1,720,974 research outputs found
Mechanical characterization of connections in seismic resistant Cross-Laminated Timber structures
Full text linkCross-Laminated Timber (CLT) structures are assembled with massive timber panels that are fastened together and to the horizontal elements (the foundations and the intermediate floors) with step joints and mechanical connections. Due to the high in-plane stiffness of CLT, the seismic behaviour of those structures markedly depends upon the connections used.
The mechanical behaviour of lateral load-resisting systems made with CLT panels and typical connection systems was the focus of a large body of research, especially in Europe and North America. Furthermore, full-scale shaking table tests were carried out on several multi-storey buildings, demonstrating a significant ductility and energy dissipation under seismic loading.
In contrast with the significant findings associated to those research projects, specific calculation methods have not yet been included either in Eurocode 5 (static design) or in Eurocode 8 (seismic design). Nowadays, the design is done using simplified calculation methods that neglect the connections stiffness and introduce some simplifications on their mechanical behaviour.
The mechanical characterization of typical connection systems for CLT structures (e.g. with angle brackets and hold-downs, nailed and bolted to the wall and floor panels) is an expensive and time-consuming process, since requires the execution of a large number of tests. Therefore, to limit the need of experimental testing to a minimum, significant effort should be devoted to develop advanced numerical models capable to predict their load-displacement response and failure mechanisms.
In the scope of this thesis, an extensive experimental programme was carried out on nailed steel-to-timber joints in CLT. The experimental results were used as input to assess the reliability of currently available calculation methods and to develop capacity-based design principles for nailed steel-to-timber joints in CLT (i.e. the overstrength factor and the strength degradation factor). In addition, analytical methods and numerical models capable to predict the mechanical properties and energy dissipation at different building levels (single fastener joint, connection, and wall system) were developed. Experimental results obtained during previous research projects served also for calibration of non-linear analyses, which were used to extend the test results to different configurations of technical interest. Outcomes of the parametric studies provided better understanding of the seismic behaviour and energy dissipation of typical connection systems for CLT buildings.
It was concluded that the numerical models presented within this thesis are a sound basis to investigate the seismic behaviour of CLT buildings. However, future research is required to further verify and improve these predictive models
Investigating the use of Targeted-Energy-Transfer devices for stay-cable vibration mitigation
Full text linkFree vibrations of a taut cable with an attached passive Targeted-Energy-Transfer (TET) device are investigated using an analytical formulation of the complex generalized eigenvalue problem. This problem is of considerable practical interest in the context of stay-cable vibration suppression in bridges, induced by wind, rain–wind and parametric excitation. The TET device is a nonlinear apparatus, which has been investigated and successfully ap- plied to the vibration suppression in several structural or mechanical systems. This study proposes, for the first time, the use of the TET device as a simple passive apparatus for stay-cable vibration mitigation. In this applica- tion, the device was modelled as a dashpot with a viscous damper in parallel with a power-law nonlinear elastic spring element and a lumped mass restrained to one end. The ‘flexibility of the support’ (imperfect anchorage to the deck) was also simulated by placing an elastic support (linear elastic spring) in series between the dashpot and the deck. The study derives a new family of ‘universal design curves’ for the TET device, by accounting for the effects of nonlinear elastic stiffness, lumped mass and flexibility of the support. To verify the adequacy of the universal curves and to evaluate the effectiveness of the TET devices, parametric numerical simulations were per- formed on a reference stay cable. As an application example, analytical results were employed to design the dampers of one flexible stay, installed on an existing cable-stayed bridge. In all the investigations, theoretical and numerical results were obtained and compared
Advanced modelling of CLT wall systems for earthquake resistant timber structures
No full textCross-Laminated Timber (CLT) is gaining a significant popularity among the structural products as a sustainable alternative to steel and concrete. In comparison with those traditional building materials, CLT has a low carbon footprint and provides comfortable living conditions; moreover, the high strength-to-weight ratio, the prefabrication process and the erection speed are additional key points of its broad diffusion. Determining the load-carrying capacity of lateral load-resisting systems made of CLT panels (such as CLT wall systems, i.e. CLT walls with mechanical connections) is crucial for both the static and seismic design of CLT structures. However, a calculation method has not yet been included in Eurocode 5 (EN 1995-1-1:2004/A2 2014). Nowadays, the load-carrying capacity of CLT wall systems is determined with forcebased design procedures. However, since the analysis neglects the connections stiffness, those methods do not ensure that the wall system behaves in accordance with the design assumptions. Furthermore, simplified methods neglect some contributions that might affect the response of a CLT wall system, like the friction between the wall and the element that is restrained to and the effect of the overlaying floor. Based on the above-mentioned issues, this paper proposes a new numerical model of a CLT wall system. The model schematizes the CLT panel as an elastic orthotropic element, while the connections (i.e. hold-downs, angle brackets, and screws) are simulated as non-linear hysteretic springs. The interaction between the CLT panel and the foundation, and between the wall element and the CLT floor restrained on top of it, were specifically addressed in the development of the model.
The study presented herein is carried out considering the experimental tests results obtained by Gavric et al. (2015a-2015b-2015c). At first, experimental and numerical results are compared. A parametric study is carried out afterwards, (a) by varying the vertical load applied on top of the CLT wall, (b) by modifying the aspect ratio of the timber panel, and (c) by simulating a CLT floor screwed on top of the wall. Results are collected in diagrams and compared with a simplified design procedure commonly adopted by practicing engineers and discussed by Pozza et al. (2016b), highlighting how those contributions influence the elastic stiffness and the load-carrying capacity of a CLT wall system
STAY-CABLE VIBRATION MITIGATION USING NONLINEAR TARGETED-ENERGY- TRANSFER DEVICES: A PARAMETRIC STUDY
No full textLarge-amplitude oscillation of inclined stays are often associated with rain-wind induced phenomena, vortex shedding and parametric excitation. To suppress the problematic vibrations and to minimize the potential high cost of maintenance or repair, passive damping system have been widely employed. In most investigations the damper was modelled as a pure linear viscous device or as a linear viscous damper with internal stiffness perfectly anchored to the deck. Other authors evaluated the effectiveness of the damping system introducing the effects of the flexibility in the damper support. The aim of this paper is to investigate the use of Targeted-Energy-Transfer (TET) devices for cable vibration suppression; a new family of devices is proposed in which a nonlinear elastic stiffness force mechanism and device, modelled as a power-law elastic spring element, is placed in parallel with a linear viscous damper. The imperfect anchorage to the deck is also taken into account by an elastic support. The free vibration of a taut-cable with an attached passive TET device is investigated using an analytical formulation of the complex generalized eigenvalue problem in the presence of two cable segments. The concept of “universal design curve” was examined. A parametric study was performed on a reference cable to verify the adequacy of the “universal design curve” and to evaluate the effectiveness of TET device; the study was subsequently extended to two real stays, taken from the Fred Hartman Bridge (Houston, Texas, USA) and from the Stonecutters Bridge (Hong Kong, China). In all the investigations, theoretical and numerical results were obtained and compared
Targeted-Energy-Transfer devices for stay-cable vibration mitigation
No full textFree vibrations of a taut-cable with an attached passive Targeted-Energy-Transfer (TET) device are investigated using an analytical formulation of the complex generalized eigenvalue problem. This problem is of considerable practical interest in the context of stay-cable vibration suppression in bridges, induced by wind, wind-rain and parametric excitation. The TET device was modelled as a dashpot with a viscous damper in parallel with a power-law elastic spring element; a linear elastic spring was also added between the dashpot and the deck. Two types of TET devices were analysed: a Linear TET and a Nonlinear TET. For both the devices a family of “universal design curves” was developed, by accounting the effect of the elastic stiffness and the flexibility of the support. To verify the adequacy of the universal curves and to evaluate the effectiveness of TET devices, numerical simulations were performed on a reference cable and subsequently extended to an existing stay, taken from the Fred Hartman Bridge (Houston, Texas, USA)
q-factor estimation for timber Blockhaus buildings
No full textThis paper investigates the structural response and vulnerability of Blockhaus buildings under seismic loads. Blockhaus systems are widely used in daily practice for the construction of wooden houses or commercial buildings (see for example (Rubner Haus AG SpA)). Native of forested areas, they are often built also in earthquakeprone regions. Several research projects have been carried out, in order to experimentally and/or numerically assess the seismic performance of full-scale structures and small components (Branco & Araújo (2012), Piazza et al (2013), Bedon et al (2015a), Bedon et al (2015b), Bedon et al (2015c), Grossi et al (2016)). However, the seismic characterization of this construction system requires further investigations and studies, since the current standards for timber structures (e.g. Eurocode 5 (2004) and Eurocode 8 (2004)) do not provide specific recommendations for the calculation of their q-behaviour factor.
In this work, based on past research contributions (Bedon et al (2015a), (2015b), (2015c)) and full-scale experimental tests (Piazza et al (2013)), Finite-Element (FE) investigations are performed on three dimensional Blockhaus buildings subjected to seismic loads by means of FE-models implemented in the ABAQUS software package (Simulia (2012)). Parametric simulations are carried out on several building archetypes, characterized by different overall geometrical properties, inter-storey floors (up to 2 levels), number and position of openings, corner joints. Design recommendations are then provided, based on the collected FE results, for a rational estimation of the corresponding q-behaviour factor
Experimental investigations and design provisions of steel-to-timber joints with annular-ringed shank nails for Cross-Laminated Timber structures
Full text linkThis paper investigates the mechanical and the hysteretic behaviour of steel-to-timber joints with annular-ringed shank nails in Cross-Laminated Timber (CLT). These fasteners are used to anchor typical metal connectors, such as hold-downs and angle brackets, to the CLT panels. The experimental pro- gramme presented in the paper was carried out at the Institute of Timber Engineering and Wood Technology, Graz University of Technology (Graz, Austria). Average and characteristic values of the exper- imental strength capacities are evaluated and compared to the analytical predictions determined accord- ing to current structural design codes and literature. Furthermore, to fulfil the requirements of the capacity-based design, the overstrength factor and the strength degradation factor are evaluated and con- servative values are recommended
Pressure coefficients for evaluating wind loads on large roofs: Comparison between Database-Assisted Design and Italian standards
No full textIn the last decade, the Database-Assisted-Design methodology has emerged as a powerful approach to estimate structural wind loads and as an alternative to prescriptive design standards, in particular for Low-Rise Buildings (e.g., Simiu et al., 2003). This work examines the applicability of this approach to wind load analysis and design as an alternative to the prescriptions currently included in the Italian design standard. In this paper, mean pressures coefficients on large roofs of industrial buildings are examined and compared to the recommendations of the current Italian national design guidelines (CNR-DT 207/2008)
Assessment of the structural stability of Blockhaus timber log-walls under in-plane compression via full-scale buckling experiments
No full textBlockhaus structural systems are obtained by assembling multiple timber logs able to interact with each other by means of simple mechanisms (e.g. contacts, tongues and grooves, and carpentry joints, also referred to as 'corner' joints). Although these systems have ancient origins, the structural behaviour of Blockhaus systems under well-defined loading and boundary conditions is still complex to predict. The paper focuses on the assessment of the typical buckling behaviour and resistance of in-plane compressed timber log-walls. The effects of various mechanical and geometrical aspects such as in-plane rigid inter-storey floors, load eccentricities, different types of lateral restraints, openings (e.g. doors or windows) or additional metal stiffeners, are investigated by means of full-scale buckling experiments. Results are then critically discussed and preliminarily assessed via analytical formulations taken from classical theory of plate buckling and column buckling. Although further advanced studies are required for the development of a generalized buckling design method, it is shown that several mechanical and geometrical aspects should be properly taken into account to correctly predict the structural capacity of Blockhaus systems under in-plane compression.Blockhaus structural systems are obtained by assembling multiple timber logs able to interact with each other by means of simple mechanisms (e.g. contacts, tongues and grooves, and carpentry joints, also referred to as ‘corner’ joints). Although these systems have ancient origins, the structural behaviour of Blockhaus systems under well-defined loading and boundary conditions is still complex to predict. The paper focuses on the assessment of the typical buckling behaviour and resistance of in-plane compressed timber log-walls. The effects of various mechanical and geometrical aspects such as in-plane rigid interstorey floors, load eccentricities, different types of lateral restraints, openings (e.g. doors or windows) or additional metal stiffeners, are investigated by means of full-scale buckling experiments. Results are then
critically discussed and preliminarily assessed via analytical formulations taken from classical theory of
plate buckling and column buckling. Although further advanced studies are required for the development of a generalized buckling design method, it is shown that several mechanical and geometrical aspects should be properly taken into account to correctly predict the structural capacity of Blockhaus systems under in-plane compression
Numerical modelling of steel-to-timber joints and connectors for CLT structures
Full text linkThe mechanical behaviour of steel-to-timber joints with annular-ringed shank nails is investigated using numerical modelling and a component approach. These joints are used in Cross-Laminated Timber (CLT) buildings to anchor metal connectors such as hold-downs and angle brackets to the timber panels. At first, a general hysteresis model is introduced, where a single fastener joint is schematized as an elasto-plastic beam embedded in a non-linear medium with a compression-only behaviour. A second hysteresis model is then presented, where the mechanical behaviour of the joint is simulated by a non-linear spring with three degrees of freedom. Both models are calibrated on the design rules prescribed by the reference standards. Moreover, average strength capacities are determined from the corresponding characteristic values assuming a standard normal distribution and suitable coefficients of variation. As first applicative examples of the proposed models, shear tests are simulated on single steel-to-timber joints with annular-ringed shank nails and on a connection made of an angle bracket and sixty nails. The scatter of mechanical properties in steel-to-timber joints is also taken into account in the simulations and a stochastic approach is proposed, demonstrating acceptable accuracy
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