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The catastrophic effects of recent earthquakes around the world have highlighted the vulnerability of civil buildings; in Europe, most of the existing buildings were built after World War II and since they have largely exceeded their useful life, are characterized by a high vulnerability linked both to the durability of the materials and to the lack of seismic provisions in design. The current trend in constructional field is to design interventions aimed at the seismic and energy improvement of these structures jointly, in order to achieve a sustainable and integrated retrofit intervention. To this aim, the use of 2D orthogonal steel exoskeletons for the seismic strengthening represents a suitable solution, providing at the same time an enhancement in thermal insulation, through a 'double-skin' solution. Due to the growing interest in the design of steel exoskeleton interventions, a detailed dynamic characterization of this external strengthening solution is needed. In this framework, the present work, aims to investigate the effectiveness of the investigated strengthening solution by means of bi-directional non-linear dynamic analyses. Extensive numerical analysis was carried out. The selected case-study is an Italian existing pre-1980 s RC frame building located in Mugnano di Napoli (NA, Italy), which is a medium-high seismic hazard site in Italy (peak ground acceleration equal to 0.156 g). Two alternative strengthening solutions were numerically investigated by means of non-linear time-history and incremental dynamic analyses performed applying fifteen bidirectional ground motions on three-dimensional numerical models. The results of the dynamic non-linear analyses allow to quantify more appropriate performance indicators, both in terms of expected demand and overall collapse capacity, useful for the development of optimization strategy and design of exoskeleton system. The results highlighted that both the investigated strengthening scenarios allow to increase the stiffness of the existing building and allow it to meet the current code safety requirements reducing also the residual inter-storey drift. The obtained results are not strictly related to the investigated case study, but can be extended to all the structures retrofitted that follows the presented design procedure. • The catastrophic effects of recent earthquakes around the world have highlighted the vulnerability of civil buildings. • Due to the interest in the design of exoskeleton, a dynamic characterization of this strengthening solution is needed. • The strengthening of an existing RC structure with EXO-S-2D placed orthogonally to the building façade is presented. • The efficiency of a displacement-based design procedure is validated by means of static and dynamic non-linear analyses. • The results of the dynamic non-linear analyses allow to quantifyperformance indicators of the strengthened intervention. [ABSTRACT FROM AUTHOR]

[Display omitted] • Tests to prequalify EC8-compliant extended stiffened end-plate joints are presented. • Design procedure is proposed and verified against tests and finite element simulations. • Two alternative dissipation mechanisms are promoted, i.e., full and equal strength joints. • Tests showed plastic deformations solely in the connected beam for full strength joints. • Tests showed plastic deformations in the beam and the connection for equal strength joints. An experimental and numerical campaign was carried out to validate the design procedure and prequalify extended stiffened end-plate (ESEP) joints for ductile seismic resistant moment-resisting frames in the framework of Eurocodes. In fact, the tested ESEP joints have been designed with the aim of guaranteeing ductile and dissipative seismic response in accordance with the principles of the hierarchy of resistances to enforce plastic deformations in ductile components of the joints. Two alternative types of dissipation mechanisms are promoted, namely either (i) plastic deformations solely in the connected beam using full strength joints or (ii) plastic deformations in both the beam and the connection using equal strength joints. The adopted design criteria and technological details are described and discussed. The results of experimental tests confirmed the expected yielding mechanisms of the joints, which satisfied the acceptance criteria for both North American and European prequalification. Finite element simulations of ESEP joints have been validated against the experimental results and have been used to investigate the local response of the joints, thus highlighting the influence of both geometrical and mechanical parameters. [ABSTRACT FROM AUTHOR]

Extended stiffened end-plate bolted joints are widely used in seismic resistant steel frames. In the United States of America (USA) this type of joint is seismically pre-qualified according to AISC 358-16. At the present time, prequalification criteria for different types of bolted joints are also under development in Europe within the framework of the EQUALJOINTS (i.e. European pre-QUALified steel JOINTS) research project. The design criteria and detailing rules proposed by this European project for extended stiffened end-plate joints differ from AISC criteria in some respects. Therefore, the aim of this work is to verify and to compare the effectiveness of both design procedures through the results of a comprehensive parametric study based on finite element (FE) simulations. The FE results show that both AISC and European design procedures can guarantee the formation of a plastic hinge in the beam under cyclic loading. However, under column loss scenarios the European connections are more ductile than those designed according to both AISC 358-16 and AISC 341-16. In addition, it is investigated the possibility to use heavy columns satisfying the resistance requirements without stiffeners (i.e. continuity plates and supplementary web plates). The comparison between the response of the joints with and without stiffened columns shows that heavy unstiffened columns can be adopted without appreciably modifying the joint response. [ABSTRACT FROM AUTHOR]

• The steel exoskeletons could be designed to significantly increase the structural performance of existing structures, providing at the same time useful support for the energy efficiency upgrading and architectural restyling. Indeed, steel exoskeletons are suitable for an integrated and reversible retrofit intervention coherently with Holistic Vision approach. • Currently, there are not encoded design methodologies properly developed for orthogonal steel exoskeletons. • A detailed step-by-step procedure was introduced to cover all the aspects related to the dimensioning of the orthogonal exoskeleton systems designed to increase the structural performance of existing structures. • The efficiency of the proposed design procedure was investigated through non-linear analyses performed on an existing industrial building. The case study is located in Nusco (Av, Italy) which is classified as in medium–high seismic intensity area. • The introduction of steel orthogonal exoskeleton allows to increase both the stiffness and the resistance of the existing building also regularising its dynamic behaviour. The steel exoskeleton systems are widespread structures applied on the external perimeter of an existing building designed to absorb the horizontal actions providing at the same time useful support for energy efficiency upgrading and architectural restyling. The growing interest in the use of exoskeletons is due to the possibility of creating an integrated retrofit (structural, energetic, formal and functional), combining "structural safety" with the concepts of "deep renovation". Among many solutions present in the literature, in the present work, the steel exoskeletons placed orthogonally to the facade of the existing building have been analysed; this structural typology has the advantage to allow rapid retrofit execution without interfering with the activities carried out within the existing structure. In this framework, the aim of the present work is the introduction of a detailed step-by-step design procedure for steel exoskeletons systems adopted for seismic retrofit of existing buildings looking also at the accessibility and operability aspects. Each step of the procedure was individually explained and it was applied for the design of a strengthening intervention on a real single-storey steel industrial building. The existing building is located in Nusco (Av, Italy) which is classified as in medium–high seismic intensity area which corresponds a peak ground acceleration (a g) equal to 0.238 g. Non-linear analyses were conducted to assess the existing structure's performance as well as to verify the effectiveness of the strengthening solutions designed. The use of the exoskeleton systems allows to strongly increase both the elastic stiffness and the resistance of the investigated industrial building without interrupting its product activity. [ABSTRACT FROM AUTHOR]

[Display omitted] • Seismic resisting joints exhibit satisfactory performance at elevated temperature. • Pre-damage of joints has limited influence on the response at high temperature. • The performance of the joints is investigated by means of finite element analyses. • The resisting temperature largely depends on the vertical loads acting on the beam. • Lateral-torsional restraints are crucial to guarantee ductility and resistance. The mechanical features of beam-to-column joints may highly affect the resistance of steel structures at elevated temperature. Therefore, the behaviour of steel joints under elevated temperature has been widely studied. However, few studies have been specifically devoted at investigating the response of bolted end-plate joints under elevated temperature that were formerly damaged by seismic actions. Hence, the study summarized in this article was addressed to: (i) to analyse the response of seismically prequalified extended stiffened end-plate joints under high temperature; (ii) study the influence of the seismic damage on performance of this type of joints at elevated temperature. Parametric finite element simulations were carried out to achieve these purposes varying the dimensions of members, the design criteria of the joints, the vertical loads, the presence of lateral-torsional restraints and the level of imposed damage. The obtained results in terms of moment-rotation-temperature characteristics and pattern of plastic deformations showed in which terms the type of cyclic damage may affect the performance of the joints under high temperature as well as the influence of the efficiency of bracings to restrain the beams against lateral-torsional buckling. [ABSTRACT FROM AUTHOR]