Research and Development for the Safety of Threatened Buildings

Objective:

To provide a technical foundation that supports improvements to building and fire codes, standards, and practices that reduce the impact of extreme threats to the safety of buildings, their occupants and emergency responders, and to move us toward the BFRL Goal of Homeland Security and Disaster Resilience.

Problem:

What is the problem? Building and fire codes in the United States exist, among other reasons, to ensure the safety of occupants in the event of anticipated excessive loads including those due to extreme winds and the likelihood of a probable worst case fire. The collapse of the World Trade Center (along with the terrorist attacks on the Pentagon, Hart Senate Office Building, and the Murrah Federal Building) has focused the general public, governments at all levels, and the construction and building products industries on the need to also understand the possible impacts of terrorist acts on building operations, structural integrity, and emergency response procedures, and on the need to develop economically justifiable strategies to mitigate the potential loss of life from future threats.

The prediction of failure modes in a closely-coupled building system is beyond our current capability, and standard test methods tell nothing of the expected performance of the building should the mechanical load (from wind) or thermal load (from a fire) exceed a prescribed value. In addition, building designers, owners, operators, occupants and first responders are faced with chemical and biological threats unforeseen prior to 2001.

Why is the problem hard? Societies dictate that buildings and other structures used routinely by civilian populations need to withstand severe, but anticipated, natural and manmade hazardous events, including fires and wind storms. The degree of severity anticipated is a choice made at the local or regional level, depending upon the state. A reliable data base has been established in many places on the occurrence and severity of natural hazards; however, setting the severity level for manmade fires or blasts (accidental or intentional) is complicated by three factors: the historical data base on these events is sparsely populated and/or unreliable, extrapolation of historical data to future events is highly uncertain due to changes in human activities, and almost all fires have the potential of being severe given the right set of circumstances that may be out of the control of the building designer. The technical challenge is to translate the impact of a specified fire into heat transfer through building materials and structural elements, accounting for breaching of building partitions, thermally-induced strain, and redistribution of loads; and eventually leading to local or global collapse. While for wind loads a more reliable data base may exist, the challenges faced by the designer are no less daunting due to complex ways that winds exert their forces on building and the close coupling of the overall building response to localized damage.

Technical issues surrounding possible chemical, biological or radiological (CBR) threats to buildings are even more ephemeral. Tough questions faced by building designers, owners, operators, regulators, and emergency service providers in response to possible CBR threats include: How should HVAC systems be designed and operated to contain a poisonous aerosol or gas? How has peoples' behavior changed since 9/11 in response to an emergency? Should the same emergency egress and fire service access techniques and strategies be used in the case of a biological threat as for a fire? Can new technologies be developed or design practices be adapted to increase the safety of the building occupants without undue economic burden on the owners/operators?

How are these problems solved today and by whom? The standard test methods and building practices upon which current building and fire codes are based rank the performance of one material, component or subsystem against alternative designs, with the expectation that some minimum rating translates into a sufficient level of safety of the material, component or subsystem when installed in the actual building. Safety factors are used to account for our ignorance abut the magnitude of actual loads, and for the uncertainty in response of the complex building system to these loads. Prior to the start of this Program, the problem in wind engineering was solved largely by using traditional, seat-of-the-pants “magic numbers” and simplifying tables and plots that were proven to be adequate for typical structures, but when applied to complex structural systems may result in inconsistencies with respect to risk that are detrimental from the safety and economy points of view.

Although performance-based building and fire codes exist on the books, they are used sparingly and limited to very specific situations. The more general problems associated with the entire building response to an extreme threat remain unsolved.

Approach:

The Safety of Threatened Buildings (STB) Program is part of the response of NIST to the events of 9/11, and has been developed through extensive discussions and partnerships with industry, academia, professional societies, codes and standards organizations, emergency services, and other government agencies. The Program is responsive to recommendations from the 2002 FEMA Building Performance Study and the 2005 NIST Federal Building and Fire Safety Investigation of the World Trade Center Disaster Final Report on the Collapse of the World Trade Center Towers. The Program is also responsive to the specific hazards posed by the introduction of a noxious aerosol into the environment such as occurred in the Hart Senate Office Building and a blast such as occurred in the Murrah Federal Building. In addition the Program is responsive to hazards posed by extreme winds and the need to develop analytical approaches and cost-effective risk management solutions to multi-hazard failure scenarios. The fruits of the research in this Program are not limited to the specific failures mentioned above, and will be applicable to the built environment in general.

Closely coordinated with the STB Program are BFRL's Advanced Fire Services Technology Program, which addresses the equipment standards and guidelines needs of emergency responders, and the National Earthquake Hazard Reduction Program (NEHRP), which addresses the problem-focused research needs for accelerating the adoption of appropriate risk-mitigation solutions. The STB Program, on the basis of the research results, also assists the model building and fire codes and standards organizations to understand and adopt, as appropriate, the recommendations in NIST NCSTAR 1, and additional recommendations resulting from technical investigations by NIST into other building failures.

What are the new technical ideas and why can we succeed now? The fundamental new idea underpinning this program is that the safety of threatened buildings can be enhanced significantly by developing a robust capability to predict the effects of hazards (fire, wind, blast, CBR aerosol releases) and competing innovative technologies/designs on the performance of building systems. This will be achieved by developing validated data to characterize the threat, validated physics-based models to predict performance, metrics for measuring performance, acceptance criteria for differing levels of performance objectives, and mitigation strategies based on evaluated performance. The convergence of three factors makes it possible to achieve success now: demand from the general public and policymakers for enhancing the safety of public buildings and reducing losses from future disasters, demand from the private sector to fill science and technology gaps, and recent advances in the relevant disciplines and in computational capabilities that allow physics-based solution of these highly computer-intensive problems.

Four general areas of research have been targeted to support near and long term improvements to reduce the vulnerability of the structure, building occupants and first responders to extreme manmade and natural threats:
• Structural Integrity
• Fire Resistance
• Emergency Egress and Access
• Emergency Building Environmental Technologies and Standards

Recent Results:

Structural Integrity