Seismic response of rigid shallow footings

This chapter concerns the seismic design of shallow footings and, more specifically, a critical analysis of the soil-structure interaction problem. To highlight the non-linear mechanical response of shallow footings, several experimental test results are reviewed and the so-called “macro-element theory” is introduced as a promising interpretative and simulation tool. Various macro-element approaches are discussed and their employment for seismic design is exemplified by means of some engineering applications. RÉSUMÉ. Cet article concerne la conception parasismique des fondations superficielles et, en particulier, une analyse critique du problème d’interaction sol-structure. Pour mettre en évidence la réponse mécanique non linéaire des fondations superficielles, plusieurs résultats expérimentaux ont été pris en compte et la théorie du « macro-élément » a été introduite comme instrument pour interpréter et simuler les résultats. Différentes approches « macroélément » sont discutées et leur utilisation pour la conception parasismique est mise en exemple à travers des applications

Discretization effects in the finite element simulation of seismic waves in elastic and elastic-plastic media

Presented here is a numerical investigation that (re-)appraises standard rules for space/time discretization in seismic wave propagation analyses. Although the issue is almost off the table of research, situations are often encountered where (established) discretization criteria are not observed and inaccurate results possibly obtained. In particular, a detailed analysis of discretization criteria is carried out for wave propagation through elastic and elastic-plastic media. The establishment of such criteria is especially important when accurate prediction of high-frequency motion is needed and/or in the presence of highly non-linear material models. Current discretization rules for wave problems in solids are critically assessed as a conditio sine qua non for improving verification/validation procedures in applied seismology and earthquake engineering. For this purpose, the propagation of shear waves through a 1D stack of 3D finite elements is considered, including the use of wide-band input motions in combination with both linear elastic and non-linear elastic-plastic material models. The blind use of usual rules of thumb is shown to be sometimes debatable, and an effort is made to provide improved discretization criteria. Possible pitfalls of wave simulations are pointed out by highlighting the dependence of discretization effects on time duration, spatial location, material model and specific output variable considered

Modelling of the mechanical interaction between anchored wire meshes and granular soils

In this paper a theoretical and numerical analysis of the mechanical interaction between an anchored deformable facing structure and the underlying granular soil is presented. The reinforcement system is mainly composed of wire meshes, geosynthetics, bars/ties, and spike steel plates. This is usually employed for stabilising potentially unstable slopes. The punching process that can occur locally is described by considering the complex interaction mechanisms arising among the different system elements and the soil. For the sake of simplicity, the soil stratum and the anchor are assumed to be horizontal and vertical, respectively. A simplified displacement-based approach is introduced with the main goal of evaluating both the maximum force that can be applied on the anchor and the correlated penetration of the spike plate within the soil. The approach is validated by means of experimental laboratory test results already available in the literature. Finally, in order to stress the role of the spike plates’ spacing and the soil stiffness in influencing the mechanical response of the system, the numerical results of parametric analyses are discussed

Shear wave propagation along infinite slopes: A theoreticallybased numerical study

In this paper, the seismic response of ‘infinitely’ long slopes is numerically analysed via the formulation of a 1D analytical/numerical model, in which the soil mechanical behaviour is assumed to be elasto-perfectly viscoplastic and simple shear (SS) kinematical constraints are imposed. In order to simplify the problem, a theoretically based procedure to set up a fully 1D shear constitutive model is defined, within which the mechanical response of a multiaxial relationship is condensed. The use of a 1D shear constitutive model is aimed at reducing the number of unknowns and, therefore, the computational costs. In particular, the case of the Mohr–Coulomb yield criterion is considered, while an enhanced Taylor–Galerkin finite element algorithm is employed to simulate the seismic wave propagation within the soil stratum. The proposed ‘condensation’/calibration procedure captures both the ‘pseudo’-hardening pre-failure behaviour and the influence of dilation on the occurrence of strain-localization, which characterize, under SS conditions, the static response of virgin perfectly plastic soils. The effectiveness of the conceived method is shown with reference to freshly deposited deposits, while, in the case of highly overconsolidated strata, some difficulties arise because of the brittle behaviour induced both by unloading and non-associativeness

Soil–foundation modelling in laterally loaded historical towers

reserved3The seismic stability analysis of historical towers requires reliable soil–structure interaction (SSI) modelling. To avoid the complexity of non-linear dynamic computations, engineers frequently resort to simplified static approaches, such as the so-called pushover analyses. Their outcomes are deeply affected by the foundation response under horizontal/moment (HM) loading. This paper encompasses a comprehensive SSI modelling process, taking as a reference case study the embedded shallow footing of the Ghirlandina tower in Modena, Italy. First, a finite-element (FE) soil–foundation model is developed with an inhomogeneous distribution of the undrained soil strength, related to the in situ inhomogeneity of the stress state. Total stress FE analyses are then performed to derive the undrained failure locus of the footing and numerical data about its pre-failure performance. This locus is found to exhibit distorted HM cross-sections, depending on the vertical load and hardly reproducible through standard analytical formulae. The FE evidences are then condensed into a simple elasto-plastic HM ME model, reproducing the outcomes of the inhomogeneous FE model, and employable for the pushover seismic analysis of historical towers.Pisanò, F.; di Prisco, C.G.; Lancellotta, R.Pisanò, F.; DI PRISCO, CLAUDIO GIULIO; Lancellotta, Renat

SULLA MODELLAZIONE DI SABBIE LIQUEFATTE COME FLUIDI NON NEWTONIANI

Le conseguenze catastrofiche della liquefazione di sabbie sono ben documentate in letteratura e riguardano molti ambiti dell’ingegneria geotecnica. L’incremento delle sovrappressioni associate all’insorgere della liquefazione fa sì che il terreno liquefatto abbia un comportamento meccanico assimilabile a quello di un fluido viscoso non-newtoniano. E’ possibile quindi ottenere previsioni riguardanti i flussi di terreni liquefatti e la loro interazione con strutture attraverso l’utilizzo di procedure numeriche legate alla fluidodinamica. In questa nota si presenta l’applicazione di un metodo numerico recentemente sviluppato per applicazioni nell’ambito dell’interazione fluido-struttura (Particle Finite Element Method, PFEM) allo scopo di simulare il comportamento di sabbie liquefatte, il cui comportamento reologico è assimilato a quello di un fluido alla Bingham. In particolare, sarà presentata una procedura per la calibrazione dei parametri del materiale attraverso l’analisi a ritroso di prove di dam breaking in piccola scala e un’applicazione riguardante l’interazione tra condotte sottomarine e il terreno liquefatto circostante

Soil-foundation modelling in laterally loaded historical towers

the discussion focuses on a methodological approach to modelling soil-structure interaction of historical tower