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 designed 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. Demonstration that robust structural systems provide benefits for mitigation of multiple hazards will facilitate adoption of enhanced connections in codes, standards, and practice. This project will focus on the hazards of extreme winds and disproportionate collapse, areas where there is a need for further experimental data and modeling approaches to enable the development of performance-based design approaches for robust structural systems.
Objective - Develop performance-based design methods for structural systems to achieve robustness against multiple hazards, including extreme winds and 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. A key technical idea in support of this work is the robustness index, a metric for structural robustness 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 push-down 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. It is recognized that robust structural systems provide benefits for mitigation of multiple hazards, not just disproportionate collapse. Demonstration of enhanced robustness against multiple hazards will facilitate adoption of enhanced connections in standards and practice. This project will focus on the hazards of extreme winds and disproportionate collapse, areas where further experimental data and modeling approaches for nonlinear structural response are needed to enable performance-based design of robust structural systems. 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. The experimental data and modeling approaches resulting from this research will also support the development of performance-based design approaches for extreme winds.
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.