On vertical bearing capacity of ring footings: finite element analysis, observations and recommendations

Ring footing is defined by a radius ratio (ri /ro) which is the ratio of the inner radius (ri) to outer radius (ro) of the ring footing. The behaviour of ring footings (ri /ro = 0 to 1) lies in-between the behaviour of circular (ri /ro = 0) and strip footings (ri /ro = 1). In this study, the variation of bearing capacity factors N’c, N’q and N’γ of smooth and rough base ring footings from axisymmetric solutions (ri /ro = 0 to 0.95) to the plane strain solutions (ri /ro = 1) is examined using finite element (FE) analysis. The bearing capacity factors of ring footings for the ri /ro = 0 to 0.75 are taken from Chavda and Dodagoudar (2019) and the same solution technique is adopted to evaluate the bearing capacity factors for ri /ro = 0.75 to 1. Based on the observations from the FE results, the guidelines are provided in regard to the following points: (a) the proper choice of the equation for ultimate bearing capacity of ring footing (b) the effect of variation of the Young’s modulus and Poisson’s ratio on the ultimate bearing capacity and (c) the dependency of N’γ on the width of ring footing. An example problem is considered to illustrate the pattern of variability in the bearing capacity of ring footing for ri /ro = 0 to 1. Based on the pattern, the ultimate bearing capacity equation for the ring footing is proposed using the bearing capacity expressions of the circular and strip footings.