SYSTEM-LEVEL SEISMIC PERFORMANCE QUANTIFICATION OF REINFORCED MASONRY BUILDINGS WITH BOUNDARY ELEMENTS
Theses
Overview
Overview
abstract
The traditional construction practice used in masonry buildings throughout the world is limited to walls with rectangular cross sections that, when reinforced with steel bars, typically accommodate only single-leg horizontal ties and a single layer of vertical reinforcement. This arrangement provides no confinement at the wall toes, and it may lead to instability in critical wall zones and significant structural damage during seismic events. Conversely, the development of a new building system, constructed with reinforced masonry (RM) walls with boundary elements, allows closed ties to be used as confinement reinforcement, thus minimizing such instability and its negative consequences. Relative to traditional walls, walls with boundary elements have enhanced performance because they enable the compression reinforcement to remain effective up to much larger displacement demands, resulting in a damage tolerant system and eventually, more resilient buildings under extreme events.
Research on the system-level (complete building) performance of RM walls with boundary elements is, at the time of publication of this dissertation, nonexistent in open literature. What little research has been published on this innovative building system has focused only on investigating the component-level performance of RM walls with boundary elements under lateral loads. To address this knowledge gap, the dissertation presents a comprehensive research program that covered: component-level performance simulation; system-level (complete building) experimental testing; seismic risk assessment tools; and simplified analytical models to facilitate adoption of the developed new building system. In addition, and in order to effectively mobilize the knowledge generated through the research program to stakeholders, the work has been directly related to building codes in Canada and the USA (NBCC and ASCE-7) as well as other standards including FEMA P695 (FEMA 2009) (Chapter 2), TMS 402 and CSA S304 (Chapter 3), FEMA P58 (FEMA 2012) (Chapter 4), and ASCE-41 (Chapter 5). Chapter 1 of the dissertation highlights its objectives, focus, scope and general organization. The simulation in Chapter 2 is focused on evaluating the component-level overstrength, period-based ductility, and seismic collapse margin ratios under the maximum considered earthquakes. Whereas previous studies have shown that traditional RM walls might not meet the collapse risk criteria established by FEMA P695, the analysis presented in this chapter clearly shows that RM shear walls with boundary elements not only meet the collapse risk criteria, but also exceed it with a significant margin. Following the component-level simulation presented in Chapter 2, Chapter 3 focused on presenting the results of a complete two-story asymmetrical RM shear wall building with boundary elements, experimentally tested under simulated seismic loading. This effort was aimed at demonstrating the discrepancies between the way engineers design buildings (as individual components) and the way these buildings actually behave as an integrated system, comprised of these components. In addition, to evaluate the enhanced resilience of the new building system, the tested building was designed to have the same lateral resistance as previously tested building with traditional RM shear walls, thus facilitating direct comparison. The experimental results yielded two valuable findings: 1) it clearly demonstrated the overall performance enhancements of the new building system in addition to its reduced reinforcement cost; and 2) it highlighted the drawbacks of the building acting as a system compared to a simple summation of its individual components. In this respect, although the slab diaphragm-wall coupling enhanced the building lateral capacity, this enhancement also meant that other unpredictable and undesirable failure modes could become the weaker links, and therefore dominate the performance of the building system. Presentation of these findings has attracted much attention of codes and standards committees (CSA S304 and TMS 402/ACI 530/ASCE 5) in Canada and the USA, as it resulted in a paradigm shift on how the next-generation of building codes (NBCC and ASCE-7) should be developed to address system-levels performance aspects. Chapter 4 introduced an innovative system-level risk assessment methodology by integrating the simulation and experimental test results of Chapters 2 and 3. In this respect, the experimentally validated simulations were used to generate new system-level fragility curves that provide a realistic assessment of the overall building risk under different levels of seismic hazard. Although, within the scope of this dissertation, the methodology has been applied only on buildings constructed with RM walls with boundary elements, the developed new methodology is expected to be adopted by stakeholders of other new and existing building systems and to be further implemented in standards based on the current FEMA P58 risk quantification approaches. Finally, and in order to translate the dissertation findings into tools that can be readily used by stakeholders to design more resilient buildings in the face of extreme events, simplified backbone and hysteretic models were developed in Chapter 5 to simulate the nonlinear response of RM shear wall buildings with different configurations. These models can be adapted to perform the nonlinear static and dynamic procedures that are specified in the ASCE-41 standards for both existing and new building systems. The research in this chapter is expected to have a major positive impact, not only in terms of providing more realistic model parameters for exiting building systems, but also through the introduction of analytical models for new more resilient building systems to be directly implemented in future editions of the ASCE-41. This dissertation presents a cohesive body of work that is expected to influence a real change in terms of how we think about, design, and construct buildings as complex systems comprised of individual components. The dissertation’s overarching hypothesis is that previous disasters have not only exposed the vulnerability of traditional building systems, but have also demonstrated the failure of the current component-by-component design approaches to produce resilient building systems and safer communities under extreme events.
