Generalised ultimate loads for pile groups

Abstract

Foundation 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 multiaxial 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 multiaxial inertial macroelement for bridge abutments

“…In addition to the vertical force transferred by the deck, the loading combination includes the self‐weight of the abutment and of the soil fill resting on the footing, W i , and the soil thrust, T s , resulting in an inclined load and a moment acting on the foundation. Under these conditions, Q 3 (+,1D) can be calculated through the conventional approaches used for shallow and deep foundations loaded by eccentric and inclined loads (for piled foundations, Gorini and Callisto 32 developed a numerical tool that allows for a prompt evaluation of Q 3 (+,1D) ). The resulting ultimate locus in the Q 1 ‐ Q 3 space for the reference soil–abutment system (Section 3.1) is illustrated in Figure 4A.…”

Seismic reliability of Italian code‐conforming bridges

“…The frequency-and amplitude-dependent effects of dynamic SFSI are considered by introducing uniaxial, inertial macroelements (1DME) at the base of each support in the global structural models. The macroelements follow the framework proposed in Gorini et al 64,65 : they are perturbed by the seismic motion computed through the site response analysis, at the effective depths, z eff , of the geotechnical systems taken as 10D (D = pile diameter) for the piers' foundations, 66 and as max{L f ,10D} for the abutments' (where L f is the longitudinal width of the abutment foundation). 64,65 To simulate amplitude-dependent effects, a linearly elastic-perfectly plastic constitutive law was assumed for the 1DMEs as an extremely manageable numerical tool readily adaptable to different configurations.…”

“…For the piers' foundations, the multiaxial ultimate resistance is computed by using the ultimate limit state surface model by Gorini and Callisto. 66 In terms of load path, in stage b) the horizontal forces, Q 1-2 , increase together with the moment transmitted to the foundation, calculated for this purpose as Q 1-2 × h p (h p = height of the pier). The mass of each 1DME-P is determined as a function of the elastic stiffness and first-mode vibration period, obtained using the commercial software DYNA.…”

“…For the piers’ foundations, the multiaxial ultimate resistance is computed by using the ultimate limit state surface model by Gorini and Callisto 66 . In terms of load path, in stage b) the horizontal forces, Q 1‐2 , increase together with the relative moment transmitted to the foundation, calculated for this purpose as Q 1‐2 × h p ( h p = height of the pier).…”

A Class of Thermodynamic Inertial Macroelements for Soil-Structure Interaction

“… deep foundations (Figure 2b): hyper-egg with super-elliptical generatrices describing the combinations of forces, Q1-Q2-Q3, and moments, QR1-QR2, producing failure of a pile group [15];  caisson foundations (Figure 2c): roto-translated hyper-ellipsoid in the generalised force space, Qi (i=1, 2, 3, R1, R2, R3), relative to the force transfer between foundation and superstructure in presence of sloping ground (developed in this work);  integral bridge abutments (Figure 2d): roto-translated hyper-ellipsoid in the generalized force space, Qi, representing the force exchange at the deck-abutment contact, as an extension of the model by Gorini et al [16] for semi-integral abutments.…”