Mitigating higher-mode effects in steel rocking core-moment resisting frame systems using viscoelastic damper connections

Abstract

The use of stiff rocking cores in moment resisting frame (MRF) systems has been shown to mitigate story drift concentration and enhance the global collapse capacity of the structure. However, this beneficial stiffening effect concurrently amplifies floor accelerations, potentially exacerbating damage to acceleration-sensitive nonstructural components and creating a design trade-off. This study aims to resolve this conflict by introducing viscoelastic dampers (VEDs) between the rocking core and the MRF. The VED connections allow relative deformation between the moment frame and the rocking core, thereby dissipating vibration energy and mitigating higher-mode effects. The design methodology for the VEDs is developed based on the second mode of the structure. A dimensionless connection factor (CF) is introduced to guide the selection of VED capacity, which can be physically interpreted as an estimate of the VEDs’ damping ratio with respect to the second mode of the structure. A total of 12 nonlinear structure models are developed in OpenSees, including one traditional bare MRF, one enhanced by a steel rocking core with hinged rigid links, and ten using VED connections with CF values ranging from 0.03 to 1.5. Nonlinear response history analyses are performed. The results show that, compared to the rigid-link system, VED connections with a CF greater than 0.5 not only maintain excellent drift control (reducing both maximum IDR and DCF) but also drastically reduce peak floor accelerations by up to 39.1 %, achieving the desired simultaneous mitigation. Larger CF values also result in lower deformation demands on the VEDs. Additionally, collapse assessments are conducted on the analyzed models using the procedure provided in FEMA P695, in conjunction with seismic risk evaluation. The results indicate improved collapse resistance for rocking core-MRF systems with VED connections, provided the CF is no less than 0.2.