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No / This study explores the lateral deflections of diagonal braces in concentrically-braced earthquake-resisting frames. The performance of this widely-used system is often compromised by the flexural buckling of slender braces in compression. In addition to reducing the compressive resistance, buckling may also cause these members to undergo sizeable lateral deflections which could damage surrounding structural components. Different approaches have been used in the past to predict the mid-length lateral deflections of cyclically loaded steel braces based on their theoretical deformed geometry or by using experimental data. Expressions have been proposed relating the mid-length lateral deflection to the axial displacement ductility of the member. Recent experiments were conducted on hollow and concrete-filled circular hollow section (CHS) braces of different lengths under cyclic loading. Very slender, concrete-filled tubular braces exhibited a highly ductile response, undergoing large axial displacements prior to failure. The presence of concrete infill did not influence the magnitude of lateral deflection in relation to the axial displacement, but did increase the number of cycles endured and the maximum axial displacement achieved. The corresponding lateral deflections exceeded the deflections observed in the majority of the previous experiments that were considered. Consequently, predictive expressions from previous research did not accurately predict the mid-height lateral deflections of these CHS members. Mid-length lateral deflections were found to be influenced by the member non-dimensional slenderness ( ) and hence a new expression was proposed for the lateral deflection in terms of member slenderness and axial displacement ductility. / TATA Steel

This thesis investigates the performance of concentrically braced zipper frames through complementary experimental and numerical simulation approaches and proposes a design methodology for an innovative bracing scheme labeled as the suspended zipper frame. The suspended zipper frame intends to ensure that the top-story hat truss remains elastic, resulting in very ductile behavior of the structure. In the first part of the work, a three-story prototype frame was designed based on a preliminary design method. Three tests were conducted on one-third scale models of this prototype to verify the design procedure and assess the system performance under very different load histories. Comparisons of the results between analyses and experiments validated the partial-height zipper mechanism envisioned, and led to refinements of the design procedure and establishment of appropriate design details for these frames. The design and performance of this structural system are illustrated with three-, nine-, and twenty-story buildings designed for the same masses as those used in the SAC studies for the Los Angeles area. The proposed design strategy results in suspended zipper frames having more ductile behavior and higher strength than typical zipper frames. In addition, the suspended zipper frames also appear to reduce the tendency of chevron-braced frames to form soft stories and to improve seismic performance without having to use overly stiff beams. Finally, an explanation of the design philosophy as well as code language format of the design procedure is given.

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Concentrically braced frames with replaceable brace modules (RBMs) have the potential of improving the constructability of braced frames, mitigating the structural damage during earthquakes, and minimizing the time of post-earthquake repairs. To fill the gaps between the component-level performance of RBMs and system-level behaviour of SCBFs with RBMs, this thesis focused on the overall system-level seismic performance of SCBFs with RBMs in three steps. Firstly, the effects of beam-column connection fixity on the behaviour of three SCBFs were investigated to determine what level of fixity, if any, is required to ensure adequate collapse capacity of an SCBF. Secondly, the effects of column design parameters on braced frame seismic performance were investigated, where two different brace-to-frame connections were considered: 1) conventional gusset plate connection and 2) the newly proposed connection detail with RBMs. Detailed numerical modelling was undertaken to develop improved provisions for designing columns in SCBFs. Finally, a large-scale experimental program was conducted to evaluate the seismic performance of braced frames with initial and replaced RBMs where realistic boundary conditions were provided. Three different beam-column connections that can be used in SCBFs with RBMs were designed and tested. Based on the current work, the recently proposed concept of replaceable brace modules, accompanied by the recommended methods for designing columns and detailing beam-column connections, appears to be a promising approach. The fabrication and installation are simpler, the seismic performance is similar to that of SCBFs with currently accepted connection detailing, and the approach can increase the post-earthquake reparability of steel concentrically braced frames. / Dissertation / Doctor of Philosophy (PhD)