Influence of the Uplifting Mechanism of Embedded Footings on the Nonlinear Static Response of Steel Concentrically Braced Frames

The aim of this paper is to evaluate the nonlinear static response of steel concentrically braced frames (CBFs) by considering the response of embedded footings in granular soils using the Beam on Nonlinear Winkler Foundation (BNWF) approach. To model the vertical stress-displacement behaviour of footings, it is possible to define a backbone curve capable of reproducing the total response by adding in series the behaviour of the compression zone with that of the tension zone. When uplift of the soil–foundation system is allowed, it has been demonstrated that it is necessary to consider the horizontal stresses of the native soil on site and the degree of compaction of the soil mass above the footing to avoid significant deviations between the analysis results and the real response. The tension zone in the backbone curves was calibrated while considering these parameters, and given the difficulty associated with their calibration, an estimation is reported that could also be used in the case of practical applications. The implementation of the model has been validated through various pushover analyses on an archetype steel CBF originally tested on a fixed base condition, and predictions were made for the flexible base condition, considering different types of soil and different embedment depths. The results suggest that there is a relationship between the nonlinear static response of steel CBF structures and the uplift mechanism of embedded footings that is influenced by the embedment depth of the footings and the shape of the tension-displacement zone of the backbone curves considered in the modelling process. The proposed model can be used to simulate the flexible base condition of CBF structures on embedded footings using nonlinear springs when carrying out performance-based design procedures.

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