Date

2014

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Civil Engineering

First Adviser

Sause, Richard

Other advisers/committee members

Wilson, John; Pessiki, Stephen; Tremblay, Robert

Abstract

Conventional steel concentrically-braced frames (CBFs) are a stiff and efficient seismic lateral force resisting system, but they have limited ductility capacity due to brace buckling, low-cycle fatigue, and fracture. Under earthquake loading, conventional CBFs dissipate energy by yielding and buckling of braces and yielding of gusset plates. This behavior results in significant damage to the CBF after the earthquake and residual lateral drift. A new seismic lateral force resisting system, known as a self-centering concentrically-braced frame (SC-CBF), has been developed and studied by researchers at Lehigh University. The SC-CBF has an arrangement of members that is similar to that of a conventional CBF. The SC-CBF columns, however, are not rigidly attached to the foundation in the vertical direction, which enables the columns to uplift and enables the SC-CBF to rock on its foundation. Steel post-tensioning bars run vertically over the height of the CBF and are stressed to clamp the CBF to the foundation. Energy is dissipated through energy dissipation devices instead of through yielding and buckling of the CBF braces. The ductility capacity of the SC-CBF is increases compared to that of a conventional CBF, by enabling the CBF to rock on its foundation.The SC-CBF lateral force resisting system has been studied and a first generation design procedure for SC-CBFs has been developed. A laboratory testing program at Lehigh University tested a 4-story, 0.6-scale SC-CBF using hybrid simulation. The 4-story SC-CBF was subjected to 31 intense earthquakes at both the design basis earthquake (DBE) intensity level as well as the maximum considered earthquake (MCE) level. The SC-CBF performed very well.The first generation design procedure was developed through the study of 6-story SC-CBFs, but it is uncertain how well this design procedure would work for SC-CBFs with a greater number of stories. It was also not well understood how the behavior of the SC-CBF system would change as the height increases. This research investigates the seismic performance of SC-CBF structures across a range of heights, studies the accuracy of the first generation design procedure, studies the parameters influencing the peak member forces and peak drift demands for SC-CBFs under the DBE, and develops and validates a second generation design procedure for SC-CBFs that adequately predicts peak member forces and peak drift demands under the DBE for SC-CBFs across a range of heights. The effect of ground motion selection on the performance of the SC-CBF system in nonlinear time-history analysis was also studied. Finally, a comparison study was performed with conventional CBFs to understand how SC-CBF behavior might impact the performance of nonstructural systems. The results of these studies indicate that, while the SC-CBF system is useful even for taller structures, its effectiveness at limiting member force demands, as currently configured, generally diminishes with increasing height. The correlation between modal responses was studied and shown in many cases to be widely varying. A new modal combination technique and associated modal correlation coefficients are proposed to account for the correlation between the modal responses and to adequately predict peak member force demands. It is shown that considering both the foundation flexibility and second order effects are important for estimating peak drift demands, particularly for SC-CBFs less than 6 stories. It is also shown that using accepted techniques for selecting and scaling ground motions to the seismic hazard may not lead to an accurate estimate of the largest SC-CBF response for a given hazard level. A second generation design procedure for SC-CBFs is proposed and validated to show that it is capable of adequately estimating peak member force demands and peak roof drift demands under the DBE across a range of heights.

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