Mechanical characterization of connections in seismic resistant Cross-Laminated Timber structures

Cross-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

Modelling the mechanical behaviour of typical wall-to-floor connection systems for cross-laminated timber structures

This paper proposes a numerical model capable of predicting the mechanical behaviour and the failure me- chanism of typical wall-to-floor connections for Cross-Laminated Timber structures. Such systems are assembled with angle brackets and hold-downs, anchored to the wall and floor panels with profiled nails and bolts. The metal connector and the elements to which it is fastened are modelled using 3D solid bodies, while the steel-to- timber joints are simulated as non-linear hysteretic springs. Shear and tension tests are reproduced on two connection systems and results are compared to the test data obtained from similar configurations. Simulations lead to accurate predictions of the mechanical behaviour (i.e. elastic stiffness, maximum load-carrying capacity, and shape of the hysteresis cycles) and energy dissipation. Finally, the performance when lateral and axial loads are applied simultaneously is investigated. Analyses are carried out by varying the inclination of the load, with respect to the axis of the connector, between 0\ub0 and 90\ub0. Results exhibit a quadratic interaction relationship between shear and tension loads, and prove that their coupled effect influences the stiffness and the maximum load-carrying capacity

Investigating the use of Targeted-Energy-Transfer devices for stay-cable vibration mitigation

Free 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\u2013wind 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 \u2018flexibility of the support\u2019 (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 \u2018universal design curves\u2019 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

Numerical modelling of steel-to-timber joints and connectors for CLT structures

The 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

Experimental investigations and design provisions of steel-to-timber joints with annular-ringed shank nails for Cross-Laminated Timber structures

4siThis 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.Link to official publication: DOI 10.1016/j.conbuildmat.2016.06.072partially_openembargoed_20180709Izzi, Matteo; Flatscher, Georg; Fragiacomo, Massimo; Schickhofer, GerhardIzzi, Matteo; Flatscher, Georg; Fragiacomo, Massimo; Schickhofer, Gerhar

A hysteresis model for timber joints with dowel-type fasteners

Predicting the mechanical behaviour and the failure mechanism of timber joints with dowel-type fasteners re- quires consideration of several factors, including the geometrical and mechanical properties of the metal fas- tener, the physical properties of timber and the interaction between such elements. This paper proposes a nu- merical model where a joint is schematized as an elasto-plastic beam in a non-linear medium with a compression-only behaviour. Unlike the differential approach adopted by most of the hysteresis models pub- lished in literature, this model predicts the load-displacement response using simple mechanical relationships and basic input parameters. Furthermore, the model is capable of reproducing the effect of the cavity formed around the fastener by timber crushing, and simulates the hysteretic behaviour and the energy dissipation under cyclic conditions. Shear tests are reproduced on nailed steel-to-timber joints in Cross-Laminated Timber and results are compared to the experimental test data obtained on similar single fastener joints. Simulations lead to accurate predictions of both the mechanical behaviour (initial stiffness, maximum load-carrying capacity, global shape of the loading curve and of the hysteresis cycles) and the total energy dissipation observed in the tests

Assessment of the structural stability of Blockhaus timber log-walls under in-plane compression via full-scale buckling experiments

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 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

Experimental tests on a hybrid timber-frame wall system

This paper presents an innovative lateral load-resisting wall system, which is an evolution of the light-timber frame (LTF) shear walls currently available on the market. In comparison to traditional LTF walls, the novelty aspect is the use of Cross-Laminated Timber (CLT) beams and studs instead of solid timber elements. Thanks to this \u2018hybrid\u2019 approach, this new system combines some peculiar aspects of LTF structures (such as the limited weight and the high dissipative behaviour) with the potentials of CLT. Moreover, the use of CLT elements limits the issues due to the compressive deformations on bottom beams and permits to employ some innovative connections with high mechanical properties. Cyclic shear tests are carried out on two configurations of interest, assembled by considering different layouts of the load-bearing elements. Test results are compared to the experimental data obtained on similar LTF systems and differences are critically discussed

Identification of bi-allelic LFNG variants in three patients and further clinical and molecular refinement of spondylocostal dysostosis 3

: Spondylocostal dysostosis (SCD), a condition characterized by multiple segmentation defects of the vertebrae and rib malformations, is caused by bi-allelic variants in one of the genes involved in the Notch signaling pathway that tunes the "segmentation clock" of somitogenesis: DLL3, HES7, LFNG, MESP2, RIPPLY2, and TBX6. To date, seven individuals with LFNG variants have been reported in the literature. In this study we describe two newborns and one fetus with SCD, who were found by trio-based exome sequencing (trio-ES) to carry homozygous (c.822-5C>T) or compound heterozygous (c.[863dup];[1063G>A]) and (c.[521G>T];[890T>G]) variants in LFNG. Notably, the c.822-5C>T change, affecting the polypyrimidine tract of intron 5, is the first non-coding variant reported in LFNG. This study further refines the clinical and molecular features of spondylocostal dysostosis 3 and adds to the numerous investigations supporting the usefulness of trio-ES approach in prenatal and neonatal settings