Daniele Zonta

Low Power Wireless Sensor Network for Building Monitoring

A wireless sensor network is proposed for monitoring buildings to assess earthquake damage. The sensor nodes use custom-developed capacitive microelectromechanical systems strain and 3-D acceleration sensors and a low power readout application-specified integrated circuit for a battery life of up to 12 years. The strain sensors are mounted at the base of the building to measure the settlement and plastic hinge activation of the building after an earthquake. They measure periodically or on-demand from the base station. The accelerometers are mounted at every floor of the building to measure the seismic response of the building during an earthquake. They record during an earthquake event using a combination of the local acceleration data and remote triggering from the base station based on the acceleration data from multiple sensors across the building. A low power network architecture was implemented over an 802.15.4 MAC in the 900-MHz band. A custom patch antenna was designed in this frequency band to obtain robust links in real-world conditions. The modules have been validated in a full-scale laboratory setup with simulated earthquakes.

Structural health monitoring of wind towers: remote damage detection using strain sensors

Exploiting wind energy in complex sites like mountain terrains implies the necessity for remote structural health monitoring of the wind towers. In fact, such slender vertical structures exposed to wind may experience large vibrations and repeated stress cycles leading to fatigue cracking. Possible strategies for remote fatigue damage detection are investigated. Specifically, this paper is focused on the use of suitable strain sensors for crack detection in critical sites of the structure, suggesting several strategies taking into account the possibility of wind direction changes and/or wind calm phases. They are based on a radial arrangement of strain sensors around the tower periphery in the vicinity of the base weld joint. The most promising strategy uses the strain difference between adjacent strain sensors as an index of the presence of a crack. The number of sensors to be installed is dictated by the minimum crack size to be detected, which in turn depends on the expected extreme wind conditions and programmed inspection/repair schedule for the structure.

A reliability-based bridge management concept

Since 2004, the Autonomous Province of Trento, Italy, has adopted a Bridge Management System entirely based on reliability concepts. The system operates on the web, and includes sections for (1) condition state evaluation, (2) safety assessment, and (3) prioritization. Condition appraisal is based on visual inspections, and acknowledges the general rules of the AASHTO Commonly Recognized Standard Element system. Normally, the system conservatively estimates the prior reliability of each bridge, based on the sole inspection data. Where the condition of the bridge gives cause for concern, its reliability is evaluated in a more formal manner using multi-step procedures of increasing refinement. Decision-making is driven by a principle whereby priority is given to those actions that, within a certain budget, will minimize the risk of occurrence of an unacceptable event in the whole network. In this paper, the operation of the system is illustrated, with the support of a number of practical cases.

Photonic crystals for monitoring fatigue phenomena in steel structures

This paper introduces the concept and development of a strain sensing system for structural application based on the properties of photonic crystals. Photonic crystals are artificially created periodic structures, usually produced in the thinfilm form, where optical properties are tailored by a periodicity in the refractive index. The idea of using the crystal as a sensor is based on the observation that a distortion in the crystal structure produces a change in the reflected bandwidth. When a photonic crystal is designed to operate in the visible part of the spectrum, a permanent distortion of the film results in a change in its apparent color. This property makes photonic crystals suitable for permanent monitoring of structural elements, as any critical changes in the strain field can be promptly and easily detected by visual inspection. A simple and low-cost example of photonic crystals consists of opals synthesized by vertical deposition. In this contribution we introduce a target application for the fatigue monitoring of wind turbines, and then provide the reader with some basic information concerning modeling of the crystal architecture and fabrication of these structures. Next we discuss their application to strain measurement, specifying how reflection and transmission properties of the opals have to be designed to satisfy the expected strain response of the sensor. Finally, we present the preliminary results of a laboratory validation carried out on thin films applied to a rubber support.

MANAGING THE HISTORICAL HERITAGE USING DISTRIBUTED TECHNOLOGIES

In this article we outline a research effort aiming to develop technological tools for real-time risk management of historic buildings. In detail, the project includes 1) the development of a prototype Historical Heritage Management System (HHMS), with 2) the technological and conceptual tools for integrating this HHMS with real-time monitoring, and 3) the demonstrative application of a pilot monitoring system, working within this framework, to a case study, the Aquila Gate in Trento. Motes network sensors are the basis of the monitoring system. We show that the risk-updating methodology proposed is able to deal with all the uncertainties involved (measurement noise, model uncertainty, inaccurate prior information) and to early-warn the manager of any possible future failure condition.

A fuzzy expert system for automatic seismic signal classification

A fuzzy rule based expert system for seismic signal classification is proposed.Relevant discriminant features are extracted from seismic signal.The system exploits the information derived from both experts knowledge and data.Adding weights to fuzzy rules generally improves the classification results.Vote by multiple rule fuzzy reasoning method shows the best performance. Automatic classification of seismic events is of great importance due to the large amount of data received continuously. Seismic analysts classify events by visual inspection and calculation of event signal characteristics. This process is subjective and demands hard work as well as a significant amount of time and considerable experience. A reliable automatic classification task considerably reduces the effort required and makes classification faster and more objective. The aim of this study is to develop a fuzzy rule based expert classification system that is able to imitate human reasoning and incorporate the analysts knowledge of seismic event classification. The fundamental idea behind using this approach was motivated by the way in which human analysts classify seismic events based on a set of experiential rules. Additionally, this approach was chosen due to its interpretability and adjustability, as well as its ability to manage the complexity of real data. Relevant discriminant features are extracted from event signal. Using these features, the classification system was built based on the vote by multiple rule fuzzy reasoning method with three types of rules. Comparison of this method with the single winner classical fuzzy reasoning model was carried out. Classification results on real seismic data showed the robustness of the classifier and its capability to operate in on-line classification.

Webshaker: Live Internet Shake-Table Experiment for Education and Research

Internet technologies are employed to allow real‐time video monitoring, control, and execution of bench‐top shake‐table experiments. Structural models of relevance to education in dynamics and earthquake engineering can be tested remotely over the internet on a 24‐h, 7‐day (24/7) basis. The underlying elements of experiment tele‐observation and tele‐operation/control are incorporated in a newly developed website http://webshaker.ucsd. edu.

In-field testing of a steel wind turbine tower

The strong drive to exploit wind energy has recently led to consideration of new types of location for wind turbines, including mountain regions. A major concern for wind farm installation in these sites is the long-term reliability of the support structures. In a flexible steel tower, a combination of vortex shedding and gusting caused fatigue cracks at the base joint. To identify the conditions critical to the development of this phenomenon, a twoblade down-wind turbine was thoroughly investigated in an in-field experimental campaign. This turbine features a 13m diameter rotor of rated power 11kW, mounted on an 18m tubular steel tower. In operation, the blades rotate at a fixed rate of 2 Hz. The tower, instrumented with 15 accelerometers, was first dynamically characterized in the absence of significant wind. Next, its spectral response to wind excitation was identified both in operation and with the rotor at rest. The outcome of the experiment suggests that the vulnerability to fatigue of this model of turbine is very sensitive to its modal behaviour, this in turn depending on the mechanical admittance of the foundations.

A Strain-Flexibility-Based Approach to Damage Location

The effectiveness of mode-shape-ba sed damage location methods is investigated, with specific regards to their application to the assessment of civil structures. Different location approaches proposed in the literature are compared and theoretically analysed. It is observed that, when damage is modelled as a loss in stiffness, the most direct way of exploiting mode-shapes information consists of estimating the flexibility matrix in a strain coordinate system. This matrix, referred to as strain-flexibility matrix, can be easily computed from the natural frequencies and the mode shapes expressed in terms of strain. Each diagonal element of the strain-flexibility matrix represents a local flexibility, thus changes in th ese quantities can be directly utilised as damage indexes. These findings have been validated through numerical and experimental examples. More specifically, the newly proposed damage indexes have been employed in the structural assessment of a 42.0 m long single-span steel bridge. The strain-flexibility indexes have been calculated comparing the experimental dynamic response, measured at 30 different locations, with that predicted through a simple FE model, representing the structure in the undamaged situation. Outcomes are consistent with the visual evidence of damage and with the outcomes of a static load test.

Direct Displacement-Based Design of Glulam Timber Frame Buildings

We introduce a direct Displacement-Based Design methodology for glued laminated timber portal frames with moment-resisting doweled joints. We propose practical expressions to estimate ultimate target displacement and equivalent viscous damping, and we demonstrate that these expressions provide prior values that are close to those obtained a posteriori using a more refined model. Applied to case studies, the method yields base-shear forces lower than those obtained using the force-based approach of Eurocode 8. This is due to the high dissipation capacity of the specific connection technology, which apparently is conservatively accounted for in the q-factor of Eurocode 8.