Dynamic behaviour of unbraced steel frames resting on soft ground

Many recent earthquakes clearly illustrate the importance of local ground properties for the dynamic response of structures. The dynamic response of an engineering structure is influenced by the medium on which it is founded. On solid rock, a fixed-base structural response occurs which can be evaluated by subjecting the foundation to the free-field ground motion occurring in the absence of the structure. However, on deformable ground, a feedback loop exists. In other words, when a feedback loop exists, the structure responds to the dynamics of the soil, while the soil also responds to the dynamics of the structure. Structural response is then governed by the interplay between the characteristics of the ground, the structure and the input motion. This study involved a numerical investigation of the dynamic behaviour of unbraced steel frames resting on soft ground. Two types of mid-rise unbraced steel frame, including 5- and 15-storey buildings on a soft soil deposit, were selected and analysed under the influence of three different earthquake acceleration records. The above-mentioned frames were analysed under two different boundary conditions: i) fixed-base (no soil-structure interaction) and ii) flexible-base (considering soil-structure interaction). The results of the analyses in terms of structural forces and lateral displacements for the above-mentioned boundary conditions are compared and discussed.