UC San Diego

Steel moment frames are commonly used in seismic regions, with wide flange steel columns serving as essential elements in their design. Compliance with drift limit requirements updated after the 1994 Northridge earthquake has led to the frequent selection of deep and slender columns, which maximizes the moment of inertia of the section for more economical design. However, such columns are susceptible to experiencing local and global buckling and subsequent axial shortening when subjected to high axial forces and cyclic lateral loads. This study presents the development and experimental observations of a full-scale testing program of cruciform beam-to-column subassemblies through quasi-static and advanced hybrid simulation.A novel framework for hybrid simulation is presented that employs a mixed displacement and force control strategy. A key feature of this framework is a controller-based displacement-to-force transformation that enables compatible displacements between the numerical and experimental model for force-controlled degrees of freedom. The framework can be applied within conventional displacement-based time-integration algorithms allowing for implementation across a variety of software and hardware architectures; it is demonstrated here using OpenSees software with detailed nonlinear numerical models. This study includes numerical verification of the proposed force control strategy and the later implementation for the hybrid simulations as part of the experimental program. In the hybrid tests, the experimental column axial load is applied in force control due to the large stiffness and when combined with lateral seismic loads, result in column buckling with significant axial shortening. The hybrid simulation results verify that the proposed mixed displacement and force control strategy effectively enforces displacement compatibility and force equilibrium between the numerical and experimental structures at the boundary degrees of freedom. Additionally, hybrid tests include an online model updating algorithm focused on the modeling parameters of plastic hinge elements representing the reduced beam sections in the numerical model. A smooth plasticity model is utilized for beam plastic hinges with updating parameters identified from on-line experimental data through a modified version of the unscented Kalman filter. The hybrid simulation shows that the numerical beam hinges based on simple hysteretic model with updated parameters are able to capture the characteristic behavior observed in experiments even for cases involving fracture. A selective updating concept is proposed to allow for updating multiple numerical component accounting for asymmetric behavior and variability in the response. The selective updating method is validated through virtual hybrid simulations that are better able to identify and isolate the effects from different simulations. The combination of results from physical and virtual tests highlights the benefits of model updating on the local and overall system-level response. Data from the present study and previous tests were employed to develop a column hinge macro-element. It allows for simulating the moment-rotation response of deep columns through fiber-based elements, while the vertical direction is modeled via an empirical expression based on the measured axial shortening. The macro-element is included in a steel moment frame model to assess the axial shortening for different seismic intensities.