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A method for the seismic design of multi-propped retaining walls|Dimensionamento sismico di paratie multi-vincolate
No full textThis paper describes and validates a novel method for the seismic design of multi-propped retaining walls. The method is conceived as an application to excavations of the static-non-linear analysis employed for structural systems: this is a decoupled method, whereby the seismic demand and the seismic capacity are derived independently and are subsequently compared onto the acceleration-displacement plane. The seismic demand is described by an elastic response spectrum, that can be either derived from a ground response analysis, or can be directly specified by a building code. Conversely, the seismic capacity is obtained by applying horizontal static forces on the same model used to analyse the static construction sequence. The method is readily applicable in engineering practice, and has the advantage of considering explicitly the following aspects: (i) the influence of the construction sequence on the seismic response of the system; (ii) the deformability of the soil-structure system; (iii) the non-linear behaviour of the soil, including the attainment of its strength during the construction stages and the ensuing earthquake loading
Seismic design of flexible cantilevered retaining walls
No full textIn this paper, the seismic behavior of embedded cantilevered retaining walls in a coarse-grained soil is studied with a number of numerical analyses, using a nonlinear hysteretic model coupled with a Mohr-Coulomb failure criterion. Two different seismic inputs are
used, consisting of acceleration time histories recorded at rock outcrops in Italy. The numerical analyses are aimed to investigate the
dynamic behavior of this
class of retaining walls, and to interpret this behavior with a pseudostatic approach, in order to provide guidance
for design. The role of the wall stiffness on the dynamic response of the system is investigated first. Then, the seismic performance of the
retaining walls under severe seismic loading is investigated, exploring the possibility of designing the system in such a way that during the earthquake the strengths of both the soil and the retaining walls are mobilized. In this way, an economic design criterion may be developed, that relies on the ductility of the system, as it is customary in the seismic design of structures
Generalised ultimate loads for pile groups
No full textFoundation piles can be used as a means for increasing the capacity of the foundations under static loads or, at the same time, can be regarded as an additional source of energy dissipation for the structure during strong motion. Under multi-axial loading, the ultimate capacity of a pile group is closely connected with the attainment of the flexural strength in the piles, which can in turn vary significantly according to the specific load path followed. Nonetheless, the design of piled foundations is still based on an independent evaluation of the vertical and horizontal capacities without accounting for the interaction between the several loads acting on the footing. To overcome this issue, in this paper a simplified numerical procedure for evaluating the capacity of piled foundations under multi-axial loading conditions is developed, which is based on the lower bound theorem of plastic limit analysis. On the basis of the numerical results, an analytical model of ultimate limit state surface is proposed, representing the force combinations that activate global plastic mechanisms of the soil–piles system. The identification of the ultimate surface necessitates a limited number of parameters having a clear physical meaning. The ultimate surface can lead to an optimised design of pile groups, allowing for a better control of the ultimate capacity as a function of the expected load patterns under static and dynamic conditions. In structural analysis, the ultimate surface can also be regarded as a bounding surface of a plasticity-based macroelement for piled foundations to account for the nonlinear features of the soil–pile system
A Class of Thermodynamic Inertial Macroelements for Soil-Structure Interaction
Full text linkThe seismic performance of structures can be significantly influenced by the interaction with the foundation soils, with effects that depend on the frequency content and the amplitudes of the ground motion. A computationally efficient method to include these effects in the structural analysis is represented by the macroelement approach, in which a geotechnical system is modelled with a single macroelement that describes the generalized force-displacement relationship of the system. While this method has been mainly developed for shallow foundations, the present study proposes a class of macroelements representing the macroscopic response of different foundation types, including abutments, piled and caisson foundations. The generalized force-displacement relationships for these models are elastic-plastic and are derived using a rigorous thermodynamic approach. The plastic responses of the macroelements are bounded by the ultimate capacities of the geotechnical systems, while the inertial effects associated with the soil mass involved in the dynamic response of the structure are simulated by introducing appropriate participating masses. The macroelements are implemented in OpenSees; in this paper they are applied to assess the seismic performance of a tall viaduct showing highly nonlinear features
Un modello strutturale semplificato per la valutazione del danno indotto dallo scavo di gallerie su edifici in muratura
No full textIn questa nota viene proposto un approccio parzialmente disaccoppiato per la valutazione del danno indotto dallo scavo di gallerie su manufatti storici in muratura, con riferimento particolare agli edifici della Roma rinascimentale. L’interazione terreno-struttura viene studiata mediante analisi agli elementi finiti in cui si utilizza un modello semplificato per l’edificio, chiamato “solido equivalente”, ottenendo un notevole sgravio dell’onere computazionale e di modellazione. Il solido equivalente ha la stessa geometria della parte interrata dell’edificio analizzato e per descrivere il suo comportamento meccanico viene utilizzato un modello costitutivo elastico lineare-trasversalmente isotropo, con moduli opportunamente calibrati. I risultati delle analisi mostrano che, se la perdita di volume generata è nell’intervallo tipico atteso per scavo meccanizzato con scudi a pressione di terra, le previsioni ottenute usando tale modello semplificato sono molto prossime a quelle fornite da un modello strutturale dettagliato. La valutazione del danno può quindi essere effettuata in maniera disaccoppiata, applicando a un modello completo dell’edificio in esame il campo di cedimenti ottenuto con il solido equivalente, in un’analisi puramente strutturale. Nello studio presentato viene valutata, mediante analisi parametriche, la adeguatezza della strategia proposta nel caso in cui sia necessario tenere conto della non-linearità del comportamento della muratura di cui è costituito l’edificio in esame, adottando per la stessa un opportuno modello costitutivo elasto-plastico
On the dynamic response of shallow foundations in saturated soils
Full text linkIn recent years, the development of macroelement approaches to include the macroscopic nonlinear response of soil-foundation systems in the assessment of structures is receiving an increasing interest by virtue of the minimal computational effort required. However, existing formulations commonly neglect any undrained or partly drained soil behaviour, that may be crucially important for simulating the response under dynamic loading. The present study provides an insight into the effects of the hydro-mechanical coupling of the soil on the macroscopic multiaxial cyclic response of shallow foundations. This is accomplished through a series of nonlinear transient analyses on a fully coupled soil-foundation numerical model implemented in OpenSees, providing an explicit description of the pore water pressure build-up induced by the nonlinear soil behaviour. The numerical study explores different assumptions for the hydraulic regime, from drained to undrained conditions. The effect of the volumetric-deviatoric coupling on the cyclic response of the reference foundation is examined, highlighting the key role played by the drainage conditions on the stiffness and dissipative features of the foundation system. The effect of non-linearity on the above effects is discussed and interpreted in terms of degradation of the system response at the macro scale
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