Response of a raft supported structure to spatially varying ground motion

Spatial variation of ground motion has a significant impact on the response of structures that extend over long distances parallel to the ground. Large, relatively rigid foundations are known to average spatially varying ground motion, within its envelope. This paper discusses the response of a typical containment structure (CS) supported by a 40m diameter circular raft in a deep soil site in the Indo Gangetic Plain, subjected to spatially varying ground motion. A three dimensional model of the structure is developed by the Sub Structuring method using the SASSI2010 program. Both foundation and near-field soil are modelled using brick elements, while the soil layers are modelled as horizontal viscoelastic layers. The methodology is evaluated using the analytical transfer functions developed by Luco and Wong (1986), and the comparison is found to be good. The coherency function proposed by Abrahamson (2006), for the deep soil site is then used in the analysis, for the deep soil site. It is found that the response of the structures when subjected to spatial variation in ground motion decreases with increase in embedment depth as well as size of the foundation. A 45degree angle of incidence of the plane wave is found to result in highest response in the structure.