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The deployment of Structural Health Monitoring (SHM) systems is a natively interdisciplinary task that involves joint research contributions from sensing technologies, data science and civil engineering. The capability to assess, also from remote stations, the working conditions of industrial plants or the structural integrity of civil buildings is widely requested in many application fields. The technological development aims to continuously provide innovative tools and approaches to satisfy these demands. As a first instance, reliable monitoring strategies are needed to detect structural damages while filtering out environmental noise. Ongoing solutions to tackle these topics are based on the exploitation of highly customized sensing technologies, such as shaped transducers for Acoustic Emission (AE) testing or Micro-Electro-Mechanical System (MEMS) accelerometers for Operational Modal Analysis (OMA) [1]. On the other hand, effective data acquisition and storage techniques must be employed to cope with the heterogeneity of the sensing devices and with the amount of data produced by collecting raw measured signals. Finally, damage detection and prediction tasks should be computed via data-driven algorithms that can complement the model-based alternatives traditionally used in civil engineering. Layered SHM architectures [2] represent straightforward approaches to address the system complexity originated by this interdisciplinary design; however, few real-world implementations have been presented so far in the literature. In this paper, we overcome these limitations by presenting an Internet of Things (IoT)-based SHM architecture for the predictive maintenance of industrial sites and civil engineering structures and infrastructures. The proposed cyber-physical system includes a monitoring layer, that consists of accelerometer-based sensor networks, a data acquisition layer, built on the recent W3C Web of Things standard [3], and a data storage and analytics layer, which leverages distributed database and Machine Learning tools. We extensively discuss the hardware/software components of the proposed SHM architecture, by stressing its advantages in terms of device versatility, data scalability and interoperability support. Finally, the effectiveness of the system is validated on a real-world use-case, i.e., the monitoring of a metallic frame structure located at the SHM research labs of the University of Bologna, Italy, within the MAC4PRO project [4].

Dissipative steel connections for cross-laminated timber buildings are often characterized by biaxial tension-shear interaction in strength and displacement capacity. Detailed continuum models with nonlinear finite elements provide faithful results in simulating the hysteretic response of complex structural systems but generally require too much computational effort. The use of nonlinear macro-elements can strongly increase the efficiency of the analysis with limited decrease of accuracy of the results. In this work, the hysteretic response of a dissipative connection for cross-laminated timber structures was simulated in the OpenSees framework with a macro-element model and compared with results from tests and a continuum finite-element model developed in the Ansys® framework. The accuracy of the macro-element in reproducing the tension-shear interaction has been evaluated in terms of displacement and force domains of the connection. A further comparison between the macro-element and the continuum finite-element modelling was provided analysing the seismic response of cross-laminated timber shear walls with different arrangement of the dissipative connections. Results show that the proposed macro-element can simulate the nonlinear biaxial behaviour of the dissipative bracket and predict the force-displacement response of CLT shear walls with slight conservative underestimation of hysteretic energy dissipation. The macro-element could be implemented in models of buildings for prediction of their dynamic response with a limited computational effort.

This article reports on the creation of an open database of piezo-actuated and piezo-received guided wave signals propagating in a composite panel of a full-scale aeronautical structure. The composite panel closes the bottom part of a wingbox that, along with the leading edge, the trailing edge, and the wingtip, forms an outer wing demonstrator approximately 4.5 m long and from 1.2 to 2.3 m wide. To create the database, a structural health monitoring system, composed of a software/hardware central unit capable of controlling a network of 160 piezoelectric transducers secondarily bonded on the composite panel, has been realized. The structural health monitoring system has been designed to (1) perform electromechanical impedance measurement at each transducer, in order to check for their reliability and bonding strength, and (2) to operate an active guided wave screening for damage detection in the composite panel. Electromechanical impedance and guided wave measurements were performed at four different testing stages: before loading, before fatigue, before impacts, and after impacts. The database, freely available at http://shm.ing.unibo.it/ , can thus be used to benchmarking, on real-scale structural data, guided wave algorithms for loading, fatigue, as well as damage detection, characterization, and sizing. As an example, in this work, a delay and sum algorithm is applied on the post-impact data to illustrate how the database can be exploited.

Ultrasonic wavefield imaging with a non-contact technology can provide detailed information about the health status of an inspected structure. However, high spatial resolution, often necessary for accurate damage quantification, typically demands a long scanning time. In this work, we investigate a novel methodology to acquire high-resolution wavefields with a reduced number of measurement points to minimize the acquisition time. Such methodology is based on the combination of compressive sensing and convolutional neural networks to recover high spatial frequency information from low-resolution images. A data set was built from 652 wavefield images acquired with a laser Doppler vibrometer describing guided ultrasonic wave propagation in eight different structures, with and without various simulated defects. Out of those 652 images, 326 cases without defect and 326 cases with defect were used as a training database for the convolutional neural network. In addition, 273 wavefield images were used as a testing database to validate the proposed methodology. For quantitative evaluation, two image quality metrics were calculated and compared to those achieved with different recovery methods or by training the convolutional neural network with non-wavefield images data set. The results demonstrate the capability of the technique for enhancing image resolution and quality, as well as similarity to the wavefield acquired on the full high-resolution grid of scan points, while reducing the number of measurement points down to 10% of the number of scan points for a full grid.

Recent Structural Health Monitoring (SHM) systems might take advantage of Internet of Things (IoT) technologies for fine-grained and autonomic sensors data management and processing. Moreover, current SHM deployments often demand for the installation of multi-type and heterogeneous sensor devices capable to perform long-term measurements; from here, the need for dedicated software platforms allowing for scalability and interoperability requirements arises. In this paper, we jointly address the two issues above by proposing MODRON, which is a SHM-dedicated IoT platform with sensor-to-cloud support. The software architecture leverages the W3C Web of Things (WoT) standard for multi-source sensors data acquisition and fusion. The platform includes an edge component, implementing the communication with the monitoring layer and the data exposition through WoT Web Things (WTs), and a cloud component, embedding sensor/WT management capabilities, which is in charge of distributed data storage, aggregation, visualization and analytics. We illustrate the abstract MODRON architecture and its current implementation that supports two different SHM sensor types (MEMS accelerometers and piezoelectric devices). In addition, we describe the system operations on a real-world SHM system, i.e. the monitoring of a metallic structure instrumented with multiple sensor networks.

The direction of arrival (DoA) estimation of Lamb waves is a fundamental task to locate acoustic events, such as those caused by impacts in plates or shells. To perform this task, a novel cluster of piezoelectric sensors is presented in this work. The designed cluster is composed by three irregularly shaped patch transducers (P1, P2 e P3). This is in contrast with the approaches that are typically presented in literature which are based on isotropic piezo-disks. In our approach, the transducers are shaped with a procedure based on the Radon Transform, so that the difference in time of arrival (DToA) of the Lamb waves at patches P1 and P2 is linearly related to the DoA, while P3 is designed so that it is possible to perform the estimation of DoA without knowing the actual wave velocity. The numerical validation shows that the performance in the DoA estimation achieved by means of the proposed cluster compares favorably with respect to clusters of conventional sensors, even in the case of noise-affected measurements.

Experts 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.

Composite materials are widely used in the industry, and the interest of this material is growing rapidly, due to its light weight, strength and various other desired mechanical properties. However, composite materials are prone to production defects and other defects originated during exploitation, which may jeopardize the safety of such a structure. Thus, non-destructive evaluation methods that are material-independent and suitable for a wide range of defects identification are needed. In this paper, a technique for damage characterization in composite plates is proposed. In the presented non-destructive testing method, guided waves are excited by a piezoelectric transducer, attached to tested specimens, and measured by a scanning laser Doppler vibrometer in a dense grid of points. By means of signal processing, irregularities in wavefield images caused by any material defects are extracted and used for damage characterization. The effectiveness of the proposed technique is validated on four different composite panels: Carbon fiber-reinforced polymer, glass fiber-reinforced polymer, composite reinforced by randomly-oriented short glass fibers and aluminum-honeycomb core sandwich composite. Obtained results confirm its versatility and efficacy in damage characterization in various types of composite plates.

Traditional connections in earthquake-resistant cross-laminated timber buildings are susceptible of brittle failures, even when buildings are designed and supposed to be ductile. This is mainly due to the large underestimation of the actual strength of the ductile components, with consequent increased strength demand for the brittle parts, which may fail if designed with insufficient overstrength. Recent studies demonstrate that the use of steel connections characterized by a well-defined mechanical behaviour can improve significantly ductility and dissipative capacity of cross-laminated timber structures and the reliability of the capacity design. In this paper, the conceptual model of capacity design is discussed, proposing some modifications to improve its reliability for traditional and high-ductility connections for CLT structures. Results from quasi-static cyclic-loading tests of an innovative ductile bracket are presented and the corresponding overstrength factors are computed using the proposed conceptual method and compared with values available in the literature for traditional connections. Finally, a comparative application of the capacity criteria to the design of the innovative bracket and of a traditional nailed connection is presented and discussed.

Guided waves in Structural Health Monitoring per-mit to detect different types of damage inside a structure, such as cracks, as well as external impacts. A precise localisation of the damage or the impact point is necessary in order to improve the reliability and accuracy of monitoring approaches. A localisation algorithm based on the estimation of the waves Direction of Arrival (DoA) by means of a DFT-Based Continuous Wavelet Transform decomposition is presented in this work. The approach exploits a multiresolution DFT-Based CWT decomposition applied to the signals acquired by cluster of three piezoelectric (PZT) transducers in 60 degrees configuration. The multiple frequency-based filtering of the signals in the wavelet domain counteracts the complexity caused by the dispersive characteristics of the material. Subsequently the cross-correlation method and the angle estimation are applied for each computed scale. Finally, the Direction of Arrival is computed by an averaging procedure across scales. The approach is validated by means of an experimental setup. The results show the accuracy of the proposed algorithm, enabling to estimate the DoA with an average error of 1.15 degrees and a maximum error of 1.74 degrees.

In this work, a novel baseline-free procedure aimed at detecting and sizing delaminations in composite plates is proposed. To such a purpose an air-coupled probe is used to measure the sound pressure in air that is generated by leaky guided waves traveling along the plate and scattered by defects. In particular, by scanning the plate surface with a proper probe tilt angle and lift-off, wavefields dominated by a single guided mode can be obtained. Then, a processing procedure based on warped time–frequency operators is used to filter out the main wavefield generated by the acoustic source in order to enhance the one scattered by defects. Such operators are designed basing on the dispersion curves of the dominant guided mode considered. Finally, it is shown how the scattered wavefield can be further processed to infer the defect position and size by means of the Circular Hough Transform. The presence of a barely visible delamination on a 4.9 mm thick composite laminate was detected by exploiting the proposed approach. The obtained results are in good agreement with those obtained by using a commercial phased-array ultrasonic C-scan device.

The seismic response of un-reinforced masonry buildings due to in-plane stiffening of their single-layer timber floors is numerically investigated by means of non-linear incremental dynamic analyses. Two independent case studies of two-storey buildings with timber floors stiffened with alternative techniques (addition of 45° oriented timber boards, of steel plates or of a 50-mm thick reinforced-concrete slab) have been analysed using three-dimensional numerical models. The non-linear elements of the models have been calibrated on experimental data available in literature to reproduce the actual hysteretic behaviour of both masonry and floors. Results of the non-linear incremental dynamic analyses are presented for different Peak Ground Acceleration values, to quantify the different seismic effectiveness of the building achieved with each stiffening method. Main findings show that, for the considered geometry and properties of the case studies, even not infinitely stiff floors are able to confer a good seismic behaviour to un-reinforced masonry buildings. On the contrary, their seismic capacity may decrease if a retrofitting method leading to excessive floor stiffening is adopted.

Timber-concrete composite (TCC) structures have become a wide-spread technique to realize new composite floors or improve existing ones. The key-point to guarantee adequate mechanical performance to these composite structures is the use of connectors characterized by adequate strength and stiffness between timber beam and concrete slab. Modern cylindrical connectors, such as self-tapping screws, are rising interest because they combine high performance mechanical properties and quickness of execution. In this paper, a simplified theoretical approach to calculate shear strength and stiffness of TCC joints made with inclined screws is suggested 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. 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.

Timber platform-frame shear walls are characterized by high ductility and diffuse energy dissipation but limited in-plane shear resistance. A novel lightweight constructive system composed of steel columns braced with oriented strand board (OSB) panels was conceived and tested. Preliminary laboratory tests were performed to study the OSB-to-column connections with self-drilling screws. Then, the seismic response of a shear wall was determined performing a quasi-static cyclic-loading test of a full-scale specimen. Results presented in this work in terms of force-displacement capacity show that this system confers to shear walls high in-plane strength and stiffness with good ductility and dissipative capacity. Therefore, the incorporation of steel columns within OSB bracing panels results in a strong and stiff platform-frame system with high potential for low- and medium-rise buildings in seismic-prone areas.

This paper reports the structural characterization of an innovative engineered aluminium beam, used as bottom rail for timber shear walls, capable of overcoming the crucial durability problemthat could affect the timber shear walls near the foundations, at the same time improving the precision and speed of installation. Traditional techniques have disadvantages such as excessive geometric inaccuracies, difficulty in achieving efficient and long-lasting solutions, compression orthogonal to the wood grain, reduced durability due to increasing dampness and problems with waterproofing. Results from laboratory tests conducted to verify the mechanical behaviour of this innovative bottom rail when subjected to tension and compression perpendicular to its axis are reported in comparison with numerical provisions. The ultimate strength of the aluminium-to-timber connections, specifically developed to be compatible with the beam and easily installable, was also obtained. The results demonstrate that this innovative technology ensures the achievement of the expected static performances. Installation and durability advantages are highlighted and the first utilization of the aluminiumbeam is illustrated.

This paper deals with the conception and characterization of an innovative connection for cross-laminated timber (CLT) panels. The connection is designed to provide an adequate level of dissipative capacity to CLT structures also when realized with large horizontal panels and therefore prone to fragile shear sliding failure. The connector, named X-bracket, has been theorized and designed by means of numerical parametric analyses. Furthermore, its cyclic behavior has been verified with experimental tests and compared to that of traditional connectors. Numerical simulations of cyclic tests of different CLT walls anchored to the foundation with X-brackets were also performed to assess their improved seismic performances. Finally, the analysis of the response of a 6 m × 3 m squat wall demonstrates that the developed connection provides good ductility and dissipation capacities also to shear walls realized with a single CLT panel.

This paper presents an innovative steel connection specifically developed to provide increased dissipative and ductility capacities to CLT buildings. The proposed element assures high ductility, negligible pinching behaviour and well defined response to combined shear and tension loads. The fixing of CLT panels by means of these elements makes the applicability of the capacity design principles more reliable. The novel joint was theorized and optimized via numerical modelling. Then, its behaviour was validated through experimental tests. Results in terms of strength, stiffness and ductility are illustrated and discussed, in comparison with commonly used connection elements. When used to connect CLT panels to foundations, to floor diaphragms or reciprocally, the proposed connection allows to reach high dissipative and ductility capacity even when large CLT shear walls without vertical joints are adopted. This has been proved via non-linear numerical simulation of a cyclic-loading test of a shear wall.