [1]. Although these shortcomings, bolted joints are a joining technique very common, but due its complex behaviour, they are still a subject of research around the world. General studies about the bolted connections strength, may include several particular research topics, such as the slip resistance of the bolted joints with the identification of respective slip factors for different surface conditions [2], the bearing strength at bolt holes with [3] and without [4] hole-bolt clearances for several connection geometries (end distance and bolt pitch). In addition to experimental tests, various authors have also been studied the behaviour of bolted joints by using numerical models. Chung [3] carried out a numerical model of bolted connections on cold-formed steel to evaluate the failure modes using lap shear tests. Silva [5] performed a numerical study with FEM using ANSYS, comparing the fatigue strength between riveted and bolted connections through experimental tests and numerical models. More recently, Rodrigues et al. [1] carried out the study on double shear bolted joints, investigating the influence of the friction coefficient and they present a discussion about the evolution of the linear-elastic stress concentration factors. In this research work, the static monotonic behaviour of double-shear bolted butt joints with different clamping stresses (e.g. snug tight bolts, high preloaded bolts) are investigated, addressing particularly the failure modes. Besides static monotonic tests, slip factors are also evaluated by slip monotonic tests. Despite being a topic intensively investigated either experimentally or numerically [6, 7], this subject is revisited with a new testing campaign using relevant materials for rack structures, exploring the following aspects: i) use of thin plates (2-3 mm), which are less common in other types of structures; ii) effect of the preload levels; iii) effect of different surface coatings; iv) FE numerical modelling. This work has been performed as a contribution for the FASTCOLD project and is considered as first step of the research preceding a fatigue characterization, under progress with similar bolted joints. O