Robust Structural Systems for Multi-Hazard Mitigation

Summary

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.

Description

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.

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. In addition to testing under column loss scenarios, the large-scale experimental setups developed for this work can be leveraged to collect data on the nonlinear performance of structural connections, components, and systems under cyclic loading protocols representative of extreme winds. There is a lack of data on nonlinear structural performance under wind loads, and such data are required to develop modeling and analysis approaches for performance-based design.
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. For nonlinear structural response to extreme winds, cyclic loading effects are important, typically with lower amplitudes but a larger number of cycles relative to seismic loading. Nonlinear structural modeling and analysis approaches previously developed for disproportionate collapse mitigation can be extended to performance-based design for wind, building on experimental data collected in this research. 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 structural systems that are robust against multiple hazards, including extreme winds and disproportionate collapse.

Major Accomplishments

Research Outcomes:

• Weigand, J.M. and Berman, J.W. (2015) "Integrity of Bolted Angle Connections Subjected to Simulated Column Removal," Journal of Structural Engineering, in review.
• Main, J.A., Weigand, J.M., Johnson, E.S., Francisco, T., Liu, J., Berman, J.W., and Fahnestock, L.A. (2015). "Analysis of a Half-Scale Composite Floor System Test under Column Loss Scenarios." Proc., ASCE Structures Congress, Portland, OR.
• Weigand, J.M. and Berman, J.W. (2015). "New steel gravity connection details for enhanced integrity." Proc., ASCE Structures Congress, April 23-25, 2015, Portland, OR.
• Lounis, Z. and T.P. McAllister (2014). "Risk-based decision making for sustainable and resilient infrastructure systems." Journal of Structural Engineering, in review.
• M. Ghosn, D.M. Frangopol, T.P. McAllister, M. Shah, S. Diniz, B.R. Ellingwood, L. Manual, F. Biondini, N. Catbas, A. Strauss, Z.L. Zhao (2014). "Part I: Reliability-based structural performance indicators for structural members." J Struct Eng, in review.
• Ghosn M., L. Dueñas-Osorio, D. M. Frangopol, T. P. McAllister, P. Bocchini, L. Manuel, B. Ellingwood, et al. (2014). "Part II: Reliability-based performance indicators for structural systems and networks." J. Stuct. Eng., in review.
• McAllister, T.P. (2014). "Research needs and gaps for developing a risk-based framework for facility and community disaster resilience." Journal of Structural Engineering, in review.
• Main, J.A., Bao, Y., Lew, H.S., and Sadek, F. (2014). "Robustness of Precast Concrete Frames: Experimental and Computational Studies," Proceedings, Structures Congress 2014, Boston, MA.
• Bao, Y. Main. J.A., Lew, H.S., and Sadek, F. (2014). "Robustness assessment of RC frame buildings under column loss scenarios." Proceedings, Structures Congress 2014, Boston, MA.
• Working group established within ASCE/SEI Disproportionate Collapse Technical Committee to develop guidelines on alternate load path analysis; project team members will lead in overall editorship and in authorship of a chapter on numerical modeling.
• A new ASCE/SEI Standards Committee on disproportionate collapse mitigation of building structures has been established.
• A new PCI Task Committee has been established to develop guidelines for design of precast concrete frame structures to resist disproportionate collapse.
• McAllister, T.P. (2013) "Developing Guidelines and Standards for Disaster Resilience of the Built Environment: A Research Needs Assessment", TN 1795.
• Developed evaluation tools, acceptance criteria, and performance metrics to be used in a performance-based design approach to mitigate disproportionate collapse.
• Developed "A Guide to Assessing Vulnerability of Buildings to Disproportionate Collapse" in collaboration with industry

Potential Research Impacts:

• Lew, H.S., Bao, Y., Pujol, S., and Sozen, M.A. (2013). "Experimental study of RC assemblies under a column removal scenario." ACI Structural Journal, in press.
• Main, J.A. and Sadek, F. (2013). "Modeling and analysis of single-plate shear connections under column loss." Journal of Structural Engineering, ASCE, in press.
• Bao, Y., Lew, H.S., and Kunnath, S.K. (2013). "Modeling of reinforced concrete assemblies under a column removal scenario." Journal of Structural Engineering, ASCE, in press.
• ASTM E2506 Guide for Developing a Cost-Effective Risk Mitigation Plan for New and Existing Constructed Facilities (Revisions adopted 2012)

Realized Research Impacts:

• Main, J.A. (2014). "Composite floor systems under column loss: Collapse resistance and tie force requirements." Journal of Structural Engineering, 10.1061/(ASCE)ST.1943-541X.0000952 , A4014003.
• Main, J.A. and Sadek, F. (2014). "Modeling and analysis of single-plate shear connections under column loss." Journal of Structural Engineering, 140(3), 04013070.
• Bao, Y., Lew, H.S., and Kunnath, S.K. (2014). "Modeling of reinforced concrete assemblies under a column removal scenario." Journal of Structural Engineering, ASCE, 140(1), 04013026.
• Bao, Y., Lew, H.S., Sadek, F., and Main, J.A., (2013). "A Simple Means for Reducing the Risk of Progressive Collapse." ACI Concrete International, 35(12), 33-38.
• Lew, H.S., Main, J.A., Robert, S.D., Sadek, F., and Chiarito, V.P., (2013). "Performance of steel moment connections under a column removal scenario. I: Experiments." Journal of Structural Engineering, ASCE, 139(1), 98-107.
• Sadek, F., Main, J. A., Lew, H.S., and El-Tawil, S. (2013). "Performance of steel moment connections under a column removal scenario. II: Analysis." Journal of Structural Engineering, ASCE, 139(1), 108-119.
• Sadek, F., Main, J. A., Bao, Y., and Lew, H. S., (2011), "Testing and Analysis of Steel and Concrete Beam-Column Assemblies under a Column Removal Scenario," Journal of Structural Engineering, ASCE, 137(9), pp. 881-892.
• Alashker, Y., El-Tawil, S., and Sadek, F., (2010), "Progressive Collapse Resistance of Steel-Concrete Composite Floors," Journal of Structural Engineering, ASCE, 136(10), pp. 1187-1196.
• Khandelwal, K., El-Tawil, S., and Sadek, F. (2009). "Progressive collapse analysis of seismically designed steel braced frames." Journal of Constructional Steel Research, 65(3), 699-708.
• Sadek, F., El-Tawil, S., and Lew, H.S. (2008). "Robustness of composite floor systems with shear connections: Modeling and evaluation." Journal of Structural Engineering, ASCE, 134(11), 1717-1725.
• Khandelwal K., El-Tawil, S., Kunnath, S., and Lew, H.S. (2008). "Macromodel-based simulation of progressive collapse: Steel frame structures." Journal of Structural Engineering, ASCE, 134(7), 1070-1078.
• Bao, Y., Kunnath, S., El-Tawil, S., and Lew, H.S. (2008). "Macromodel-based simulation of progressive collapse: RC frame structures." Journal of Structural Engineering, ASCE, 134(7), 1079-1091.

Impact of Standards and Tools:

• A new ASCE/SEI Standards Committee on disproportionate collapse mitigation of building structures has been established, based on a proposal by NIST. NIST is leading the development of a chapter on acceptance criteria for structural performance and making substantial contributions to a chapter on design and analysis approaches. Project team prepared white papers outlining the scope and content for both chapters. (FY13)
• A new PCI Task Committee has been established to develop guidelines for design of precast concrete frame structures to resist disproportionate collapse based on the outcome of NIST research that revealed a vulnerability of precast concrete connections. NIST is participating in the committee and is tasked with examining the effectiveness of proposed connection configurations in reducing vulnerabilities to disproportionate collapse. (Committee established in FY12)
• Developed evaluation tools, acceptance criteria, and performance metrics to be used in a performance-based design approach to mitigate disproportionate collapse. (FY12)
• Developed "A Guide to Assessing Vulnerability of Buildings to Disproportionate Collapse" in collaboration with industry. (Draft completed in FY12, to be published in FY13)
• McAllister, T.P. (2013) "Developing Guidelines and Standards for Disaster Resilience of the Built Environment: A Research Needs Assessment", TN 1795.
• ASTM E2506 Guide for Developing a Cost-Effective Risk Mitigation Plan for New and Existing Constructed Facilities (Revisions adopted 2012)
• Project team wrote a section on structural systems for the ASCE/SEI Standard 59-11 on Blast Protection of Buildings. (published in FY11)
• Structural integrity requirements for tie reinforcement submitted by NIST based on experimental and analytical research have been incorporated in the ACI 318-09 Building Code. (published in FY09)
• Structural integrity requirements proposed by the Ad Hoc Joint Industry Committee on Structural Integrity have been adopted for the 2009 IBC (published in FY09)
• Best practices guide for preventing progressive collapse in buildings (NISTIR 7396) published and widely cited, including adoption in the ASCE 7-10 Standard as part of the commentary section on general structural integrity. (published in FY07)