1,574 research outputs found
Soil-caisson-bridge pier-deck dynamic interaction under strong earthquakes: the influence of primary and secondary soil nonlinearities
No full textIn the last few years, the ever-increasing computing power of personal computers has allowed to explore beyond the assumption of linear viscous-elastic behaviour usually made for foundation soils, even when subjected to strong seismic excitations. In this context, Gaudio and Rampello [1] performed a parametric study focusing on the seismic performance of massive caisson foundations supporting bridge piers subjected to strong one-directional earthquakes, capable of triggering the nonlinear and inelastic soil behaviour. 3D dynamic Finite Element (FE) analyses were performed twice in the time domain, once assuming the soil to behave as an elastic-plastic and once as a linear viscous-elastic medium: through the comparison of results, the observed reduction of inertial forces transmitted to the superstructure was mainly attributed to the energy dissipation occurring in the foundation soils, due to the attainment of their inelastic behaviour. These nonlinearities can be classified as “primary”, developing in the free-field soil, and “secondary”, resulting from the oscillating foundation [2]: these were not distinguished in the parametric study. In this paper a step further is made, as the relative influence of the “primary” and “secondary” nonlinearities is evaluated. A simple 3-degree-of-freedom plane-strain model simulating the soil-foundation-bridge pier-deck system is subjected to the horizontal acceleration time histories computed at the depth of the foundation centroid from preliminary 1D inelastic ground response analyses performed in free-field conditions. The influence of “primary” nonlinearities is assessed by comparing these results with those obtained after applying seismic inputs coming from 1D nonlinear viscous-elastic free-field analyses. The comparison is performed in terms of some performance indexes, such as the deck drift and the bending moment acting at the base of the pier. A fair estimate of the influence of “secondary” nonlinearities is finally provided comparing the results obtained applying the 1D inelastic free-field motion with those computed in the 3D nonlinear dynamic FE analyses
IGS Student Award 2020
No full textGeosynthetic-reinforced earth (GRE) retaining walls show a better performance than conventionally-designed walls during destructive earthquakes, due to their capability of redistributing seismic-induced deformations within the reinforced zone. In this paper, a recently-proposed method to design GRE walls is first recalled, where the wall is designed to trigger an internal plastic mechanism in the presence of strong earthquakes. Following a pseudo-static approach, the seismic coefficient k is therefore assumed equal to the internal seismic resistance of the wall kcint. The seismic coefficient is then calibrated against given seismic wall performance, expressed in terms of limit values of earthquake-induced displacements. Permanent displacements are evaluated through empirical relationships that were previously developed on the basis of a parametric integration of an updated Italian seismic database. Effectiveness of the proposed procedure is then demonstrated by assessing, through Finite Difference nonlinear dynamic analyses, the seismic performance of two walls, namely a GRE and a conventional gravity wall, characterised by the same seismic resistance but triggering an internal and external plastic mechanism, respectively. They are both subjected to a real strong motion, capable of activating a plastic mechanism. Results showed that lower permanent displacements are accumulated in the GRE wall where internal mechanisms are triggered
On the assessment of seismic performance of bridge piers on caisson foundations subjected to strong ground motions
No full textSubstructure method is widely used to evaluate the seismic performance of caisson foundations supporting bridge piers subjected to strong ground motions, mainly because of its simplicity. However, the strongly-simplifying assumption of linear viscous-elastic behaviour for the foundation soil limits its applicability to flexible systems subjected to low-intensity earthquakes, for which irreversible strains and pore water pressure build-up are not anticipated. Furthermore, lumped-parameter models are typically adopted in calculations in which soil-foundation compliance is reproduced via dynamic impedance functions, whose dependency on frequency of excitation is often neglected. Modification of Free-Field Motion leading to Foundation Input Motion (FIM), due to the presence of caisson embedment, is also mostly ignored. The influence of these simplifying assumptions on the seismic performance of bridge piers on caisson foundations is assessed in this paper through a parametric study, where soil-caisson-bridge pier-deck systems differing in geometric and mechanical properties are subjected to real seismic records. Dynamic analyses were carried out in the time domain with the finite element method, using a 3D continuum and a lumped-parameter model for the foundation soil. In the 3D model both the linear viscous-elastic and the nonlinear soil behaviour were assumed, while a linear viscous-elastic behaviour was assumed in the lumped-parameter model. The influence of inelastic soil behaviour was assessed by comparing the seismic performance of the systems obtained with the 3D model, while the role of FIM was evaluated by comparing the results of the dynamic analyses computed assimilating the soil to a linear elastic medium
Seismic behaviour of a caisson supporting a bridge pier
No full textThis paper shows the results of 3D FE coupled dynamic analyses carried out in the time domain to study the seismic performance of a rigid and massive caisson foundation supporting a slender bridge pier. The system is subjected to a real acceleration time history. To evaluate the permanent displacements induced by seismic loading soil behav-iour was described using an elastic-plastic hysteretic model capable to provide a fair estimate of non linear soil behaviour and hysteretic damping under cyclic loading conditions. The analyses were carried out in terms of effective stresses in undrained conditions, thus evaluat-ing excess pore water pressure induced by earthquake loading. Dis-placement and rotation time histories obtained with and without the bridge pier are presented. These results are compared with those com-puted from linear equivalent free-field analyses
The pictorial wit of Domenico Tiepolo
No full textThis thesis takes a new approach to Domenico Tiepolo’s (1727-1804), Divertimento Per li Regazzi (c.1795-1804), it is arguably the artists most enigmatic graphic work, which features the commedia dell’arte character Pulcinella. The drawings have hitherto been subject to rigorous connoisseurial analysis. Indeed, in his introduction to ten of the drawings in a catalogue of Italian Eighteenth-Century Drawings in The Robert Lehman Collection at the Metropolitan Museum of Art, New York, James Byam Shaw states that this particular series of drawings has now become so famous ‘that it is hardly necessary to add to the literature of the series.’1 In my opinion it would be a great pity if future generations of scholars were discouraged by this remark, for I believe the drawings still have much to ‘tell’ the contemporary art historian and would further benefit from increasingly interpretative readings. Previously, scholars have regarded Domenico Tiepolo as an imitator of his father, Giambattista Tiepolo (1696-1770), and interpreted the re-appropriation of motifs in the Divertimento as signs of old age and fatigue. I suggest, on the contrary, that in this series of drawings in particular, Domenico was an innovator.
This project carves out new territories within the study of the series in that it focuses on the playful nature of the drawings, and how the suite can be understood in relation to contemporary theory concerning games and play, and ludic musical/improvisatory forms. Additionally, the drawings are discussed as a case history in a now popular emerging dialectic on the late works of aged artists: here I consider how these drawings, often funny, poignant, sensitive and delicate reveal how the elderly painter reconciles himself not only to the passing of his own life and the extinction of his family line but to an entire political, cultural and visual tradition
Dynamic soil-structure interaction of bridge-pier caisson foundations
No full textThis paper presents the main results of 3D FE dynamic analyses carried out in the time domain to assess the seismic performance of rigid and massive circular caisson foundations supporting bridge piers. Various foundations systems are subjected to a real acceleration time history. Soil behaviour is described by an elastic-plastic model capable to provide a fair estimate of nonlinear soil behaviour and hysteretic damping under cyclic loading conditions. The coupled dynamic analyses are carried out in terms of effective stresses, thus evaluating the excess pore water pressures induced by earthquake loadings. Caisson construction stages are reproduced in a simplified way. The influence of pier height and caisson slenderness on maximum and permanent displacement and rotation attained during and at the end of the seismic shaking is considered. The equivalent seismic coefficient to be adopted in a pseudo-static analysis to check the safety of the foundation against geotechnical limit states is also evaluated
Influenza dell’input sismico sulla prestazione di pile da ponte fondate su pozzi
No full textIn questa nota viene valutata l’influenza del contenuto in frequenza e della durata della fase di strong-motion dell’input sismico sulla prestazione di pile da ponte fondate su pozzi. A tal fine vengono presentati alcuni risultati di uno studio parametrico, nel quale diversi sistemi pila-pozzo-terreno sono stati sottoposti a 3 differenti accelerogrammi relativi a eventi sismici reali, scalati per ottenere valori simili dell’intensità di Arias. Lo studio è stato condotto svolgendo analisi dinamiche accoppiate in condizioni tridimensionali con il metodo degli elementi finiti (FEM); tali analisi sono state svolte nel dominio del tempo in termini di tensioni efficaci, descrivendo il comportamento meccanico dei terreni di fondazione mediante un modello costitutivo elasto-plastico con incrudimento isotropo. I diversi sistemi pila-pozzo-terreno sono caratterizzati dalle medesime condizioni iniziali, al fine di poter isolare l’effetto che le proprietà del moto sismico hanno sulla prestazione dell’opera. A tale scopo il valore iniziale del coefficiente di sicurezza al carico limite è stato fissato e posto pari a FSv = 5.5
Progettazione sismica di muri in terra rinforzata secondo un approccio prestazionale
No full textI sistemi geotecnici soggetti a violenti terremoti tipicamente coinvolgono il comportamento irreversibile e isteretico dei terreni di fondazione. In presenza di eventi sismici intensi tale evenienza non può essere evitata: al contrario, essa può essere addirittura favorita, purché la resistenza del sistema non si riduca durante il sisma ma si mantenga stabile denotando un comportamento duttile. Questo paradigma di progettazione (Capacity Design), inizialmente concepito per l’ingegneria strutturale (Paulay e Priestley, 1992) sta acquisendo una sempre maggiore risonanza anche nel campo dell’ingegneria geotecnica. Infatti, tale approccio conduce a una forte riduzione delle forze inerziali trasmesse alla struttura, al prezzo di un accumulo di spostamenti permanenti fino al termine dell’evento sismico. Nel contesto di un approccio prestazionale (Performance-Based Design), la verifica di sicurezza degli elementi duttili è soddisfatta quando tali spostamenti risultino minori di determinati valori di soglia, questi ultimi funzione dello Stato Limite considerato e della Vita Utile del sistema in analisi.
I concetti sopra richiamati sono stati recentemente applicati nello sviluppo di una procedura di progetto di muri in terra rinforzata con geosintetici (Cazzuffi, 1983) soggetti ad azioni sismiche intense (Gaudio et al., 2018a, 2021), la cui prestazione sismica soddisfacente durante terremoti intensi è stata ampiamente documentata in letteratura (Koseki et al., 2009; di Filippo et al., 2019). La principale ragione per la quale i muri in terra rinforzata mostrano un comportamento migliore di quello dei muri a gravità convenzionali è da attribuire alla loro capacità di ridistribuire le deformazioni indotte dal sisma nella zona rinforzata, purché gli elementi di rinforzo siano adeguatamente duttili, come nel caso dei geosintetici.
In questo articolo viene innanzitutto richiamato un metodo proposto di recente per il progetto di muri in terra rinforzata con geogriglie e risvolto in facciata, in cui viene adottato un approccio prestazionale e nel quale l’azione sismica è rappresentata attraverso un approccio pseudo-statico (Gaudio et al., 2018a). In tale metodo i muri vengono progettati per attivare meccanismi plastici interni durante eventi sismici intensi, così da coinvolgere il comportamento duttile degli elementi di rinforzo. A tal fine, il coefficiente sismico k da utilizzare nel metodo dell’equilibrio limite viene assunto pari al coefficiente sismico legato ai meccanismi plastici interni, kcint, che a sua volta deve risultare inferiore al coefficiente sismico calcolato assumendo meccanismi plastici esterni, kcext; ciò per garantire l’attivazione di meccanismi plastici che coinvolgano gli elementi di rinforzo. Nell’ambito di un approccio prestazionale, il soddisfacimento della verifica in termini di spostamenti permanenti viene assicurata calibrando il coefficiente sismico k su determinati valori della prestazione sismica, espressa in termini di valori di soglia dello spostamento permanente, dy. Tali spostamenti permanenti sono stati preliminarmente calcolati svolgendo una integrazione parametrica del database sismico italiano attraverso il metodo del blocco rigido di Newmark (Newmark, 1965).
L’efficacia della procedura proposta viene di seguito dimostrata discutendo i risultati di analisi dinamiche non lineari svolte con il metodo delle differenze finite, nelle quali viene valutata la prestazione sismica fornita da due diversi muri, l’uno in terra rinforzata progettato per attivare meccanismi plastici interni, l’altro a gravità progettato per attivare meccanismi plastici esterni. I due muri sono caratterizzati dalla medesima resistenza sismica e sono soggetti alla stessa registrazione di un evento sismico intenso occorso durante la sequenza sismica che ha colpito il centro Italia nel 2016. I risultati delle analisi mostrano che la prestazione sismica del muro in terra rinforzata è notevolmente migliore di quella fornita dal muro convenzionale, poiché caratterizzata da valori dello spostamento permanente notevolmente inferiori
Valutazione del coefficiente sismico equivalente per i pozzi di fondazione di pile da ponte
No full textIn questa nota vengono discussi i criteri di definizione dell’azione statica equivalente da portare in conto nelle verifiche di sicurezza dei pozzi di fondazione di pile di ponti e viadotti. A tale scopo si sono utilizzati i risultati di una serie di analisi dinamiche condotte in condizioni 3D su 31 schemi di pile fondate su pozzi, tutti caratterizzati dallo stesso coefficiente di sicurezza ai carichi verticali. Nelle analisi sono state variate le caratteristiche geometriche del pozzo e della pila e sono stati applicati differenti input sismici, variando il contenuto in frequenza e la durata significativa a parità di intensità di Arias. Il comportamento meccanico dei terreni di fondazione è stato descritto da un modello costitutivo elasto – plastico incrudente per ottenere una buona stima della nonlinearità e della dissipazione di energia in condizioni cicliche. Le analisi dinamiche sono state svolte in condizioni non drenate, ma in termini di tensioni efficaci, al fine di ottenere una stima del-le sovrappressioni interstiziali indotte dal sisma; sono stati inoltre simulati gli effetti delle fasi costruttive del pozzo di fondazione.Inertial forces induced by earthquakes into bridge-pier caisson foundations add to the self-weight of the foundation and to the actions applied by the superstructure, bringing the foundation closer to geotechnical ultimate limit states. The safety of the foundation can then be checked by performing pseudo-static analyses, in which inertial forces are proportional to the self-weight of the foundation through the equivalent seismic coefficient, that represent the global response of the foundation during the seismic event.
In this paper, the equivalent seismic coefficient to be used in a pseudo-static analysis to check the safety of bridge-pier caisson foundations against geotechnical ultimate limit states was evaluated. The equivalent seismic coefficient was obtained by performing a parametric study, for different caisson diameters and slenderness ratios, D = 8 and 12 m and H/D = 0,5–1–2, pier heights, hs = 15–30–60 m, and properties of the input motion, specifically the frequency content and the significant duration, for similar values of the Arias intensity. All systems were characterised by the same value of the static safety factor under drained conditions, FSv = 5,5, to evaluate the seismic coefficient of different systems characterised by the same initial conditions. The 3D FEM dynamic analyses were performed in the time domain and in terms of effective stresses, describing soil behaviour by an elastic – plastic model (HS small). Caisson construction stages were simulated in a simplified way.
The time histories of the equivalent seismic coefficient were evaluated from the dynamic analyses to represent the horizontal forces that foundation soils apply to the caisson foundation: the maximum value reached by kh eq(t) during the seismic event was defined as the equivalent seismic coefficient. Values of kh eq were always smaller of the maximum seismic coefficients of the input motions (kh inp max), as well as smaller of the seismic coefficients computed, at the ground surface, through seismic ground response analyses carried out in free–field conditions using the LE method (kh max (L.E., p.c.)), or by using the simplified procedure proposed by the Italian Building Code (kh max (NTC-08)). The ratios of the equivalent seismic coefficient to maximum coefficient at the ground surface, kh eq/kh max, range between 0,19 and 0,57 referring to the Italian Building Code and between 0,15 and 0,45 referring to the results obtained using the LE method. The minimum values of these ratios are obtained for high values of the non-dimensional parameter 0, that is for strong asynchronous effects and flexible bridge piers. In these conditions, high frequency removal leads to lower values of the equivalent seismic coefficient, to an increase of the mean period Tm and to a decrease of the Arias intensity IA of the time histories of the equivalent seismic coefficient
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