Innovative technologies for the vibration control of equipment in critical facilities

The present thesis settles in the framework of the dynamic modelling, seismic mitigation and permanent health monitoring of equipment in critical facilities. The main objective is to assess the feasibility and the effectiveness of innovative mitigation Technologies including: the passive isolation of a single equipment (equipment isolation systems); the passive isolation of a raised floor, on which a group of several equipment is anchored (isolated raised-floor systems); a non-conventional Tuned Mass Damper (TMD), realized by converting a large mass already present on the structure, e.g. the mass of an equipment, into a tuned mass, in order to reduce the structural response. The area of research is believed to have the potential for a wide application, since a still limited number of authors has addressed the theoretical and technical issues associated with the seismic design and protection of equipment. As a result, international seismic codes and standards also appear to be inadequate in this regard. Furthermore, the proposed technologies give the opportunity of addressing topics of significant scientific interest: the analysis of the dynamic interaction between an equipment and its supporting structure, generally neglected in current Literature and seismic codes; the definition of reduced-order generalized models, introduced for design purposes; the constitutive modelling of nonlinear hysteretic isolation systems; the dynamic analysis of non-proportionally damped systems and nonlinear hysteretic systems; the stochastic analysis of the dynamic response, given the probabilistic nature of the earthquake excitation; the proper definition of energy indices to assess, in a synthetic and effective way, the performance of the proposed mitigation technologies. A distinctive aspect of the work is eventually given by the use of shaking table tests on reduced scale models, for the sake of dynamic identification and seismic analysis. In the end, the thesis aims at defining criteria and methodologies to harmonize the seismic design of equipment with the one of the structure, in a joint and coordinated effort to achieve the operational performance objectives prescribed to critical facilities by international seismic codes

Innovative technologies for the vibration control of equipment in critical facilities

The present thesis settles in the framework of the dynamic modelling, seismic mitigation and permanent health monitoring of equipment in critical facilities. The main objective is to assess the feasibility and the effectiveness of innovative mitigation Technologies including: the passive isolation of a single equipment (equipment isolation systems); the passive isolation of a raised floor, on which a group of several equipment is anchored (isolated raised-floor systems); a non-conventional Tuned Mass Damper (TMD), realized by converting a large mass already present on the structure, e.g. the mass of an equipment, into a tuned mass, in order to reduce the structural response. The area of research is believed to have the potential for a wide application, since a still limited number of authors has addressed the theoretical and technical issues associated with the seismic design and protection of equipment. As a result, international seismic codes and standards also appear to be inadequate in this regard. Furthermore, the proposed technologies give the opportunity of addressing topics of significant scientific interest: the analysis of the dynamic interaction between an equipment and its supporting structure, generally neglected in current Literature and seismic codes; the definition of reduced-order generalized models, introduced for design purposes; the constitutive modelling of nonlinear hysteretic isolation systems; the dynamic analysis of non-proportionally damped systems and nonlinear hysteretic systems; the stochastic analysis of the dynamic response, given the probabilistic nature of the earthquake excitation; the proper definition of energy indices to assess, in a synthetic and effective way, the performance of the proposed mitigation technologies. A distinctive aspect of the work is eventually given by the use of shaking table tests on reduced scale models, for the sake of dynamic identification and seismic analysis. In the end, the thesis aims at defining criteria and methodologies to harmonize the seismic design of equipment with the one of the structure, in a joint and coordinated effort to achieve the operational performance objectives prescribed to critical facilities by international seismic codes

feasibility and effectiveness of exoskeleton structures for seismic protection

Abstract In this study, a self-supporting structure, namely an exoskeleton, is considered as set outside a main structure and suitably connected to it. From the structural point of view, the exoskeleton is conceived as a "sacrificial" appendage, called to absorb seismic loads in order to increase the performance of the main structure. From the architectural and technological point of view, additional functions may be associated through an integrated design approach, combining seismic with urban and energy retrofitting. Particular and attractive applications can therefore be envisaged for existing buildings. A reduced-order dynamic model is introduced, in which two coupled linear viscoelastic oscillators represent the main structure and the exoskeleton structure, respectively, while either a rigid link or a dissipative viscoelastic connection is considered for the coupling. The equations of motion are set in non-dimensional form and a parametric study is carried out in the frequency domain to confirm that exoskeleton structures can be feasible and effective in reducing earthquake-induced dynamic responses

Nondimensional Shape Optimization of Nonprismatic Beams with Sinusoidal Lateral Profile

The present paper deals with the optimal design of nonprismatic beams, i.e., beams with variable cross section. To set the optimization problem, Euler-Bernoulli unshearable beam theory is considered and the elastica equation expressing the transverse displacement as a function of the applied loads is reformulated into a system of four differential equations involving kinematic components and internal forces. The optimal solution (in terms of volume) must satisfy two constraints: the maximum Von Mises equivalent stress must not exceed an (ideal) strength, and the maximum vertical displacement is limited to a fraction of beam length. To evaluate the maximum equivalent stress in the beam, normal and shear stresses have been considered. The former was evaluated through the Navier formula, and the latter through a formula derived from Jourawsky and holding for straight and untwisted beams with bisymmetric variable cross sections. The optimal solutions as function of material unit weight, maximum strength, and applied load are presented and discussed. It is shown that the binding constraint is usually represented by the maximum stress in the beam, and that applied load and strength affect the solution more than material unit weight. To maintain the generality of the solution, the nondimensionalization according to Buckingham pi-theorem is implemented and a design abacus is proposed

Stochastic Multi-objective Optimisation of Exoskeleton Structures

In this study, a structural optimisation problem, addressed through a stochastic multi-objective approach, is formulated and solved. The problem deals with the optimal design of exoskeleton structures, conceived as vibration control systems under seismic loading. The exoskeleton structure is assumed to be coupled to an existing primary inner structure for seismic retrofit: the aim is to limit the dynamic response of the primary structure to prevent structural damage. A non-stationary filtered Gaussian white noise stochastic process is taken as the seismic input. Design variables pertain to the mechanical properties (stiffness, damping) of the exoskeleton structure. Two concurrent and competing objective functions are introduced, in order to take into account not only safety performance but also economic cost considerations. The resulting trade-off is solved searching the Pareto front by way of a controlled elitist genetic algorithm, derived from the Non-dominated Sorting Genetic Algorithm-II. Sensitivities of Pareto fronts and Pareto optimal sets to different system parameters are finally investigated by way of a numerical application

Cyclic uniaxial testing and constitutive modelling of cementitious composite materials

Innovative cementitious composite materials are drawing considerable interest due to their substantially improved mechanical properties, as compared to ordinary cement-based materials: among the others, higher tensile strength, tensile strain hardening, flexural strength, fracture toughness [1] and resistance to fatigue. Their enhanced ductility appears to be promising and particularly suited to structural applications under severe dynamic loading conditions (earthquake, impact, blast) [2]. Accurate constitutive models to simulate the dynamic behaviour of cementitious composites are hence needed, as well as corresponding appropriate testing protocols for their experimental characterisation [3]. In this study, the response of cementitious composites to cyclic uniaxial loadings has been investigated. Cyclic response is essential to understand the effects of unloading and reloading on the material, to examine how it behaves in the transition from tension to compression and to characterise its properties in terms of energy dissipation and strain-rate sensitivity. Different loading schemes have been considered, including reversed cyclic tension/compression loadings, in order to identify the complete stress-strain curve and the transition behaviour, which can occur, for instance, under seismic, fatigue and wind loads. Monotonic quasi-static tension and compression tests have been also performed, to provide a benchmark for the evaluation of the envelope curve of cyclic response. The experimental campaign was carried out on cylindrical specimens, a standard geometry in compression testing of cement-based materials. Several series of homothetic specimens (height to diameter ratio fixed as 2) with different dimensions were tested, to evaluate the influence of scale effects. Variability and reproducibility of the testing results have been taking into account by employing a minimum number of three specimens per loading condition. All the tests were performed, under deformation-controlled regime, on an MTS servo-hydraulic testing machine with 250 kN load cell. The testing machine was customised with accessories designed to meet specific test requirements, avoiding instability and bending moments during the alternating phases of uniaxial compression and tension. Linear variable displacement transducers (LVDT) and strain gauges were used to measure vertical displacements and lateral deformations, respectively. The results obtained experimentally represent a reliable basis for the development of constitutive models suited to numerical simulation. References [1] Restuccia, L., Reggio, A., Ferro, G.A., Kamranirad, R., “Fractal analysis of crack paths into innovative carbon-based cementitious composites”, Theoretical and Applied Fracture Mechanics, 90, 133-141, 2017. [2] Yoo, D. Y., Banthia N. “Mechanical properties of ultra-high-performance fiber-reinforced concrete: A review”, Cement and Concrete Composites, 73, 267-280, 2016. [3] Kesner, K.E., Billington, S.L., Douglas K.S. “Cyclic response of highly ductile fiber-reinforced cement-based composites”, ACI Materials Journal, 100(5), 381-390, 2003

new self healing techniques for cement based materials

Abstract: In recent years, researches concerning cement-based materials has been focused not only on the strength and the toughness but also on the durability. In fact, the interest on concrete's self-healing process is increasing, due to the rapidly deterioration of that material which tends to crack and thus quickly deteriorate. In this paper, a new self-healing technology for cement-based materials is proposed. This technology is based on the encapsulation method of repairing agent inserted in randomly distributed shell inside the material during its preparation. Two different kind of shells were used: glass spheres and pharmaceutical capsules. The material the shells are made of has to be endowed with a series of fundamental characteristics. That material has to be inert with respect to the repair agent so that it doesn't react with it, resisting to the severe stress condition that the shells undergo during the mixing, and at the same time being capable of breaking down when the crack intercept them, having a good compatibility with the cement mixture. The results demonstrate that it is possible to use this kind of shell to encapsulate the repairing agent: the crack breaks them and they release the healing agent, which allows patching up the crack

Integrated, sustainable, low-impact retrofitting through exoskeleton structures : a case study

Improvement of safety and eco-efficiency of existing buildings is an interdisciplinary problem: this is an established, although recent, research and policy acquisition. The age profile analysis of the EU's building heritage reveals that the main part of this 27 billion m^2 stock was built between 1961 and 1990, and a significant percentage before 1960. Poor thermal and environmental performances, as well as the failure to comply with modern seismic design codes, are common problems that require an integrated solution approach (Caverzan et al. 2016). In this context, exoskeleton structures appear to be a promising retrofitting strategy due to a number of reasons: the potential for a multifunctional design combining structural safety, energy efficiency and environmental sustainability; limited interference with existing structural and nonstructural components; minimal service or business downtime during the retrofitting process (Reggio et al. 2018). In this paper, a case study is presented, dealing with the integrated, seismic and energy, retrofitting of a mid-rise building, located in the city of Torino (Italy). The existing structure, a non-ductile reinforced concrete frame, is coupled via a rigid connection to an exoskeleton structure, realised as a steel braced frame. The exoskeleton structure is set adjacent to the existing structure and designed in order to reduce the seismic response of the latter, in terms of displacements and internal forces. In the perspective of an integrated design approach, the exoskeleton structure is further used to support external thermal insulation panels, aimed at the energy upgrading of the building envelope. Possible interference and synergies between seismic and energy retrofitting requirements are highlighted and discussed