Stati limite ultimi per spalle da ponte

Nonostante la risposta dinamica delle spalle possa influenzare la prestazione di un ponte, la valutazione di questo effetto è complicata per l’elevatissima domanda computazionale dei modelli numerici completi, che includano esplicitamente nel modello strutturale la spalla e il terreno che interagisce con essa. In questa nota si fornisce un contributo alla definizione di un approccio semplificato di analisi basato sulla simulazione del comportamento delle spalle tramite macro-elementi, focalizzando in particolare l’attenzione sulle condizioni ultime dell’insieme spalla-terreno. Tramite l’applicazione dei teoremi di estremo dell’analisi limite, attraverso il metodo degli elementi finiti con mesh adattiva, si individuano i potenziali meccanismi plastici del sistema spalla-terreno, considerando anche meccanismi combinati in cui i pali di fondazione della spalla possano raggiungere la propria resistenza insieme al terreno. Si propone quindi un modello per la previsione della resistenza ultima delle spalle in condizioni di carico multi-assiali, identificato tramite la calibrazione di un numero limitato di parametri costitutivi. È mostrato come gli effetti inerziali associati all’azione sismica possano essere incorporati nel modello tramite una contrazione e rotazione della superficie che rappresenta le condizioni ultime della spalla, definita nello spazio delle forze. La superficie così definita si presta a essere inclusa in una rappresentazione generale della risposta sismica di una spalla attraverso la definizione di un macro-elemento per le spalle, ma è anche utilizzabile più immediatamente come uno strumento di verifica della resistenza delle spalle in un approccio alla progettazione sismica in termini di forze statiche equivalenti

A non-linear static approach for the prediction of earthquake-induced deformation of geotechnical systems

This paper illustrates an original and simple method to predict earthquake-induced deformations of geotechnical systems. The method is an extension of static non-linear analysis, and is conceived to predict the behaviour of geotechnical systems, like supported and unsupported excavations, that during the seismic motion accumulate displacements in a single direction. The seismic capacity of the system is described by its capacity curve, obtained either from a numerical push-over analysis or through a simplified procedure. The corresponding seismic demand is described by a combination of the elastic response spectrum, including basic information on the maximum amplitudes of the seismic motion, and a cyclic demand spectrum, that provides additional information about the equivalent number of cycles that contribute to the accumulation of displacements. In the paper, the method is described in detail and is validated through different procedures, namely: comparisons with experimental results obtained in the geotechnical centrifuge; comparison with results of advanced numerical analyses; extensive comparison, using a large database of seismic records, with the results of non-linear time-domain analyses. In its final part, the paper provides guidance for the practical use of the method for design

Capacity design of retaining structures and bridge abutments with deep foundations

This paper examines a seismic capacity design approach for retaining structures with piled foundations, which assumes full-strength mobilization in the soil and in the foundation piles during the earthquake. The plastic mechanism activated by the seismic forces consists of a horizontal movement of the structure, involving plastic hinging in the piles. This mechanism is triggered when the seismic inertial forces acting within the structure and the soil mass equal the overall strength of the soil-pile foundation system. The paper describes an iterative procedure for evaluating the critical seismic acceleration that activates the plastic mechanism. The seismic performance of the structure is expressed by its permanent displacements and the corresponding curvature ductility demand in the foundation piles. With reference to an idealized bridge abutment, this procedure is expressed in a fully consistent nondimensional form and is applied to a reference case, to show its potentiality and to discuss the influence of a number of key parameters, such as the soil strength and the foundation geometry on the seismic performance of the structure. © 2013 American Society of Civil Engineers

Soil-structure interaction for bridge abutments: two complementary macro-elements

In recent years, the designers of girder bridges in seismic areas have frequently opted for a continuous structural scheme, in which the abutments are called to carry large seismic forces engaging the dynamic response of the soil-abutment system. It follows that the abutment response assumes a central role in evaluating the seismic performance of a bridge as an effect of its strong interaction with both the soil and the superstructure. This consideration introduces the cardinal question pursued in the present research: how and to what extent can the dynamic response of the abutments alter the global behaviour of a bridge and vice versa? To this end, this study proposes a method of analysis based on two complementary macro-elements, which simulate the salient aspects of the dynamic soil-abutment-superstructure interaction in the structural and geotechnical analyses of the bridge, preserving a manageable computational demand of the numerical soil-structure models. The two models consist of a macro-element of the soil-abutment system, developed as a useful tool for the structural analysis, and a macro-element of the superstructure to be included in the local model of the abutment instead. The internal responses of the macro-elements define a link between the dynamic response of the soil-abutment system and the global response of the superstructure, representing a step forward to a semi-direct approach for the study of the dynamic soil-structure interaction. The macro-elements were coded in the open-source finite element analysis framework OpenSees and validated against the results obtained with advanced nonlinear dynamic analyses of fully coupled soil-structure interaction models implemented in OpenSees