Progettazione Strutturale ed Architettonica di una Passerella Pedonale in Alluminio e Vetro sull'Arno a Pisa

La presente Tesi di Laurea raccoglie tutte quelle cosiderazioni, confronti, disegni, calcoli, verifiche e discussioni sviluppate in merito alla progettazione Strutturale ed Architettonica di una Passerella Pedonale in Alluminio e Vetro sul Fiume Arno a Pisa. Il lavoro ha avuto come principali obbiettivi la ricerca sia del minimo impatto architettonico che del soddisfaciento di requisiti dei sicurezza e confort strutturale. La fase di form-finding architettonica è stata svolta ricercando una soluzione che riduca il più possibile l'impatto urbano, architettonico ed idraulico della passerella nei confronti del delicato contesto circostante. Dopo lo studio di alcune soluzioni preliminari si è giunti ad una forma finale caratterizzata da un'apprezzabile leggerezza e trasparenza, salvaguardando la visuale dei passanti verso i limitrofi "Lungarni" e verso l'antistante "Chiesa della Spina". Il comportamento strutturale dell'intera costruzione è stato studiato, sia localmente che globalmente, mediante analisi non-lineari di modelli numerici agli elementi finiti dotati di appropriate imperfezioni tecnologiche iniziali. Le simulazioni sono state eseguite sia in campo statico (mediante analisi incrementaleinon-lineari quasi-statiche) che in campo dinamico (mediante analisi incrementali time-history) rispettivamente con lo scopo di determinare il grado di sicurezza nei confronti dell'instabilità globale e di monitorare le vibrazioni indotte dal traffico pedonale. Le azioni trasmesse al sistema fondazione-terreno, la dipendenza della struttura dai cedimenti del terreno e le conseguenti cadute di spinta risultano sensibilmente ridotte grazie all'inserimento di un sistema di pre-sollecitazione innovativo. Esso prevede la tesatura dei cavi "a scomparsa" mediante accorciamento verticale-radiale, dando luogo ad una quasi totale riduzione delle reazioni orizzontali del terreno indotte dai carichi permanenti. L'adozione di pannelli verticali in vetro come elementi irrigidenti a taglio - in ausilio alla struttura principale in alluminio - ha permesso di ottenere una struttura caratterizzata da elevate performance nei confronti delle critiche vibrazioni indotte dal passaggio dei pedoni, pur mantenendo un ridotto ingombro visivo. L'utilizzo di tali elementi ha rivestito tuttavia un ruolo non indispensabile nei confronti della sicurezza della passerella in esame. Infatti, anche in caso di una catastrofica rottura vandalica-accidentale di alcuni o tutti gli elementi in vetro, la passerella non giunge a collasso ma conserva un'apprezzabile sicurezza strutturale a seguito della ridistribuzione delle sollecitazioni interne della struttura principale in alluminio, grazie al notevole grado di ridondanza che la caratterizza. Se ne conlude che, per questa applicazione, l'utilizzo del vetro come materiale strutturale risulta essere confinato al soddisfacimento dei requisiti prestazionali nel campo degli Stati Limite di Esercizio, in particolare nei confronti delle vibrazioni indotte dal passaggio dei pedoni, che neli ultimi anni hanno influenzato in modo dominante la fattibilità e la progettazione strutturale di moderne passerelle pedonali

Energy-based Approach for Dissipative Structural Glass System in Seismic Regions

Current design codes and standards provide limited indications for advanced analysis and earthquake engineering of structural glass applications in seismic regions. This work provides an energy-based approach for efficient design and structural performance evaluation of structural glass systems in seismic regions. The analytical formulation of the energy-based approach for dissipative non-linear structural glass systems is firstly presented. A practical application is then described by means of analytical and numerical studies. The results show that the combination of appropriate structural design with advanced non-linear analysis allows the achievement of highly efficient design and satisfactory performances comparable to the ones of other common structural systems

Novel Laminated Connections

As an alternative to standard adhesive connections, the PhD research project ‘Novel Laminated Connections for Structural Glass Applications’, which recently started at the ICOM-EPFL, focuses on the use of polymer foil/interlayer materials to laminate (metal) connections to glass. Two interlayer materials, namely the ionomer SentryGlas (SG) and the structural transparent addition-cured silicon (TSSA) seem good candidates for this lamination connection technique, and are investigated in this research. The research includes experimental and numerical studies, and aims at developing a model for predicting the deformation and failure behaviour of laminated connections

Experimental and numerical analysis of thick embedded laminated glass connections

Laminated glass components are usually realized by bonding glass plates using interlayer polymers that develop adhesion forces during lamination. Recently, these adhesion forces have been used also to realize special adhesive connections for structural glass components and assemblies. The typical example of such a joining technique is conventionally known as \u201cembedded laminated connection\u201d, where a metal insert is encapsulated in multi-ply laminated glass components. In this study, careful consideration is paid for the investigation of the mechanical behaviour of embedded laminated connections with thick metal insert. To this aim, small-scale laboratory tests, Finite Element (FE) numerical models and analytical considerations are presented. Firstly, the results of experimental investigations at different temperatures are discussed, giving evidence of the geometrical and mechanical parameter effects on the so observed performances. It is observed, in particular, that the temperature markedly affects not only the maximum load carrying capacity but also the failure mode of the studied connection typology. Non-linear numerical simulations are then developed in ABAQUS on refined FE models, able to account for the geometrical and mechanical properties of the reference connection specimens. Further analytical considerations are also presented, in support of the observed experimental findings. It is shown, in particular, that as far as high temperatures are not attained, the mechanical performance and failure mode of the examined connections is strictly related to glass breakage. In addition it is also observed that at high temperature, failure mode (i.e. bubble formation) and failure location are in line with the expectations. Rather close correlation can be also found for the same embedded connections between test results, FE numerical simulations and analytical assumptions

Edge-laminated Transparent Structural Silicone Adhesive (TSSA) Steel-to-Glass Connections

The connections between glass components are very critical aspects of structural glass design. Laminated steel-to-glass connections have recently been developed that combine high strength and transparency. This work focuses on the Transparent Structural Silicone Adhesive (TSSA), produced by Dow Corning. TSSA is typically used for the realization of circular point connections on the glass surface. An alternative approach of using TSSA is considered in this study, by laminating stainless steel connectors on the edge of the glass. These connections are experimentally and numerically investigated. The edge bonded specimens are tested in shear and the stress distribution of the adhesive is analyzed by means of a three-dimensional finite element model. The distribution of stresses in the adhesive is non-linear showing significant stress peaks towards the free edges of the adhesive. A parametric study is conducted to relate the magnitude of the shear stress peaks and bending stresses with the eccentricity of the applied load. The occurrence of failure at lower engineering stresses than the ones recorded for circular point connections is explained using the theory of bending-shear interaction laws. Based on these failure criteria, shear stress peaks that occur due to the eccentricity of the applied load have an important influence on the global resistance of the connection

Laminated connections under tensile load at different temperatures and strain rates

In the last years, a novel typology of adhesive connections for structural glass application has emerged, known as laminated adhesive connections, which makes use of the transparent ionomer SentryGlasa (R) (SG) from Kuraray and the Transparent Structural Silicon Adhesive (TSSA) from Dow Corning. Despite being used in several projects, limited information is available in literature on their mechanical behaviour and on the effects of strain rate and temperature. In this work the behaviour of laminated connections under tensile loading is studied by means of experimental, analytical and numerical analyses. The experimental investigations show that temperature and strain rate variations have important effects on the mechanical response of the connections. Two main interesting phenomena are also observed: the whitening phenomenon in TSSA and the development of bubble within the SG adhesive. The analytical studies of the stress state show that confinement state of the adhesive induces a non-uniform three-dimensional stress distribution in the adhesive with a dominant hydrostatic component of the stress tensor, which is observed to be in agreement with the experimental results. Three-dimensional finite numerical analyses show that the stress field deviates from the uniform distribution with a large gradient of hydrostatic and deviatoric stresses over the adhesive area. The output of the finite numerical model are then compared with the observations of the experimental campaigns. Herein, the full set of numerical results is synthetized by the definition of so-called stress factors. The latter allow to derive the three-dimensional stress state in the adhesive at different temperatures and to compute the stress peak in the non-linear stress field distribution. Finally, prediction models are proposed for the tensile resistance of TSSA and SG laminated connections. A logarithmic law is proposed for the strain rate effects for both TSSA and SG connections. Linear and inverse hyperbolic-tangent-based laws are instead proposed for the TSSA and SG temperature effects, respectively

Laminated Connections for Structural Glass Applications

The demand for architectural transparency has drastically increased structural use of glass in buildings and constructions. Connections between structural glass components represent one of the main critical aspects of glass engineering. In the last years, the use of adhesive connections in structural applications has been considered promising because avoids the drilling process and the high contact stress intensifications of bolted connections. This work focuses on a novel typology of adhesive connections known as laminated adhesive connections. Two transparent adhesive materials are used: the ionomer SentryGlas® (SG) from Kuraray and the Transparent Structural Silicon Adhesive (TSSA) from Dow Corning. In this work, the mechanical behaviour of laminated connections at varying temperature, strain rate and loading condition is studied by experimental, analytical and numerical investigations. The mechanical behaviour of laminated connections under shear loading is firstly experimentally investigated at varying strain rate and temperature. A variational approach is proposed to analytically describe the non-linear stress field distribution in the adhesive. Numerical analyses are then performed to quantify stress peaks and the non-linear the stress field distribution in the connection over the three dimensions. Prediction models are proposed for the computation of the shear resistance of laminated connections. The mechanical behaviour of laminated connections under tensile loading is then experimentally investigated at varying strain rate and temperature. An analytical study is performed to derive the effect of the confinement state on the stress state of the adhesive, with particular attention to the hydrostatic component of the stress tensor. FEM analyses are then used to compute the 3D temperature-dependent stress field distribution, the deviatoric and hydrostatic components of the stress tensor and to quantify the stress peaks in the adhesive. Prediction models are proposed for the computation of the tensile resistance of laminated connections. The analytical development of a new generalized failure criterion is then presented. In this study, a novel four-dimensional model that account for a generic stress state is derived. The governing equation is expressed as a function of the three-dimensional stress tensor, the temperature and the strain rate. Both deviatoric and hydrostatic energetic components are taken into consideration by means of a non-linear function of the two contributions. The proposed model is defined over four independent dimensions: the equivalent stress, the hydrostatic stress, the strain rate and the temperature. At varying strain rate and temperature, the surface evolves as a function of the strain rate and temperature values, following either a linear or logarithmic or hyperbolic-tangent law depending on the material. Additional experimental investigations are also performed by tensile-torsion tests to validate the proposed model at varying value of triaxiality. The mechanical behaviour of embedded laminated connections under pull-out force is finally experimentally investigated at different temperatures. Numerical finite element analyses are carried out and the results compared to the varying failure mode and failure location at different temperatures. A new prototype of a glass components with embedded laminated connections is proposed, focusing on post-breakage behaviour and redundancy

Analytical and numerical estimation of the q-behaviour factor of structural glass frames

In the Italian and international scenario, current design standards for seismic resistant buildings provide recommendations for the advanced analysis of several structures subjected to earthquakes, but no specific details are given for structural glass systems. There, critical design issues for glazed structures may derive from the lack of appropriate resistance but also from the limited accommodation of displacement demands. Consequently, joints and restraints can have a key role for design optimization purposes. This paper presents an energy-based analytical study and a refined Finite Element (FE) numerical analysis for a case-study glass frame, showing the potentials, limits and issues of the calculations approaches, towards the definition of preliminary design estimates for earthquake resistant glass frames. Special care is spent for the potential ductile and dissipative behaviour of structural glass frames, with some recommendations for the q-behaviour factor

A Contribution to the Theoretical Prediction of Life-Time in Glass Structures

In order to assess safety levels in glass structures a scattered and inhomogeneous variety of mostly complicated resistance criteria is presently available, very often requiring specially developed softwares. For this reason engineers who wants to assess with reliability the actual safety level of glass structures of relevant economical importance are still obliged to undertake expensive experimental tests. In the attempt to overcome this problem, it was formulated a new semi-probabilistic failure prediction method called "Design Crack Method” (DCM), which is a compromise between the necessity to accurately model the complex mechanical behaviour of glass at breakage and the need to reduce the analytic complexity of the calculations. On the basis of Linear Elastic Fracture Mechanics, such aim has been analitically reached in the present work by defining a new quantity called Design Crack, characterized by a mathematical expression that depends on the probability of failure and on the surface damaging level. The proposed method, which is in accordance with the basic principles of the Structural Eurocodes, allows to predict glass lifetime taking into due account the influence of parameters like the surface extension and the loading time-history of the structural element. The results obtained by some applications on the D.C.M. have been numerically compared in this paper with those of the existing most frequently used theoretical methods

Preliminary studies on the mechanical behaviour of thick embedded laminated connections

Laminated glass components are usually realized bonding glass plates by means of interlayer polymers that develop adhesion forces during lamination. Recently, these adhesion forces have been used also to realize adhesive connections for structural glass components. An example of this joining technique is the so-called embedded laminated connection. In embedded connections a metal insert is encapsulated in laminated components between the glass plates. In this work the mechanical behaviour of embedded laminated connections with thick metal insert is studied by means of modeling and laboratory testing. Firstly, the results of experimental investigations at different temperatures are presented. It is observed that the temperature variations affect not only the maximum load carrying capacity but also the failure mode. In order to explain the mechanical behaviour and the failure mode of this joining technique, the results and the experimental observations of the testing campaign are then analysed and compared to numerical simulation and analytical studies. Finally, the application of laminated connections to large scale testing is here presented