Collapse Assessment of Buildings under Seismic Loading

Summary

NIST recently completed a multi-year study of three structural steel framing systems to evaluate the correlation between ASCE/SEI 7 (ASCE 2010) and ASCE/SEI 41 (ASCE 2014) as a part of its studies of "first-generation" performance-based seismic engineering (PBSE) techniques. A major conclusion was that there is a need for a more complete understanding of the collapse likelihood of current code-compliant (ASCE/SEI 7) buildings subjected to strong earthquake ground motions. To begin satisfying this need, this project focuses on analytically determining the collapse probabilities for a series of increasingly severe earthquake ground motions for a suite of code-compliant (ASCE/SEI 7) steel buildings that were designed for the ASCE/SEI 7 - ASCE/SEI 41 recently completed study.  The probabilities of collapse will be determined using the methodology outlined in FEMA P695 (FEMA 2009).

Description

Objective - To evaluate the collapse probabilities of a suite of steel buildings for a series of increasingly severe earthquake ground motions, and to compare these probabilities to the collapse objective presumed in ASCE/SEI 7.  The probabilities of collapse will be determined using the methodology outlined in FEMA P695 (FEMA 2009). This assessment will either validate the basis of current code (ASCE/SEI 7) provisions or provide the technical basis for targeted changes to those provisions

What is the new technical idea? Performance-based seismic engineering (PBSE) is intended to let the structural engineer develop earthquake-resistant building design solutions that are more efficient and cost effective than those obtained using the prescriptive building code requirements found in ASCE/SEI 7, which is intended for general use. NIST GCR 09-917-2 (NIST 2009) identified the need for further research and refinement of the PBSE methodologies in U.S. model building codes. NIST GCR 12-917-20 (NIST 2012) then identified the need for advanced seismic design criteria so that a structural system designed using ASCE/SEI 7 would more accurately meet the intended collapse objective in the design basis earthquake. To address these needs, a multi-year project was undertaken to assess the correlation between ASCE/SEI 7 and its PBSE counterpart, ASCE/SEI 41, which was initially created for existing building assessment (Harris and Speicher 2015a, 2015b, and 2015c). One key recommendation from this project and from NIST (2012) is the need to conduct collapse assessments of building suites to determine whether the intended performance objectives of ASCE/SEI 7 is being met.

This collapse assessment will be performed following the methodology developed in FEMA P695 (FEMA 2009) to systematically quantify and assess the nonlinear performance of a building, and to determine the probability of collapse in a “standardized” manner. NIST GCR 10-917-8 (2010) studied the FEMA P695 methodology and concluded that it is a valuable tool for analyzing a structural system to support its properties' inclusion in model buildings codes. As a consequence, FEMA P695 has been accepted as the best currently available methodology to establish the collapse probability of a seismic force-resisting system and the methodology will be referenced in the upcoming ASCE/SEI 7-16 standard.

In this research, the previously mentioned suite of archetype buildings will be evaluated, with the probability of collapse established for each building and compared with the intended design performance objective. This is accomplished by running nonlinear dynamic analyses using a suite of ground motions scaled at various intensity levels to predict the responses of the buildings from elastic into the nonlinear range and finally to collapse. Final results will be presented in terms of a probability of collapse at different ground motion intensities. The probability of collapse at a maximum intensity level (the maximum considered earthquake event) will be quantified for each building, and then compared with the intended target collapse probability prescribed in ASCE/SEI 7. This comparison will complete the next step toward investigating the consistency between ASCE/SEI 7 and ASCE/SEI 41. Using these results, recommendations made in Harris and Speicher (2015a, 2015b, and 2015c) will be calibrated and extended to further improve current PBSE provisions in ASCE/SEI 41 that target equivalent performance to that of a new building.

What is the research plan? The FEMA P695 methodology will be exercised to measure the collapse probabilities of the suite of steel archetype buildings designed for the Harris and Speicher study. The heart of the P695 process is incremental dynamic analysis (IDA) (Vamvatsikos and Cornell 2002). In IDA, the nonlinear building model is first developed and dynamically analyzed for a scaled selected ground motion. The response of the building (usually in terms of the roof drift ratio) is recorded. The nonlinear dynamic analysis is repeated by incrementally increasing the scale factor of the input ground motion, until the building collapses in the analysis. The same analysis is then repeated for a suite of 44 ground motions. The result of all these analyses is a cumulative distribution function, known as a "fragility curve," for the probability of exceeding a specific damage state (such as collapse) for a given ground motions intensity.

An initial investigation will center on whether two-dimensional (2-D) or three-dimensional (3-D) models are required for this work. Many researchers investigating the probability of collapse via IDA use 2-D models to avoid the complexity of analyzing 3-D models. This issue will be investigated in relation to the building suite to determine whether 2-D or 3-D modeling should more appropriately be used.

Next, the 4-story steel special moment frame (SMF) model will be developed and calibrated in OpenSees (McKenna 2000).  This process will then be repeated for the 8- and 16-story SMFs.  The effects of the various design and analysis assumptions will also be investigated in relation to the seismic performance and associated collapse probabilities to complement the IDA results.  These items will include investigating the effects of linear static and dynamic design approaches, the sensitivity to the modeling and solutions algorithm assumptions, and the effects of the manner in which IDA is performed. 

A complementary task will be completed using the results of the FEMA P695 analysis to characterize the lateral force distribution along the height of the building. Performing FEMA P695 analyses will provide a large data set on the shape and variability of the force distribution at various intensity levels ranges from the elastic response up to the collapse. Compiling this data improves the understanding from the lateral demand distribution, and provides insights on the potential improvements for the lateral force distribution method prescribed in ASCE/SEI 7. 

After the completion of the above-mentioned tasks, other lateral load-resisting systems, starting with buckling-restrained braced frames, will be investigated in a similar manner. These systems have been studied by Harris and Speicher using the same approach as that for SMF buildings.

Additionally, this project will share designs, models, and analysis results with collaborators at the University of Illinois (Prof. Fahnestock) that are investigating the effects of seismic loads in relation to current stability design approaches.  Confusion exists in current practice about how to implement current stability provisions for building controlled by seismic events. To address this need, Prof. Fahnestock is proposing to identify a rigorous, transparent and straightforward approach for considering seismic stability in steel lateral force-resisting systems (LFRS).  The suite of building designs created in previous NIST work (i.e., ASCE/SEI 7 – ASCE/SEI 41 study) will be used as case studies to explore the stability of modern buildings.  These additional activities will leverage the completed work to the fullest.

Given that data from this research can be used to develop improved fragility functions, NEHRP researchers will follow closely the ongoing research at the Community Resilience Center of Excellence to assess possible areas of future collaboration.