How to balance passive and active fire protection systems in building design
Both passive and active fire protection features should be included in the design to ensure protection of the building and its occupants.
- Know the key elements of active fire protection and passive fire protection.
- Understand which codes and standards assist fire protection engineers in their design.
- Learn about the various building systems that impact the fire protection system.
The safety of building occupants and the protection of property is maintained throughout construction of a building in accordance with applicable building and fire code regulations at the time of construction and through the life of the building. The basic premise of the model building codes, such as the International Building Code (IBC) and NFPA 5000: Building Construction and Safety Code, is to protect building occupants and property by providing minimum levels of protection.
The protection of the building and its occupants is maintained through the specification, installation, and maintenance of a comprehensive set of requirements that include both passive and active fire protection features.
Passive fire protection features are generally understood to include the fire-resistance-rated construction of a building, compartmentation of the building, and building elements that contain the fire to a specific floor or area. Horizontal fire-resistance-rated assemblies (floors) contain the fire to the floor of origin. Larger buildings may be further subdivided using vertical fire-resistance-rated assemblies (walls) to contain a fire to a smaller area on a floor. The fire-resistance rating of vertical and horizontal assemblies are determined from testing conducted in accordance with ASTM E119: Standard Test Methods for Fire Tests of Building Construction and Materials.
Emergency egress paths (corridors and vertical exit-stair enclosures) provide a protected means for building occupants to reach an emergency stair and safely leave the building. Fire-resistance-rated areas of refuge may be included so that building occupants can safely gather in a protected space during a fire event in lieu of exiting the building. The means of construction (types of materials and assemblies) provide this inherent level of passive protection.
Active fire protection systems are the automatic suppression and detection systems that detect a fire, alert the building occupants, and control the fire in its early incipient stages. Also included in active systems are smoke control systems, which are designed to evacuate smoke from a building to maintain minimum levels of visibility and allow for safe building-occupant egress.
Proper design, installation, and maintenance for all passive and active fire protection systems over the life of the building allow for redundant levels of protection to safeguard building occupants and property during a fire. No individual system is 100% reliable, therefore buildings with redundant levels of protection ensure that should a fire occur, one or more of the installed systems will function as designed to contain/control the fire and ultimately protect the building occupants. Proper operation also will allow for safe firefighting operations by first responders.
Modernizing building design
In the late 1800s and early 1900s, building construction moved away from the use of combustible construction materials that resulted in large city-wide conflagrations. Traditional building materials that have a high thermal mass, structural stability, and excellent fire resistance—such as brick, concrete, stone, masonry, plaster, and others—formed the framework for this new era of building construction.
Early in the 20th century, the benefits of an automatic sprinkler system became more understood, and these active systems started to be integrated into building construction. The concept of balanced building construction design using traditional building materials to passively contain a fire to the area of origin and automatic sprinkler systems to control the fire. Working in concert, the large city-wide conflagrations became a distant part of history while the fire-safety improvements led to a reduction in the loss of life and significant drops in property losses.
Modern building design is trending toward taller buildings that cannot be built as easily using the traditional heavy, thermally massive building materials. Lighter building materials have been developed such that the more thermally massive brick, stone, masonry, and plaster building elements are used less often and more fire-“efficient” products, such as sprayed fire-resistive materials (SFRM), gypsum wallboard, and light gauge metal framing, are common building materials. These modern building construction products can be used in assemblies to provide the same level of protection at a fraction of the weight, cost, and installation time as compared with the more traditional products.
However, building codes have been shifting to add greater reliance on automatic sprinkler system installations and less on passive fire-resistance-rated construction. When the automatic suppression and detection systems function as designed, and they do the vast majority of the time, then the goal of protecting property and building occupants has been met.
Over the past 10 or 15 years, there has been a dramatic shift from a balanced fire protection set of building codes to regulations that add increasingly more reliance on automatic suppression systems to provide the minimum levels of protection. The original reason for this shift was to provide building owners and developers with an incentive to install automatic suppression systems in buildings. Reducing the requirements for passive fire protection systems resulted in a cost savings, which could be applied to the sprinkler installation.
These trade-offs—commonly referred to “sprinkler trade-offs”—are not necessarily a bad trend, except for the fact that automatic suppression systems are not 100% reliable. No single system (passive or active) is 100% reliable. When you rely on one system or design feature to provide the vast majority of your building protection, a failure of that system can lead to catastrophic results.
The design process
Using the IBC, for example, the building design process starts with defining the use and occupancy of the building in Chapter 3, followed by building heights and area requirements in Chapter 5, and then defining the types of construction in Chapter 6. Special-occupancy requirements (high-rise buildings, underground structures, institutional occupancies, high-hazard occupancies, etc.) are provided in Chapter 4. The minimum requirements for passive and active fire protection features are provided in Chapters 7, 8, and 9, with Chapters 7 and 8 dedicated to passive fire protection features and Chapter 9 dedicated to active fire protection systems.
Some of the first code allowances for reductions in fire-resistance ratings are located in Chapters 4 and 5. For example, minimum fire-resistance ratings in certain high-rise buildings are permitted in Chapter 4 to be reduced when suppression systems with specific design features are provided (e.g., sprinkler control valves equipped with supervisory initiating devices and water-flow initiating devices are installed on each floor). Building heights and areas listed in Chapter 5 can be increased in the presence of automatic suppression systems within a building.
Per Table 601 of the IBC, the fire-resistance ratings of certain building elements, including the primary structural frame, for Types IIA, IIIA, and VA constructions are permitted to be zero (unprotected) when the building is protected throughout with an automatic suppression system. Numerous other sprinkler trade-offs are included throughout Chapter 7, where reductions in the minimum fire-resistance ratings of different types of passive fire protection features, such as exterior walls, firewalls, fire barriers, and draftstopping, are permitted. In Chapter 8, the minimum interior-finish rating requirements can be reduced when automatic suppression systems are installed in, for example, exit stairways, passageways, and corridors.
The sprinkler trade-offs in the early sections of the building codes cited above may seem extreme to some, but so far, the loss history does not seem to indicate that this trend is resulting in an increase in property loss or loss of life. It may, however, take many more years for the effects of these trends to be recognized as the building requirements are implemented in a larger number of buildings.
One area where sprinkler trade-offs can be readily identified as potentially putting the general public in significant harm is the steps some states and jurisdictions have taken regarding controlling the installation of combustible cladding materials on the exterior of high-rise buildings. Various jurisdictions, such as Washington, D.C., Virginia, and Indiana, have included exceptions to the model code whereby compliance with NFPA 285: Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-Load-Bearing Wall Assemblies Containing Combustible Components is not required for certain applications if the building is fully protected by an automatic sprinkler system.
An example of this modification is provided in Section 2603.5.5 of the Virginia Uniform Statewide Building Code (USBC). This section of the code requires exterior walls containing foam plastic materials to be tested in accordance with, and comply with, the acceptance criteria of NFPA 285. However, Exception 3 in the 2012 Edition of the Virginia USBC states that buildings equipped throughout with an automatic sprinkler system would not require compliance with NFPA 285. This type of sprinkler trade-off has been similarly incorporated into the Washington, D.C., building codes and the Indiana Building Code.
High-rise buildings are unique in that a large number of people are located in a building with a relatively small footprint, which is very difficult to access from a firefighting standpoint. If the outside of the building is clad in a combustible material whose flammability is uncontrolled, significant vertical and lateral flame spread can occur over large areas of the building surface should a fire occur. In the United States, compliance with NFPA 285 is required for all exterior wall assemblies containing combustible components (insulation materials and/or cladding materials) installed on buildings more than 40 ft in height, with some exceptions. Requirements limiting the installation of metal composite material systems, high-pressure laminate panel systems, and exterior insulation finish systems are provided in Chapter 14 of the IBC. Exterior wall systems containing plastic materials including insulated metal panel systems and glass-reinforced plastic panels must comply with the specific Chapter 26 requirements.
The NFPA 285 test method subjects an exterior wall assembly to prescribed fire exposure conditions representative of a fire originating in an interior room. This fire is then assumed to transition to flashover with the fire plume exiting a simulated window opening and exposing the exterior face of the building to direct flame impingement. This interior-originating fire scenario, which transitions to flashover with the fire plume extending out a window opening, has become a point of contention as it relates to current local code amendments and sprinkler protection.
During the original exterior-wall fire-testing program conducted at Southwest Research Institute (SwRI) in San Antonio during the early 1980s, both an interior-originating fire and an exterior-originating fire were evaluated. It was decided that an interior-originating fire was the worst-case fire scenario, thus it was used as the basis for developing the fire exposure conditions specified in NFPA 285.
The interior-originating fire would subject both the interior and the exterior sides of a wall assembly to the high-temperature fire exposure conditions. In contrast, an exterior-originating fire exposure would only subject the exterior face of the wall assembly to the direct flame impingement. From a testing and qualification standpoint, any wall assembly that was capable of meeting the performance requirements of the interior-originating fire exposure conditions also would meet the minimum fire-performance requirements of an exterior-originating fire.
This may not necessarily hold true if the test method only incorporated an exterior fire exposure. Combustible materials located behind heavy exterior veneers (such as brick or concrete) or combustible insulation material installed in the wall-stud cavity would not see any heat effects from the exterior fire, potentially providing a false sense of security as to the acceptable fire performance. Further supporting this position, the Canadian (CAN)/ULC S-134: Standard Method of Fire Test of Exterior Wall Assemblies and the British Standard (BS) 8414: Fire Performance of External Cladding Systems, Part 1 and Part 2, test standards use an interior fire exposure source that extends out the simulated window opening. While the testing conditions are different from NFPA 285, the fire exposure concept is the same.
Significant fires have occurred in the Middle East, Eastern Europe, Asia, and most recently in London. In these cases, it appears that a combustible cladding material not intended to be installed on a high-rise building (at least per U.S. codes) ignited, resulting in nearly the entire exterior wall surface burning. Many of the fires that occurred internationally involved buildings that were apparently clad with what is typically termed as “non-NFPA 285-compliant” metal panel systems.
It is too early to know for certain what contributed to the recent fire in London, but to date, we fortunately have not experienced this type of fire in the U.S. The reason why we have not experienced these fires is that the building officials, architects, designers, and engineers of projects incorporating combustible materials in the exterior wall construction have been doing an exceptional job of complying with the building code regulations
[subhead] Code adoption
In the U.S., the model codes (such as the IBC) form a framework that state and local jurisdictions adopt for their specific use. Many jurisdictions simply adopt the IBC and enforce the requirements as currently written. Some states and jurisdictions modify the model codes to meet their specific needs. Larger jurisdictions, such as New York City, may take the framework of the model code and substantially modify the code requirements to match their specific needs and historical approach to building construction and fire safety.
The rationale put forth for adopting amendments to the state and local building codes for requiring compliance with NFPA 285 is that the NFPA 285 test is designed for a fire originating inside the building. The theory is that in a fully sprinklered building, the sprinklers would operate and control the fire in its incipient stage, thus preventing the fire from growing to sufficient size to transition to flashover and impact the exterior wall.
There are a number of fallacies associated with this rationale. First, sprinklers will only control a fire that originates within the building. It appears that many of the international fires originated from ignition sources located on exterior balconies. For reasons unknown at this time, the fires ignited the exterior wall cladding and quickly spread to the surrounding balcony wall surfaces, progressing up the side of the building and eventually fully encompassing the entire exterior-building wall surface. Interior sprinklers did not come into play in controlling the fire in its early development stages, as the balconies were not sprinklered.
Some of these buildings were considered by local codes to be classified as being equipped throughout with an automatic sprinkler system. However, these sprinklers were just able to control fires that ignited in perimeter rooms on multiple floors. Secondly, other plausible external ignition sources include trash dumpsters or accumulated trash fires, car fires located adjacent to the building, and burning vegetation.
Many high-rise buildings incorporate 1st-floor retail spaces and/or outdoor dining. A fire originating in one of these spaces could also impinge on the exterior-building wall surface. Any potential exterior ignition source could easily involve a vast majority of the building’s wall surface. Additionally, a fire occurring within an occupied space in a building could be a shielded fire whereby the installed sprinklers do not control the fire, permitting even a relatively small initiating fire source to impinge on the building exterior.
The historical fire loss has indicated that a large initiating fire source is not required to start a major, full-building conflagration. This was the case in the Address Hotel fire and the Torch Tower fires in the Middle East, for example. Thus, a small interior-initiating fire source could plausibly transition to the exterior of the building.
Another issue associated with relying on the automatic sprinkler system is that once ignition of the exterior wall panels occurs, the fire quickly spreads up the face of the building with the thick fire plume covering large portions of the building’s exterior wall surface. Radiant energy from the fire plume will be sufficient to ignite interior combustibles (curtains/drapes, furniture, carpet) within each perimeter room, potentially on multiple floors.
In the recent fires in the Middle East, the interior sprinklers did control the individual room fires, but the system was pushed to its limits. The same thing was observed in the Monte Carlo Fire in Las Vegas, which occurred in January 2008. An external ignition source (welders on the roof) ignited the combustible façade, resulting in significant burning on the building’s exterior due to improperly protected shapes. During the fire event, separate fires ignited in rooms on different floors and the sprinklers in these individual rooms were just able to control the fires.
However, if sprinklers in multiple rooms activate, then the number of operating sprinklers could exceed the design area and water supply. Should this occur, additional sprinklers would not have sufficient water supply and will not be able to control the individual fires. This may lead to multiple fires occurring on multiple floors in what is now essentially an unsprinklered building.
Fortunately, the fire scenario has not occurred here in the U.S. Whether an exterior fire would be able to ignite multiple fires on multiple floors and overpower the installed sprinkler system is not the primary point. The fact that an exterior ignition source could ignite a large fire on the exterior of a building is possible. A large exterior fire, even if it does not progress into the interior of the building, is obviously not a desirable outcome for any engineer, architect, building owner, or designer that participated in the design and construction. This specific sprinkler trade-off to allow non-NFPA 285-compliant, combustible, exterior cladding materials to be installed in high-rise buildings has the ability to result in large-loss fires, leading to scrutiny and questioning in the public forum, especially given what we currently know about exterior building fires.
The current state of U.S. building and fire codes is very good. However, the introduction of code exceptions that allow the use of non-NFPA 285-compliant, combustible, exterior wall-cladding materials on high-rise buildings creates potential risks and hazards, and they are not adequately adhering to maintaining the balance of the building’s fire protection design. As a result, these code exceptions lead to an unintended compromise in building safety. These sprinkler trade-offs that allow for the use of non-NFPA 285-compliant, combustible, exterior wall systems on high-rise buildings need to be reversed. These current amendments to the local codes must be reviewed carefully when considering other future trade-offs, to ensure that the balance in the passive and active fire protection building design is maintained and we do not put building occupants at an increased level of risk.
Arthur J. Parker is a senior fire protection engineer at JENSEN HUGHES. He is an active member on ASTM E05: Fire Standards and the NFPA Fire Test Committee. He has more than 25 years of experience in fire testing and analysis of rated assemblies to determine compliance with building code regulations.