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Ductility and behaviour factor of wood structural systems - Theoretical and experimental development of a high ductility wood-concrete shearwall system
Full text linkThis dissertation focuses on the seismic behavior, ductility and dissipative capacity of modern timber buildings. A number of innovations in the field of timber structures are reported with special regard to the modeling techniques suitable for timber joints and to the characterization of the seismic behavior of modern timber systems.
A preliminary overview on the seismic-resistant timber building technology and on their evolution from the past to nowadays is reported in the introduction of this thesis work. A review of the state of art about the available seismic codes is also reported and the main lack and incongruence with the current constructive practice are pointed out.
The basic terms and concepts used in structural modeling and nonlinear analysis of timber structure are provided in the first part of this dissertation. The specific behavior of wood joints under cyclic actions and therefore under earthquakes is described with emphasis to the pinching effect and strength and stiffness degrading. A literature review on the main numerical models proposed to reproduce the hysteretic load-slip curve of single fasteners, joints and whole wooden elements is presented and discussed. A proposal for a new wood joint numerical model that can be easily implemented into a standard commercial Finite Element code is reported. The reliability of such new developed model to reproduce the fasteners hysteresis behavior is presented and critically discussed in comparison with experimental results.
The second part of this thesis work is based on the evidence that the growing spread of the use of timber structures has led to the development of numerous innovative construction systems but at the same time a lack of code provisions for seismic timber structure still remains, in particular concerning the ductility (or behavior) factor q to be used for the design of different timber systems. This part of dissertation analyzes the definitions of the q-factor given in the scientific literature and its relevance in the design of seismic resistant structures. The traditional methods for estimating the q-factor are investigated and an innovative procedure for expeditious q-factor estimation is presented. The theoretical aspects of this new analytical-experimental procedure are reported and the main advantages and limitations are critically discussed.
The seismic behavior of the Cross Laminated Timber structure is in deep studied in the third part of this dissertation. Such building system is largely spreading in the constructive practice but no design guidelines are provided in the seismic codes yet, especially for what concerning the definition of their sound behavior factor. Aim of this part of dissertation is to define the influence of some significant building characteristics, such as building technology, storeys number, slenderness, design criteria etc.., on the q-factor value. Such influences were studied referring to a numbers of building configuration and by means of nonlinear analyses carried out using specific hysteretic spring lamp-mass models. Based on such numerical assessment a proposal for an analytical formulation suitable to calculate the q-factor of CrossLam buildings has been developed and is presented. The validation and the applicability limits of the proposed formulation are presented and critically discussed.
The final part of the dissertation investigates from the structural efficacy of newly developed construction technology which uses an external concrete shelter made of precast R.C. slabs to improve the performance of standard platform-frame shear walls. The idea consists of external plating made of thin reinforced concrete slabs screwed to the wooden frame of the walls. The concrete slab acts as a diaphragm against the horizontal forces. The structural response of this shearwalls under monotonic and cyclic loading conditions has been assessed by means of experimental tests. The tests outcomes are presented and compared with those from code provisions. Fulfillment of the requirements given by current codes as regards the attribution to the Higher Ductility Class is also verified. The influence of concrete skin on the seismic response of the shearwalls is also evaluated by means of numerical analysis and the assured “q” ductility factor is estimated.In questo lavoro di tesi si analizzano il comportamento sismico, la duttilità e la capacità dissipativa dei moderni edifici con struttura di legno. Le principali innovazioni sviluppate in questa tesi di dottorato riguardano le tecniche di modellazione dei sistemi di connessione usati nelle strutture lignee e la caratterizzazione sismica dei moderni edifici in legno.
L’introduzione della tesi evidenzia le caratteristiche che rendono le strutture in legno idonee per l’impiego in zona sismica e riporta una analisi storica delle principali tipologie di edifici sismo-resistenti a struttura in legno e la loro evoluzione dal passato ai giorni nostri. L’introduzione riporta inoltre un’analisi critica dello stato normativo Europeo ed Extraeuropeo sulle progettazione sismica degli edifici a struttura in legno evidenziando le principali lacune e incongruenze con la pratica costruttiva corrente.
Il lavoro di tesi sviluppato affronta sostanzialmente quattro argomenti dettagliati in parti indipendenti. Le prime due sono di carattere generale e riguardano tutte le strutture in legno mentre le rimanenti sono specifiche di sistemi costruttivi innovativi e non ancora completamente caratterizzati sismicamente.
La prima parte della tesi è dedicata alla descrizione del comportamento isteretico che caratterizza le connessioni utilizzate nelle strutture in legno e dei modelli numerici disponibili in letteratura per una riproduzione fedele di tale comportamento evidenziandone le potenzialità, i limiti di applicazione e l’efficienza numerica. Viene inoltre proposto un modello isteretico innovativo per riprodurre il comportamento delle connessioni tipicamente utilizzate nelle strutture in legno riproducibile anche mediante codici agli elementi finiti di tipo commerciale e non specificatamente orientati alla ricerca. Questa prima parte della tesi si conclude con la validazione e la descrizione dei principali vantaggi e limiti di applicazione della modello numerico proposto.
La seconda parte della tesi riguarda la definizione del fattore di struttura q dei sistemi costruttivi in legno innovati e di recente diffusione che non sono annoverati nelle normative sismiche. In questa parte della tesi vengono descritti i metodi tradizionali utilizzati per la stima del fattore di struttura evidenziandone i vantaggi e i principali limiti. Viene proposta una procedura innovativa di tipo misto analitico-sperimentale che consente una valutazione speditiva del valore del fattore di comportamento q.. Questa parte del lavoro di tesi si conclude riportando la validazione della procedura proposta nonché gli aspetti teorici i limiti di applicabilità.
La terza parte della tesi approfondisse lo studio sul sistema costruttivo a parete massiccia del tipo CrossLam. Preliminarmente viene riportatolo lo stato dell’arte sull’attività di ricerca sinora svolta su tale sistema costruttivo. L’obiettivo di questa parte del lavoro di tesi consiste nella definizione dell’effetto di determinate caratteristiche dell’edificio come il numero di piani, la snellezza, la composizione delle pareti, i criteri di progetto ecc. sul valore del fattore di struttura da utilizzare nella progettazione sismica dell’edificio stesso. Tale correlazione viene studiata mediante una serie di simulazioni numeriche su diverse configurazioni di edifici. I risultati ottenuti sono stati sintetizzati in una nuova formulazione analitica per la definizione del fattore di struttura q a partire dalle specifiche caratteristiche dell’edificio. Infine si riporta la validazione di tale formulazione analitica e si descrivono i principali vantaggi e limitazioni.
L’ultima parte di questo lavoro di tesi consiste nello sviluppo teorico e sperimentale di un nuovo sistema costruttivo misto legno-calcestruzzo ad alta duttilità e performance anti-sismiche. Il sistema sviluppato consiste nell’applicazione di un rivestimento esterno in lastre di calcestruzzo alle tradizionali pareti di taglio a telaio. La riposta strutturale, la duttilità e il comportamento isteretico è stato verificato mediante dei test sperimentali condotti su differenti configurazioni di pareti. Infine sono state condotte delle simulazioni numeriche, con modelli numerici appositamente sviluppati e tarati sulla base dei test sperimentali, mediane le quali è stato possibile stimare il valore del fattore di struttura q da utilizzare per il progetto sismico di questo nuovo sistema costruttivo
Struttura di base per parete edilizia leggera
No full textL'invenzione riguarda un elemento tecnologico di base idoneo a creare il cordolo di base di raccordo fra la fondazione in c.a. e la parte in elevazione di edifici a struttura leggera, quali quelli in legno, acciaio o alluminio, edificati con qualsiasi tecnica costruttiva, sia temporanei che definitivi. Tale elemento di base realizza un rialzo di fondazione con funzione strutturale, ma assolve anche funzioni di isolamento termico grazie ad elementi del tipo “a taglio termico” che separano sia la fondazione in c.a. dal cordolo che il cordolo dalla struttura soprastante.
Grazie a delle particolari staffe metalliche dotate di dispositivi di regolazione in altezza e in pianta, il sistema garantisce una posa perfettamente planare e rettilinea del cordolo di fondazione indipendentemente dalla conformazione della fondazione di base.
La particolare conformazione geometrica delle staffe di collegamento del cordolo con la soprastante struttura assicura un efficace collegamento a terra ed una elevata capacità di dissipazione energetica per azioni di tipo ciclico o anche la possibilità di realizzare un isolamento sismico alla base degli edifici
Wall base structure for light buildings
No full textL'invenzione riguarda un elemento tecnologico di base idoneo a creare il cordolo di base di raccordo fra la fondazione in c.a. e la parte in elevazione di edifici a struttura leggera, quali quelli in legno, acciaio o alluminio, edificati con qualsiasi tecnica costruttiva, sia temporanei che definitivi. Tale elemento di base realizza un rialzo di fondazione con funzione strutturale, ma assolve anche funzioni di isolamento termico grazie ad elementi del tipo “a taglio termico” che separano sia la fondazione in c.a. dal cordolo che il cordolo dalla struttura soprastante. Grazie a delle particolari staffe metalliche dotate di dispositivi di regolazione in altezza e in pianta, il sistema garantisce una posa perfettamente planare e rettilinea del cordolo di fondazione indipendentemente dalla conformazione della fondazione di base. La particolare conformazione geometrica delle staffe di collegamento del cordolo con la soprastante struttura assicura un efficace collegamento a terra ed una elevata capacità di dissipazione energetica per azioni di tipo ciclico o anche la possibilità di realizzare un isolamento sismico alla base degli edifici
A Non Linear Numerical Model For The Assessment of the Seismic Behavior and Ductility Factor of X-Lam Timber Structures
No full textThis paper reports the development of a three-dimensional finite-element model suitable for the investigation of the seismic response of massive timber structures and for the determination of the “ductility factor q”, which is defined as the factor used for design purposes to reduce the forces obtained from a linear analysis, accounting for the non-linear response of a structure.
In the X-Lam buildings, the capability of energetic dissipation through inelastic behavior is concentrated in the connections between the panels and footings. The development of specific non linear spring element, able to represent the hysteretic response of load-displacement curves of such connections, is reported in this paper.
By using the non linear spring within finite element models has been possible to reproduce the experimental results of monotonic and cyclic tests on single panels and plane wall and also of a three-storey cross-laminated wooden building tested by means of a shaking table. All the results from the numerical models fit well with that from the experiments.
Supported from these preliminary validations, the model has been also used to predict the displacements and forces on the structure subjected to seismic excitations and therefore their most appropriate ‘ductility factor q’
Wood-based solutions to improve quality and safety against seismic events in conservation of historical buildings
Full text linkHistorical buildings can be highly vulnerable to earthquakes if In-Plane strength and stiffness of floors and roofs are not sufficient to limit Out-Of-Plane deformation of walls and to transmit the seismic forces among walls efficiently. In fact, floors and roofs in existing masonry buildings are normally realized with timber beams, purlins and a single layer of timber boards, and their In-Plane stiffness can be limited when subjected to shear forces. Various retrofitting techniques are available, whose effectiveness can be very different and not easily predictable theoretically. In this paper, the behavior of different strengthening criteria for historical buildings, involving the use of wood and wood based products, is illustrated and critically discussed. Code provisions, design rules and calculation methods for strengthening interventions are described. In-Plane behavior of floor and roof structures and their interaction and connection to the seismic resistant wall systems are considered. Finally, a case study of restoration intervention on a historic barn is presented, which was damaged during the 2012 earthquake in Emilia. The illustration is focused on the improvement of the whole safety of the construction by means of specifically designed interventions on the wooden roof
Procedures for seismic characterization of traditional and modern wooden building types
Full text linkThe paper analyzes different wooden buildings types used in past and nowadays to realized Low-Rise and Mid-Rise timber structures from the seismic point of view. A preliminary overview about the procedures prescribed by codes for the seismic characterization of the timber building systems is given. Then the definition of the behaviour Q-Factor in the literature and its relevance in design of structures in seismic areas is treated. Available research methods for estimating the Q-Factor based on the verification of the nonlinear seismic response of entire buildings by means of experimental tests and numerical simulations are presented and analyzed. The relevance of a proper definition of the yielding limit and of failure condition in the seismic characterization of wooden building systems is treated. Moreover, a comparison between the Q-Factor estimations obtained using different calculation methods is presented. Lastly, the appropriate Q-Factor values are given for a reliable and safe seismic design of buildings realized using the examined wooden constructive systems
Prove sperimentali monotone e cicliche su pareti realizzate con un sistema costruttivo ibrido in legno - calcestruzzo
No full textProposition of this work is the investigation of the structural behavior of a newly developed timber construction system. It is a modular prefabricated system which combines a typical Platform Frame system with an external small thickness concrete slabs having the function of diaphragm against the horizontal actions and also thermal ad acoustic functions. Walls are formed by in situ connection of the single modules preassembled in the factory, with considerable advantages from the point of view of the site management. The structural design is similar to that of the Platform Frame, where the bracing stiffness and strength assured by nailing the OSB panels to the wooden structure cumulates with that given by the concrete slabs screwed to the vertical structure with large diameter connectors. The use of large diameter connectors assure to the investigated system a considerable stiffness and in the meantime a great capacity of energy dissipation in case of seismic events. In order to verify the real performance of the construction system under seismic actions several monotonic and cyclic tests on single panel modules and on an assembled walls was carried out. Such tests were carried out in accordance with the testing protocols specified in EN 12512. Various configurations of connection to the footing beam and between timber and concrete slab were tested. In this work the results of the experimental tests are reported and the good seismic performance of the structural system has been proved. The analytical expressions that best fit the experimental monotonic load displacement curve are reported and the comparison of the measured results with that calculated according with the national and international codes are given
Shear-Compression Test on Masonry Walls with an Innovative Experimental Setup
Full text linkThe behavior and failure mechanism of a masonry wall subject to in-plane shear depends on several factors,
such as the wall geometry, the boundary conditions, the acting stresses, and the masonry mechanical parameters. The
objective of this research was the study of the in-plane shear behavior of full-scale masonry panels through an innovative
experimental setup, purposely designed to reproduce a double fixed boundary condition in order to induce a diagonal
cracking failure mode. Such a boundary condition was ensured by the presence of an upper horizontal rigid steel beam,
combined with the possibility of modulating the compressive load applied to the masonry panel, while increasing the
horizontal displacement. Nonlinear numerical simulations were carried out to analyze the capability of the experimental
setup of reproducing the desired loading and restraint conditions and to predict the shear behavior of a clay brick masonry
panel. A finite element model was realized, in which all the components of the experimental setup were included to
account for all possible failure modes, and the masonry panel was modelled according to a macro-modelling approach.
The results of the numerical predictions were compared with the results of a shear-compression test on a masonry panel,
which will be presented in the paper. The good agreement obtained between the numerical and the experimental results,
both in terms of load vs displacement curve and development of the cracking process, confirmed the suitability of the
setup in reproducing the assumed boundary conditions and shear failure mode
Analytical formulation based on extensive numerical simulations of behavior factor q for CLT buildings
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