Structural collapse demand and capacity are not clearly defined or directly simulated in current Performance-Based Seismic Engineering (PBSE) methodology. Collapse is currently "assessed" indirectly based on lateral displacement analysis results that exceed engineering judgment-based predefined limits, or collapse is implied when the numerical analysis solution algorithm fails to converge. This creates a critical gap in structural collapse assessment accuracy. This project will: (1) identify and assess critical issues concerning nonlinear mechanics of structures at collapse or near-collapse, to improve analytical accuracy and stability; (2) utilize high-fidelity LS-DYNA software analyses to benchmark the accuracy of practitioner-oriented commercial structural analysis software in simulating collapse; and, (3) develop a methodology using an energy balance approach to quantify the limits of acceptance criteria in the current ASCE/SEI 41 Standard. Current ASCE/SEI 41 acceptance criteria were established without a theoretical basis. Outcomes will be validated and verified measurement science tools and guidelines for improving simulation and assessment procedures for evaluating seismic collapse of structures using commercial nonlinear dynamic analysis software. This will improve how practitioners perform structural analysis, design, and assessment, advancing acceptance of the PBSE design philosophy by practicing structural engineers.
Objective: To develop a measurement science tool by FY 2016 that is capable of directly, accurately, and efficiently simulating the initiation and progression of structural collapse, and to develop a methodology for quantifying acceptance criteria limits found in the ASCE/SEI 41 Standard through the use of the collapse prediction capabilities, with adoption of the methodology in the Standard no later than its planned 2019 revision cycle.
What is the new technical idea? NIST GCR 09-917-2 identified improving analytical models and demand assessment for buildings near collapse from seismic loading as the second highest priority research area in support of full implementation of PBSE. NIST GCR 13-917-23 identified improving analytical models and simulation capabilities for buildings in near-collapse seismic loading as a high priority research topic. Commonly used practitioner collapse assessment analysis does not directly simulate collapse but instead requires monitoring of other demands (e.g., drift) that are indirectly associated with collapse, such as analytical results reported in FEMA P695. In spite of this imprecision, the ASCE/SEI 7 Standard uses the probability of structural collapse as a parameter to define seismic risks and corresponding minimum design loads. This project posits the new technical idea of assessing collapse behavior based on study of basic nonlinear structural collapse mechanics concepts, together with high-fidelity non-linear modeling and simulation. Calibration of these collapse mechanisms with maximum drifts and energy dissipated during strong response obtained using (less sophisticated) practitioner-oriented software packages will be performed to improve the current collapse prediction capabilities. The knowledge gained will then be applied to develop a rational, efficient, and rigorous methodology to quantify performance limits for acceptance criteria currently used in the ASCE/SEI 41 Standard that were developed in FEMA 273 over 15 years ago. The FEMA 273 limits used a small number of experimental results and engineering judgment that was based on limited post-earthquake observations; they thus lacked a strong theoretical basis. The methodology proposed here will be based on an energy approach, whereby the energy capacities of members based on existing experimental results will be compared with the demands obtained from the energy balance analysis of the nonlinear dynamic response of the entire structure.
What is the research plan? The first phase of this project will involve study of basic structural collapse mechanics to identify the fundamental issues that must be included in an analytical model that can capture such behavior. Key elements of this work will be to identify the means to simulate the behavior theoretically and then to evaluate the methods and associated accuracies obtained from current software packages. Simple structural steel moment frames and frame components will be studied initially to evaluate the simulation and modeling approaches theoretically necessary, as compared with those currently employed in analysis software. Of specific interest are the following issues:
The second phase of the research will develop high fidelity LS-DYNA models to simulate collapse of structures subjected to seismic events. Archetypical steel moment frame structures that have been designed and modeled in previous and ongoing EL projects will be used in this phase. The goal is to capture as accurately as possible those non-simulated collapse modes that would have otherwise been ignored if the structures were modeled using lower-end, practitioner-oriented software packages. The results will then be calibrated with those obtained using such lower-end software packages as SAP2000, Perform-3D, OpenSees, and Ruaumoko.
Once the mechanism of collapse in the modeling and simulation process is understood, the third phase of the research will develop a methodology to quantify the acceptance criteria with respect to the collapse performance level. This methodology will be based on a theoretical energy balance approach, with energy dissipation assessed in all simulated and non-simulated collapse modes; member energy capacity will be computed to the extent possible based on existing experimental results. The member energy capacity will be a function of the enclosed area of hysteresis curves obtained by experiment. Both monotonic and cyclic testing results (e.g., from a large amount of published testing results at NEES within the past decade, as well as other sources from published literature) will be used to establish energy dissipation capacity, as has been done in earlier work by others. The limits in defining acceptance criteria will then be verified by comparing energy capacity with energy demand at each yielding location during seismic response. Energy demand will be computed based on energy balance equations using results obtained from nonlinear dynamic analysis. This work will focus on structural steel systems, including moment and braced frames, which have been studied in other EL projects. Leveraging these existing building models will allow project time to be spent on investigation of collapse assessment of these same buildings. A steering committee will be formed through extramural contract to guide the development of the limits of acceptance criteria, and address the needs of the practitioners on performing ASCE/SEI 41 nonlinear assessments.
 Seismic Rehabilitation of Existing Buildings, ASCE/SEI 41-06, American Society of Civil Engineers, Reston, VA, 2006.
 Research Required to Support Full Implementation of Performance-Based Seismic Design, NIST GCR 09-917-2, National Institute of Standards and Technology, Gaithersburg, MD, 2009.
 Development of NIST Measurements Science R&D Roadmap: Earthquake Risk Reduction in Buildings, NIST GCR 13-917-23, National Institute of Standards and Technology, Gaithersburg, MD, 2013.
 Quantification of Building Seismic Performance Factors, FEMA P695, Federal Emergency Management Agency, Washington, DC, 2009.
 Minimum Design Loads for Buildings and Other Structures, ASCE/SEI 7-10, American Society of Civil Engineers, 2010.
 NEHRP Guidelines for the Seismic Rehabilitation of Buildings, FEMA 273, Federal Emergency Management Agency, Washington, DC, 1997.
 Research performed within the George E. Brown, Jr. Network for Earthquake Engineering Simulation.
 Fajfar P. and Vidic T., "Consistent inelastic design spectra: Hysteretic and input energy," Earthquake Engineering and Structural Dynamics, Vol. 23, No. 5, pp. 523-537, 1994.