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Structures susceptible to soft story mechanisms are particularly vulnerable to earthquakes because damage concentrated at a single story can lead to premature failure of the structure. The strongback, a stiff vertical spine pinned at the structure’s base and running its height, has been proposed as a way to impose a more uniform pattern of floor displacements and prevent soft story mechanisms. However, changes in the impact of strongbacks on the performance of structures remain unclear when considering vertical stiffness irregularities at different positions along the height of a structure and different arrangements of energy dissipation devices in a structure. This study aims to address these gaps through an extensive parametric experimental investigation varying the location of vertical stiffness irregularities and the arrangement of dampers in a small-scale four-story elastic structure with and without a strongback. For this study, each configuration of the structure is loaded with shake table-produced seismic ground motion. The results of this study show that, regardless of which story a stiffness irregularity is located, the strongback significantly reduces the maximum story drift in the structure. Furthermore, with the strongback, the maximum story and roof drift are insensitive to damper position and distribution, whereas, without it, the damper position significantly impacts the structural performance. The strongback’s ability to protect against soft story vertical irregularities, regardless of their locations, and the insensitivity of structural performance to damper arrangement when utilizing a strongback, presents promising new options for structural design, architectural design, and remediation efforts. 

Recent advances in passive structural control systems have included devices that exploit nonlinear behavior. The explicit inclusion of nonlinearities allows these passive devices to be designed to have behavior and performance that varies with different load types and amplitudes. The variable inertial rotational mechanism (VIRM) is an example of a nonlinear passive control device and consists of a mechanism that converts linear motion into rotational motion and an attached flywheel that includes masses that can move radially inside the flywheel. The radial motion of the VIRM flywheel masses results in the flywheel moment of inertia continuously varying during the response of the device. Despite a potentially small physical mass, the VIRM can provide to a system large added mass effects that can vary greatly depending on the flywheel moment of inertia. The large and variable mass effects provided by the VIRM can significantly shift the natural frequency and reduce the response amplitude of an underlying structure. While the VIRM has been investigated numerically by a number of authors, the experimental study of these devices has been limited. Moreover, most of the studies have considered semi-active or active variable inertia flywheels. The investigation of passive VIRMs are rare. This study aims to address these gaps in knowledge and experimentally investigate the response modification and pseudo resonance frequency changes of an underlying structure produced by the VIRM considering different loading conditions. For this experimental investigation, a VIRM was designed and fabricated that utilizes a lead screw and a flywheel that contains masses connected to springs that can move radially in the flywheel. This VIRM was then attached to a single-degree-of-freedom structure and subjected to different excitation types using a shake table. With data from these experimental tests, the overall fundamental frequency and the response of the system was evaluated using the experimentally estimated system transfer functions. The results of this study shows that the inclusion of the VIRM reduces the response amplitude and significantly shifted the pseudo resonance frequency of the underlying structure and that these shifts in pseudo resonance frequency are highly dependent on the loading amplitude. 

Many structural systems are susceptible to soft-story instabilities during earthquakes that are lifethreatening and can lead to damage that is too costly to repair. One way to mitigate damage and reduce the potential for soft-story instability is through the addition of an elastic spine that distributes drifts across the height of a structure. One such system is the strongback braced frame, which replaces one side of a buckling-restrained braced frame with a strongback truss. With the strongback providing vertical continuity, an expanded design space is made available for the arrangement of buckling-restrained braces (BRBs) or other energy-dissipating members. An example of this expanded design space is that a designer could opt to not include BRBs at every story. Methods for proportioning the energy-dissipating resistance in strongback braced frames have been proposed. However, most methods don't allow exploitation of the full design space. The objective of this work is to propose and evaluate a potential method of proportioning energy-dissipating members for arbitrary vertical arrangements within strongback braced frames. For a prototypical building, the BRBs are designed in various configurations using existing methods and with the new method. Nonlinear time history analyses of the resulting designs coupled with a rigid strongback are performed and the results are compared. The impacts of overstrength and P-Δ effects are quantified. The findings support the proposed method of BRB design that enables exploration of the wide design space made available by the strongback.