Seismic Response of Short Period Structures and the Development of a Self-Centering Truss Moment Frame with Energy Dissipating Elements for Improved Performance
Darling, Scott Christian
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Traditionally, earthquake engineering has focused on protecting the lives of building occupants by utilizing inelasticity in structural members and connections to dissipate seismic energy and provide protection against collapse. This design concept is partially based on the equal displacement concept, which states that peak drifts for an inelastic system will be approximately equal to the peak drifts of an elastic system with the same initial stiffness for a given dynamic loading. This is a concept that has been shown to work for structures with natural period greater than about 1.0 seconds, but does not hold true for shorter period structures. An additional consequence of this design methodology is that conventional seismic systems do not explicitly limit the amount of structural damage, or offer a repair method that allows continued use of a structure after an earthquake. In fact, the structural damage distributed throughout a building and permanent residual drifts can make a conventional structure difficult if not financially unreasonable to repair after a large earthquake. These are both concerns facing the seismic design community that are investigated as a part of this thesis. First, a computational study was conducted on short period structural systems to investigate the relationship between initial structural period and collapse potential. The investigation utilizes a statistically based analysis methodology to investigate a study of single degree of freedom (SDOF) systems with periods between 0.1 seconds and 1.0 seconds. The SDOF models were developed using an elastic-linear hardening model with post-yield stiffness ranging between -10% and +10% of the initial stiffness. This part of the study was done to gain a general understanding of the influence of natural period and post-yield behavior on the collapse performance of structural systems and appropriate response modification factors. Next, a study of multi-degree of freedom (MDOF) masonry structures with short periods was conducted to examine how the SDOF trends translated to realistic MDOF structures. Based on these two studies, recommendations were made for how current U.S. building codes could be modified to account for the behavior of short period structures. Next, a new self-centering system that builds on the concepts of previous self-centering systems is developed. The self-centering truss moment frame (SC-TMF) was developed with the goal of providing self-centering capability while concentrating inelastic deformation in replaceable structural fuses. These goals are accomplished while mitigating a number of issues seen in other self-centering systems, such as deformation incompatibility with gravity framing, limited deformation capacity, and unusual field construction techniques. The development of the SC-TMF includes a set of preliminary monotonic pushover analyses and nonlinear time history analyses to confirm the expected behavior of the system. Next, a mechanics investigation was undertaken where static pushover analyses (monotonic and cyclic) were used to help derive equations to predict system behavior, such as strength and stiffness. Finally, a parametric study was conducted to gain a better understanding of how various design decisions influence structural behavior. It was shown that the SC-TMF was a viable seismic system for controlling residual drifts and concentrating inelasticity in replaceable fuse elements while mitigating the issues seen in other conventional self-centering systems.
- Masters Theses