To develop the technical means to mitigate the risk to the built environment from earthquake ground motions through improvements to building codes, applied research to identify new design approaches, development of fundamental information regarding behavior under strong earthquake shaking, and to lead the multi-agency National Earthquake Hazards Reduction Program (NEHRP) in providing information and tools for engineers, policy makers and the public to make informed decisions through a multi-faceted, coordinated outreach efforts.
What is the problem? Earthquakes represent one of the most destructive natural hazards on earth and while infrequent in any one location, are a high consequence, low-probability event of significant threat to the built environment. With the on-going concentration of global populations in cities, threats from strong shaking to these urban centers represent a growing problem. In the United States, strong earthquakes such as the 1811-1812 New Madrid, MO series and the 1906 San Francisco temblor raised some awareness, the real era of earthquake risk mitigation began with the 1933 Long Beach earthquake. Significant damage to schools resulted in the Field Act being enacted in California which improved structural design of K-12 schools and charted the course for improvements to building codes to mitigate risk. The 1971 San Fernando earthquake resulted in significant and unexpected damage to engineered structures designed to recent codes. The result was the National Earthquake Hazards Reduction Program enacted by Congress in 1977 to coordinate improvements to building codes and the built environment.
Significant progress has been made by the United States over the past 40 years to improve design methods and building codes, to increase knowledge of structural behavior under extreme loads, to improve the characterization of seismic hazards and to take advantage of the greatly enhanced analytical abilities now available with modern computers and software. That said, much remains to be done. Knowledge of system level behavior of structures is limited principally to analytical simulations. With increased costs, economic considerations can constrain engineers using prescriptive building codes and force the use of performance based seismic engineering (PBSE), yet this approach is relatively new and requires on-going research to calibrate its methodologies. Furthermore, these same economic realities point to the need for improved knowledge of structural element behavior under loading together with the need for new, improved materials to provide cost-effective design solutions.
The development of improved design approaches for new buildings has improved the quality of the building stock designed since the dawn of the NEHRP era in the late 1970’s. A larger and significant problem remains, that of existing buildings designed to older building codes. Studies published by the LA Times (2014) have shown the LA metropolitan region possesses over 2000 older reinforced concrete buildings that are considered structurally suspect given their age. A similar issue exists in Seattle where the Seattle Times (2016) has published extensively on the large numbers of unreinforced masonry buildings that remain in service. These examples point to the urgent need for improved methods to identify existing buildings needing assessment and to accurately estimate their likely performance under strong shaking and in turn to develop cost-effective means to strengthen these buildings.
While buildings may perform adequately under strong shaking, the performance of interior systems also is important. Damage to nonstructural systems in buildings can represent economic losses and can require significant repair times to bring the building back into operation. Thus, these systems also must perform adequately to meet the intended performance of the building as a whole both during the event and after.
Knowledge of the performance of soils under strong ground shaking remains an area of needed research. The 2011 Christchurch, NZ earthquake destroyed the central business district of the city when the soil liquefied under relatively modest shaking. Threats from liquefaction exist in many areas of the country including Memphis where the Mississippi River has deposited alluvial soil subject to liquefaction. Knowledge of the interaction of soil-foundation-structure systems remains a significant problem requiring on-going research and representing a significant opportunity for improvement by incorporating systems-level thinking in design.
Recent work at NIST into how to improve community resilience has dramatically shown the central role of lifelines in modern life. Improving the performance of isolated structures is important, yet without essential building services, these buildings may not be functional following an earthquake. Thus, improved knowledge of how lifelines perform under strong shaking is also needed. This knowledge includes not only the lifeline structure itself but the supporting geotechnical systems as well.
Lastly, there is a shift in philosophy that has slowly developed in the United States since the Loma Prieta (1989) and Northridge (1994) earthquakes. In these two events, in general newer buildings performed adequately with a few exceptions. However, economic losses including direct losses from damage and indirect losses from business interruptions were significant. Historically, seismic design of buildings to ensure life safety has been the approach over time. With these two events, the question of business interruption and the need to improve building performance to minimize post-earthquake business interruption is now a common consideration. Moreover, with the greater awareness of resilience, the need for post-event building operation is gaining attention. How to achieve not only life safety but now an immediate occupancy level of building performance is an open question requiring consideration. The solution is not one of simply increasing design forces but requires systems level evaluation to achieve the needed performance levels. Adoption of immediate occupancy performance will represent a drastic shift in thinking requiring thoughtful development of supporting technical knowledge.
Public policy also is an aspect of earthquake risk mitigation. While improvements to design methods and building codes are essential, adoption of stricter building codes and their enforcement also is important. Moreover, the existing building problem is a national issue that all jurisdictions face whether in areas of seismic risk or not. Existing buildings built to earlier, perhaps less stringent standards, are not required to be retrofitted unless repurposed or modified. Los Angeles is the first major city to address this issue with their landmark City Ordinance 183893 (LA 2017) requiring that buildings be evaluated within three years and brought up to minimum performance levels within 25 years. Public policy issues are not technical per se, but are at the heart of the existing building problem.
What is the technical idea?
Improving the performance of new and existing buildings and lifelines to earthquake shaking is the overall goal of the program. That said, with finite resources a program to address this problem in a logical and effective manner has been developed in large part since NIST became NEHRP lead agency in 2004. What is proposed here is a modification of this approach to better utilize existing resources and focus on critical research needs.
The technical idea involves four major efforts all aimed at improving the seismic performance of the built environment. These four efforts complement one another and are integrated so as to reach the overall program goal and include (1) Internal applied research efforts on specific topics, (2) extramural work to support NIST on specific topics, (3) leveraging NIST research efforts with partner organizations and (4) NEHRP lead agency roles and responsibilities including community outreach. The overall effort will continue to address important technical issues such as PBSE, developing basic performance data on structural elements and determination of the impact of uncertainty on design performance. New areas will include examination of collapse probabilities for buildings designed using PBSE or traditional prescriptive approaches, evaluation of next generation reinforcing steels and retrofit options for steel and concrete buildings. Working with partners such as the NIST-funded Center of Excellence in areas of common interest is an important way to leverage internal expertise. Lastly, study of public expectations of the condition of the post-earthquake built environment also will be begun.
What is the research plan? There are two major components in the NIST Earthquake Risk Reduction in Buildings and Infrastructure: (1) development of measurement science tools for design of new buildings design and evaluation and retrofit of existing buildings, (2) NEHRP lead agency responsibilities including community outreach.