On the assessment of seismic performance of bridge piers on caisson foundations subjected to strong ground motions

Abstract

Substructure 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 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.

Soil-Caisson-Bridge Pier-Deck Dynamic Interaction Under Strong Earthquakes: The Influence of Primary and Secondary Soil Nonlinearities

“…They also discussed the influence of Foundation Input Motion (FIM) and compared it with that of irreversible soil behaviour. Although the study [1] provides some useful empirical relationships that can be used to get the overestimation of peak displacements and forces that is typically obtained when irreversible and hysteretic soil behaviour is neglected, as a function of the period ratio Teq/T0 and of the significant duration of the earthquake, TD inp [6], it does not detail the contribution of the so-called "primary" and "secondary" soil nonlinearities (even if "nonlinearities" should be replaced with "plasticity" in this context, more properly). The term "primary" stands for the development of permanent deformations and dissipation of energy in the free-field soil column, whereas "secondary" indicates the same but caused by the oscillating foundation and by the structure as a whole.…”

“…The dynamic analyses were first performed using the FE code PLAXIS 3D [13] with the numerical domain shown in Figure 2a [1]. Each analysis was performed twice, adopting two different constitutive models for foundation soils: the elastic-plastic constitutive model Hardening Soil with Small-Strain Stiffness (HS small, [14]) and a linear viscous-elastic model, the former with the mechanical parameters given in Table 1, the latter with "mobilised" values of shear modulus G and of damping ratio { resulting from preliminary 1D free-field analyses carried out with the LE method.…”

“…In the above-mentioned context, Gaudio and Rampello [1] recently performed a parametric study through 3D nonlinear dynamic FE analyses on caisson foundations of bridge piers subjected to strong ground motions, clearly showing that the influence of soil inelastic behaviour is to be considered when assessing their seismic performance, especially when the fundamental period of the whole soil-caisson-pier-deck system (i.e. considering lengthening of the period due to soil-structure interaction), Teq, approaches the period of the free-field 1D soil column, T0.…”

“…In this paper, the relative influence of the primary and secondary nonlinearities is evaluated, following the work that can be found in [1]. Specifically, a 3-degree-of-freedom (3DoF) plane-strain model representing several soil-foundation-bridge pier-deck systems is subjected to the horizontal acceleration time histories computed at the depth of the foundation centroid from preliminary 1D inelastic free-field ground response analyses.…”

Incomplete soil‐foundation contact and foundation input motion: From numerical analysis to field evidence

“…38 In case any research should include soil-foundation nonlinear interaction, both the kinematic and inertial parts of the interaction should be considered simultaneously. 39,40…”