Kinematic effects on Interaction Diagrams for Caisson Foundations

Caisson foundations are typically designed against combined loading since they are usually adopted for long-span bridges and viaducts. Their capacity can be usefully expressed in terms of Interaction Diagrams (IDs), which represent the locus of the generalised forces (N-Q-M) bringing the soil-caisson system to limit conditions. Caisson IDs have been recently proposed for static conditions only, meaning that the limit generalised forces were computed assuming no inertial forces acting within the foundation soils. However, it is worth investigating whether these inertial forces may bring to a reduction of the system resistance, what is usually referred to as kinematic effect. In this paper, the kinematic effects on the IDs for caisson foundations are assessed from the results of a parametric study, where 3D Finite Element pushover analyses were performed. A pseudo-static approach was followed, which implies that the inertial forces within the soil deposit were proportional to the self-weight of the soil via the seismic coefficient, kh, which was assumed as constant and uniform over the calculation. The parametric study involved different caisson slenderness ratios, H/D, and initial loading factors, , the latter representing the degree of mobilisation of the soil-caisson strength against vertical loading before the earthquake: nonetheless, here only the results obtained with the values H/D = 1 and  = 0.21 are discussed, for the sake of space. An analytical formulation of the IDs taking kinematic effects into account is finally provided, which may be adopted in the seismic design of caisson foundations.