UBC Theses and Dissertations
An Experimental and analytical study of chevron braced frames with vertical slotted connections Bubela, Rozlyn Kathleen
This thesis presents the findings of an experimental and analytical study into the seismic performance of a special steel chevron bracing system. Although very popular as a lateral load resisting system, it is known that concentric chevron braced frames can exhibit poor performance under severe load demands. A vertical slotted connection (VSC) detail was introduced between the top of the braces and floor beam above to prevent vertical load transfer to the beam and limit brace forces to the compressive resistance of the members. An experimental program was devised to study VSC chevron braced frame behaviour under quasi-static cyclic loading conditions. Full-scale tests were performed on two specimens with hollow tube braces. One of the specimens had the braces filled with concrete. Both frames exhibited stable, predictable behaviour under cyclic loading. The concrete-filled tube specimen sustained higher peak loads, demonstrated greater residual strength and dissipated more energy than the unfilled tube specimen. This was due to the partial inhibition of local buckling by the concrete core. The unfilled braces experienced severe local buckling, which lead to early fracture and eventual failure of the specimen. For both tests, the VSC detail provided free vertical movement of the brace assembly and out-of-plane buckling of the braces did not seem to provide significant restraint. An analytical study of the VSC chevron braced frame revealed that the flexibility of the connections needed to be included in the computer model, especially for the slotted bolted detail. Models of the frame were developed using two different computer programs, RUAUMOKO and SAP2000, and calibrated to the observed experimental results. The SAP2000 model was able to better represent the backbone curve of the hysteretic behaviour observed in testing. In both models, the use of a horizontal spring element for the brace-to-beam connection was an effective way to model the flexibility in the VSC detail while maintaining a vertical release in the connection. The results of an illustrative example comparing a conventional and VSC chevron braced frame further demonstrated the vulnerability of the gravity frame in a conventional system and highlighted some of the benefits of the VSC system.
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