Month: March 2020

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 Editorial

The use of steel and composites structures for seismic resistant building has always been a fundamental topic of seismic engineering. During the last Italian Conference of Steel Structures – CTA 2019, held in Bologna, a great part of the conference proceedings was focused on seismic issues regarding the design and evaluation of seismic performances in new buildings and the use of steel for the retrofitting of existing buildings. Steel construction in seismic areas provides many advantages as the weight reduction with a consequent reduction in seismic actions demand. In addition, metalic members are usually able to develop wide and stable hysteresis loops under seismic loadings, thus affecting the global ductility of buildings [Chao et al., 2019; Mitsui et al., 2018; Montuori et al., 2020; Piluso et al., 2019a; Castaldo et al.; 2017a; 2017b] whose is of paramount importance for the correct evaluation of the building capacity [Giordano et al., 2017; Chisari et al., 2017; Montuori et al. 2019a]. Moment resisting frames behavior under seismic loadings can be strongly affected by the degradation phenomena occurring in dissipative zones [Bernuzzi et al., 2018; Bernuzzi et al., 2019; Dell’Aglio et al., 2017; Ferraioli et al. 2018a; 2018b; Sandoli et al., 2019; Pengfei et al., 2019; Wang et al., 2019]. Moment Resisting Frames are usually cheaper than other steel typologies and assures an adequate seismic dissipation, provided that, connections are appropriately detailed and able to support the required strength and behaving as rigid connections [Tartaglia et al., 2018b; 2019; D’Aniello et al., 2017; Tenchini et al., 2018, Xu et al., 2018]. However, Moment Resisting Frames could not be used for high-rise buildings because of their high deformability that makes this structural typology very sensitive against serviceability and second order effects [Tartaglia et al., 2018a; 2018c; Montuori et al., 2019b], therefore, braced frames or dual systems are preferred [Costanzo et al. 2017; Costanzo et al., 2018; Jia et al., 2019]. In the last years, also cold formed profiles are getting head in the seismic field, used not only as a system for the strengthening and retrofitting of existing buildings [Totter et al., 2018; Formisano et al., 2016; Barbagallo et al., 2020; Ferraioli et al., 2020; Di Lorenzo et al., 2020] or for pallet racks [Gabbianelli et al., 2017; Montuori et al., 2019b] but also as the main structural system of new buildings, [Poursadrollah et al., 2020; Monsef Ahmadi and De Matteis, 2020; Campiche et al. 2018; Fiorino et al., 2018; 2019; 2012; 2017a; 2017b; Landolfo et al., 2010] or as an innovative bracing system [De Matteis et al., 2018]. Other research fields regard the use of dissipative devices in place of traditional dissipative zones for both steel structures and composites structures [Lemos et al. 2018; Latour et al. 2018; Titirla et al., 2017; Di Lauro et al., 2019; Piluso et al., 2019b; Nastri et al., 2019; Farzampour et al., 2019; Colajanni et al., 2020]. The use of these devices, properly located in points where the high displacement demand is expected allow the structure to remain in service also after the seismic event and an adequate reparability, benefit the maintenance costs. From the other side, the use of base seismic isolation remains a useful strategy to limit the plastic excursion and the structural damage [Avossa et al., 2017; Fraternali et al., 2018; Palazzo and Ferrentino, 2019]. However, the attentions of the research and many efforts are focused on designing and retrofitting buildings showing, after a seismicevent, the chance to exhibit a residual drift compatible with a convenient reparability cost.

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Nowadays, many buildings with RC framed structure need to be seismically upgraded. The insertion of steel braces equipped with friction dampers within the framed structure is a promising seismic upgrading technique. In fact, steel braces and friction dampers reduce the storey drift demand providing additional lateral stiffness and energy dissipation. Furthermore, friction dampers cap the forces transmitted by braces avoiding that the upgrading system overload the existing structure. In this paper a design procedure of the bracing-friction damper system is formulated. The design procedure is applied to a case study frame considering different combinations of the design parameters. The analysis of the seismic response of the bare and rehabilitated frames provides information on the effectiveness of the upgrading technique and proper setting of the design parameters.

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A study on the seismic retrofit of RC buildings using hysteretic dissipative braces is presented in the paper. By following a displacement-based design procedure, a fully multimodal approach based on an adaptive version of the capacity spectrum method is followed. Then, a dual RC-damped brace system idealized as bilinear is considered thus accounting for the effects of frame-damped braces interaction. Finally, the optimal distribution of dampers is determined using an iterative procedure. The proposed method is validated using nonlinear static and dynamic analyses. The results have shown the effectiveness of the proposed procedure to address the main issues of seismic design of damped braces: effect of force demands to the frame due to the dampers, higher modes contribution and effect of soft story irregularities.

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Since ‘80s the use of external additive structures, commonly called exoskeletons, is considered one of the possible alternatives for seismic retrofit of existing r.c. structures with low dissipative capacity. The first Japanese and American codes dealing with structura  rehabilitation issues, as well as many applications on the use of steel devices at the international level, are testimony of this trend, especially in high seismic hazard areas. Nowadays, the use of this intervention strategy has become of great actuality, not only because it can be implemented in a safe way without interrupting the building use, but also because it can be effectively adopted, in cases of restructuring operations with lateral addition, for the integrated (formal, energetic and functional) retrofit of the entire construction. In the present work, after a thorough state-of-the art of the main researches and applications on steel exoskeletons, their typological classification into families and the definition of the key project parameters, indispensable to both properly conceive and design such systems, have been performed.

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In the last thirty years, Hybrid Steel-Trussed Concrete Beams (HSTCBs) have been widely used in civil and industrial constructions and, therefore, their mechanical performance must be evaluated with the aim of guaranteeing adequate dissipation of the seismic energy particularly in the beam-to-column joints. However, one of the most frequent peculiarities of HSTCBs is that of using their own steel joist to cover large spans with reduced depth and, in the case of traditional beam-to-column connections, this requires large amount of steel reinforcement inside the panel zone, often made with large diameter rebars. These characteristics make both the panel zone and the beam end potentially vulnerable to the effects of the cyclic actions induced by the earthquake and dramatically reduce the dissipative capacity of the entire structure. For this purpose, this study investigates the possibility of introducing friction damper devices in the HSTCB-to-column joint of framed structures in seismic areas. The use of friction dampers prevents damage to the structural elements, improves the expected performance and limits damage to the panel zone. The feasibility study is conducted through the development of design criteria for the dimensioning of the device and numerical validation of the proposed solution.

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Among various architectural non-structural components in a building, Lightweight Steel (LWS) drywall partitions are one of the most widely used partition systems. A set of simplified numerical models are proposed here for LWS drywall partitions to simulate their in-plane response, that can be easily integrated with the building models and possess the ability to better estimate damages in them, when linked to their fragility information. Models are calibrated experimentally using the quasi-static cyclic test results, available from the recent research carried out at University of Naples “Federico II”. The models are developed in OpenSees software by using a single discretized spring to simulate the lumped behaviour of the walls for the twelve individual different configurations of the tested partitions. The accuracy of model is demonstrated by comparing the experimental and the numerical results in terms of the hysteretic response curves and the cumulative energy dissipated.

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A novel lateral force resisting system, namely steel shear panels with Butterfly-Shaped (BS) links is proposed, which is a promising alternative for improving the buckling stability and seismic behaviour of conventional steel plate shear walls. To illustrate the advantages of such a device, specimens with different number of butterfly links have been numerically investigated. To this purpose, a FE model, which has been preliminary calibrated on conventional shear panels on the basis of recent experimental tests, has been defined. Then, the influence on the hysteretic performance of some non-dimensional ratios, which characterize the butterfly link geometry, namely the slenderness ratio and the taper ratio, is evaluated. In the whole, the results demonstrate that the shear strength, initial stiffness and dissipation energy are controlled by the number of butterfly shaped links. In addition, due to buckling phenomena of butterfly shaped links, the energy dissipation of shear panels with BS links may significantly decrease. For this reason, three different limit states are evaluated and specimens which have an improved cyclic behaviour in terms of energy dissipated are suggested

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Hybrid structures are efficient and economical systems that can be successfully adopted for modular buildings. Novel hybrid industrialized and modular buildings with truss girder made of lightweight steel profiles and connected to tubular columns by means of innovative plug-and-play joints has been recently proposed within the RFCS INNO3DJOINTS with the aim to develop solution ensuring fast-track construction and increasing the quality of the finished product. In this paper, a preliminary study on the seismic behaviour of these innovative hybrid frames equipped with plug-and-play joints is summarized. Nonlinear static and dynamic analyses were performed on 2D models of a reference structural archetype. The obtained results showed that this type of structure may also be used in low seismicity zones.

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