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Progettazione sismica di tipo prestazionale: l’importanza della scelta dell’input sismico
No full textL’obiettivo principale del Performance Based Seismic Design (PBSD) consiste nella stima probabilistica delle prestazioni di una struttura soggetta all’azione sismica (PSDHA). Questa memoria concerne, in particolare, le procedure atte ad una corretta identificazione degli input dinamici da utilizzarsi in questo tipo di analisi. In dettaglio, (a) viene proposto un quadro generale per una trattazione organizzata e razionale degli ultimi contributi di ricerca finalizzati ad una corretta identificazione degli inputs sismici e (b) viene formulata una proposta per l’utilizzo di un parametro di tipo vettoriale identificativo dei sismi di progetto, composto da Peak Ground Acceleration (PGA) e Peak Ground Velocity (PGV). Vengono inoltre elaborati gli spettri di risposta (di accelerazione e di spostamento, sia elastico lineari che elasto-plastici) ottenuti a partire da gruppi di sismi identificati con la metodologia proposta e confrontati con gli spettri ottenuti da gruppi di sismi identificati in base alla sola PGA (o PGV), come indicato dalle normative vigenti
Maximum rotational response of non linear asymmetric systems
No full textStructures characterized by eccentricity in plan between the centre of mass and the centre of stiffness, under seismic excitations, develop coupled lateral-torsional response which may increase the deformation level of the structural elements which are more distant from the centre of stiffness. In this paper, first, the basic findings are briefly recalled of previous research works by the authors which have led to the identification of a synthetic structural parameter (called α) capable of capturing the essence of the torsional behaviour of linear-elastic systems and allowing effective maximum rotational response estimations. Second, preliminary non-linear parametric analysis are conducted through one-storey models by varying some key structural parameters which control elastic and inelastic torsional response in order to investigate the rotational trends of inelastic asymmetric systems under seismic excitation. The results indicate that (i) the ratio of the maximum rotation over the maximum displacement for non-linear systems is generally inferior to the corresponding ratio for linear-elastic systems, and that (ii) a useful upper bound and a good estimation for the maximum rotation developed by non-linear asymmetric systems under seismic excitation can be obtained using the α parameter
Effectiveness of the genetic approach for the optimal insertion of viscous dampers into torsionally coupled structures considering nonlinear behaviour
No full textIn a companion paper, the authors have proposed a procedure based upon the use of genetic algorithms to numerically identify the system of added viscous dampers which minimises simultaneously the variance of both the longitudinal and the rotational responses to a stochastic white noise input of asymmetric one-storey structures.
The goal of this paper is twofold. The first one is to perform an extensive parametric study (with respect to characteristics of the eccentric structures here analysed) in order to provide support to the effectiveness of the proposed procedure for optimal damper arrangement (for both linear dampers and structures). The second goal is to verify how the proposed procedure still leads to acceptable results also when more accurate non-linear modeling of the dampers is taken into account.
The parametric studies here illustrated have been performed with reference to one-storey stiffness eccentric structures subjected to a series of historical records of earthquake ground motions. The genetic algorithm approach here tested is capable of leading, in both cases of linear and non linear modeling of the dampers, to the identification of damping systems which lead to a reduced dynamic response when subjected to earthquake excitation only when in the optimisation search are used as objective functions performance indexes based upon the lateral (longitudinal + transversal) response of the system. Minimisation of the torsional response of the system only is capable of reducing substantially the torsional response of the system but may lead (in both cases of linear and non linear modeling of the dampers) to large deformations at the center of mass of the system. The investigations carried out also indicate that, in general, structures equipped with Newtonian dampers exhibit dissipative performances similar to those exhibited by structures equipped with corresponding (as obtained following the procedure here proposed) pseudoplastic dampers
A GENETIC APPROACH FOR THE OPTIMAL INSERTION OF VISCOUS DAMPERS INTO TORSIONALLY COUPLED STRUCTURES
No full textStructures characterized by non coincident center of mass and center of stiffness (eccentric structures) develop a coupled lateral-torsional response when subjected to seismic excitation. This coupled behaviour may increase significantly the local peak dynamic response of such a structure, as compared to that of an equivalent system with coincident center of mass and center of stiffness (equivalent non-eccentric system). Among earthquake protection systems, added viscous dampers have proven so far to be easily applicable and highly effective in mitigating the earthquake-induced effects. The investigation of how the insertion of systems of added viscous dampers into eccentric structures is capable of reducing the local peak dynamic response of such structures has been the object of recent studies.
The goal of the research study presented in this paper is the development of a procedure for the identification of the “optimised” systems of added viscous dampers which minimise the dynamic response of the eccentric systems. In detail, the optimisation procedure here proposed is based upon genetic algorithms and makes use of selected response indexes based upon the mean square response to white noise stochastic inputs of the eccentric systems. The analyses are developed with reference to linear elastic one-storey stiffness eccentric structures and to Newtonian (linear) viscous dampers. In addition to the introduction of the suggested procedure for the identification of the “optimised” systems, the paper presents a numerical application. The results obtained indicate that, in order to obtain eccentric systems characterised by a reduced local peak response, it is advisable to search for systems of added viscous dampers which minimise the lateral (longitudinal and/or transversal) response. As a matter of fact, systems of added viscous dampers which minimise the torsional response of the structure may indeed lead to a large lateral response which, in turn, leads to a large local peak response
Numerical verification of the effectiveness of the “alpha method” for the estimation of the maximum rotational response of eccentric systems
No full textIn previous research works, the authors, starting from the governing equations of motion of one-storey linear elastic asymmetric systems, have identified a key system parameter which controls the maximum rotational response of such systems under free and forced vibration. This parameter (called ALPHA) has also led to the identification of a simplified procedure (called ALPHA method) for the estimation of the maximum rotational response of such systems.
Main goal of this paper is to illustrate the results of a comprehensive set of 11600 numerical simulations carried out (with reference to a number of representative structures, encompassing also torsionally flexible ones, subjected to historically recorded earthquake ground motions) to verify the properties of the ALPHA parameter, with special attention devoted to the identification of the sensitivity of the proposed ALPHA method upon the fundamental period of vibration of the structure (not yet considered in previous research works). The results obtained (a) confirm the effectiveness of the ALPHA parameter to capture the intrinsic propensity of an eccentric system to develop a torsional response, (b) confirm the capacity of the ALPHA method to effectively estimate the maximum rotational response of a given eccentric system under seismic excitation, and (c) indicate how the ALPHA method is only weakly sensitive upon the period of vibration of the structure.
The paper also introduces a mechanical analog for three-dimensional one-storey asymmetric systems which provides useful insight to the dynamic properties of such systems
Numerical verification of the effectiveness of the “ALPHA” method for the estimation of the maximum rotational response of eccentric systems
No full textIn previous research works, the authors have identified a key system parameter which controls the maximum rotational response under free and forced vibrations of one-storey linear-elastic systems representative of asymmetric seismic isolated building structures. This parameter (called “ALPHA”) has also led to the identification of a simplified procedure (called “ALPHA method”) for the estimation of the maximum rotational response of such systems. Main goal of this paper is to verify the properties of the ALPHA parameter and the predictive capabilities of the ALPHA method, when applied to one-storey systems representative of generic asymmetric building structures. The verification is carried out through a comprehensive set of 11600 numerical simulations developed with reference to different representative structures subjected to historically recorded ground motions, with special attention devoted to the identification of the sensitivity of the ALPHA parameter and the ALPHA method upon the fundamental period of vibration of the structure (not yet considered in previous research works). The results obtained (a) confirm the effectiveness of the ALPHA parameter to capture the intrinsic propensity of an eccentric system to develop a torsional response, (b) confirm the capacity of the ALPHA method to effectively estimate the maximum rotational response of a given eccentric system under seismic excitation, and (c) indicate that the ALPHA method is only weakly sensitive upon the period of vibration of the structure. The paper also introduces a simple code-like provision for conservative estimations of the maximum rotation developed under seismic input by asymmetric structures based upon the confidence interval concepts
On the anchoring of timber walls to foundations: Available strategies to prevent wood deterioration and on-site installation problems
No full textExperts are aware that the critical and until now unsolved problem of timber wall buildings lies in proper anchoring of the structure to foundations. Geometric inaccuracies and discrepancies between the concrete surface and the timber structure often lead to incorrect alignment of walls, which is normally solved with provisional and inaccurate solutions, in contrast with prefabrication. However, the most important issue is the compromised durability of timber due to rising dampness and fungicide attack, which are a result of inaccurate waterproofing as moisture is absorbed and trapped at the wall base. Difficulties in restoring such damages arise together with maintenance costs of the structures. This work provides an overview on the crucial problems that affect the anchoring of timber structures to foundations. Disadvantages of traditional techniques are presented and critically discussed. An innovative aluminium bottom rail designed for Cross-Laminated Timber, light-frame and Blockhaus technologies is presented, evidencing main structural properties and capacities in assuring timber durability and long-lasting fixing. This aluminium beam is made of an extruded profile with shape and size optimized for load-bearing capacity and lateral stability. Special grooves at both sides are designed to obtain fast and efficient fixing of shear-resisting plates and hold-downs. Main results from latest experimental campaign are given to characterize the system in terms of load-bearing capacity and case-study applications in newly-realized timber buildings are presented. Finally, a proposal for possible restoring intervention to replace the damaged timber at the base of the wall is hypothesized
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A specific out-of-plane model for the dynamic analysis of masonry façades and estimation of cumulative seismic damage
No full textThe seismic damage suffered by masonry buildings, especially historical monuments, depends on a multiplicity of factors that determine their structural vulnerability and, of course, on the seismic hazard. Engineering approaches need a manageable and effective modeling of these aspects, but it is hard to admit that hazard and vulnerability can be easily synthesized by scalar measures. An advanced and rationale approach for the assessment needs to manage the mechanical complexity of masonry, whose seismic performance depends on the overall structural geometry; the masonry texture, often consisting of multiple layers; quality of blocks and mortar. It must also cope with the manifold dynamic features of earthquakes, including: amplitude of ground acceleration; frequency content; duration of the most intense part. Furthermore, it should be considered the possible occurrence of destructive events in a close sequence, which is instead rarely considered. The numerical study of these aspects requires extensive nonlinear dynamic analyses by means of specific models. Restricting the attention to the out-of-plane response of church façades, the paper presents the background and choices adopted in the formulation of a computational heuristic model that manages the hysteretic mechanical response by accounting for internal texture. It is shown that it is essential to carry out dynamic analyses to understand the different damage mechanisms and contribution of higher vibration modes. Finally, the availability of a model with excellent computational performance is exploited to study the cumulative damage in the case of a sequence of destructive seismic events
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