Mass Protection

Performance-based fire-safety design, still in its infancy in the United States, has created an environment in which more architects, engineers and building owners are asking, "Is it safe?" rather than, "Does it comply with code?" This was the original intent of performance-based design and it appears to be successful in its initial stages.

05/01/2001


Performance-based fire-safety design, still in its infancy in the United States, has created an environment in which more architects, engineers and building owners are asking, "Is it safe?" rather than, "Does it comply with code?" This was the original intent of performance-based design and it appears to be successful in its initial stages. However, another trend outside of fire-related performance-based design—chemical/biological (CB) building protection—is emerging. The relaxing of the traditionally prescriptive-based approach to fire protection is making it a better partner with the traditionally performance-based disciplines of building security design and CB building protection. It is useful to highlight the natural convergence of these fields that so often share similar goals.

Fire safety and security—with or without CB building protection—are often significant parts of any major construction effort. However, a disconnect often exists between life-safety requirements—getting every occupant out of a building safely—and security—preventing unauthorized access. One simple example is the fact that doors are required to be open and accessible for life safety in a fire emergency. However, these same doors may also be required to be locked and inaccessible due to security considerations. While there are prescriptive code requirements that address this general situation, there is little consideration for new technologies that may ultimately provide a superior solution.

This example typifies the challenges that often arise when fire and life-safety enforcement strictly adheres to prescriptive code requirements that leave little margin for interpretation. Security and CB building-protection requirements, however, are often based upon the desire and funding of the owner, whether the facilities are commercial or government-owned. Unfortunately, the rigidity of prescriptive codes can make security and CB building-protection measures difficult or impossible to implement. This conflict is especially frustrating, because the goals of these disciplines are often very similar.

In addition, the development and adoption of prescriptive codes can be a lengthy process, thereby limiting the impact of emerging technologies or approaches. Thus, the code-development and -adoption process that defines the construction and design of the vast majority of U.S. buildings is a rigid one and a process with inherent difficulties in reacting to changing conditions and circumstances.

One example is the rapidly emerging threat posed by terrorist and other chemical or biological attacks. The growing concern over such threats and significant technological advances have exposed the discipline of CB building protection to a wider base of projects. While dealing with terrorist attacks is clearly not within the scope of traditional building-code and life-safety documents, many buildings that are susceptible to such threats are being designed accordingly. Unfortunately, building and life-safety codes have no means for dealing with these situations; so the implementation of effective security measures can often be frustrating and self-defeating.

Fortunately, the advent of performance-based design has created more opportunities for fire, security and CB building protection to be developed in coordination with one another. As a direct result of this convergence, the tools and analyses used for performance-based design are being applied to CB building protection.

A performance-based alternative

Performance-based fire-protection design is an engineering process that establishes and evaluates protection options based upon the inherent features of a building or site (see "Codes and Creativity," on page 48). Overall, project goals are determined to allow designers to describe a protection approach that meets certain objectives, such as allowing enough egress time. The objectives are quantified in terms of such variables as smoke concentration or obscuration, and tested by means of deterministic or probabilistic quantitative analyses. Design fire scenarios and trial designs are then developed and evaluated against performance criteria utilizing deterministic or probabilistic analysis of fire scenarios. Successful designs can then be implemented. This approach has been used on numerous U.S. projects of varying complexity.

Of course, the performance-based methodology varies drastically from the more widely utilized prescriptive approach used by major U.S. building codes, in which strict adherence to specified criteria forms the basis of the safety level. Of course, many unique structures may have difficulty meeting these minimum requirements, but that doesn't necessarily mean that they do not provide an acceptable level of life-safety measures. It can, however, cause a breakdown of the prescriptive codes, while a performance-based design can evaluate the level of safety based upon the building's inherent configuration and use.

While performance-based design approaches have been used for years in such code-mandated disciplines as structural engineering, its application to security is unique. First of all, building and life-safety codes, for the most part, do not regulate security. Furthermore, the implementation of security approaches has been driven by market needs and has increased with public concern. So, unlike such traditional, regulated fields as fire-protection engineering, the level of security design often relates directly to the building owner's desired capital expenditure.

Attractive targets

This is even more apparent in the field of CB building protection. Addressing defense against CB attacks requires a performance-based approach that not only addresses current hazards, but also protects against the widest array of future threats. In practice, therefore, the field of CB building protection may be defined as the true cutting edge of performance-based design. Internal or external to a building, the release of a CB threat agent—or toxic industrial material (TIM), such as chlorine—could have serious impacts upon the health of occupants and equipment within the facility. Buildings are attractive and vulnerable targets for several reasons:

  • Containment of CB agents and TIMS within a confined space allows threat-agent concentrations to remain at high levels for extended periods of time.

  • Threat agents can betransported by mechanical systems . Air exchange via mechanical ventilation and natural infiltration could serve to increase exposures and intensify CB decontamination problems.

  • Population densities are high in buildings. Day and night, people spend the majority of their time indoors, making such targets ideal.

  • There is potential to deliver agents covertly, via mail packages, standoff attacks—external releases of CB agents—and in food or water.

  • Terrorists can even target specific individuals or resources.

  • Terrorists can remotely control building systems.

  • Numerous absorbing surfaces—carpet, insulation, and gypsum board—make restoration of operations difficult in buildings.

These vulnerabilities have caused high-risk government and commercial facilities to begin taking action. Battelle Memorial Institute, for example, has worked with security personnel and building owners to conduct threat, vulnerability or protection assessment at over 100 high-risk U.S. facilities.

The greatest protection challenges are posed by internal releases and biological threats. Internal threats are more difficult to address than external threats because even the smallest quantities of certain threat agents can create a lethal environment throughout a building. This intensifies the decontamination problem and makes potential protection solutions more complex. In a recent study, researchers at Battelle Memorial Institute conducted a field test in a small office building showing that just a few ounces of a simulated vaporized chemical agent could produce lethal levels throughout the entire facility within minutes.

Biological threats are distinct from such chemical threats: they are more difficult to rapidly and positively identify; their effective dose is generally significantly smaller; and their physiological effects are typically not discernible until after a lengthy incubation period. These factors—coupled with the inherent variety of architectural designs and the abundance of protective technologies available from military equipment vendors—call for a rigorous performance-based approach to arrive at a cost-effective solution that meets occupant needs.

The best approach for the unique protection requirements posed by CB threats involves developing a performance-based process consisting of threat and vulnerability assessments followed by detailed protection assessments. These form the technical basis for the design and implementation of detailed protective solutions. The assessments utilize a wide variety of tools and methodologies that are applicable across protection disciplines. For example, the process can employ computational-fluid-dynamics (CFD) codes developed to analyze smoke transport, such as the National Institute of Standards and Technology's Fire Dynamics Simulator, or FDS, to analyze the movement of chemical agents in buildings.

By defining the protection performance required for each application, solutions can be selected based on technology-implementation cost rather than on the basis of required level of protection. Low-cost, minimum-protection solutions include evacuation and sheltering-in-place . Expedient protection devices —also known as rapid-deployment devices—and enhanced sheltering-in-place options offer a higher level of protection at a moderate cost level. For high-risk assets, higher-cost, maximum-protection solutions such as stand-by and continuous positive-pressure filtration systems can be implemented. Given the vast difference in levels of protection and their associated costs—from hundreds of dollars to millions of dollars—the use of performance-based design effectively evaluates the level of protection required based upon the perceived threat, facility configuration and operational requirements.

Performance-based protection

Clearly, the advent of performance-based design for fire-protection and life-safety systems offers engineers an opportunity to enhance the flexibility, practicality and cost-effectiveness of a project—when used judiciously. However, the role of performance-based design should not end. An opportunity is missed if other performance-based disciplines, such as security and CB threat assessment, are not integrated into the entire master plan as appropriate.

It would be a great advantage for the overall success of any project—where the threat of a chemical or biological attack is present—for the engineer and the project team to consider using a performance-based design approach. Furthermore, the the complete integration of all fire-protection, life-safety, security and CB threat-assessment systems should be developed as part of the performance-based design strategy. Otherwise, the maximum potential benefits of a true performance-based approach may not be realized.



Chemical/Biological Defense: The Cutting Edge of Performance-Based Design

Performance-based design is not solely applied to fire-protection engineering; it has been used in other building-code-mandated disciplines—such as structural engineering—for many years. Unlike these disciplines, however, security design is not regulated by prescriptive building and life-safety codes, so it is inherently performance-based.

Moreover, the implementation of security systems and procedures has been primarily market driven and has tended to increase with growing public concern over security. The building owner's desired capital expenditure, therefore, drives the level of security. This makes security design quite distinct from such traditional fields of endeavor as structural engineering and fire-protection design.

The twin drivers of performance and cost are even more apparent in the field of chemical and biological (CB) building protection. Protecting structures from CB attacks requires a performance-based design approach that not only addresses current hazards, but also protects against the widest array of future threats.

In practice, therefore, CB building protection may be said to define the true cutting edge of performance-based engineering. Through its application, engineers can more fully appreciate the challenges and opportunities of performance-based work.

Codes and Creativity: Design Approaches for Life Safety

Performance-based design for fire-protection engineers is a process that establishes, documents and evaluates design and protection options based upon the inherent features of a building or site. In the past, there were few formalized methods for conducting a thorough performance-based design analysis. However, much-needed structure has been offered recently by the Engineering Guide to Performance-Based Fire-Protection Analysis and Design of Buildings —published by the Bethesda, Md.-based Society of Fire Protection Engineers (SFPE)—and the "Performance-Based Design Option" in the Quincy, Mass.-based National Fire Protection Association's 2000 Life-Safety Code . These documents have also helped improve the practice and influence of the performance-based approach for life-safety system designs.

A simplified description of the performance-based approach to fire-protection design is to develop overall project goals, such as minimizing fire-related injuries and preventing loss of life for individuals not intimate with the origin of the

fire. From those goals, basic objectives are developed; a typical example might be providing adequate time for building occupants to reach a safe place before being exposed to untenable conditions. These objectives are quantified through the use of basic performance criteria, such as:

Temperature of materials or gases.

Smoke-concentration levels or smoke-obscuration levels.

Carboxyhemoglobin levels or radiant-flux levels.

Design-fire scenarios and trial designs are then developed and evaluated against the performance criteria by means of deterministic or probabilistic analyses. Based on the results, successful designs can be developed and implemented.

On the other hand, the prescriptive approach that undergirds most major U.S. building codes attempts to achieve a minimum level of safety within a building through strict adherence to specified criteria. An example is the maximum distance an occupant must travel to reach an exit. These distances—which vary between 150 and 400 feet depending upon the type of occupancy—may be reasonable for many buildings, but unique structures may have difficulty meeting these minimum prescriptive requirements.

Does this mean that nonconforming buildings don't or can't offer acceptable levels of life safety? Not necessarily—and this is where the breakdown of prescriptive codes can often occur.

A performance-based design, alternatively, evaluates safety levels based upon a given buildings' inherent configuration, circumstances and uses.



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