• Current Issue
  • Number 2 – 2018 Complete

    DESIGN AND ASSESSMENT OF STEEL STRUCTURES
    IN SEISMIC AREAS: OUTCOMES OF THE LAST ITALIAN
    CONFERENCE OF STEEL STRUCTURES

    Elide Nastri

    Department of Civil Engineering, University of Salerno, Fisciano (SA), Italy

    Editorial

    The use of steel for seismic resistant structures is a hot-topic of seismic engineering. During the last Italian Conference of Steel Structures – CTA 2017, held in Venice, the largest 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 retrofit of existing buildings. The use of steel in seismic areas is not new as testified by many patents starting from the first decades of the Italian Kingdom [Cocco, 2018]. In fact, as it is known, steel construction in seismic areas provides many advantages as the weight reduction with a consequent reduction in seismic actions demand. In addition, steel members are usually able to develop wide and stable hysteresis loops under seismic loadings, thus affecting the global ductility of buildings [Mitsui et al., 2018]. For this reason, the need to accurately model the actual cyclic behaviour of steel member by means of simplified model becomes an urgent need. In particular, recent research efforts have been devoted to the modelling of degradation phenomena and pinching effects affecting the ultimate behaviour of dissipative zones and the overall seismic response of the structure [Giordano et al., 2017; Chisari et al., 2017]. In particular, moment resisting frames behavior under seismic loadings can be strongly affected by the degradation phenomena occurring in dissipative zones [Bernuzzi et al., 2018; Montuori et al., 2016a; Dell’Aglio et al., 2017; Ferraioli et al., 2016; Ferraioli et al. 2018a; Ferraioli et al., 2018b]. 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., 2018; D’Aniello et al., 2017; Tenchini et al., 2018; Pecce, 2016]. However, Moment Resisting Frames could not be used for high-rise buildings because of their high deformability for which braced frames or dual systems are preferred [Bosco et al., 2016; Faggiano et al., 2016; Costanzo et al., 2016; Montuori et al. 2016b; Montuori et al. 2016c; Montuori et al. 2017b; Costanzo et al. 2017; Costanzo et al., 2018; Mastrandrea et al., 2013]. 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 [De Matteis et al., 2016; Castaldo et al., 2016; Totter et al., 2018; Formisano et al.; 2016] or for pallet racks [Gabbianelli et al., 2016] but also as the main structural system of new buildings, showing very small interstorey drift [Campiche et al. 2018] 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 such as beam ends for moment resting frames or link for eccentrically braced frames [Lemos et al. 2018; Latour et al. 2018; Titirla et al., 2017; Fabbrocino et al., 2016]. 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, as a consequence, the structural damage [Avossa et al., 2017; Castaldo et al., 2016b]. The finish line is now the design and construction of buildings able not only to assure an adequate level of dissipation and reparability but also the chance to control the residual drift after both the single seismic event and seismic sequences.

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  • Current Issue
  • EXPERIMENTAL AND NUMERICAL ANALYSIS OF THE ULTIMATE BEHAVIOUR OF SQUARE HOLLOW SECTIONS UNDER COMBINED AXIAL AND BENDING LOADS

    The seismic behaviour of moment resisting steel frames (MRFs) is strongly dependent on the plastic response of beams and columns. In this typology, it is common practice to foster the hysteresis in the beams ends and at the base of the columns of the first storey, thus relying on the resources of local ductility and energy dissipation capacity of the profiles. Therefore, in order to perform seismic non-linear analyses (pushover or time-history) mathematical models able to predict the inelastic response of the steel profile in terms of resistance and rotation capacity are usually needed. Currently, in technical literature, also due to the higher simplicity of the testing rigs, the available studies mainly regard the behavior of beams subjected to non-uniform bending, while the case of columns, which are subjected simultaneously to axial and bending loads, is frequently neglected.
    Within this framework, in this paper an experimental and numerical work devoted to the monotonic behavior of square hollow sections (SHSs) (which are shapes frequently employed to realize columns) subjected to combined bending and axial loads, is presented. Results of experimental tests on steel profiles subjected to bending only or to simultaneous axial and bending loads are reported and, subsequently, with the aid of FE, a numerical analysis regarding about two thousand different cases is conducted to cope for variation of the influent parameters. The results of the experimental and FE studies are summarized in regression models expressing the ultimate resistance and rotational capacity of SHSs as a function of the main geometrical and mechanical parameters. The presented models represent a basic tool to run non-linear analysis of steel MRFs. Indeed, the maximum resistance of the member influences the redistribution of the actions in the structure, while the rotation capacity governs the local and global ductility of the structure. Both parameters are estimated properly accounting for the influence of non-dimensional axial load.

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  • Current Issue
  • CRITICAL DISCUSSION ON SEISMIC DESIGN CRITERIA FOR CROSS CONCENTRICALLY BRACED FRAMES

    Recent studies show that the application of EC8 design procedure for X-CBFs, leads to uneconomical and ineffective structural systems, characterized by massive structural members,  large lateral overstrength  and poor plastic engagement.  In this  paper, the EC8- compliant design procedure is summarized and critically discussed also comparing  the EC8 detailing  rules with those recommended  in US Code. In order to assess the influence of the different  codified  requirements,  a six-storey  residential  building  is selected  as benchmark case  and  alternatively   designed  according  to  both  European  and  US  code.  Nonlinear dynamic analyses are performed and the seismic performances of EC8 and AISC-compliant frames are discussed and compared.

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  • Current Issue
  • REMARKS ON THE APPROACHES FOR SEISMIC DESIGN OF MOMENT-RESISTING STEEL FRAMES

    Nowadays, moment-resisting steel frames are one of the most attractive typologies of structural systems resisting to earthquakes. According to the Eurocode for seismic design (EC8) four alternatives can be used, that differ in the type of seismic analysis and for the verification approaches. A research project is in progress at the Politecnico di Milano on the investigation of these seismic design methods. At first, this paper briefly discusses these approaches. Then attention is focused on a case-study, a one-bay four-stories rigid frame for which nine different seismic inputs have been defined. For each of them, the four admitted methods of seismic analysis are applied. The research outcomes underline nonnegligible differences from a structural point of view, suggesting the need of a deeper investigation of the range of applicability of the single methods.

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  • Current Issue
  • BETWEEN INNOVATION AND INTUITION: PATENTS FOR ANTI-SEISMIC BUILDINGS IN THE FIRST 50 YEARS OF KINGDOM OF ITALY

    This study proposes an analysis of the anti-seismic buildings patents granted in the first 50 years after the unification of Italy. The purpose is to expand and systematize the comprehensive knowledge framework in this particular field through the technological scenario reconstruction, referring to the patents for the anti-seismic residences. This particular field has been chosen because of the will to trace back, within a specific time span of the recent past, any potential technological precursors of the most common construction solutions in the Twentieth century. Among the identified solutions, albeit there are few of them, it is possible to trace back isolation systems, metal frame structures and reinforced masonry systems.

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  • Current Issue
  • ASSESSMENT OF DYNAMIC INCREASE FACTORS FOR PROGRESSIVE COLLAPSE ANALYSIS OF STEEL FRAMES SUBJECTED TO COLUMN FAILURE

    The nonlinear static force method and its dynamic increase factor (DIF) were investigated analysing steel frames subjected to column failure. The assessment was performed in terms of the ability of the nonlinear static method to predict the peak structural response observed in the dynamic analyses. The values of dynamic increase factor thus obtained were expressed as a function of the vertical displacement in the location of column removal.
    Variability in these results is then assessed showing that they vary more significantly depending on variables such as the number of spans, the location of column removal, the number of  building stories and the level of seismic design load. The values obtained were finally compared with the formulation of the dynamic increase factor proposed by GSA and based on the ductility factor. It was observed that using standard DIF formulae might lead to inaccurate results. In fact, these empirical formulae are based on the hypothesis of a constant value or a monotonic decreasing with ductility. On the contrary, in the cases where the hardening behavior of the catenary action is fully developed, the analytical curve of DIF first decreases and then increases with ductility.

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  • Current Issue
  • EFFECTIVENESS OF MULTI-MODE PUSHOVER ANALYSIS PROCEDURE FOR THE ESTIMATION OF SEISMIC DEMANDS OF STEEL MOMENT FRAMES

    A multi-run and multi-mode pushover procedure for the approximate analysis of the seismic response of steel moment-resisting frames is developed and evaluated. A set of generalized force vector is defined as the combination of modal forces in order to reproduce the inertial load pattern applied to the structure when the maximum drift occurs under the earthquake ground motion. The proposed procedure is easy to implement and reduces the computational cost when compared to nonlinear dynamic analysis and adaptive nonlinear static procedures. Furthermore, it overcomes the disadvantages in the use of the modal combination rules commonly used. The paper presents a parametric analysis of the response of steel moment resisting frames under various earthquake ground motions with increasing peak ground accelerations. The advantages of the proposed procedure compared to other nonlinear static procedures in literature are highlighted.

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  • Current Issue
  • SEISMIC UPGRADING OF RC STRUCTURES WITH ONLY BEAM CONNECTED STEEL PLATE SHEAR WALLS

    Steel Plate Shear Walls (SPSWs) connected to the boundary frame beams only are suitable structural systems to resist lateral loads. These systems allow for designers to avoid the high flexural demands over columns imposed by conventional SPSWs connected to both beams and columns of the surrounding frame. They can be successfully applied for seismic upgrading of existing RC structures to provide them higher levels of strength, ductility and energy dissipation capacity. This paper presents a numerical assessment of the seismic response of an existing RC structure upgraded with only beam connected SPSWs. The structure, designed for resisting vertical loads only, was erected in the district of Bagnoli in Naples. The used numerical approach is based on the simplified strip model with concentrated plasticity links. The achieved results are focused on fundamental seismic performance issues of the upgraded structure, which are compared to the upgrading solution experimentally applied to the same case study.

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  • Current Issue
  • SEISMIC BEHAVIOUR OF SHEATHED CFS BUILDINGS: SHAKE TABLE TESTS AND NUMERICAL MODELLING

    The objective of this paper is to describe the shake table tests and the numerical modelling of a cold-formed steel (CFS) building. The two-storey full scale building, named as ELISSA mock-up, was subjected to shake table tests at University of Naples “Federico II”, as a part of the European project Energy efficient LIghtweight- Sustainable-SAfe steel construction (ELISSA). The tests were performed at two phases of the building construction process. In the first phase, only structural part of mock-up was subjected to dynamic identification tests, while in the second phase, the architectonic nonstructural elements were added and the mock-up was tested under the action of natural earthquake input with the varying scaling factors. Test results showed that the investigated construction system has a good seismic performance, with very low inter-storey drifts and negligible damages. Moreover, the non-structural elements influenced greatly the seismic response of building. Furthermore, a numerical model was also developed for the experimental mock-up at both construction phases using the OpenSees software. Models were able to simulate with a good accuracy the nonlinear dynamic response of the whole building at both phases.

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  • Current Issue
  • SEISMIC PERFORMANCE OF DUAL STEEL FRAMES WITH DISSIPATIVE METAL SHEAR PANELS

    This paper deals with seismic response of dual steel frames with dissipative metal shear panels. The main goal is to provide design indications accounting for the main behavioural aspects of metal shear panels. To this purpose, numerical Incremental Dynamic Analyses (IDAs) using historical records have been carried out on two different steel frames, which have been properly designed in order to comply with specific design criteria. The outcomes highlight that metal shear panels allow a significant increment of collapse peak ground accelerations as well as a general reduction of both permanent and transient story drifts. The paper concludes with the evaluation of the equivalent q-behaviour factors to be used for simplified analysis. As it could be expected, a significant increment (up to 150%) have been determined in case of structures endowed with metal shear panels.

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