SOIL-FOUNDATION-STRUCTURE INTERACTION FOR BUILDINGS ON SHALLOW FOUNDATIONS IN THE CHRISTCHURCH EARTHQUAKE

There are a number of multi-story buildings on shallow foundations in the city of Christchurch, New Zealand that performed satisfactorily during the Christchurch Earthquake of February 22, 2011. Structural assessments following the earthquake have concluded that some buildings have performed significantly better than would have been expected given the intensity of the recorded ground motions in and around the central city. Soil-foundation-structure interaction (SFSI) provides a possible explanation for the good performance of these buildings. SFSI incorporates nonlinear geometrical effects and nonlinear soil deformation effects into earthquake analysis of structure-foundation systems. For shallow foundations, this involves uplift of the foundation and plastic deformation of the underlying soil during large earthquake shaking. These effects can potentially reduce the forces transmitted to a structure and improve performance. Structure-foundation models of generic 5, 10 and 15 story buildings on beds of nonlinear springs have been used to investigate the potential influence of SFSI in the performance of multi-story buildings on shallow foundations in Christchurch. Existing features of structural analysis software were used to capture the nonlinear effects of foundation uplift and plastic soil deformation in the springs and the models were subjected to relevant time history data from the Christchurch Earthquake. The results show that SFSI has potentially had a significant influence on the earthquake response of multi-story buildings on shallow foundations in Christchurch. Uplift of the foundation was likely to be the dominant mechanism, however, only small extents of uplift and foundation rotations were required to have a large effect on structural response.