Valutazione del coefficiente sismico equivalente per i pozzi di fondazione di pile da ponte

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.

In 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.