Alessandro Palmeri

Monitoring Dynamic Structural Tests Using Image Deblurring Techniques

Photogrammetric techniques have demonstrated their suitability for monitoring static structural tests. Advantages include scalability, reduced cost, and three dimensional monitoring of very high numbers of points without direct contact with the test element. Commercial measuring instruments now exist which use this approach. Dynamic testing is becoming a convenient approach for long-term structural health monitoring. If image based methods could be applied to the dynamic case, then the above advantages could prove beneficial. Past work has been successful where the vibration has either large amplitude or low frequency, as even specialist imaging sensors are limited by an inherent compromise between image resolution and imaging frequency. Judgement in sensor selection is therefore critical. Monitoring of structures in real-time is possible only at a reduced resolution, and although imaging and computer processing hardware continuously improves, so the accuracy demands of researchers and engineers increase. A new approach to measuring vibration is introduced here, whereby a long-exposure photograph is used to capture a blurred image of the vibrating structure. The high resolution blurred image showing the whole vibration interval is measured with no need for high-speed imaging. Results are presented for a series of small-scale laboratory models, as well as a larger scale test, which demonstrate the flexibility of the proposed technique. Different image processing strategies are presented and compared, as well as the effects of exposure, aperture and sensitivity selection. Image processing time appears much faster, increasing suitability for real-time monitoring.

Identification of Passive Devices for Vibration Control by Evolutionary Algorithms

Passive devices for vibration control (i.e., dampers and isolators) are widely adopted in many areas of engineering. For instance, such devices now provide reliable and affordable solutions in the seismic protection of industrial machines, technical equipment, buildings, and bridges. Their main advantages are simplicity and limited costs if compared to other strategies of vibration control, which also explains why the use of passive devices is becoming so popular in many civil engineering applications. The design of earthquake-resistant structures requires the assessment of the protection system’s performance by assuming specific mathematical laws for the adopted devices. These, in turn, depend on the mechanical parameters that have to be tuned properly. As a consequence, a reliable parametric identification of passive devices for vibration control is a critical point in the design process. In this chapter, a procedure is described for the dynamic identification of passive control devices through laboratory tests and evolutionary algorithms. The methodology consists of first, using standardized experimental tests, where a predefined loading condition is imposed by an external actuator; second, using soft computing numerical techniques to identify the mechanical parameters of the candidate mathematical law by minimizing the difference between the time histories of the experimental and analytical dynamic loads. The proposed procedure can be applied to a wide range of models because of its inherent stability and low computational cost and allows comparing different mechanical laws by ranking their agreement with experimental data. Final results demonstrate the effectiveness of the proposed strategy.

Wear in large diameter grouted connections for offshore wind energy converters

The paper provides a critical appraisal of the events surrounding the unexpected settlement of large-diameter monopile foundations for offshore wind energy converters, and will detail experimentation that is to be undertaken to determine the extent of the wear failure mode in such structures. The potential for wear deterioration at the grout-steel interface has been caused by the lack of axial load transfer of the grout and steel and so relative movement between the grout and steel is occurring. Hence, a better understanding of this phenomenon is of key importance to determine the long term integrity of the structure. The remedial solution currently favoured within industry consists of the installation of elastomeric bearings, which however may further exasperate the wear due to its lower stiffness than steel contact currently occurring, also because of the presence of water in the grouted connection. The aim of the testing is therefore to identify the risk to offshore foundation integrity due to grout wear under conditions typically observed during life-time operation of offshore wind turbines. The main factors influencing the wear rate of grout will be identified, and the possibility to define indicators for the integrity of the grout that could be used for offshore inspection will be also considered.

A State-Space Viscoelastic Model of Double-Beam Systems toward the Dynamic Analysis of Wind Turbine Blades

A novel state-space formulation is presented for the dynamic analysis of rotating doublebeam systems, consisting of two elastic beams continuously joined by viscoelastic springs. Although very simple, the proposed model lends itself to be used in studying the transverse vibration of wind turbine blades provided with a viscoelastic core. An efficient numerical scheme of solution is proposed, which can be further extended to include the effects of shear strain within the core, along with geometrical and material nonlinearities. INTRODUCTION A very effective strategy for mitigating dynamic effects on wind turbine blades is the inclusion of a viscoelastic layer between the outer plies (Manwell et al., 2002; Chaviaropoulos et al., 2006; Hansen et al., 2006). This sandwich configuration is usually analyzed relying on somehow equivalent values of elastic stiffness and viscous damping for the viscoelastic components (Johnson and Kienholz, 1982; Rao and He, 1993). Although straightforward, this approach may overestimate the actual dissipation provided by the viscoelastic materials, as demonstrated by recent investigations on the vibration of viscoelastically damped structures (Palmeri et al., 2003; Palmeri and Ricciardelli, 2006). These studies have also pointed out that enlarged state-space models allow overcoming the traditional concepts of equivalent stiffness and damping, and lend themselves to be implemented in computationally efficient time-domain numerical schemes. The latter feature can be very useful in the fatigue analysis of rotor blades, when cycle counting techniques require time histories of internal forces at critical points for a number of operational conditions (Fuglsang and Madsen, 1999; Ragan and Manuel, 2007; Hansen, 2008). Within this framework, a novel state-space formulation is presented for studying the transverse vibration of double-beam systems, made of two outer elastic beams continuously joined by an inner viscoelastic layer. As apposite to other methods available in the literature (Vu et al., 2000; Oniszczuk, 2000 and 2003), the proposed technique enables one to considerer beams with varying cross section and rate-dependent constitutive law for the inner layer, as typical in the case of rotor blades. Although the double-beam representation is quite crude for actual systems, the proposed state-space approach proves to be accurate and versatile, and is well suited to be extended to more sophisticated models for wind turbine applications. 1973 Earth and Space 2010: Engineering, Science, Construction, and Operations in Challenging Environments

Shaking Table Tests Validating Two Strengthening Interventions on Masonry Buildings

Masonry buildings constitute quite often a precious cultural heritage for our cities. In order to future generations can enjoy this heritage, thence, effective projects of protection should be developed against all the anthropical and natural actions which may irreparably damage old masonry buildings. However, the strengthening interventions on these constructions have to respect their authenticity, without altering the original conception, not only functionally and aesthetically of course, but also statically. These issues are of central interests in the Messina area, where the seismic protection of new and existing constructions is a primary demand. It is well known, in fact, that the city of Messina lies in a highly seismic zone, and has been subjected to two destructive earthquakes in slightly more than one century, the 1783 Calabria earthquake and the more famous 1908 Messina‐Reggio Calabria earthquake. It follows that the retrofitting projects on buildings which survived these two events should be d...

Performance-based seismic analysis of light SDoF secondary substructures

A novel procedure is presented for the application of the PBE (performance-based engineering) methodology to the seismic analysis and design of light secondary substructures. In the proposed technique, uncertainty is conveniently represented in the reduced modal subspace rather than geometric domain, which significantly reduces the number of uncertain parameters. The random response of a primary structure under earthquake excitation is investigated, various cases of linear and nonlinear secondary subsystems are examined and the propagation of uncertainty from the dynamic properties of the primary structure to the seismic performance of the secondary subsystems is quantified.

A NOVEL ANALYTICAL MODEL OF POWER SPECTRAL DENSITY FUNCTION COHERENT WITH EARTHQUAKE RESPONSE SPECTRA

In the most advanced seismic codes earthquake loads are often defined by means of pseudo-acceleration Response Spectra (RS) and the use of modal superposition analysis method is strongly encouraged. The effectiveness of the design procedures is thus limited by the underlying hypotheses, such as the linearity of the system and the reliability of the modal correlation coefficients used to combine the modal responses for MDOF systems. On the other hand, linear systems response statistics could be easily computed by using stochastic analysis tools, once a stochastic characterization of the seismic action is provided. In this paper a few-parameters analytical model for the definition of Power Spectral Density functions (PSD) coherent with Response Spectra is proposed. Closed-form relationships between the parameters involved in the definition of the PSD and the RS defined by several international seismic codes are provided. The reliability of this tool is assessed by means of a numerical campaign by comparing stochastic analysis and Monte-Carlo simulations. By using the proposed approach, the seismic action can be defined both in terms of RS and in terms of PSD, and, therefore, the engineer can choose the most appropriate analysis tool for his purpose.

Lateral Stability of Prestressed Precast Concrete Girders During Lifting: Study Case

Improvements in concrete technology, reinforcing systems and manufacturing processes enable the use of increasingly long reinforced precast concrete girders, contributing to the competitiveness of girders in concrete in comparison with other alternatives. The weight of the girders should be limited however, in order to achieve an optimum between span length and lifting and transportations costs. The current tendency in design is to minimize the width of the flanges, thus the girder becoming more flexible laterally and more prone to suffer instability phenomena during transient loading situations. An increasing number of accidents and damages associated with this instability problem are reported in the technical literature (e.g., Hurff 2010; Rose 2013). The main objective of this study is to describe a real case of lateral instability of a long prestressed concrete bridge girder during lifting as well asto perform a parametric study to understand the limits of the problem observed. Special attention is paid to the evaluation of the provisions gathered in the Model Code 2010 (MC - 2010) regarding the lateral stability, since these might not be sufficient to cover limit cases.

Stochastic analysis of fractional oscillators by equivalent system definition

Fractional oscillators have been recently proposed as damping devices under the configuration of Fractional Tuned Mass Dampers (FTMD), realized by connecting an oscil- lating mass to the primary structure through a viscoelastic link with inherent fractional con- stitutive law. The characteristic tuning frequency for the FTMD has been identified with the Damped Fractional Frequency (DFF), defined as the frequency at which the squared abso- lute value of the transfer function of the device attains its relative maximum. The definition of the DFF constitutes an interesting step towards the analysis of fractional oscillators in the frequency domain. In this paper, a simplified frequency domain approach is presented for the design of fractional oscillators subjected to stationary white noise. The analysis of the frac- tional oscillator is performed by using an equivalent single degree of freedom system with linear viscous damping. The aim is to obtain a clear understanding of the physical dynamic effects of the variations in the fractional oscillator parameters, in terms of damping and natu- ral frequencies. Moreover, the use of an equivalent system allows for the straightforward ap- plications of stochastic analysis to determine an approximate closed-form expression of the response variance.

A Numerical Method for the Dynamic Analysis of Buildings Provided with Viscoelastic Devices

Improved methods of analysis and design, along with refinements in the device hardware, make the use of viscoelastic devices completely suitable for consideration in both new and retrofitted buildings in earthquake- and wind- prone areas. In this framework, a novel numerical scheme is presented for the time-domain dynamic analysis of MDoF linear structures provided with linear viscoelastic dampers of know relaxation function. The computational burden is dramatically reduced by using a suitable modal transformation of coordinates, coupled with the Laguerre Polynomial Approximation (LPA) technique. A representative numerical example is included for the validation purposes.