Structural performance during a fire is an issue that has reached a higher public profile as a result of Sept. 11, and increasingly, infrastructure stakeholders are demanding answers to fire safety. The tragic events and subsequent collapse of the World Trade Center towers was a set of events well outside the normal design parameters considered by building code authorities. However, many professional organizations immediately felt the need to better understand the effects of fire on structural stability and to share this knowledge with the design community.

As fire protection engineers and structural engineers work together in response to these events, they have been contemplating fire as a structural load. Currently, our codes, standards, and design methods do not consider fire as a structural load, but instead simply prescribe fire resistance ratings and other related, prescriptive requirements to counter the potential effects of fire upon a structure. The purpose of this article is to provide a brief overview of the scope of fire protection engineering, a history of structural fire engineering, and a discussion of the current activities of professional organizations that will affect the future of structural fire engineering.

Fire Engineerng

Fire engineering addresses many objectives that include, but are not limited to, life safety, emergency responder safety, property protection, public welfare, historic preservation, business continuity, design flexibility, and cost effectiveness. All of these goals are not necessarily regulatory, but may be expectations of building owners. The International Building Code (IBC) does not encompass all of these goals. In fact, the IBC’s main focus is life safety, emergency responder safety, and, to a certain extent, property protection. It is not focused on the flexible use of a building or cost effectiveness. The NFPA’s 550 Guide to the Fire Safety Concepts Tree© is helpful in putting together fire protection strategies to achieve many of the above objectives. NFPA 550 focuses the user both on preventative and management techniques.

Fire protection engineering incorporates many areas of engineering and building design. For instance, a smoke control system likely is going to affect the design of HVAC systems within a building. And in many cases, the active fire protection systems will affect the water supply and the electrical systems necessary for a building. There are many examples of these interactions, however, this integration currently does not occur with structural engineering. Generally, fire is dealt with as an entirely separate aspect to structural design, and in most cases, architects address fire resistance through prescriptions in the building code. Fire protection engineers get involved with structural design only in highly complex buildings or when strict code compliance is not practical.

Fire engineering strategies focus on prevention and management. Prevention may occur with properly designed electrical systems, separation of combustibles from heat sources, or through more comprehensive prevention strategies for a facility through training and planning. If prevention cannot be achieved, the exposures to fire must be addressed and fires must be managed. There are three, overall alternatives for managing a fire: suppressing the fire, controlling combustion, and controlling the movement of the fire by construction. In most cases, these will occur in combination. Primary features that can be used in fire management are divided into active and passive protection. Active fire protection systems include fire detectors and alarms, sprinklers and other fire suppressors, mechanical smoke controls, standpipes, and fire department responders. Passive fire protection mechanisms include fire-resistant materials, compartmentalization, penetration protection, material property control and flame spread limits, separation of combustibles and fire breaks (such as aisles in high-piled storage), and means of egress layout.

These systems and mechanisms are only tools to be used in the overall design. The final strategy chosen for a facility will depend upon a variety of factors such as occupant and building characteristics.

Historically, the building codes prescribe these elements in a component-oriented style for each occupancy—for example, one chapter for fire resistance, another for egress, another for active fire protection systems. Some building codes combine requirements by occupancies, but in general, there is only a judgment, based upon experience, made about the interaction of such features. Recently, these issues are being considered in a more systematic engineering way. This is related to the national and international trends toward performance design and the need to justify the economic benefits of the regulations. Also, a building code’s intent is being evaluated. These motivating factors and the increased level of technology available have brought about the development of engineering methodologies, equations, and computer models to more accurately evaluate fire.

Structural fire engineering

Structural fire engineering is one aspect of a comprehensive fire protection design. Other decisions made regarding the overall fire protection strategy for a building or facility may affect the design as it relates to structural integrity during fire. For instance, installing sprinklers may reduce the likelihood of a fire reaching temperatures that could threaten structural components.

Currently, structural fire engineering centers on the standard fire resistance test, which is not considered by many to be engineering. The two, primary tests used internationally include the American Society for Testing and Material’s Standard Test Methods for Fire Tests of Building Construction and Materials (ASTM E119) and the International Organization for Standardization’ s Fire-resistance tests—Elements of building construction (ISO 834). These tests rely upon results from a structural member or assemblies—such as a wall or floor/ceiling assembly—that are placed into a furnace apparatus and tested for fire resistance, integrity, and insulation qualities based upon a specified time/temperature curve (see Figure 1). The major criticism with this test is that the conditions do not relate to real fires. The results simply are comparative in nature and provide little information about actual performance. In other words, the results only compare if one wall assembly is better than another or if this column performs better than that column. Also, this test does not address connections between structural members in most cases. In fact, several full-scale tests by researcher Jef Robinson during the 1990s, along with field experience, have shown that structures often perform better than the furnace test would have predicted. Essentially, the structures were able to redistribute the loads and transfer the heat.

Figure 1: Time/temperature curves are used to compare structural members or assemblies for fire resistance, integrity, and insulation qualities.

 

 

 

The disadvantages of the furnace test have been well-known for a long time, but it has been a difficult and costly issue to resolve. As early as 1928, researchers attempted to devise some relation between the furnace test and actual fires. These methods are called fire severity methods. The premise is to provide fire resistance greater than the anticipated fire severity. The more recent correlations take into account not only the fuel load, but also the thermal properties of the walls and most importantly ventilation openings. In many cases, the size of the openings and their configuration has a greater effect on the severity of a fire than does the fuel load. This is especially true in smaller compartments. Fuel load tends to have the largest affect upon the duration of a fire. In addition to the standard fire test, the American Society of Civil Engineers and Society for Fire Protection Engineer’s Standard Calculation Methods for Structural Fire Protection (ASCE/SFPE 29-99) allow the use of materials based upon a calculated relationship. This allows more unique protection features for structural elements that have not specifically been tested as an assembly within the furnace. The IBC also has calculation methods available.

The only way to make this a more engineered approach is to more closely integrate fire protection engineering with structural engineering. This means understanding the true fire load/design fire (design load) and the ability to verify compliance through comparison to failure criteria (limit states).

Implementation of such methods beyond the basic equivalency process within the building and fire codes has been fairly limited. This is a result of the lack of integration of such methods into the normal application of codes in the way that structural engineering has done. Much of this neglect has related to the lack of available data and guidelines for input into analysis tools. Statistics are scarce and may be highly unreliable for broad application. In addition, there is constant argument as to what is considered failure? The only method available to verify compliance in most cases is comparison back to the prescriptive codes. Such a comparison becomes very difficult when the design departs greatly from the building code.

Who’s doing what?

There is a movement to include fire as a structural load to be considered during structural design. For structural engineers, this would require a deeper understanding of fire hazards and how temperatures affect structural elements. Ultimately, engineers need to know what effect those temperatures have upon individual members and the overall structural system design. In the United States, there are several organizations active within the area of structures and fire and the movement to more closely link the two. They include the National Institute of Standards and Technology (NIST), SFPE, ASCE, and the American Institute of Steel Construction (AISC). The Europeans also have been active at looking at fire as more of a structural load in the development of the structural Eurocodes. Following is a brief look at each organization’s efforts to address structural fire engineering.

NIST

After the World Trade Center collapse, the NIST was charged with the investigation of the events and to initiate activities to improve our understanding of the underlying failure mechanisms. One of the key areas of concern is the effect of fires on structural stability. To address this concern, the NIST launched a research project that will create tools and practical guidance for design of structures. This includes both new and existing structures. Specifically, the organization aims to produce the following key products over a multi-year period:

• Structural Fire Safety Design of New Buildings—which will include best practices and guidelines/ pre-standards for fire safety design of structures.
• Analysis of Structural Fire Performance— which will include best practices tools for analyzing structure fire performance; load and resistance factor methodology for structural fire safety; and selected, verified, and validated predictive tools for analyzing structural performance in real fires.
• Structural Fire Safety Evaluation and Retrofit of Existing Buildings— which will include guidelines/ pre-standards for structural fire safety evaluation of existing buildings, and best practices and guidelines/pre-standards for fire safety retrofit of structures.

The NIST commissioned the SFPE to organize and facilitate a workshop—The NIST Workshop on National Research & Develop- ment Roadmap for Fire Safety Design and Retrofit of Structures (NIST workshop)—to advance these initiatives. The workshop was held on Oct. 2-3, 2003, in Baltimore. The conference was carefully organized by obtaining nine white papers to focus the discussion, and attendees were invited who represented various sectors of the fire protection industry. These representatives included fire safety designers, structural designers, architects, code enforcement officials, academia, researchers, and professional and industry associations. The workshop was designed with several breakout sessions that focused on design fires, thermal analysis, structural design, and existing buildings.

From this conference, various priorities and needs were identified for best practices/design guides and standards to be properly developed. (The details of these priorities can be found in the 2004 NIST workshop report.) The participants identified several areas where design data is lacking, including the following:

• Design fires. Design fires are the input that allows the analysis to occur. Significant research must be conducted to understand fire loads in buildings. In addition, little is known about how systems, such as sprinklers, probabilistically affect the occurrence and magnitude of design fires.
• Thermal properties. Limited material property data at elevated temperatures exists.
• Failure/limit states. Generally accepted or standardized failure criteria and/or limit states are not available.

One of the most significant conclusions of the meeting was that for any of the methods of analysis and supporting data to be accepted, information, such as design fires and failure criteria, need to be codified or held within a widely accepted standard or design guide. This data also needs to be supported by a strong methodology to apply the values. Participants also agreed that most designs do not need a complex structural fire analysis and that a simplified approach would be appropriate for many situations.

SFPE

This organization has been active in the NIST initiatives on several fronts. It has drafted many documents to assist with performance design such as the SFPE Engineering Guide to Performance-Based Fire Protection, Analysis and Design of Buildings (SFPE 2000), and the recent release of the 2004 Code Officials Guide to Performance Based Design Review, which was completed in collaboration with the International Code Council (ICC).

To address structural fire engineering, it released a discussion paper that addresses the role of the design professional in structural fire design titled, "Structural Design for the Fire Condition—the Role of the Design Professional." This paper raises many similar questions and concerns that are addressed in this article.

Additionally, it has worked with the Structural Engineering Institute (SEI) of ASCE to develop ASCE/SFPE Standard 29-99, which provides methods of calculating the fire resistance of selected structural members and barrier assemblies using structural steel, plain concrete, reinforced concrete, timber and wood, concrete masonry, and clay masonry. The standard is intended to have calculation methods that give fire resistance results equivalent to those in ASTM E119. This standard increases design flexibility, but still bases the fire loading on the standard fire resistance test.

Further, the SFPE now is embarking on a project with the ASCE that will take a closer look at performance approaches for structural design. As part of that activity, the SFPE has a task group looking at fire exposures to structural elements. The results of this work will provide guidance on calculating the fire boundary conditions to structural elements. It should be noted that the ICC’s 20 03 International Building Code (IBC) currently references ASCE/SFPE 29 for steel assemblies.

The SFPE also has hosted many technical conferences related to the full range of fire issues, and most recently held a conference specific to structural fire engineering topics. This conference—2003 Designing Structures for Fire Conference—was co-hosted with ASCE/SEI and proceedings are available from the SFPE.

ASCE

As discussed above, the ASCE has been active in this area of structural fire engineering with the release of ASCE 29-99 and through the new project with the SFPE to develop a performance-based standard. In addition, the SEI hosts technical tracks at its annual conferences related to fire resistance of buildings.

AISC

In early 2001, the AISC initiated a long-term project aimed at advancing the engineered design for fire safety of structures. This effort was motivated by large-scale fire tests on steel-framed buildings in the United Kingdom that demonstrated superior, overall structural performance compared with a single member within the test furnace. The Fire Safety Engineering Committee of AISC held its first meeting in May 2001 and later commissioned a study to create a comprehensive strategy for the integration of structural and fire engineering. This long-term strategy identifies the gaps in current knowledge and education, and the limitations of contemporary design methods and regulatory practices.

The organization further proposes specific research, development, and educational programs that must be implemented to achieve higher quality of fire-resistant designs. The issues include fire load densities, fire temperature modeling, thermal and mechanical properties of materials at elevated temperatures, characteristic structural behavior and failure criteria, and risk assessment techniques. More importantly, regulatory acceptance and adequate education of designers and building officials were identified as key issues.

The strategies are broad in scope and likely will feed into the overall effort to advance the concepts of structural fire engineering in the United States.

Eurocodes

The Eurocodes are intended to replace national standards within the member states of the European Union. There is a transitional period where these codes will be an alternative to the current, national stand a r d s. After that transitional period, the national standards must be replaced. Each of these standards will have nationally determined parameter allowances to adjust for differences in regulatory control preferred in each member state. The structural Eurocodes have made a substantial effort to integrate fire with the structural provisions.

Table 1 summarizes the range of approaches that have been integrated into the Eurocodes. The "Tabulated data" methods are prescribed fire resistance ratings—this is similar in approach to Chapter 7 of the IBC. When using the more advanced methods within the Eurocodes, failure appears to be measured against modified structural loads for fire. There is continuing debate as to the design fires and failure criteria used as a basis for these methods.

Currently, all the methods embodied in the Eurocodes can occur in the United States through the use of the equivalency clause. The major difference is the level of integration with the structural aspects of the document and, to a certain extent, the inclusion of codified design fires and failure criteria.

The future

The term fire protection engineering had not had such wide-spread mention before the tragic events of Sept. 11. Most people look at the World Trade Center tower failures as an extreme event for which designers cannot be expected to account, and one that normal building codes never were intended to—and may never—address. At the same time, this event has led to a stronger demand by the public and building industry to better understand why our buildings perform the way they do. In addition, there is a need from some industries to recognize that looking at the systematic performance of structures yields different and often more positive results than the standard fire resistance test. There are many activities occurring both within the United States and abroad that are affecting the future of structural fire protection and related engineering methods. This movement will more directly deal with fire as a structural load instead of as an independent variable. There are many issues, both technical and policy related, that will affect the success of this transition.

One of the questions that has arisen is whether these advancements in structural fire engineering are intended simply to provide tools for unique and complex buildings or to rethink how we deal with fires and structures overall. The mood by some seems to be rethinking the overall approach. An analogy might be made to the transformation of sprinkler design from pipe scheduling to hydraulic design. This, of course, is a simplistic analogy as there are many more complicating factors such as how fire loads and failure criteria are determined. More than likely, there would be various levels of design allowed. These may range from the more traditional prescriptive approach (conventional) to the more advanced, performancebased (first principles) design approach.

What would this mean for the IBC? Would Chapter 7 be integrated within Chapter 16 and the associated material-specific chapters? Before such drastic steps are taken, there are many factors to consider, such as the specific role of each of the requirements of Chapter 7, as well as additional analysis to ensure that all issues are addressed properly. For now, the conclusions as determined during the NIST workshop—the lack of design fires for input into the design methodologies, how features such as sprinkler systems affect these inputs, along with agreedupon failure criteria—will be the largest barrier to full implementation of more advanced structural fire engineering methods.

Conclusion

The practice of fire engineering is not a new one. However, in light of recent demands by the professional community, as well as the public, structural design may be growing to include more engineered approaches to fire protection. Many professional organizations are working hard to address these issues and to develop design approaches that make sense.

Beth Tubbs, P. E ., is a senior staff engineer in the International Code Council’s Codes and Standards Development department. She is located in the Boston office and can be reached at btubbs@iccsafe.org.

Sidebar: Who else is concerned?

By Jennifer Goupil, P.E.

The Alliance for Fire Safety (AFS)—a nonprofit advocacy group comprised of concerned members of the fire- and smoke-resistant construction industry, fire code experts and consultants, and former firefighters—is another group advocating fire protection within buildings. The organization supports a balanced design approach to providing fire and life safety code requirements, particularly passive fire protection measures.

"The AFS is taking a two-pronged approach to accomplishing its mission," said Communications Director Bernie Allmayer. "First, we are proactive ly promoting the issue of balanced fire protection, and second, our technical committee is dedicated to advising municipal jurisdictions on proposed amendments to existing building codes."

According to Allmayer, the ASF feels strongly that recent building codes lower the overall level of fire safety in the constructed environment by overly relying on active systems such as sprinklers.