Study of steel buildings under multicomponent near-field ground motions and nonlinear soil-structure interaction

Common building regulations typically establish the seismic design criteria based on earthquake movements occurring in far-field regions. For the areas close to faults, the codes have introduced specialized criteria or coefficients aimed at incorporating the influence of the related seismic effects into the design spectrum. While the application of these criteria is straightforward, the inherent uncertainty associated with the proposed methodologies hinders the ability to conduct a precise evaluation of the seismic performance of structures. This challenge is more pronounced concerning the distribution of drift and inelastic behavior of buildings in these regions, especially when they are influenced by the concurrent effects of three translational components of an earthquake and the flexibility of the foundation. Consequently, there is a necessity for more comprehensive investigations. In light of this, the present study conducts three-dimensional nonlinear time history analyses of 32 steel building models in OpenSees software, varying in the number of stories (ranging from 3 to 12), structural system types (special cross braced frames, SCBF and special moment resisting frames, SMRF), soil classifications (D and E based on ASCE 7–22), and base conditions (fixed and flexible). The analyses consider the simultaneous influence of three translational components of suitably selected near-field earthquake ground motions. Modeling of the soil flexibility is conducted using the Winkler approach. The comparative study of the fixed- and flexible-base structures indicates that soil-structure interaction significantly contributes to increased inter-story drifts, particularly in taller braced frames, with the first story experiencing increases of as much as 80%. Despite the decrease in the base shear due to the consideration of soil-structure interaction, it is responsible for increasing the plastic hinge rotations and the permanent displacement of the stories, particularly in the SCBF buildings. In the worst-case scenario, for the braces which are the key elements to controlling the seismic performance of the SCBF buildings, the plastic hinge rotations increase by as much as 5 times. Moreover, the permanent lateral displacement of the models can also increase by a factor of 3. In most cases, the maximum increase of the story drift due to base flexibility corresponds to the story where the story drift is the lowest in the fixed-base condition. Using the obtained results, an equation is proposed to convert the lateral displacements of a fixed-base building to those for the flexible-base case.