Evaluation of seismic energy dissipation systems for application to narrow wall piers in soft-story wood-frame structures

Abstract

Recent experimental testing of a full-scale building, with dampers installed in toggle type displacement amplification frames, was conducted as part of the NEES-Soft Project and demonstrated that this type of energy dissipation retrofit is effective at protecting the ground story (limiting peak ground story drift) while preventing the transfer of excessive force into the upper stories. In this thesis, two alternative displacement amplification frames are examined and their effectiveness is compared, via numerical simulations, with that of the experimentally tested toggle frame. To preserve the large openings in the soft ground story, each frame was examined for three distinct aspect ratios, including very narrow frames that would allow placement behind narrow wall segments. Analytical relationships were derived to define the displacement-dependent displacement amplification provided by each frame and were subsequently validated via kinematic analysis using the program SolidWorks. The displacement amplification factor was then used to derive the effective damping coefficient provided by the frame which was in turn utilized to model the damping added to the ground story of a numerical model of a four story building that had previously been experimentally tested under seismic loading at NEES-UCSD as a part of the NEES-Soft Project. Numerical simulations of the seismic response of the building were performed using the program SAWS with earthquake records scaled to DBE (Design Basis Earthquake) and MCE levels. The simulated response of the building is examined from the perspective of: 1) the effect of various damper frame aspect ratios and 2) the effectiveness of the different frames relative to one another.

Wood-framed structures with a soft ground story (typically the result of large openings in the ground story) are a common type of structure that has performed poorly under seismic loading. Recently, efforts have been made in some U.S. cities in seismically active regions to mandate the retrofit of such structures. Retrofit methods described in current design guidelines focus on modifications to the ground story only (adding elements to the ground story to increase strength and provide additional stiffness), resulting in a transfer of seismic forces to the upper stories. The objective is to provide a level of performance defined as "shelter-in-place" under MCE (Maximum Considered Earthquake) level ground shaking. Such performance can be achieved by alternative methods that focus on the addition of energy dissipation mechanisms in the ground story. In addition, higher levels of performance, such as "immediate occupancy" under MCE level ground shaking, can be achieved via the addition of energy dissipation mechanisms in the ground story along with the conversion of selected partition walls to shear walls in the upper stories. To avoid impeding on the large openings in the ground story, it is desirable to position the energy dissipation mechanisms (damping devices installed in light steel framing) behind the narrow wall segments (piers) that exist between the large openings. As such, to provide a sufficient amount of damping, the damping devices should be installed in a framing system that provides amplification of the damping effect.