Cold formed steel trusses. 

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... steel framing demonstrates extensive development, even if is a relatively new system, due to some great advantages, such as high strength-to-weight ratio, reduced labor costs and fast erection due to the light weight of cold-formed members. The cold-formed steel trusses represent an economical option to the classical wood trusses used mainly in residential buildings, and to the hot-rolled trusses used for industrial applications. Several proprietary products have been developed, considering C, Z, hat, or more particular sections for chords and webs. The connections may be realized by welding, by using adhesives, with mechanical connectors as bolts or screws, or by some innovative mechanical connecting techniques such as press joining or rosette joining. The mechanical connections are among the most suitable, taking into account the production costs and rapidity of execution. Initiated by the more widespread use of the cold-formed steel in the residential construction market, systematic research to investigate the behaviour of cold-formed steel trusses was carried out at the University of Missouri-Rolla. On the basis of full-scale tests on fink C-section truss assemblies, Harper [10] studied the buckling lengths for the top chord members, realized by a single C-section. Riemann [22] developed a computer analysis model and conducted full-scale truss tests in order to determine the capacity of compression web members, and suggested an interaction equation for the design of compression webs as beam–columns. Ibrahim et al. [11] made another series of tests on full-scale truss assemblies, considering the same system, i.e. single C-sections for chords and webs, connected by self-drilling screws. The authors proposed an interaction equation for checking unreinforced top chords subjected to axial compression, bending and web crippling. LaBoube and Yu [13] synthesized the above-presented research conducted at the University of Missouri-Rolla, which strongly influenced the design recommendations contained in the Standard for Cold-Formed Steel Truss Design issued recently by AISI [1]. This standard is intended to be a response to the problems that these particular systems raise for the designer, and applies to the design, quality assurance, installation and testing of cold-formed steel trusses used for load carrying purposes in buildings. As shown in the Standard Commentary [2], even if the structural analysis requirements are based on available information concerning the behaviour of cold-formed steel single C-section truss assemblies [13], these requirements do not preclude the use of more rigorous analysis or design assumptions, as determined by rational analysis and/or testing. As regards innovative mechanical connection techniques, two interesting research areas are worth mentioning here. Pedreschi et al. [17] demonstrated the efficiency of press joining, by making tests on single lap joints or groups of press joins and on full-scale pitched trusses made from cold- formed Z-sections. Mäkeläinen and Kesti [15] studied the behaviour of the rosette joining system and its possibilities in roof-truss structures, by making tests on simple joints in shear or in cross-tension, and also by making tests on sub-assemblies. The authors concluded that the rosette joint has very good capacity to resist tensile forces and that the shear capacity seems to be sufficient for applications in lightweight steel trusses. The research on cold-formed steel trusses is generally focused on systems for residential roofs, having relatively reduced spans. For larger spans, efficient solutions can be achieved with higher resistance members, made for example with optimized cross-sections, like “pentagon” sections, studied by Blumel and Fontana [3]. The authors showed that the use of this particular cross-section with a large radius of gyration for both axes offers statical and constructional advantages for the chords. However, the low-cost design of the truss joints using a gusset plate welded onto the ridge of the cross-section can lead to important section deformations. The authors developed a calculation model for the local load- bearing behaviour of this particular type of cold-formed truss joint, and validated this model by means of numerical and experimental analysis on truss segments. The trusses built of cold-formed steel sections with bolted connections, made from built-up C-sections for chords and single C-sections for webs, represent another possible constructive system for residential buildings, also reliable for larger spans. In this system, the webs are connected to the chords by means of bolts placed on both flanges of the C-section of the web member. Fig. 1 shows two applications using such trusses. Fig. 1(a) shows the structure of a supplementary storey built for the Alcatel Company building in Timisoara, Romania, while Fig. 1(b) shows the trusses used to build the roof of a church in Bucharest, Romania. As regards the design of this type of cold-formed truss system, two problems arise: the stability behaviour of the compressed chord, taking into account that for these kinds of built-up members no design recommendations exists in the norms, and the real behaviour of the joints. Studies concerning the stability behaviour of the built-up C-profiles connected by bolted C-stitches were performed by Niazi [16]: based on the results obtained by Johnston [12], for hot-rolled columns in which the battens are attached to the chords by hinged connections, the compressed built- up C-section is supposed to work on an elastic foundation, provided by the roof purlins. The authors of the present paper calibrated a finite element model suitable for predicting the ultimate load for such built-up elements, used for the compressed chords of cold-formed steel trusses, but also as columns in cold-formed steel framing [7]. The numerical model showed good results compared with the above procedure and experimental results. As regards the analysis of the web members, it must be emphasized that the use of two or more bolts for each flange of the C-section, in relation with the element slenderness, is supposed to modify the classical assumption of pinned connections, used in case of truss structures. Moreover, the eccentricity of joints could not be avoided, and this fact should also be considered into a global analysis, because it may require additional efforts. For these reasons, the analysis of trusses built of cold-formed steel sections with bolted connections should consider the real behaviour of the joints. This may lead to reduced buckling lengths of the web members, but at the same time, to supplementary bending moments in these elements. In chapter D3 “Analysis” of the latest AISI Cold-Formed Steel Truss Design Standard [1] it is shown that: “in lieu of a rigorous analysis to define joint flexibility, the following analysis model assumptions should be assumed: . . . (b) web members are assumed to have pinned connections at each end. Use of a specific joint stiffness other than the complete rotational freedom of a pin for a connection shall be permitted if the connection is designed for the forces resulting from a structural analysis with this specific joint stiffness”. This means that the Standard allows for joint stiffness to be considered in the design of a truss, even if no specific equations for calculating this parameter are given. This should be in fact difficult, considering the number of different cold-formed steel truss systems. In the case of cold- formed trusses with bolted connections, a rigid behaviour of the joints is also not realistic, taking into account the deformability of the joint due to the bearing deformation produced by the bolt in the thin plate, associated with the hole elongation, bolt tilting and slippage due to the hole clearance. Generally, the research in the field of bolted connections in cold-formed steel framing is focused on determining their bearing resistance. For the first time, Zadanfarrokh and Bryan [25] analysed both experimentally and theoretically the flexibility of bolted connections in cold-formed steel sections, and gave a formula for the flexibility of a single lap bolted joint, but this approach was not included in any design recommendation. More recently, [24] investigated experimentally some particular column base connections and beam–column sub-frames, made from cold-formed steel sections, in different bolted connection configurations, in order to assess their strength and stiffness. The study identified different failure modes and concluded that the bolted moment connections were effective in transmitting moment between the connected sections, enabling effective moment framing in cold-formed steel structures. Another recent study concerning the stiffness of bolted connections in a steel portal framing system was made by Lim and Nethercot [14]. The authors described a finite element model that can be used to determine the stiffness of the individual bolt joint. Using this stiffness, a beam idealization of a cold- formed steel bolted moment connection was determined, in order to predict the initial stiffness of the apex joints. The numerical and theoretical study was validated through tests on full-scale joints. The research presented in this paper summarizes the work performed for several years at the University of Timisoara, Romania, aimed at evaluating the real behaviour of joints in cold-formed steel trusses connected by bolts, and at proposing a theoretical model for the joint stiffness. The experimental programme was developed in three steps. First, the rotational rigidity of some truss connections was ...