Structural engineers are increasingly using Performance-Based Seismic Engineering (PBSE), instead of building code-based prescriptive procedures, in the design of new buildings in earthquake-prone areas. There is currently no national consensus on how PBSE is to be implemented. With no specific "PBSE technique" presented in ASCE/SEI 7 (ASCE 2010), practitioners who design new buildings using performance-based methodologies often apply the techniques developed for evaluating existing buildings that are detailed in ASCE/SEI 41 (ASCE 2014). Taking such an approach is typically referred to as "first-generation" PBSE. However, potential problems arise with this approach because the developers of ASCE/SEI 41 adopted a number of conservative assumptions to account for the large uncertainties in evaluating existing (often older) buildings. How well this approach and the assumptions it contains relate to new building design is not clear. There has been no comprehensive study conducted to correlate the expected performance levels targeted by ASCE/SEI 7 and ASCE/SEI 41.
NIST has previously assessed the "first-generation" PBSE techniques in the design of new steel special moment frames, special concentrically braced frames, and eccentrically braced frames (Harris and Speicher 2015a, b, and c).
The current project continues the assessment of the applicability and accuracy of adopting "first-generation" PBSE analysis methods developed for evaluating existing buildings for use in performance-based design of new buildings. This work includes two main topics: (1) the completion of an investigation concerning PBSE for steel buckling-restrained braced frames (BRBFs); and, (2) the continuation of an investigation concerning PBSE for reinforced concrete special moment frames. As with the previous assessments, the objective is to advance "first-generation" PBSE methods by evaluating the performance of a set of code-compliant buildings using ASCE/SEI 41 methodologies now in use by many practitioners. The evaluations will be used to validate these first-generation PBSE methods and identify needed changes that will improve model building code provisions and will advance the state-of-practice.
Objective - The objective of this study is to advance first-generation performance-based seismic engineering (PBSE) methods by assessing the performance of a set of code-compliant buildings using ASCE/SEI 41 methodologies. This study focuses on steel buckling-restrained braced frames (BRBFs) and reinforced concrete (RC) special moment frames (SMFs).
What is the new technical idea? Structural engineers are increasingly using Performance-Based Seismic Engineering (PBSE), instead of building code-based prescriptive procedures, in the design of new buildings in earthquake-prone areas. With no specific "PBSE technique" presented in ASCE/SEI 7 (ASCE 2010), practitioners who design new buildings using performance-based methodologies often apply the techniques developed for evaluating existing buildings that are detailed in ASCE/SEI 41 (ASCE 2014). Taking such an approach is typically referred to as "first-generation" PBSE. Potential problems arise with this approach because the developers of ASCE/SEI 41 adopted a number of conservative assumptions to account for the large uncertainties in evaluating existing (often older) buildings. How this evaluation framework and the assumptions it contains relate to new building design is not clear, and there has been no comprehensive study conducted to correlate the expected performance levels targeted by ASCE/SEI 7 and ASCE/SEI 41. Additionally, the General Services Administration recently adopted ASCE/SEI 41 as the de-facto design standard for use in design of new buildings in regions of high seismicity, without validation of this approach for new building design (GSA, 2012).
To begin the correlation research, an earlier phase of this project thoroughly evaluated three sets of archetypical steel buildings (special moment frames, special concentrically brace frames, and eccentrically braced frames), which were designed using ASCE/SEI 7 and then assessed using ASCE/SEI 41. The resulting three volume report has been published. The report shows the need for improvements to make ASCE/SEI 41 more consistent with ASCE/SEI 7 for new building design (Harris and Speicher 2015a, b, and c).
With the current phase of the research, the initial project scope was extended in FY 2014 by adding the assessment of buckling-restrained braced frames (BRBFs), a newly developed and increasingly used structural system, and reinforced concrete (RC) special moment frames (SMFs). Both of these systems are commonly constructed in earthquake-prone regions. The same research strategy that led to the original three reports has been implemented for the BRBFs and RC SMFs. The mutual goal of these new parallel studies is again to assess how well the "first-generation" PBSE design approaches found in ASCE/SEI 41 align with the performance levels found in ASCE/SEI 7, and to suggest measures to improve the alignment.
A closely related study supporting the primary effort will be initiated to assess the impact of the ground motion selection and site condition characterization on the assessment results for the nonlinear dynamic procedure (NDP) in ASCE/SEI 41. The procedure for selecting ground motions prescribed in ASCE/SEI 41 may not represent the state-of-the-art in ground motion selection approaches, and can potentially be improved. Additionally, the impact of the site condition on the assessment results has not been comprehensively studied. These two factors will be assessed by evaluating the performance of a set of the archetypical steel SMFs designed and modeled in the previous phase of this project when they are subjected to various sets of ground motion that have been selected for different site conditions.
What is the research plan? A research framework similar to that already used to assess the suite of steel lateral force resisting systems by Harris and Speicher (2015a, b, and c) is being applied to a suite of six archetypical steel BRBFs and a suite of four to six archetypical RC SMFs. The building layouts are based on adapting the archetypical building configurations already developed in the previous PBSE-ASCE/SEI 41 steel frame phase of this project, which includes 4, 8, and 16-story buildings. Each building is designed using both the equivalent lateral force (ELF) procedure and the response spectrum analysis (RSA) procedure of ASCE/SEI 7, to give a range of potential member sizes and strengths to then compare the prescriptive ASCE/SEI 7 approach with the "first generation" PBSE approach. Each system will then be subjected to rigorous evaluation using all four of the ASCE/SEI 41 assessment procedures, with particular emphasis given to nonlinear dynamic analysis, to assess compliance with seismic performance objectives found in ASCE/SEI 41. The buildings are being modeled in sufficient detail (including material and geometric nonlinearities) to capture all important response quantities. The design and assessment results will be vetted with small, highly qualified external peer review panels (PRPs) for both the BRBF and the RC SMF systems.
Six steel BRBFs have already been designed and the assessment process is well underway; publication of the complete assessment is projected to be finished in Q2 of FY 2016.
Four- and eight-story RC SMFs are being designed in accordance with the provisions in ASCE/SEI 7 and ACI 318-11 (ACI 2011). The design and assessment of the four-story building is complete, and a similar effort is in progress for the eight-story building, which will be completed by end of FY 2015. The study on the concrete four- and eight-story buildings will be reviewed by an extramural peer review panel (PRP) in Q1 of FY2016. A decision concerning proceeding with the design and assessment sixteen-story RC SMF building will be made following feedback from the PRP. Some leading practitioners have anecdotally questioned the need for examining RC SMF's of such heights, because it is perceived that RC SMF's are no longer being used for those heights. Assuming that a decision to proceed is made, the design and assessment of the sixteen story RC SMF will be conducted starting in Q2 FY 2016; completion would be anticipated in Q4 FY2016.
Based on the ASCE/SEI 41 assessment and comparison with the ASCE/SEI 7 design expectations, the applicability of using ASCE/SEI 41 for the design of new BRBF and RC SMF building structures will be assessed, giving potential insight into how well the ASCE/SEI 41 methodology meets the performance levels and design intent of ASCE/SEI 7. The final assessment will be the first of its kind, and will be disseminated through NIST Technical Notes (TN's), one for each building type, and two peer-reviewed journal papers as indicated in the project milestones.
The related study of the impacts of site conditions and ground motion selection approaches has been initiated in the summer of 2015. Two Pathways summer interns have worked on the project, under supervision of Drs. Sattar and Speicher. This phase of the work is anticipated to be completed in Q3 of FY 2016 but is partially dependent on the availabilities of student interns in 2016. If the current schedule is maintained, a journal paper will be submitted in Q4 of FY 2016.