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.

11/17/2017


This article has been peer-reviewed.

Learning objectives:

  • 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.

Figure 1: This shows a high-rise fully sprinklered building that meets current building code requirements. All graphics courtesy: JENSEN HUGHESPassive 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

Figure 2: A large-scale fire-resistance test of a rated wall assembly was conducted in accordance with ASTM E119. 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

Figure 3: This fire-performance test of an exterior wall assembly containing combustible materials was conducted in accordance with NFPA 285.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.


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