Enhancing Seismic Resiliency of Steel Buildings through Three-Dimensional Modeling of Diaphragm System Interaction with Braced Frame

The design objectives of early seismic codes were mainly achieved by using acceptable construction materials, and minimum levels of strength and stiffness largely derived based on roughly estimated demands. Early seismic design provisions resulted in sometimes adequate levels of performance for buildings, efficient and reliable building performance was not accomplished with the simplistic demand-based design process adopted. Consequently, capacity-based design approaches and later performance-based design approaches have been adopted and developed for new seismic design provisions. To achieve a certain performance level for conventional steel buildings during seismic events, all the members should be designed to meet the component deformation and force capacities. The lateral-force resisting system, key for developing successful seismic performance, has two main parts: 1) the Vertical Lateral-force resisting system (VLFRS) of which a common example in steel buildings is the braced frame, and 2) the Horizontal lateral-force resisting system (HLFRS) consisting of the floor and roof diaphragm. The floor diaphragm acts as a critical element that distributes the demands developed in a building during an extreme event to the vertical lateral-force resisting systems and eventually to the building foundation. Compared to the VLFRS, far less attention has been paid to the role of the diaphragm in seismic building response, particularly for steel framed buildings. The first part of this dissertation contributes to developing fundamental understanding of steel deck diaphragms as structural systems integrated within the overall building performance and improved strategies for accurate modeling of floor systems within three-dimensional building models to enhance the overall structural resilience of a building. In this study, the effect of different diaphragm designs on the behavior of steel buildings is investigated using three-dimensional computational building models that consider nonlinear behavior in both the vertical and horizontal elements of the seismic force resisting system. Different diaphragm design scenarios based on ASCE 7-16, are investigated for a series of 1, 4, 8, and 12-story archetype buildings with special concentrically braced frames (SCBFs) as the vertical lateral force resisting system: 1) traditional design, 2) alternative design with Rs = 1.0, 3) alternative design with Rs = 2.0 or 2.5, and 4) alternative design with Rs = 3.0 are all considered. For the studied buildings first, modal analyses were conducted to study their basic dynamic properties. Second, nonlinear pushover analyses were investigated to analyze their static overstrength and ductility. Third, nonlinear response history analyses were conducted to evaluate building seismic performance. Finally, the FEMA P-695 methodology is used to assess the seismic performance and propose a reasonable Rs values for conventional SCBF steel buildings. The second part of this dissertation addresses rod bracing, which has a wide application in metal buildings as a truss diaphragm including the sidewalls to provide the lateral stiffness in longitudinal direction, and also the roof by providing the lateral bracing for the rafters in the out-of-plane direction. In this part of the dissertation, an experimental program is conducted to establish the stiffness, strength and applicable limit states for rod bracing. Twelve rod brace assemblages with differing details related to the geometry of the primary frame members, anchorage of the rod into the frame member, and angle of the rod relative to the framing member were tested in tension until failure. Finally, a framework is presented for future evaluation of seismic performance for rod-braced metal buildings by implementing the experimentally established stiffness and strength values and three-dimensional computational modeling of metal building systems