Inelastic Analysis of Foundation Structures

Abstract

The earthquake response of shallow and raft foundations is having a significant importance due to its complex behavior because of the semi-infinite soil media. Winkler’s model is the simplest model to deal with the structure and soil. The Winkler model represents the foundation reaction as proportional to the soil displacement at a particular point, which results in the elasticity of the soil being the only parameter in consideration. But in reality the soil cohesiveness is having a significant contribution in soil structure interaction, and therefore the consideration of coupling effects of Winkler springs need to be accounted for. Most of the existing elements either consider certain parameters of the foundation or assume an elastic beam and foundation response. In this paper a new finite element formulation was developed in which these limitations were eliminated. This improved model can be viewed as a soil with a combination of cohesive behavior which transmits the rotation due to bending in addition to the Winkler effect. The non linear response of structures resting on this improved foundation model can be analyzed by assuming that the foundation resists compression and tension. In reality soil is very weak in tension and its tension capacity needs to be neglected, which leads to lift-off regions at different locations. This phenomenon becomes much more complicated by considering the inelastic soil structure behavior, which leads to a highly nonlinear problem. In order to estimate the necessary nonlinear soil parameters, an analytical procedure based on the Vlasov model is proposed. Parametric analyses of an inelastic reinforced concrete beam have been carried out and comparisons were made between different foundation parameters, and displacement and mixed finite element formulations. The presented solutions and applications show the superiority of the proposed nonlinear foundation model. Introduction The earthquake response of shallow and raft foundations is having a significant importance due to the complex behavior of the surrounding semi-infinite soil media. Winkler’s model (1867) is the simplest model to deal with the structure and soil. The Winkler model represents the soil beneath the foundation as a system of similar but mutually independent elastic springs. But in reality these springs should be dependent on each other and need to be nonlinear. Some other models provide greater information on the stress-strain relations of the soil mass than the Winkler model but are complex in their mathematical view and difficult to implement in the real world. To address these drawbacks, some modified approaches were proposed. In the Winkler foundation model, it is assumed that the foundation reaction at a particular point is proportional 2676 Structures 2009: Don't Mess with Structural Engineers © 2009 ASCE