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Severe earthquakes cause damage to buildings. One measure of damage is the residual drift. Large residual drifts suggest expensive repairs and could lead to complete loss of the building. As such, research has been conducted on how to reduce the residual drift. Recent research has focused on self-centering frames and dual systems, both of which increase the post-yield stiffness of the building during and after an earthquake. Self-centering systems have yet to be adopted into standard practice but dual systems are used regularly. Dual systems in steel buildings typically combine two types of traditional lateral force resisting systems such as bucking restrained braced frames (BRBFs) and moment resisting frames (MRFs). However, the cost of making the moment connections for the MRFs can make dual systems costly. An alternative to MRFs is to use gravity columns as the secondary system in a dual system. The gravity columns can be used to help resist the lateral loads and limit the residual drifts if the lateral stiffness of the gravity columns can be activated. By restraining the displacement of the gravity columns, the stiffness of the columns adds to the stiffness of the brace frame, thus engaging the lateral stiffness of the gravity columns. Three methods of engaging the stiffness of the gravity columns are investigated in this thesis; one, fixed ground connections, two, a heavy elastic brace in the top story, and three, a heavy elastic brace in the middle bay. Single and multiple degree of freedom models were analyzed to determine if gravity columns can be effective in reducing residual drift. In the single degree of freedom system (SDOF) models, the brace size was varied to get a range of periods. The column size was varied based on a predetermined range of post-yield stiffness to determine if the residual drift decreased with higher post-yield stiffness. Three and five story models were analyzed with a variety of brace and column sizes and with three different configurations to activate the gravity columns. Using gravity columns as part of a dual system decreases the residual drift in buildings. The results from the SDOF system show that the residual drift decreased with increased post-yield stiffness. The three and five story models showed similar results with less residual drift when larger columns were used. Further, the models with a heavy gravity column in the top story had the best results.

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Post-tensioning (PT) strands have been employed in a number of self-centering seismic force resisting systems as part of the restoring force mechanism which virtually eliminates residual building drifts following seismic loading. As a result of the PT strands large elastic deformation capability, they have been proven to work efficiently in these types of systems. Although typically designed to stay elastic during design basis earthquake events, strands may experience inelastic cyclic loading during extreme earthquakes. Furthermore, the yielding and fracture behavior of PT strand systems is central to the collapse behavior of self-centering systems. The loading conditions to which PT strands are typically subjected in prestressed/post-tensioned concrete applications are vastly dissimilar, and only limited research has explored the behavior of PT strands as subjected to inelastic cyclic loading. The testing program conducted to characterize the behavior of PT strand systems as they might be applied in self-centering applications incorporated more than fifty tests, including monotonic and cyclic tests to failure. Variations in the test configuration included strand obtained from two manufacturers, single-use and multiple-use anchorage systems, and variations in initial post-tensioning strand stress. Characteristics of the response that were investigated included seating losses, deformation capacity prior to initial fracture, additional deformation capacity after initial fracture, and the overall load-deformation behavior. Data was analyzed to provide recommendations for PT strand system usage in self-centering seismic force resisting systems. It was concluded that significant strength and ductility allow PT strand systems to consistently provide self-centering systems with reliable restoring force capability. / Master of Science

In the past two decades, there have been significant advances in the development of self-centering (SC) seismic force resisting systems. However, examples of SC systems used in practice are limited due to unusual field construction practices, high initial cost premiums and deformation incompatibility with the gravity framing. A self-centering beam moment frame (SCB-MF) has been developed that virtually eliminates residual drifts and concentrates the majority of structural damage in replaceable fuse elements. The SCB consists of a I-shaped steel beam augmented with a restoring force mechanism attached to the bottom flange and can be shop fabricated. Additionally, the SCB has been designed to eliminate the deformation incompatibility associated with the self-centering mechanism. The SCB-MF system is investigated and developed through analytical, computational, and experimental means. The first phase of the work involves the development of the SCB concepts and the experimental program on five two-thirds scale SCB specimens. Key parameters were varied to investigate their effect on global system hysteretic response and their effect on system components. These large-scale experiments validated the performance of the system, allowed the investigation of detailing and construction methods, provided information on the behavior of the individual components of the system. The experimental results also provided data to confirm and calibrate computational models that can capable of capturing the salient features of the SCB-MF response on global and component level. As a part of the second phase, a set of archetype buildings was designed using the self-centering beam moment frame (SCB-MF) to conduct a non-linear response history study. The study was conducted on a set of 9 archetype buildings. Four, twelve and twenty story frames, each with three levels of self-centering ratios representing partial and fully self-centering systems, were subjected to 44 ground motions scaled to two hazard levels. This study evaluated the performance of SCB-MFs in multi-story structures and investigated the probabilities of reaching limit states for earthquake events with varying recurrence period. The experimental and computational studies described in this dissertation demonstrate that the SCB-MF for steel-framed buildings can satisfy the performance goals of virtually eliminating residual drift and concentrating structural damage in replaceable fuses even during large earthquakes. / Ph. D.

Olofsfors AB är ett företag som tillverkar olika typer av stålprodukter. En av produkterna är deras vägstål, vilket de har tillverkningsproblem med i dagsläget. Råmaterialet i form av plattstång behöver centreras av hydraulcylindrar under transport samt under hålstansningsprocessen. I dagsläget centrerar cylindrarna inte materialet på ett korrektsätt. I detta arbete har brister i det nuvarande systemet undersökts och förslag till förbättringar för det hydrauliska centreringssystemet tagits fram. Syftet med arbetet var att få en bättre funktion för centreringen av plattstång ivägstålsproduktionen, samt att möjliggöra en kortare cykeltid. För att hitta problem i nuvarande system studerades hydraulschemat. När misstankar om en dålig systemdesign uppstod, användes ett program som heter FluidSim 5 för att simulera och validera misstankarna. En slutsats som kunde dras var att det nuvarande hydrauliska systemet inte kan garantera en tillförlitlig centrering av råmaterialet. Lösningsförslaget som tagits fram innefattar en roterande flödesdelare med ett säkerhetssystem för tryckintensifiering och kavitation. Om en flödesdelare med 2% delningsnoggrannhet används, blir centreringsnoggrannheten runt 1—2 mm, beroende på vilka cylindrar som studeras. Då en flödesdelare används, är noggrannheten hos cylindrarna starkt beroende av den aktiva slaglängden. När slaglängden minskas uppnås snävare toleranser hos centreringen. / Olofsfors AB is a company that manufactures different steel products. For one of their products, road grading steel, they currently experience problems in the manufacturing process. The raw material in the form of flat bars needs to be centered by hydraulic cylinders during transport and during hole punching. In the current situation, thecylinders do not center the material in a correct way. In this report, functional errors are identified and improvements in the hydraulic centering system are proposed. The purpose of this study was to improve centering of the flat bar for road grading steel production, and to enable a shorter cycle time. To identify problems in the existing system, the hydraulic scheme was studied. When suspicions of a poor system design arose, a program called FluidSim 5 was used to simulate and validate the suspicions. One conclusion that could be drawn was that thecurrent hydraulic system could not guarantee a reliable centering of the raw material. The proposed solution comprises a gear type flow divider, with a safety system for pressure intensification and cavitation. Using a flow divider with 2% dividing accuracy, the centering accuracy is around 1—2 mm, depending on the cylinders selected. When using a flow divider, the accuracy of the cylinders is strongly dependent of the active stroke length. When the stroke length is reduced, tighter tolerances are achieved in the centering.

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. / Master of Science