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Innovative connection systems for timber structures
Full text linkConnections and fasteners play an essential role in the determination of strength and stability, ductility and robustness, i.e., the overall behaviour of timber structures. In particular, connections subjected to static loads are to be investigated in terms of strength and stiffness, whereas the ones designed to withstand cyclic (e.g., seismic) loads need also the definition of their complete hysteretic response. This Ph.D. dissertation focuses on the behaviour of modern connections being developed and employed in timber engineering.
An initial overview on mechanical connections employed in timber structures and their evolution is reported in the introductive section of this thesis. Advantages as well as critical issues of traditional connections are the motivations for the evolution and the improvements brought by innovative connections. Two different applications of innovative timber connections are analysed and hereby discussed, each one facing different issues. The first one claims to give an insight into modern screws employed in Timber-concrete composite (TCC) structures, where the major objective is to achieve maximum strength and above all stiffness. The second is directly focused on the cyclic performance of modern connections employed in Cross Laminated Timber (CLT) structures where dissipative capacity and structural damping are of utmost importance. Consequently, the present manuscript is subdivided into two main parts.
The first part deals with TCC joints realized with modern screws. The key-point to guarantee adequate mechanical performance to these composite structures is the use of connectors that demonstrate sufficient shear strength and stiffness at the interface between timber beam and concrete slab independently of the presence of an intermediate layer. Modern cylindrical connectors, such as self-tapping screws, are rising interest because they combine remarkable performance, when their withdrawal capacity is exploited, and quickness of execution especially in case of onsite installation. In this paper, a theoretical approach to calculate shear strength and stiffness of TCC joints made with inclined screws is discussed and compared to current design procedures. Furthermore, a report on short-term push-out tests of TCC joints realized with inclined self-tapping screws carried out varying fastener arrangement, diameter and concrete type is given. Consequently, a comparison between the results obtained with the theoretical method and experimental tests is reported and critically discussed in terms of both strength and stiffness.
The last section of the first part present the design of an innovative connector that combines the use of self-tapping screws and a glass-fibre reinforced polymer (GFRP) element as components to realize structural TCC joints. FRP is being used in civil engineering since decades, but most of these applications utilize pre-impregnated thermosetting composites, the most common of which is carbon fibre-reinforced polymer (CFRP). On the contrary, injection moulded thermoplastic materials are relatively new and lack of history of their use in civil infrastructures. The aim is to develop a connection that solves typical installation issues of inclined screws and avoids stress concentration issues that may occur in the concrete layer. Numerical simulations, carried out to design this particular joint and exploiting a hybrid approach, are described in detail. Then, results from the experimental tests conducted to investigate the behaviour of the device subjected to shear loading conditions are compared with the analytical predictions valid for inclined screws previously described.
The second part of this work focuses on the developing of an innovative earthquake-resistant connections employed for CLT structures. The seismic performance of CLT buildings is mainly related to the capability of joints to perform plastic work, since timber elements have limited capability to deform inelastically. Nowadays, the use of hold-down and angle bracket connections, which were originally developed for platform-frame constructions, has been extended also to CLT buildings. Nevertheless, the dissipative capacity of light-frame buildings is mainly diffused in nailing between frames and panels while, in CLT walls, the dissipative contribution is exclusively assured by ductile joints connecting the panels. The need of more reliable connections that provides well predictable and stable hysteretic behaviour, reduced pinching phenomenon (caused by the wood embedment) and strength degradation, justifies the continuous development of “innovative” connections. In this work, a newly developed connection element that overcomes the aforementioned issues and works for both tensile or shear loads is designed and assessed, and various significant aspects are discussed.
Initially, the design procedure of the connection element and preliminary experimental tests that validates the numerical predictions are illustrated. Then, improved versions of the device are illustrated and their experimental results reported with particular attention in describing their hysteretic response and coupled shear-tension strength domain. In this work, an important role is also given to the application of the capacity design criteria applied at the joint level in order to guarantee the best exploitation of the connection’s dissipative capacity. Therefore, theoretical concepts, which describe the overstrength of traditional and innovative connections, confirmed by experimental tests of the brackets anchored to a CLT panel, are also given. In the last chapter is presented a numerical model that, following a macro-element approach, reproduces the actual cyclic response of the investigated device when subjected to combined shear-tension loads. Finally, the results of Non-Linear Dynamic Analyses of a case study CLT building realized which such model are reported and the seismic capacity of the case study building is evaluated.
With these two examples, this thesis aims to give an original contribution in the evaluation of performance of innovative connection systems for timber structures, combining the use of theoretical, numerical and experimental models, and highlighting the emerging differences with respect to the use of traditional fasteners and connections
Sushi Drop - SUstainable fiSHeries wIth DROnes data Processing
No full textWithin SUSHI-DROP, a customized unmanned underwater vehicles will be developed and equipped with acoustical and optical technologies in order to implement a non-invasive mean to assess environmental status of habitats, fish stocks population and, in general, to monitor the biodiversity of marine ecosystems. We are planning to assess the accuracy of the opto-acoustic surveys in deriving single-species abundance indices (in numbers or weight) for direct input into stock assessments, and to evaluate the benefits brought by these new technologies with respect to the classical procedures based on fish sampling. Moreover, a dedicated open-access database system will be created to collect, maintain and share the scientific data acquired by the UUVs
Sviluppo e progettazione di sistemi innovativi per il monitoraggio in tempo reale dell’integrità strutturale di parti di veicoli racing/automotive e confronto con tecniche diagnostiche tradizionali
No full textSviluppo e progettazione di sistemi innovativi per il monitoraggio in tempo reale dell’integrità strutturale di parti di veicoli racing/automotive e confronto con tecniche diagnostiche tradizional
Sensors
No full textSensors provides an advanced forum for the science and technology of sensors and biosensors. It publishes reviews (including comprehensive reviews on the complete sensors products), regular research papers and short notes
Timber-concrete composite connections using GFRP notches fastened with self-tapping screws: Conceiving, numerical modelling and testing
No full textIn the structural engineering field, injection moulded Glass Fibre Reinforced Polymer (GFRP) components are typically used as parts or accessories of fastening systems. In this paper, an extended numerical and experimental study on a Timber-Concrete Composite Connection (TCCC) realized with an injection-moulded GFRP notch fastened to the timber beam with self-tapping screws is presented. Numerical simulations performed to predict the performance of the investigated TCCC in terms of force-displacement curves are compared with the outcomes from forty push-out tests. Results demonstrate how the GFRP notch assure the full exploitation of the screws withdrawal capacity without incurring in sudden concrete failures independently from the type of concrete used (normal and light-weight)
A piezoelectric coded-excitation scanning acoustic transducer for Lamb wave inspections
Full text linkSeveral techniques for Guided Wave (GW) inspections have already been developed. Most of them rely on extensive sensor deployment and damage localization algorithms characterized by significant computational costs. However, there is a growing demand for simpler, more autonomous, and more affordable systems across various fields. In particular, the implementation of wireless, battery-powered systems with a reduced number of sensor nodes and simpler processing would greatly facilitate the transition of this inspection technology on the field. Following this direction, this work presents the design of a novel piezoelectric transducer composed of four different patches, i.e. with only four input/output channels, to scan a given area. The peculiar piezo-load distributions allow the association of different spectral binary sequences for each 6∘ discrete angular step. By evaluating the distance-of-flights (DoFs) of detected peaks, the range coordinates for multiple defects are identified. Meanwhile, the angular information is extracted by demodulating binary sequences of peaks with comparable DoFs across several frequency bands. Since the transducer is designed as an encoder, it is referred to as coded-excitation scanning acoustic transducer (CESAT). More specifically, the Gray code is used to generate spectral patterns to reduce the uncertainty between two adjacent angular steps. A new quantization procedure for the optimal generation of the piezo distributions is also proposed. Ad hoc signal processing algorithms, suitable for embedded applications, were developed to extract multi-target range and angle information. The processing is based on the frequency decomposition of the recorded signal using an FIR filter bank and on dispersion compensation procedures for pulse 're-compression'. The transducer encoder behavior is validated through a finite element analysis. Finally, numerical simulations were performed to assess the effectiveness of the CESAT and associated signal processing in multi-defect detection and localization tasks
Transducer-to-Transducer Communication in Guided Wave Based Structural Health Monitoring
No full textSystems for guided wave (GW) based structural health monitoring are limited in the frequency bandwidth of the excitation signal due to the underlying wave’s dispersion that causes a broadening of the waveform along the waveguide. Hence, bandlimited waveforms such as toneburst or chirp signals are widely employed. Recently, the authors have investigated the potential of phase-modulated signals, known from CDMA-communication systems, as a possible way to overcome such limitation. It was found that this class of excitation enables a parallel transmission and reception of all piezoelectric transducers which may lead to a significant reduction in the overall system complexity, because of channel switching is not required anymore. In particular, by means of matched filtering a pulse compression and signal demodulation can be achieved. In addition, the signal-to-noise ratio can be improved by considering phase modulation signals of longer code length. In this paper, we investigate additional properties of phase-modulated signals in terms of their capability to transmit digital information on the health status of the structure through the structure itself [1]. This may lead to autonomous GW-based SHM system where the sensor nodes do not communicate with each other through a wireless module, but where the information are being delivered through the waveguide. A proof-of-concept will be demonstrated here on an isotropic plate using both simulated signals and experimental measurements. The implementation of the concept requires to compensate the guided wave signals from dispersion. Here a suitable Warped Frequency transform [2] designed on the group velocity of the guided modes, is applied. Such transformation has two beneficial effects: 1) it compensates for the detrimental effect of dispersion, 2) it preserves the pseudo-orthogonality of the encoded pulses, because it is computed with a unitary operator
Improving Damage Localization Through Gaussian Process Regression and Enhanced Acoustic Emissions Time of Arrival Estimation
No full textAccurate localization of structural flaws through time of arrival (ToA) estimation is fundamental for structural health monitoring (SHM). However, current methods suffer from inaccuracies in noisy environments, particularly when using standard statistical approaches like the Akaike information criterion (AIC). To overcome these limitations, we propose a new approach built on an adaptive picking technique (APT) combined with gaussian process (GP) regression. APT employs frequency-based mode separation and adaptive envelope analysis to reliably identify first arrivals without prior modal knowledge, while GP regression provides probabilistic damage localization with confidence intervals. Experimental validation on an aluminum plate instrumented with six sensors demonstrates superior performance across multiple conditions, achieving a median error of 13.7 mm with 4.9× improvement over AIC-only alternatives. Under noise conditions down to 18 dB), representative of operational scenarios, our approach not only maintains significant robustness, improving up to 8.2× with respect to AIC, but also generalizes across training and testing conditions, confirming its reliability for real-world SHM applications
A design strategy to improve the directivity of wavenumber-spiral frequency-steerable acoustic transducers
Full text linkDrastic hardware simplification and cost reduction of Guided Wave (GW) based systems can be achieved by using piezoelectric transducers that, shaped according to a frequency based beam steering concept, present inherent directional capabilities exhibiting preferential radiation/sensing directions. In particular, directional GWs generation and sensing can be achieved by patterning the piezoelectric material lay-out and the electrodes. Frequency steerable acoustic transducers (FSATs) peculiar electrodes' shape produces a spatial filtering effect which is frequency-dependent, so that a direct relationship can be established between the direction of propagation (wavenumber) and the spectral content of the transmitted/received signal. In this paper we present a novel design strategy of FSAT that enables enhanced sensor directivity by using a local variation of the density of the piezoelectric material
A stamp size, 40mA, 5 grams sensor node for impact detection and location
No full textIn this work, a stamp-size footprint, low power, and light weight sensor node for impact detection and location on laminate composite and metallic structures is presented. This device passively exploits the guided waves originated by the impacts and is meant as a basic building block for smart structure sensor networks development. It draws power from a two-wire differential data-over-power (DoP) network communication interface, which is also used for half-duplex communication at 200 kbps with a gateway device and the other nodes in the network. Each node measures roughly 30mm×23mm, consumes less than 35mA, and weighs less than 5 g, making it attractive for aerospace systems where size, power and weight reduction are crucial. Elastic waves generated from impacts and propagating on the structure
to be monitored are converted to electrical signals by an innovative, patent-pending, spiral shaped piezoelectric transducer. Signals are simultaneously acquired at 1 Msps on each channel and processed by an embedded STM32F3 low-voltage 32-bit mixed-signal MCU with DSP and FPU instructions. A 128 KiB SPI serial SRAM is used for data storage while program instructions are stored in the MCU embedded 256 KiB flash. A low-voltage, high-speed, half-duplex RS485 transceiver is used to interface the MCU to the bus through a filtering mesh of passive components. This mesh also connects to a low-dropout voltage regulator, allowing it to draw power from the DoP bus without interfering with data transmission. A separate gateway device is also presented: it is capable to simultaneously interface and
feed the DoP bus by drawing power either from a common PC USB2.0 port or from an external power supply. A network counting up to 64 nodes on a single wire can be implemented and interfaced to a PC for real-time impact detection applications. This network can be extended to an arbitrary number of nodes by means of signal repeaters
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