Building-Level Seismic Performance Assessment

Summary

This project focuses on two research tasks promoting enhanced seismic building performance. Research Task 1 investigates the use of low-damage rocking systems as a cost-effective solution to minimize earthquake damage and enable timely recovery after major earthquake events. This research task consists of analytical studies, outreach and collaboration, and dissemination. The analytical component will focus on evaluating the seismic performance of a suite of archetype buildings, designed with conventional and low-damage approaches, to further assess the feasibility of rocking systems. The outreach component will involve a workshop convening experts in research and practice to inform recommendations on how to increase the use of rocking in building designs and to foster collaborations with industry stakeholders. The third component will focus on creating a technical guideline that will provide a tool for practicing engineers to implement rocking more readily into traditional building designs. This overall effort is envisioned as an initial attempt to promote wider use of low-damage technologies, synergistically supporting the goals of immediate occupancy, functional recovery and other general resilience-based goals widely circulating in the current public and technical discourse.

In research task 2 of this project, we focus on creating an enhanced understanding of the seismic performance of buildings typical of the central and eastern United States (CEUS). In certain regions of moderate seismicity, particularly CEUS, member sizes of the seismic force-resisting systems (SFRS) can be governed by wind requirements. If these buildings are assigned to Seismic Design Category (SDC) D, as defined in model building codes, seismic detailing of members and connections is required because of anticipated ductility demands on the SFRS. These detailing requirements may result in a significant increase in construction costs.

The effect of wind-dominant design on the seismic performance of steel buildings built in the CEUS has not been formally quantified. Also, irrespective of wind, the seismic performance (and associated fragility functions) of buildings in the CEUS is largely unknown. This project will investigate the seismic collapse performance of steel buildings in the CEUS through the following synergistic activities: (1) a NIST-sponsored workshop to identify overarching research needs for buildings in the CEUS, (2) the development of seismic fragility curves for new and existing (circa 1980) steel systems for use in community resilience efforts by the NIST-funded Community Resilience Center of Excellence (CoE) and (3) evaluation of the system strength provided by wind design that results in new steel systems that satisfy the current seismic collapse objective and assessment of cost savings via reduced seismic connection detailing. Activities (2) and (3) are contingent on the workshop findings.

Description

Objective:

To better understand the seismic performance of buildings designed in areas of medium-to-high seismicity and to identify ways to create enhanced performance through low-damage approaches, improved code provisions, and improved analysis procedures.

What is the Problem?

Despite progress made in the development of building codes that protect lives, communities continue to face costly and sometimes lengthy recoveries following major earthquakes. Earthquake-resistant buildings have traditionally been designed like an electrical circuit. Given an overload (strong earthquake), predetermined fuses (structural elements) of the building will be damaged and require repair. In some cases, damage is severe enough that the building is permanently deformed and may need to be demolished. For example, in the aftermath of the 2011 Christchurch Earthquake, the central business district (CBD) was cordoned off for several years due to extensive damage and approximately 70% of the building stock was demolished, including many newly designed structures (Pampanin 2015). The Christchurch experience clearly demonstrated that societal expectations for the performance of the built environment do not match the objectives set by current seismic design codes. With the recent publication of the FEMA P-2090 / NIST SP-1254 Functional Recovery Report, NIST researchers are leading efforts to advance recovery-based design objectives that prioritize post-earthquake building functionality in addition to occupant safety. A recommendation of the report is that new buildings be designed to support acceptable functional recovery times following a major earthquake. However, designing buildings for limited earthquake damage and rapid recovery using conventional construction practices may be uneconomical and/or challenging. New options for low-damage design need to be more widely available for the building design community.

What is the Technical Idea?

The project consists of the two research tasks (RT), which will be described separately:

RT1: Seismic Performance of Low-Damage Rocking Systems

The present surge of interest in designing structures with consideration of post-earthquake functionality has created the opportunity for innovative approaches to be brought into the mainstream of structural design. Rocking structural systems can be implemented to limit damage and preserve building functionality, while also ensuring occupant safety, offering a potentially inexpensive solution for earthquake-resistant construction that better aligns with societal expectations. The structural components in rocking systems are designed to uplift once the resistance provided by post-tensioning and/or their self-weight is exceeded. The ingenuity of the system is that the uplifting behavior protects the rocking elements from damage while utilizing gravity to re-center and prevent permanent deformation. Rocking systems have been implemented in a small number of buildings with little additional cost (ECQ 2020). Despite extensive research demonstrating their feasibility, wider implementation of rocking systems has been hindered, presumably due to a lack of knowledge, experience, technical design guidelines, and other factors. To bridge the gap between research and practice, this project will provide new information to the professional community that enables the wider adoption of low-damage rocking technologies in buildings.

RT2: Seismic Performance of the Built Environment in the Central and Eastern United States

Seismic design and assessment provisions in model building codes and standards have been developed primarily due to performance requirements for buildings and lifeline infrastructure located in regions of high seismicity along the western United States (WUS). Some provisions have been modified or extrapolated for consideration of anticipated regional design, construction, and seismological differences between the CEUS and the WUS, but little research has been performed to validate these extrapolations. Therefore, it is possible that some aspects of such extrapolations may be inadequate in terms of safety and/or cost-effectiveness, notably when high return period windstorms govern the design. To achieve a more accurate measure of resilience for the built environment in the CEUS, a comprehensive evaluation of current design and construction practices and needs is required. In this context, resilience addresses preparedness, mitigation, response, and recovery actions.

The first phase of this research task involved creating a suite of sixteen steel buildings designed for three locations in the CEUS for both the mid-1980s and the 2018 International Building Code. The two-building code designs provide an understanding of the seismic design, construction, and performance changes over time. These buildings have been added to the database of steel buildings to enable dissemination to the research community to further the understanding of building performance in the CEUS. 

The second phase of this research task will engage a contractor to develop and host a theme-focused workshop to develop research topics that detail the anticipated engineering principles, basic and applied research, and implementation activities required to advance seismic design and construction practices for new and existing buildings and lifeline infrastructure located in the CEUS. The impact of the efforts associated with this topical report is to improve the performance of the built environment in the CEUS for a major earthquake. The topical report will outline a systematic process with balanced, applicable topics that EEG can conduct in the future. This report represents the first effort required to evaluate and provide regionally accepted guidelines for seismically designing, evaluating, or retrofitting these systems to support the role they play in enhancing the resilience of the community they serve. Tentative topics include, but are not limited to, the following:

• Policies and practices for the design and construction of new buildings and lifeline infrastructure;
• Policies and practices for the evaluation and retrofit of existing buildings and lifeline infrastructure;
  • Identification of vulnerable typologies of the built environment and retrofit solutions
  • Identification of pilot areas/cities to be evaluated for seismic retrofit
• Analysis of regional hazards, including their temporal, consequential, and spatial occurrences;
  • Regionally adjusted ground motions for design and geo-hazards evaluation
• Decision support frameworks incorporating life cycle considerations and data collection;
• Societal needs to enhance resilience; and
• Recommendations for prioritizing topics.

What is the Research Plan?

The project consists of the two research tasks (RT), which will be described separately:

RT1: Seismic Performance of Low-Damage Rocking Systems

The project is focused around three major components: (1) an analytical investigation to evaluate and demonstrate benefits for a broad set of archetype buildings; (2) a workshop to understand impediments to and establish a roadmap for wider implementation; and (3) design guidelines to support industry uptake.

The analytical investigation will focus on developing a comprehensive suite of archetype buildings designed using conventional and low-damage rocking seismic force-resisting systems. The archetype space will include varying building heights and locations to assess the parameters that make rocking most advantageous. This includes exploring the sensitivity of rocking behavior to various ground motion characteristics. Nonlinear computer models will be developed to enable performance comparisons. Findings will be documented in a research report that includes a comprehensive assessment of the cost-benefit of rocking systems and helps identify the design space best suited for implementation of rocking. There are potential benefits of rocking in both the superstructure and foundation. The initial project efforts will focus on rocking of the superstructure. A plan will also be developed to incorporate foundation rocking.

In parallel with the analytical investigation, a workshop on low-damage rocking systems will be initiated. The workshop will invite leading practitioners and researchers to formally identify and document impediments to wider implementation of low-damage rocking technologies in practice.  Prior to the workshop, a roadmap outline will be developed to map out a plan for coordinated efforts to produce design guidelines and standards for rocking systems.  This effort will be carried out in collaboration with the current NEHRP Provisions Update Committee (PUC) Issue Teams working on rocking systems, as appropriate.  The outline will be disseminated as a read-ahead document for the workshop.  The findings of the workshop will be documented in a report.

The third component of this project will be to develop preliminary design guidelines for rocking reinforced concrete shear walls and/or steel braced frames.  Currently, design guidelines specific to U.S. practice do not exist for these rocking systems. These guidelines will be developed in collaboration with a group of external experts. The guidelines will ultimately help promote practical implementation of low-damage rocking systems.  The guidelines will also help pave the way for development of a pre-standard.

RT2: Seismic Performance of Buildings and Lifelines in the Central-Eastern United States

The first task resulted in a report of the building suite, with designs and associated information published and made available via www.NEHRP.gov. The second task is related to developing a workshop. A contractor is being engaged to develop and lead a team of experts to develop a comprehensive, concise report of research topics that detail needed research required to advance seismic design and construction practices for new and existing buildings and lifeline infrastructure located in the CEUS. In so doing, the contractor will develop and host a theme-focused workshop to finalize and prioritize the needed research topics.

Recommendations following the workshop will be considered to chart the path forward for this project.  This will include evaluating research needs on Risk Category III and IV buildings (e.g., hospitals). This will also give a clearer vision of the potential studies for the CEUS building suite.  This work will include investigating retrofit strategies or other important aspects of existing buildings in the CEUS as identified in the workshop.

---

References:

AISC (2016). Seismic Provisions for Structural Steel Buildings – ANSI/AISC 341-16. American Institute of Steel Construction, Chicago, IL.

ASCE (2016). Minimum Design Loads for Buildings and Other Structures. ASCE/SEI 7-16. American Society of Civil Engineers, Reston, VA.

ASCE (2017). Seismic Evaluation and Retrofit of Existing Buildings. ASCE/SEI 41-17. American Society of Civil Engineers, Reston, VA.

Dassault Systemes (2014). ABAQUS Unified Finite Element Analysis 6.14 [Software]. Waltham, MA.

ECQ (2020). “US Engineer Designs Resilient Building with a Little Help from his Kiwi Friends,”  The Earthquake Commission, https://www.eqc.govt.nz/news/us-engineer-designs-resilient-building-with-a-little-help-from-his-kiwi-friends

FEMA (2009). Quantification of Building Seismic Performance Factors. FEMA P695. Federal Emergency Management Agency, Washington, D.C.

FEMA-NIST (2021). “Recommended Options for Improving the Built Environment for Post-Earthquake Reoccupancy and Functional Recovery Time,” FEMA-NIST Special Publication FEMA P-2090/NIST SP-1254, 135 pp.

ICC (2008). “Strategic Plan for the National Earthquake Hazards Reduction Program,” Prepared by the Interagency Coordinating Committee (ICC) of NEHRP, www.nehrp.gov/pdf/strategic_plan_2008.pdf

Harris, J.L. and M.S. Speicher (2015a). Assessment of First Generation Performance-Based Seismic Design Methods for New Steel Buildings Volume 1: Special Moment Frames. NIST TN 1863-1. National Institute of Standards and Technology. Gaithersburg, MD.

Harris, J.L and M.S. Speicher (2015b). Assessment of First Generation Performance-Based Seismic Design Methods for New Steel Buildings Volume 2: Special Concentrically Braced Frames. NIST TN 1863 2. National Institute of Standards and Technology. Gaithersburg, MD.

Harris, J.L and M.S. Speicher (2015c). Assessment of First Generation Performance-Based Seismic Design Methods for New Steel Buildings Volume 3: Eccentrically Braced Frames. NIST TN 1863 3. National Institute of Standards and Technology. Gaithersburg, MD.

LSTC (2015). LS-DYNA R8.0 [Computer Software]. Livermore Software Technology Corp., Livermore, CA.