Robust Structural Systems for Disproportionate Collapse Mitigation
In the context of codes and standards for structural design, the term robustness has been used to indicate the ability of a structural system to resist damage under extreme loads. Since the early 1980s, U.S. codes and standards have included requirements intended to prevent an initial local failure from spreading progressively and resulting in the collapse of a disproportionately large part of a structure. However, these codes and standards have either lacked specific provisions or criteria to achieve this goal or have adopted prescriptive requirements that provide minimum levels of force continuity but do not ensure resistance to disproportionate collapse. Consequently, vulnerabilities to disproportionate collapse are widespread in current U.S. construction practice, particularly for gravity framing systems, which are design to carry vertical loads only. Motivated by these considerations, this project will develop and demonstrate effective strategies for enhancing the robustness of various structural systems, along with provisions for codes and standards to enable engineers to effectively design structures with enhanced robustness.
Develop and demonstrate effective strategies for enhancing the robustness of conventional structural systems, and develop provisions for codes and standards to mitigate disproportionate collapse.
What is the new technical idea?
The new technical idea is to develop and demonstrate effective strategies for enhancing the robustness of conventional structural systems, along with analysis guidelines and acceptance criteria to enable engineers to effectively design structures with enhanced robustness. As an example of an innovative strategy for enhancing robustness, preliminary NIST research has demonstrated that local debonding of reinforcing bars at critical locations can delay fracture and enhance the capacity of reinforced concrete moment frames by as much as 30 %. A key technical idea in support of this work is the robustness index, a metric for structural robustness recently developed by NIST researchers that represents the ratio between the ultimate capacity of the damaged structural system and the applicable gravity loads acting on the system. The NIST robustness index is obtained from nonlinear static pushdown analyses under local failure scenarios, accounting for dynamic effects through an energy-based analysis. The NIST robustness index will provide the basis for evaluating and comparing the effectiveness of various strategies for enhancing structural robustness, including innovative structural systems that will be developed in this research. The development of analysis guidelines and acceptance criteria for enhanced structural systems will build on experimentally validated reduced-order modeling approaches developed for conventional structural systems in previous NIST research. The results of this research will provide the technical basis for performance-based design provisions in a new standard for mitigation of disproportionate collapse, which is being developed through the Structural Engineering Institute of the American Society of Civil Engineers (SEI/ASCE) in response to a proposal by NIST.
What is the research plan?
The research plan has two primary components, which will be carried out in parallel as outlined in the following paragraphs.
1. Development and demonstration of effective strategies for enhancing structural robustness: The primary focus of this research will be on enhancing the robustness of gravity framing systems, where vulnerabilities to disproportionate collapse have been identified in previous NIST research. NIST research has demonstrated the vulnerability of steel gravity frame systems with shear connections to disproportionate collapse under column loss scenarios, and similar vulnerabilities exist in precast concrete gravity frame systems. Seismically designed moment-resisting frames have been shown to be robust against column loss in previous experimental and computational studies by NIST. However, because of their substantial cost, these systems are typically used only at selected locations within a building, to resist lateral loads, while the remainder of the building uses gravity framing. Enhancement of seismically designed moment frames will also be considered in cases where modifications to conventional connection details can significantly enhance robustness. NIST research on precast concrete moment frames has shown that eccentricities in welded connection details can cause premature failure. Modified connection details for enhanced ductility will be investigated. Candidate strategies for enhanced robustness will be developed in close partnership with industry to ensure their feasibility and cost-effectiveness. Alternative strategies will be evaluated through computational modeling, using the robustness index to quantify and compare their effectiveness. Testing of selected structural components and assemblies will be conducted to characterize component-level performance and calibrate modeling approaches. Ultimately, testing of structural systems will be carried out in partnership with industry, other government agencies, and academic collaborators, to demonstrate the robustness of enhanced structural systems.
2. Development of analysis guidelines and acceptance criteria for the design of robust structural systems: This research will focus initially on developing simplified modeling parameters and acceptance criteria for seismically designed steel and concrete moment connections for use in routine structural design. These results will enable designers to more fully exploit the robustness of moment-resisting frames for mitigation of disproportionate collapse, because previous experimental and computational research by NIST has shown that current acceptance criteria for these connections are overly conservative. While current modeling parameters and acceptance criteria were based primarily on seismic test data and consider only flexural behavior, the performance of structural connections under column loss scenarios can be strongly influenced by the development of axial forces associated with arching action and catenary action. This research will characterize the influence of axial forces on connection performance, as influenced by factors such as span length, connection depth, and the degree of axial restraint provided by the surrounding structure. This will be achieved by conducting parametric studies using experimentally validated modeling approaches developed in previous NIST research. An additional factor that will be studied for reinforced concrete structures is the influence of creep, which can progress rapidly under the high-stress conditions following column loss, as recent large-scale tests have shown, resulting in lower resistance than if creep effects were neglected. A second phase of this research will focus on the development of modeling and analysis guidelines for the enhanced structural systems to be developed in this project, to enable designers to take advantage of these enhanced systems in achieving robust structural systems for disproportionate collapse mitigation.
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