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Eccentrically Braced Frames Designed According to the Second Generation of Eurocode 8
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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.
Steel moment resisting frames (MRFs) are drift‐sensitive structures and their design is largely influenced by code requirements to limit the lateral displacements and to control the second order effects. Current and next generation of Eurocode 8 recommend different rules to verify the lateral rigidity and resistance of steel MRFs that lead to structures that are substantially different in terms of size of members and overall performance. In order to investigate the influence of the design rules on the seismic behaviour of ductile steel MRFs a parametric numerical study was carried out on 48 structures designed according to Eurocodes as well as US codes, the latter analysed as benchmark, varying the number of storeys, moment resisting spans and seismic intensity. The results of non‐linear static and dynamic analyses show that the structures complaint with current Eurocode 8 are the most expensive and characterised by the greater lateral rigidity and resistance, while MRFs designed in accordance with ASCE7 rules have the smaller profiles and the lower overstrength. The frames designed with rules of the latest version of next generation of Eurocode 8 have an intermediate behaviour even if closer to ASCE7‐compliant structures. These results are also confirmed by the probability of collapse that is smaller than the minimum value specified by EN1990 for the considered consequence class of the examined buildings.
Industrial single-storey buildings are the most diffuse typology of steel construction located in Italy. Most of these existing buildings were erected prior to the enforcement of adequate seismic provisions; hence, crucial attention is paid nowadays to the design of low-impact retrofit interventions which can restore a proper structural performance without interrupting productive activities. Within this framework, an existing industrial single-storey steel building located in Nusco (Italy) is selected in this paper as a case-study. The structure, which features moment resisting (MR) truss frames in both directions, is highly deformable and presents undersized MR bolted connections. Structural performance of the case-study was assessed by means of both global and local refined numerical analyses. As expected, the inadequate performance of connections, which fail due to brittle mechanisms, detrimentally affects the global response of the structure both in terms of lateral stiffness and resistance. This effect was accounted for in global analyses by means of properly calibrated non-linear links. Thus, both local and global retrofit interventions were designed and numerically investigated. Namely, lower chord connections were strengthened by means of rib stiffeners and additional rows of M20 10.9 bolts, whereas concentrically braced frames (CBFs) were placed on both directions’ facades. Designed interventions proved to be effective for the full structural retrofitting against both seismic and wind actions without limiting building accessibility.
This paper presents an extensive numerical study on the seismic performance of steel eccentrically braced frames (EBFs) designed according to the ASCE 7-16 standard. A series of EBF building structures ranging from 2 to 12 stories located in downtown Los Angeles are designed with different yielding links, including conventional and replaceable links. The performance of the structures is investigated through nonlinear time-history analyses (NLTHAs) under a suite of 40 ground motions, selected and scaled to match the target uniform hazard spectrum. The peak deformations are first examined to assess the performance of the buildings as well as the design procedure. Residual deformations are also evaluated and critically compared with acceptable limits, to provide insight into the downtime and recovery time of EBFs after major earthquakes. Previous studies have demonstrated that the response of stable yielding systems to major earthquakes is often accompanied by significant residual deformations, and that residual drifts exceeding 0.5% can cause hindrance to the buildings occupants. In EBFs, in addition to residual drifts, residual link rotations are also relevant and expected to be more severe due to the localization of inelastic deformations in the yielding link. For replaceable modular yielding links, significant residual link rotations after major earthquakes will hinder repairs and the link replacement process, which is one of the important design objectives. The performance of EBFs is also compared with what was observed in previous studies for buckling restrained braced frames and special moment resisting frames. In the end, the results from the extensive NLTHAs are used to establish a relationship between peak drifts and peak link rotations in EBFs.
In the present paper, the influence of the mean stress on the fatigue behavior of smooth and notched medium carbon steel components (C45 grade) is investigated through conventional and more advanced design methodologies. Original experimental data are provided for two notched specimen geometries and three different high load ratios (R = {0.7, 0.8, 0.9}). To assess the effect of the mean stress in a larger range, the bulk of fatigue data has been widened through more than 150 tests on C45 steel drawn from literature. The considered experimental data ranged between a load ratio of −2 and 0.9. The mean stress effect has been assessed in a first attempt through well‐established literature models resulting in a data correlation characterized by a large scatter. Finally, an attempt to uniquely consider both geometrical and load ratio effects has been performed through a numerical application of the strain energy density (SED) method. Elevated load ratios (R ≥ 0.7) prove to be highly detrimental for C45 steel with a fatigue life reduction of up to 3 orders of magnitude; however, this effect can be considered with a sufficient degree of accuracy through the SED method.
Steel eccentrically braced frames (EBFs) combine the advantages of both moment-resisting frames and concentrically braced frames and provide an attractive seismic design solution. Shear-critical replaceable links were developed about a decade ago to improve the performance, design process, construction, and postearthquake repair process in EBFs. However, replaceable shear links are still susceptible to the commonly observed limit states in EBF links such as web fracture close to web stiffeners, local buckling, and lateral torsional buckling. This paper presents the experimental validation of a new generation of optimized cast steel replaceable yielding links for use in EBFs, which address the shortcomings of conventional and fabricated replaceable links. The performance of the proposed cast steel links is investigated through nine large-scale component-level and system-level experiments, covering a range of link sizes and loading conditions, while also studying the impact of other phenomena influencing the response such as the presence of axial load, the level of axial restraint in the links and the presence of a concrete slab. The tested series of cast links demonstrated enhanced ductility levels and low-cycle fatigue life when compared to fabricated links, achieving up to 0.21 radians of link rotation under the AISC protocol used for the qualification of EBF links.
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 (ag) 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.
The current edition of Eurocode 8 (EN 1998 (CEN (2004))) does not explicitly cover the earthquake-resistant design of the Lightweight steel (LWS) buildings made with cold-formed steel (CFS) frame. Extensive research has been carried out in the past decade on this topic that provides a solid background for an update of the current provisions and the development of a new edition of the code. As part of the revision process of Eurocode 8, several activities are currently under development. Within this framework, this paper presents a set of design rules for the seismic design of LWS buildings based on background studies carried out at the University of Naples “Federico II”. A critical review of the provisions given by international design standards for the seismic design of LWS systems is also presented. The proposed design rules include a set of provisions for achieving the dissipative behaviour of Lateral Force Resisting System (LFRS) and predicting their design strength, together with behaviour factor values and geometrical and mechanical limitations. Different types of LFRS are analysed, namely CFS strap-braced walls and CFS shear walls with steel sheets, wood, or gypsum sheathing. Worked examples are also presented to show the applicability of the proposed design rules on a set of building archetypes. Validation numerical study is performed using both nonlinear static and incremental dynamic analysis approaches. Finally, the collapse performance of the archetypes is assessed following the seismic performance evaluation protocol of FEMA P695.
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.
The recent seismic events have highlighted the need of tools for predicting the performance until collapse and hence assisting to minimize the damages and losses. A European project, named ELISSA, has been recently finished, in which the University of Naples Federico II has explored the seismic behaviour of a Cold Formed Steel (CFS) full-scale two-storey building, via shake-table tests on bare structure and complete construction phases of specimen. Starting from experimental results, advanced numerical models have been developed using the OpenSees software, which consider both structural and non-structural elements and their contribution to the response. The comparison between experimental and numerical results showed that the proposed approach can capture the seismic behaviour of tested building, with acceptable prediction of the response in terms of inter-storey drift peaks and time history if structural and non-structural components are explicitly considered. A further task showed that the earthquake sequence effect can affect the results and it should be taken into account for a better numerical prediction.
The behavior of different length wide flange (WF) links in eccentrically braced steel frames with or without axial force application, were experimentally investigated using a subassemblage system simulating the frame action. The results are examined with emphasis on the influence of link length on the performance. Moment, shear, and axial force interaction, as well as the effect of strain hardening on moment redistribution, energy dissipation, and flange and web buckling are considered. Design and analysis methods for cyclically loaded links are proposed.
Eccentrically braced steel framing in seismic applications can provide high elastic stiffness and large inelastic energy dissipation capacity. The performance of this framing system depends to a great extent on the behavior of short active link sections of the beams. The results of an experimental investigation of the effects of inelastic loading history, connection details, and web stiffener details on active link behavior are presented. The test results re then evaluated using energy dissipation as the basic parameter. A design procedure for active links which yield primarily in shear is then outlined. This procedure includes recommendations on the determination of structural configuration, member sizes, link connection details, and web stiffener details. Suggested connection and stiffener details are illustrated.
This paper provides an introduction and an overview of the design and behavior of seismic-resistant eccentrically braced frames (EBFs). Within the last ten years, EBFs have become a widely recognized lateral load-resisting system for steel buildings in areas of high seismicity. The primary purpose of this paper is to present design recommendations for links and connections in EBFs. Some basic concepts on the behavior of EBFs are reviewed, and highlights of significant experimental results are presented. The important effects of link length on both the elastic and inelastic response of EBFs under lateral load are emphasized. The paper focuses on EBFs constructed with shear links, as these provide for the maximum stiffness, strength, ductility and energy dissipation capacity of an EBF. Suggested details are provided for links and for selected connections in EBFs.
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