Structural Performance Under Multi-Hazards Program

This program addresses the gap between basic research and building codes, standards, and practice through measurement science research to: (1) predict structural performance to failure under extreme loading conditions: (2) predict disaster resilience at the building and community scale; (3) assess and evaluate the ability of existing structures to withstand extreme loads; (4) design new buildings and retrofit existing buildings using cost-effective, performance-based methods; and (5) derive lessons learned from disasters and failures involving structures.

Objective:  To develop and deploy advances in measurement science to enhance the resilience of buildings and infrastructure to natural and manmade hazards by 2016.

What is the problem?   Natural and manmade disasters cause an estimated $57 billion in average annual costs (and growing), with catastrophes like Hurricane Katrina and future “Kobe” earthquakes causing mega-losses exceeding $100B. Existing extreme load-related prescriptive requirements of building codes, standards, and practices stifle design and construction innovation and increase construction costs. The risk in large disaster-prone regions of the Nation is substantially greater now than ever before due to the combined effects of development and population growth. As noted by the National Science and Technology Council, “…a primary focus on response and recovery is an impractical and inefficient strategy for dealing with [natural disasters]. Instead, communities must break the cycle of destruction and recovery by enhancing disaster resilience.”^[1]

The link between basic research and building codes, standards, and practices is weak. Further, the measurement science is lacking to: (1) predict structural performance to failure under extreme loading conditions: (2) predict disaster resilience at the building and community scale; (3) assess and evaluate the ability of existing structures to withstand extreme loads; (4) design new buildings and retrofit existing buildings using cost-effective, performance-based methods; and (5) derive lessons learned from disasters and failures involving structures.

Why is it hard to solve?   The natural processes that produce risks in the built environment and the information relative to those risks for use by design professionals, standards developers, and emergency planners are not well understood. Cost-effective mitigation strategies that improve the performance of structural systems are complex, often lying outside the breadth of the prescriptive procedures that dominate building codes, standards, and practices. Methods for transferring basic research results into practice are limited. The engineering community lacks standard methods of predicting, evaluating, and assessing the disaster resilience of structures as they respond to extreme loads. Communities lack standard methods of assessing disaster resilience at the community scale for use in making disaster preparedness and mitigation decisions.

How is it solved today, and by whom?   The problem is not solved today although progress is being made. The disaster resistance of structures and disaster resilience of communities is determined by building codes, standards, and practices used when structures were built – most older structures have only minimal resistance. Most codes, standards, and practices are highly prescriptive, simplified, and inconsistent with respect to risk – stifling innovation and increasing cost. There is a lack of validated tools and metrics to evaluate structural and community performance, as well as the risks to which they are exposed – the lack of accurate models increases conservatism and decreases cost-effectiveness. Codes and standards are developed by private sector organizations that often lack the resources needed to develop the technical bases to improve them – practices, codes, standards used in design, construction, and retrofit are based largely on research performed or supported by the government.

Why NIST?    This program supports the EL mission of promoting U.S. innovation and competitiveness by anticipating and meeting the measurement science, standards, and technology needs of the U.S. building and fire safety industries in ways that enhance economic security and improve the quality of life. The program supports the EL core competency in resilience and reliability of structures under multi-hazards. The program further fulfills a national knowledge transfer role that is not well-supported by a fragmented U.S. construction industry (ACI 318, AISC, ASCE 7). Finally, NIST has statutory responsibilities including: (1) the National Windstorm Hazards Reduction Act (2004); (2) the Fire Prevention and Control Act (1974); and (3) the National Construction Safety Team Act (2002).

What is the new technical idea?  The fundamental new idea is that disaster resilience can be enhanced significantly by developing a robust capability to predict the effects of hazards on the performance of complex structural systems and on community-wide response. This will be achieved by developing: (1) validated data to characterize the hazard environment; (2) validated physics-based models to predict performance of structures to failure; (3) metrics for measuring performance; (4) acceptance criteria for differing levels of performance objectives; (5) mitigation strategies based on evaluated performance; and (6) science-based tools to estimate losses and predict resilience at the community scale.

The scope of the research includes extreme wind engineering and structural fire resistance with progressive collapse and multi-hazard failure analysis being cross-cutting research topics. Development of cost-effectiveness tools for evaluating multi-hazard risks at the community scale and implementation of the World Trade Center recommendations for research and development are also a part of this program.

Why can we succeed now?  It is possible to succeed now because there is strong demand from the general public and policy makers for enhancing disaster resilience of communities and reducing losses from future disasters as well as demand from the private sector to fill science and technology gaps. Recent advances in the relevant technical disciplines and in computational capabilities make possible significant advances in the component research topics. Finally, there is an increasing body of fundamental structural behavior knowledge available from NSF-supported basic research.

What is the research plan?   The program consists of five research thrusts: 

(1) Develop validated tools that predict structural performance to failure under extreme loading conditions. This research thrust consists of three elements:

• Develop an improved understanding of the hazards to the built environment. The outcomes of this element will include: innovative methods for defining design wind speeds; risk-based storm surge maps; and improved understanding of fire loads on structures.
• Develop validated structural response models that characterize structural response from initial loading to failure for individual hazards (e.g., wind, fire) and within a multi-hazard context. The outcome of this element will be rational assessment of safety and reliability of structures at specified performance levels for individual hazards and within a multi-hazard context.
• Develop validated, simplified tools to characterize structural response from initial loading to failure for individual hazards and within a multi-hazard context. The outcome of this element will be validated, simplified tools that can be used by practicing structural engineers in routine structural design.  

(2) Develop community-scale loss estimation tools to predict consequences of disasters, leading in turn to increased resilience. This research thrust consists of two elements:

• Develop science-based tools to assess disaster resilience and estimate losses at the community scale due to individual hazards and within a multi-hazard context. The outcome of this element will be rational assessment of potential community-scale losses due to individual hazards and within a multi-hazard context.
• Integrate science-based tools for loss estimation at the community scale with cost-effectiveness tools for risk management technologies. The outcome of this element will be decision support tools for risk mitigation at the community scale.

(3) Develop validated tools to assess and evaluate the capabilities of existing structures to withstand extreme loads. This research thrust consists of three elements:

• Develop validated tools for use in initial visual evaluation and in simplified analyses. The outcome of this element will be rapid visual screening methodologies and simplified analytical tools to evaluate the ability of existing structures to withstand extreme loads.
• Develop validated models for detailed analysis from initial loading to projected failure. The outcome of this element will be experimentally validated, high-fidelity models of the behavior of existing structures in response to extreme loads, from initial loading through collapse.
• Develop validated, simplified models for routine analysis from initial loading to failure. The outcome of this element will be validated, simplified tools that can be used by structural engineers in routine practice for analysis of the response of existing structures to extreme loads from initial loading through collapse.

(4) Develop performance-based guidelines for cost-effective design of new buildings and, where warranted, rehabilitation of existing buildings. This research thrust consists of four elements:

• Develop acceptance criteria for different performance levels. The outcome of this element will be published performance criteria for structures subjected to extreme loads and under progressive collapse.
• Develop performance-based design guidelines for new structures. The outcome of this element will be published design guidelines for new structures to address individual hazards, multi-hazards, and progressive collapse.
• Develop cost-effective mitigation strategies for existing structures. The outcome of this element will be published guidelines for mitigation strategies for existing structures to individual and multi-hazards and progressive collapse.
• Develop performance-based pre-standards for new and existing structures. The outcome of this element will be to provide to standards bodies performance-based pre-standards to address individual and multi-hazards and progressive collapse as well as for testing of fire resistive materials and fire resistant steel used for building construction. 

(5) Derive lessons learned from disasters and failures involving structures. This research thrust consists of three elements:

• Develop and implement procedures for initial site reconnaissance and perform initial site reconnaissance as required. The outcome of this element will be to conduct initial site reconnaissance efforts using uniform procedures for site access, data collection and archiving, reporting on findings, and criteria for recommending more detailed investigations.
• Perform and report on comprehensive technical studies, when warranted, involving specific structures or classes of structures. The outcome of this element will be documented findings and conclusions from the studies, recommendations for changes to practices, standards, and codes to reduce the potential for similar failures in the future.
• Develop national data archiving capabilities and implement information dissemination technologies. The outcome of this element will be a national resource database to store and broadly disseminate findings from studies of disaster and failure events. 

How will teamwork be ensured?   Within each of the component projects, the individual team members have been assigned based upon their specific expertise and have well-defined, complementary roles within their projects. This program is highly synergistic with EL’s National Earthquake Hazard Reduction Program (NEHRP) and there will be close collaboration between the two programs. Established collaborations with the Fire Research Division, the Office of Applied Economics, the Statistical Research Division, and Mathematical and Computational Sciences Division (ITL) will bring important capabilities to bear on the component research projects. Partnerships with other Federal agencies complement the capabilities of the NIST team (e.g., large-scale experiments).

What is the impact if successful?   The impact of this program will be significantly enhanced disaster resilience (with respect to extreme loads) of the nation’s communities and built environment. This will result in reduced societal risk and reduced cost and operational impacts of disasters on individuals, businesses, and government. The program will also foster a transformation from prescriptive to performance-based design codes and standards. This transformation will enable innovation in materials, technologies, and system designs and foster cost-effectiveness, thus enhancing the U.S. construction industry’s international competitiveness.

A number of key stakeholder groups will have an interest in the outcomes of this program. At-risk communities and the American public are a key stakeholder and beneficiary of this program. Government at all levels that is responsible for pre-disaster mitigation and for response, recovery, and rebuilding in the aftermath of catastrophic disasters will also have a keen interest in the products of this research program. Design and construction practitioners, facility owners and operators, standards and codes developers, state and local building officials, and property risk insurers will all benefit from the results of this research.

Impacts already achieved by the program include the 40 model building and fire code changes consistent with the NIST WTC investigation recommendations now required by the International Code Council’s (ICC) I-Codes. Similarly, the National Fire Protection Association (NFPA) has adopted 15 changes responsive to the World Trade Center Recommendations for inclusion in the 2009 Editions of the NFPA 5000 Building Code, NFPA 1 Fire Code, and NFPA 101 Life Safety Code.

Specific EL Products

Near-Term (by 2012):

• Guidelines for vulnerability assessment of new and existing buildings for disproportionate collapse potential – these will help identify the needs for cost-effective solutions for designing new buildings or retrofitting existing buildings to reduce the potential for disproportionate collapse. (70% draft complete; final guidelines to be issued in FY2012 following industry workshop.)
• Best practices guide for fire resistance design of concrete and steel building structures – the guide will broadly disseminate current best practices to design professionals leading to more cost-efficient and rational approaches for structural fire resistance in new buildings. (Complete)
• Software tool and user guide for the database-assisted design of steel and reinforced concrete tall buildings for wind – this will provide design professionals with a tool for more efficient and cost-effective design of buildings for wind. (Complete)
• Map-based software tool for computing the hazard associated with combined hurricane wind/storm surge events – this tool will enable the development of realistic hazard maps for coastal communities at risk of exposure to hurricane winds and storm surge. (Complete)

Medium-Term (3-8 years):

• A first-of-its-kind facility in the U.S. for conducting large-scale tests of structures exposed simultaneously to realistic structural loads and fire conditions – this will provide experimental validation of performance-based design tools leading to more efficient, cost-effective, and safer designs for structural fire resistance. (Under construction; building to be complete in FY2012.)
• Standard for the design of new buildings and retrofit of existing buildings to resist disproportionate collapse – this will lead to safer buildings by providing the standard for designing buildings to prevent disproportionate collapse. (In progress.)
• Guidelines for performance-based design of structures for fire – this will lead to safer and more cost-effective buildings by providing the technical basis and fire scenarios needed to consider fire as a design condition.
• New standard wind speed hazard map – this will lead to large reductions in annual losses from windstorms and to more efficient design through more realistic estimation of the wind hazards throughout the United States. (To be proposed for 2016 edition of ASCE 7.)
• Decision-support tool for estimating community-scale losses – this tool will lead to the reduction of uncertainty related to the performance of new building codes and standards on the disaster resilience of structures and communities.

Long-Term (beyond 8 years):

• Validated predictive tools for fire safety design and retrofit of structures – this will lead to more efficient design of new buildings and retrofit of existing buildings through experimentally validated, performance-based design tools.
• Standards for performance-based design of structures for fire – this will lead to safer and more cost-effective buildings by providing the validated technical basis, supporting technical and cost data, and detailed fire scenarios to consider fire as a design condition.
• Integrated software tool combining computational fluid dynamics (CFD) and database-assisted design of buildings to resist wind loads – this would lead to safer, more cost-effective, and repeatable design of buildings by reducing reliance on wind tunnel testing since the CFD model will be used to represent realistic extreme wind conditions.
• New maps of hazards due to combined hurricane wind speed/storm surge events for selected high-risk U.S coastal areas – the new hazard maps will lead to more resilient coastal communities by providing a realistic basis for building design standards.
• Standard for robustness-based design of buildings – this will lead to improved system safety performance and resilience of building structures under multi-hazards.
• Standard method to assess resilience of the built environment – deliver an enhanced science-based standard method that can be used to evaluate disaster resilience for multiple hazards at the community or regional scale.

What is the standards strategy?  The component projects that make up the Structural Performance Under Multi-Hazards program have been structured to produce specific major products that can be transferred directly to standards development organizations. Project team members have established themselves in key standards committee positions within ASCE/SEI 7, AISC, ACI, and ISO that are directly relevant to the current research areas and consistent with the EL mission. Furthermore, the program has recently submitted an application to participate as a member of the IBC-Structural committee, which will strengthen the ability of the program to work directly with the code bodies to affect improvements to the code where appropriate and relevant to the results of our research and consistent with the EL mission and the desire of ICC to work more closely with NIST.

Within the structural engineering community, ASCE/SEI 7 is the standard for minimum design loads for buildings and other structures. It is referenced by the International Building Code and in turn references ACI 318 and AISC Design Specifications for Structural Steel. Focusing on these standards ensures that the provisions based on NIST research will have broad application in the industry.

Top standards development needs for the program include the following:

1. Development of standards for design of new buildings to resist disproportionate collapse. (pre-standard planned for FY2013)
2. Development of standards for performance-based design of structures for fire. (pre-standard planned for FY2013)
3. Development of performance-based guidelines, criteria, and metrics for resilient structures.
4. Development of new wind hazard map. (planned for FY 2013)
5. Development of a performance-based standard to assess structural fire resistance in existing buildings. (planned for FY2013)

The above standards are generally updated on 5-year basis. For example, the next opportunity to introduce new or updated standards into ASCE/SEI 7 is in the 2016 edition. The major research products that lead to new or significantly improved code provisions are introduced during these major code cycles. In some cases, opportunities may be available on a different cycle to implement supporting provisions in standards documents that are referenced by the above standards. 

How will knowledge transfer be achieved?  Knowledge transfer will be accomplished through several mechanisms. Research results will be disseminated to key standards, codes, and industry organizations including: American Society of Civil Engineers/Structural Engineering Institute, American Institute of Steel Construction/American Iron and Steel Institute, American Concrete Institute, ASTM International, International Standards Organization, International Code Council, and the National Fire Protection Association, through active participation by NIST staff and submittal of pre-standards for consideration by the appropriate organizations. NIST also collaborates with industry and academia through grants and other mechanisms to conduct complementary research. The NIST guest researcher program and other vehicles allow collaboration with experts through joint research efforts. NIST provides guidance to industry on best practices, design, and rehabilitation techniques to withstand extreme loads, and assessing disaster resilience at the community scale. Finally, NIST publishes research results in peer-reviewed journals and conference proceedings.

Major Accomplishments:

Prevention of Disproportionate Structural Collapse

• Structural integrity requirements for tie reinforcement submitted by NIST based on experimental and analytical research have been incorporated in the ACI 318-08 Building Code. (Impact)
• Structural integrity requirements proposed by the Ad Hoc Joint Industry Committee on Structural Integrity (Lew, committee member) have been adopted for the 2009 IBC. (Impact)
• Developed a set of robust structural integrity requirements for steel and reinforced concrete structures for incorporation into the AISC and ACI design specifications. (Outcome)

• Developed draft of “A Guide to Assessing Vulnerability of Buildings to Disproportionate Collapse”^[2] in collaboration with industry. Document will be finalized in 2012. (Outcome)
• Developed experimentally validated 3D models of steel and reinforced concrete frame buildings for assessment of reserve capacity and vulnerability to disproportionate collapse. (Outcome)
• Developed a methodology for assessing robustness of steel and concrete framed buildings based on the quantification of reserve capacity and the ability of the structural system redistribute loads using a series of push-down analyses. (Outcome)

Fire Resistance Design and Retrofit of Structures

• Code change regarding fire protection of secondary structural members (structural frame requirements), based on the WTC recommendations, was adopted to the 2009 IBC code. (Impact)
• Submitted stress-strain relationship for structural steels at elevated temperature for adoption by the 2016 AISC Specification, June 2010. (Outcome)
• Conducted 4 seminars on the draft Best Practice Guidelines for Fire Resistance Design of Concrete and Steel Buildings^[3]and published the final version on the NIST website. (Outcome)

Wind Engineering and Multi-Hazard Failure Analysis

• Development of the American Nuclear Society (ANS) Standard 2.3, Standard for Estimating Tornado, Hurricane, and Extreme Straight Winds (Simiu, Contributor, 2010) (Impact)
• Development of ASCE 7-10 Standard (Major Contributor, 2010), including incorporation of Database-Assisted Design techniques in ASCE 7-10 Commentary; Chapter C31; incorporation of correspondence between Saffir/Simpson scale and ASCE 7-10 Standard basic speeds; incorporation of requirement on directional wind climatological/aerodynamics interface. (Impact)
• NIST developed database-assisted design procedures that substantially improve modeling of wind effects on both low- and high-rise structures. The procedures incorporate:
  • Software for tall buildings (steel and reinforced concrete (EL near-term strategic outcome).
  • Novel synchronous pressure measurement technologies, capable of capturing the imperfect spatial coherence of pressures on the building facades.
  • Modern computational capabilities, including wavelet-based data compression techniques that enable the use of large datasets by design practitioners.
  • Findings based on NIST-led investigations that established the need to improve the physical and probabilistic modeling of design criteria, including a world-wide inter-laboratory comparison of wind effects estimates.
  • Multi-hazard approaches to developing mixed probability distributions of hurricane, large-scale extratropical storms, thunderstorm winds, and to wind and seismic effects.
  • Innovative methods for establishing the heretofore unknown strength reserves of engineering structures due to post-linear behavior under wind loads.
  • Accurate, transparent, user-friendly approaches to calculating wind effects on tall buildings, made possible by the capability to solve simultaneously differential equations of dynamic motion and to replace obsolete and impractical spectral methods developed in the 1960s and still used to date.
  • Novel probabilistic approaches to establishing design values consistent with societally-acceptable reliability levels.
  • Relations between the Saffir-Simpson hurricane classification and specified wind speeds required in the design process. (Outcome)
• NIST developed a methodology for incorporating effects of waves on total storm surge calculation using NOAA’s SLOSH model. (Outcome)
• Map-based software tool for extracting joint probabilities of combined hurricane wind speed and storm surge hazards, report in progress, 2010.^[4]  (Outcome)
• NIST developed a methodology for estimating the risk posed by the combined effect of hurricane wind and storm surge on specific coastal location (accounting for local topography). (Outcome)

Fire Resistive Materials for Structural Steel

• Measured a more than 50% drop in adhesion for commercial SFRM when exposed to high humidity. (Outcome)
• Developed a new sample geometry (trapazoid) that greatly simplifies the measurement of adhesion. (Outcome)

Standard Methods to Assess the Resilience of the Built Environment

• An ASTM-based approach for developing standards that model losses and help stakeholders assess disaster resilience built around ASTM’s E06 Committee on Performance of Buildings and ASTM’s E54 Committee on Homeland Security. (Output)
• Revisions to both E 2506 and E 2541 have been approved for balloting by subcommittees E06.81 and E54.02. (Outcome)

Recognition of EL:   Department of Commerce Special Act Award presented to staff members involved in the investigation of the World Trade Center 7 Collapse Investigation.  Department of Commerce Silver Medal awarded to Stephen Cauffman, Long Phan, Fahim Sadek, and Dat Duthinh for conducting the reconnaissance of the performance of physical structures following Hurricane Katrina and Rita (2007). Department of Commerce Bronze Medal awarded to Long Phan and Emil Simiu for work leading to the development of the Enhanced Fujita Tornado Intensity Scale (2006). Department of Commerce Gold Medal awarded to the World Trade Center Investigation Team (2005). Invited lectures at international institutions.