Geodynamic significance of ophiolites within the Calabrian Arc

Abstract Slices of oceanic lithosphere belonging to the neo‐Tethys realm crop out discontinuously in the northern Calabrian Arc, Southern Apennines. They consist of high‐pressure–low‐temperature metamorphic ophiolitic sequences formed from metaultramafics, metabasites and alternating metapelites, metarenites, marbles and calcschist. Ophiolites occupy an intermediate position in the northern Calabrian Arc nappe pile, situated between overlying Hercynian continental crust and the underlying Apenninic limestone units. In the literature, these ophiolitic sequences are subdivided into several tectonometamorphic units. Geochemical characteristics indicate that metabasites were derived from subalkaline basalts with tholeiitic affinity (transitional mid‐oceanic ridge basalt type), and a harzburgitic‐lherzolitic protolith is suggested for the serpentinites. The pressure–temperature‐deformation paths of the metabasites from different outcrops display similar features: (i) the prograde segment follows a typical Alpine geothermal gradient up to a metamorphic climax at 350°C and 0.9 GPa and crystallization of the high‐pressure mineral assemblage occurs along a pervasive foliation developed during a compressive tectonic event; and (ii) the retrogression path can be subdivided in two segments, the first is characterized by nearly isothermal decompression to approximately 400°C and 0.3 GPa and the second follows a cooling trajectory. During low‐pressure conditions, a second deformation event produces millimetric to decametric scale asymmetric folds that describe west‐verging major structures. The third deformation event is characterized by brittle extensional structures. The tectonometamorphic evolution of the ophiolitic sequences from the different outcrops is similar. Both thermobarometric modeling and tectonic history indicate that the studied rocks underwent Alpine subduction and exhumation processes as tectonic slices inside a west‐verging accretionary wedge. The subduction of oceanic lithosphere was towards the present east; therefore, the Hercynian continental crust, overthrusted on the ophiolitic accretionary wedge after the neo‐Tethys closure, was part of the African paleomargin or a continental microplate between Africa and Europe.     

Theory of plastic mechanism control for MRF–CBF dual systems and its validation

In this paper the theory of plastic mechanism control is presented for moment resisting frame–concentrically braced frames dual systems, i.e. for structural systems combined by moment resisting frames and concentrically braced frames. It is aimed at the design of structures failing in global mode, i.e. whose collapse mechanism is characterised by the yielding of all the tensile diagonals and the occurrence of buckling in the compressed ones, and by plastic hinge formation at all the beam ends and at the base of first storey columns. The proposed methodology is based on the application of the kinematic theorem of plastic collapse, by imposing that the global mechanism equilibrium curve has to lie below all the other equilibrium curves corresponding to undesired mechanisms. The practical application of the design methodology is illustrated by means of a worked example. In addition, the results of a non-linear static pushover and dynamic analyses of the designed structure are also discussed in order to demonstrate the effectiveness of the proposed design procedure.     

Seismic reliability of V‐braced frames: Influence of design methodologies

According to the most modern trend, performance‐based seismic design is aimed at the evaluation of the seismic structural reliability defined as the mean annual frequency (MAF) of exceeding a threshold level of damage, i.e. a limit state. The methodology for the evaluation of the MAF of exceeding a limit state is herein applied with reference to concentrically ‘V’‐braced steel frames designed according to different criteria. In particular, two design approaches are examined. The first approach corresponds to the provisions suggested by Eurocode 8 (prEN 1998—Eurocode 8: design of structures for earthquake resistance. Part 1: general rules, seismic actions and rules for buildings), while the second approach is based on a rigorous application of capacity design criteria aiming at the control of the failure mode (J. Earthquake Eng. 2008; 12 :1246–1266; J. Earthquake Eng. 2008; 12 :728–759). The aim of the presented work is to focus on the seismic reliability obtained through these design methodologies. The probabilistic performance evaluation is based on an appropriate combination of probabilistic seismic hazard analysis, probabilistic seismic demand analysis (PSDA) and probabilistic seismic capacity analysis. Regarding PSDA, nonlinear dynamic analyses have been carried out in order to obtain the parameters describing the probability distribution laws of demand, conditioned to given values of the earthquake intensity measure. Copyright © 2009 John Wiley & Sons, Ltd.     

Seismic reliability of traditional and innovative concentrically braced frames

Structural engineering problems are always affected by many sources of uncertainty, such as aleatory of material properties, applied loads and earthquake intensity, therefore, seismic assessment of structures should be based on probabilistic methods. Since PBSD (Performance‐based Seismic Design) philosophy was formulated, many researches have been conducted in this field in order to develop simple and accurate procedures for evaluating structural reliability. An important contribution has been provided by Jalayer and Cornell, who have developed a closed‐form expression to evaluate the mean annual frequency of exceeding a defined limit state. In this paper, by assuming the record‐to‐record variability as the only source of uncertainty, the seismic reliability of concentrically braced frames designed according to traditional and innovative methodologies is investigated, and a comparison between their performances is presented. In particular, two design methodologies have been applied: Eurocode 8 provisions and a new design methodology based on a rigorous application of ‘capacity design’ criteria. The innovative reduced section solution strategy, based on the reduction of cross sections at bracing member ends, has also been analysed. Copyright © 2011 John Wiley & Sons, Ltd.     

Advances in theory of plastic mechanism control: closed form solution for MR‐Frames

In this paper new advances in the application of ‘Theory of Plastic Mechanism Control’ (TPMC) are presented. TPMC is aimed at the design of structures assuring a collapse mechanism of global type. The theory has been developed in the nineties with reference to moment‐resisting frames (MRFs) and progressively extended to all the main structural typologies commonly adopted as seismic‐resistant structural systems. In particular, the outcome of the theory is the sum of the plastic moments of the columns required, at each storey, to prevent undesired failure modes, i.e. partial mechanisms and soft‐storey mechanisms. The theory is used to provide the design conditions to be satisfied, in the form of a set of inequalities where the unknowns are constituted by the column plastic moments. This set of inequalities was originally solved by means of an algorithm requiring an iterative procedure. The advances presented in this paper are constituted by the identification of a ‘closed form solution’ and by the use of TPMC in a more systematic design approach. This result is very important, because the practical application of TPMC can now be carried out even with very simple hand calculations. The practical application of TPMC is herein presented with reference to the design of a multi‐storey frame whose pattern of yielding is validated by means of both push‐over analysis and incremental dynamic analyses. Copyright © 2014 John Wiley & Sons, Ltd.     

Asbestos and other fibrous minerals contained in the serpentinites of the Gimigliano-Mount Reventino Unit (Calabria,S-Italy)

Serpentinites are metamorphic rocks with good technological properties and valuable ornamental characteristics, which have been exploited since ancient times. Actually, their use is limited and monitored in several countries worldwide because they can contain fibrous asbestos minerals that may be carcinogenic. Furthermore, certain types of fibrous minerals can be confused with asbestos, and must therefore be carefully investigated. We have investigated the possible presence of the asbestos and non-asbestos fibrous phases contained in serpentinitic rocks in a meta-ophiolitic sequence from the Gimigliano-Mount Reventino Unit (Southern Italy), which had not been previously assessed. The detection and quantification of asbestos and the correct distinction of the fibrous non-asbestos minerals are very important not only from a scientific point of view, but also from a legislative one. This is especially the case for the administrative agencies that have to take decisions with regards to the implementation of public and occupational health protection measures (e.g., in road yards and quarry excavations). As a consequence of this, serpentinitic rock samples have been characterized in detail through X-ray powder diffraction, scanning and transmission electron microscopy combined with energy-dispersive spectrometry, analytical electron microscopy (SEM–EDS and TEM–AEM), differential scanning calorimetry, thermogravimetry and micro-Raman spectroscopy. Two kinds of asbestos and four kinds of non-asbestos fibrous silicates have been detected in the examined samples. In order of decreasing abundance these are polygonal serpentine, chrysotile, fibrous antigorite, tremolite, gedrite and magnesiohornblende. The size, morphology, crystallinity and chemical composition of the fibres were also discussed, in the light of the possible role these properties could play in the carcinogenic effect on human health.     

Influence of connection typology on seismic response of MR‐Frames with and without ‘set‐backs’

The work presented is aimed at the investigation of the influence of beam‐to‐column connections on the seismic response of MR‐Frames, with and without ‘set‐backs’, designed according to the Theory of Plastic Mechanism Control. The investigated connection typologies are four partial strength connections whose structural details have been designed to obtain the same flexural resistance. The first three joints are designed by means of hierarchy criteria based on the component approach and are characterized by different location of the weakest joint component, leading to different values of joint rotational stiffness and plastic rotation supply and affecting the shape of the hysteresis loops governing the dissipative capacity. The last typology is a beam‐to‐column connection equipped with friction pads devoted to the dissipation of the earthquake input energy, thus preventing the connection damage. An appropriate modelling is needed to accurately represent both strength and deformation characteristics, especially with reference to partial‐strength connections where the dissipation of the earthquake input energy occurs. To this aim, beam‐to‐column joints are modelled by means of rotational inelastic springs located at the ends of the beams whose moment‐rotation curve is characterized by a cyclic behaviour which accounts for stiffness and strength degradation and pinching phenomena. The parameters characterizing the cyclic hysteretic behaviour have been calibrated on the base of experimental results aiming to the best fitting. Successively, the prediction of the structural response of MR‐Frames, both regular frames and frames with set‐backs, equipped with such connections has been carried out by means of both push‐over and Incremental Dynamic Analyses. Copyright © 2016 John Wiley & Sons, Ltd.