Striking a balance between passive and active fire protection

Occupant protection, structural safety, and proper suppression systems designed in compliance with the 2012 International Building Code can reduce the risk of damages from fire exposure in a building.

03/23/2012


Fire protection, in terms of construction, is the means to inhibit or mitigate the ignition, growth, and spread of fire and its effects through the built environment. The insurance industry has struggled for centuries to reduce risk from fire, and only in the past 50 years have life safety systems and building codes matured to the point that compliance with applicable codes and standards provides a significant reduction in exposure. When it comes to reducing the risk of growth and spread of fire and, equally important, smoke generated by fire, the model codes offer the following methodologies:

1.     Compartmentalize the space with construction capable of limiting the effects of fire

2.     Control the growth of the fire via automatic or manual suppression

3.     Manage the products of combustion such that transmission to critical spaces is inhibited.

The first two are easily recognized as passive and active fire protection, respectively. The third is a hybrid better known as smoke control, which can be active or passive. The goal of this article is to discuss the roles and relationship of each method within the 2012 International Building Code (IBC).

Passive protection

Passive protection is the creation of floor, ceiling, roof, and wall assemblies that are constructed to limit the transfer of heat or smoke from one portion of a building to another. Passive fire protection has the following functions:

  • Protect occupants during emergencies
  • Protect the spread of fire and smoke via vertical openings (shafts)
  • Separate hazards
  • Protect building systems and construction.

In all cases, the goal of passive protection is to buy time for the evacuation or relocation of occupants to a place of relative safety or to allow firefighters the time to evaluate and manage an emergency and conduct rescue and suppression operations. Gone are the days when buildings were advertized and insured as “fireproof” or “absolutely fireproof.” Through science and experience we have come to understand that with enough heat steel will melt, concrete will spall, and wood will burn or char. Since the late 1960s the construction and insurance industries have employed the term “fire-resistive” and more recently “fire-resistance-rated” to reflect the understanding that protected assemblies will only resist the effects of fire for a certain amount of time.

Protecting occupants during emergencies

In most buildings, occupants are encouraged to leave a building when there is an emergency, especially a fire, so fire-resistance-rated construction is used to protect the means of egress. But certain use conditions, such as healthcare and correctional occupancies, seek to relocate occupants using smoke compartments and smoke barriers. Accessible egress is provided in some cases by areas of refuge.

Methods of protecting the means of egress, especially the exit component, are generally rigid and independent of the use of the building, the hazards present, or the presence of active fire protection. For example, if a building is four stories or more in height, IBC 1022.2 will require a stair enclosure with a two-hour fire-resistance-rated assembly. This level of protection is integral to the philosophy of the egress system, which dictates that as occupants move along the egress system, they become more directed to a place of safety and more protected from the origin of the fire. This is illustrated by the figure below. Note that these terms are distinctly defined in the codes and not interchangeable.

Figure 1: This diagram illustrates the level of protection provided by the egress system of a building in which occupants progress through in order to reach safety in the event of a fire. Courtesy: ccrd

The exit access includes all the occupied spaces and corridors leading to the exit, and it may or may not be protected. However, once an exit is reached, one- or two-hour fire-resistance-rating is expected. Exits include stair enclosures, exit passageways, horizontal exits, and exterior exit doors.

An important feature to understand is that the goal of successive passive protection mirrors firefighting efforts in terms of methodology. The first goal of a firefighting effort, once rescue operations have been completed, is to limit the fire to the room of origin, or space of origin if within a large room. If that cannot be accomplished, firefighting efforts will focus on a wing or compartment of a building, or a floor. If the fire cannot be contained to a floor and has spread to multiple levels, the goal will be to limit the fire to those levels. In progressive fashion therefore, a fire is attempted to be limited to:

  • The room of origin
  • The suite, compartment or wing of a floor
  • The floor of a building
  • A limited number of floors
  • The building
  • A campus, neighborhood or city block.

Occupants are more likely to be aware of their immediate surroundings and those in the direct vicinity, such as an office suite or floor on which they work, than another floor of a building. With few exceptions, occupants must be afforded greater levels of protection the further committed they are to the exit system.

Protection of vertical openings

When it comes to the protection of vertical openings or shafts and the reason for the maintenance of detailed requirements regarding their construction, one needs to look no further than the MGM Grand Hotel fire in 1980. This fire was responsible for the loss of life on floors high above the origin of the fire, and as a result floor-to-floor integrity became a paramount concern. It should be noted that shaft requirements were in the code prior to 1980, but this fire specifically illustrated how buoyant gases can quickly travel through a building where protection of floor penetrations does not exist or is not maintained. The design of the HVAC system, including a huge plenum, seismic joints, and lack of complete shaft enclosure, contributed to the vertical spread of fire and smoke. It should also be noted that the fire started on and consumed an entire floor that was not sprinkler protected, based on an exception at the time that casinos did not need to be sprinkler protected because they were always occupied.

In the 2012 IBC, shafts and their exceptions have been split into two new sections — IBC 712: Vertical openings and IBC 713: Shaft enclosures. Many conditions exist that do not present the same risk associated with an extended series of openings between successive floors. Examples include parking garages, escalators, and exit access stairs that connect only two floors; therefore, these are called vertical openings. Shafts must still be protected with one- or two-hour fire-resistance-rated assemblies very much like exit stairways because of their vertical configuration. Because the spread of both heat and smoke are a concern, penetration by ducts must be protected with combination fire/smoke dampers. IBC 717: Ducts and air transfer openings, previously IBC 716, is very similar and still provides the same exceptions for smoke control systems and sub-ducts.

Separation of hazards

Certain areas of a building have a higher risk of ignition and fire growth than others. This could be because of the hazard, fuel load, source of ignition, lack of regular occupancy, or a combination of these. Examples include mechanical rooms with gas-fired equipment, waste collection rooms, hazardous control areas, and Group H occupancies. Conversely, certain spaces could be so critical that the authority having jurisdiction (AHJ) or owner may want to protect them from exposure to a fire adjacent to the subject use. These may include medical gas rooms or sensitive data rooms.

There is still a tendency to separate many mechanical, electrical, or communications rooms with fire-resistance-rated construction, but this is typically not required by code. The designer should carefully consider incidental uses in IBC 509: Incidental uses, or research the applicability of accessory and non-separated occupancies in IBC 508: Mixed use and occupancy, before assuming separation is required. Some electrical rooms do require fire-resistance-rated enclosures based on the equipment, but this is not the case for most electrical rooms with standard voltage or transformers of 112.5 kVA or less. The local AHJ and owner should also be consulted to see if there are specific requirements to protect hazards or critical areas.

If a separation from a hazard is required, it is typically required to be constructed as a fire barrier in accordance with IBC 707: Fire barriers, which then only requires fire dampers per IBC 717.5.2. Many times, combination fire/smoke dampers are specified when only a fire damper is required. Coordination within the design team is critical to determine which types of assemblies require fire, smoke, or combination fire/smoke dampers.

Another type of separation is the protection of exterior walls, which is based on proximity to a lot line or other buildings. The history of this requirement dates back to urban conflagrations that have occurred over the centuries and the desire to provide either a defensible space between buildings or an elevated level of fire protection to buy time for firefighters to control a fire. IBC Table 602 provides the requisite fire protection ratings based on hazard and fire separation distance. Additionally, IBC Table 705.8 should be consulted for the allowable percentage of openings based on fire separation distance. This percentage applies to the protection of exterior openings, such as windows and doors, but does not apply to vents per IBC 714: Penetrations.

Protection of building systems

Many specialty building services, such as emergency equipment and life safety systems, must be protected with fire-resistance-rated construction to ensure system integrity during a fire event. This is similar to hazard protection but applies to the specific protection of equipment, wiring, or duct runs used to serve the specialty systems. An example would be the survivability of an emergency voice/alarm communication system as required by NFPA 72-2010: National fire alarm and signaling code, Section 12.4: Pathway survivability, or the protection of emergency generators per NFPA 110-2010: Standard for emergency and standby power systems, Section 7.2.1.1.

Lastly, structural elements used to establish type of construction may also be fire-resistance-rated. IBC Table 601 prescribes different rating requirements for these elements. For example, interior bearing walls of Type IA construction require three-hour protection, whereas bearing walls of Type II-B or Type V-B construction require no protection. This type of rated assembly is there to protect itself, not to provide a separation and inhibit the spread of fire, but to protect the structure so that it stays in place during firefighter operations. Generally the larger, taller, or more hazardous the building, the higher level of inherent fire protection it will require. The important thing to note is that penetrations in these types of fire-resistance-rated assemblies do not require opening protection. Doors, dampers, windows—none of these are rated for these types of assemblies.

Active fire protection

Active fire protection depends on the initiation of a system or human action to suppress or manage a fire event. The system that most often comes to mind is an automatic sprinkler system, but active fire protection can also include gaseous, foam, or chemical agent systems and can include manual means such as fire extinguishers and standpipes. Smoke control can also be a means of active fire protection as discussed later. For the purposes of this discussion, a sprinkler system is assumed to mean an automatic sprinkler system installed in accordance with NFPA 13-2010: Standard for the installation of sprinkler systems.

Detection and notification portions of a fire alarm system are not considered active fire-protection systems because they serve a life safety function. However, a fire alarm system may initiate an active fire protection system such as a pre-action sprinkler system; it may close smoke dampers or magnetically held-open doors or shut down an HVAC system; or it may activate a smoke control system sequence.

Over the past 100 years, and to a greater extent in the past 50 years as technology, use, and awareness have progressed, sprinkler systems have demonstrated significant reductions to the risk of fire growth and spread within buildings. Therefore, many areas of the code offer reduced requirements based on the presence of an approved sprinkler system. It is important to note that sprinklers are often required by the code simply because the risk is considered too high. This includes hazards based on quantity or type of fuel (i.e., high-pile storage or hazardous materials) or it can be because of the extended time it takes to evacuate or relocate occupants within certain occupancies (i.e., Group I and R occupancies).

Even when sprinklers are required due to risk alone, reductions in code requirements are still available. Users of the Uniform Building Code (UBC) will remember a time when designers had to carefully select the application of sprinklers for height, area, or substitution for rated construction. With the exception of substitution, there is no longer any limit as to the combined application of sprinkler reductions. Some of these applications include:

  • Increased or unlimited area (IBC 506.3 and 507)
  • Increased height and number of stories (IBC 504.2)
  • Increased egress capacity (IBC 1005.3)
  • Longer dead ends, common paths, and travel distances (IBC 1018.4, 1014.3, and 1016.2)
  • Reduction in fire-resistance-rated construction.

 

This article will elaborate on this last point to show the relationship between passive and active fire protection. Where portions of the code require protected fire or smoke assemblies, the following items illustrate conditions where the presence of sprinkler protection can be used to reduce or eliminate these requirements:

1.     Increase in the quantities of hazardous materials, thereby reducing or eliminating the requirement for Group H occupancies or control areas (IBC Tables 307.1(1), 307.1(2), 414.2.5(1), and 414.2.5(2)).

2.     Reduction of type of construction from I-A to I-B and shaft construction from two-hour to one-hour in high-rise buildings less than 420 ft in height (IBC 403.2.1).

3.     Elimination of the rating for atrium glass walls with provisions to wet the entire surface of the glass without obstructions (IBC 404.6, Ex. 1).

4.     Elimination of the rating for exit stair or ramp enclosure glazing or security glazing in a Group I-3 with similar provisions to wet the entire surface of the glass (IBC 408.2.8 and 408.7).

5.     Elimination of the rating for combustible storage in attics, under-floor, and concealed spaces (IBC 413).

6.     Reduction in the rating required to separate hazards including:

a.     Control areas (IBC 414.2.4)

b.     Separation of occupancies (IBC Table 508.4)

c.     Incidental use areas (IBC 509.4 and Table 509).

7.     Substitution of one-hour fire-resistance-rated construction for Types IIA, IIIA, and VA (IBC Table 601).

8.     Increase in the percentage of openings in exterior walls (IBC Table 705.8)

9.     Unlimited unprotected openings in exterior walls when the building is sprinkler protected and exterior openings are protected by a water curtain (IBC 705.8.2 exception)

10.  Elimination of the enclosure of exit access stairways serving four stories in other than B or M occupancies or an unlimited number of stories for Group B or M occupancies with draft curtains and closely spaced sprinklers (IBC 1009.3 Ex. 3 and Ex. 4) Note that the 2012 IBC permits this for required exit access stairways where the 2009 IBC did not.

11.  Reduction in or elimination of rating of corridors (IBC Table 1018.1).

Beyond the IBC, the 2011 edition of the National Electrical Code (NEC) also permits reduction of fire-resistance rating of assemblies based on sprinkler protection as follows:

1.     Electrical vaults may be protected with one-hour construction in lieu of three-hour construction (NEC 110.31).

2.     Electrical nonmetallic tubing (ENT) may be used exposed or concealed in building exceeding three stories in lieu of requiring a thermal barrier (NEC 362.10).

3.     Transformer vaults may be protected with one-hour construction in lieu of three-hour construction (NEC 450.42)

4.     Emergency feeder-circuit wiring and spaces housing emergency feeder-circuit equipment may be unprotected in lieu of two-hour protection (NEC 700.10(D)).

5.     Equipment and connections for sources of power such as storage batteries, generator sets, UPS (uninterruptible power supplies), separate services, and fuel cell systems may be unprotected in lieu of one-hour or two-hour construction (NEC 700.12 and 708.20).

Smoke Control

Smoke control can be active or passive depending on the geometry, fuel load, and occupant conditions. The goal of smoke control is much more specific in that it seeks to direct the products of combustion into a defined volume via physical separation and/or pressurization or it seeks to exhaust the products of combustion via natural or mechanical venting to allow a tenable environment for the time it takes occupants to reach relative safety.

Simple smoke control systems may include activation of building elements such as smoke or fire dampers, release of magnetically held-open doors, HVAC shut down, and pressurization of exit enclosures to bolster the resistance of a passive barrier. More complex systems could include mechanical, electrical, suppression, and fire alarm systems along with passive barriers to ensure a tenable environment during a fire emergency, thus requiring a coordinated design effort and experienced engineering.

Although a full discussion of smoke control is outside the scope of this article, designers should be aware that smoke control is required by the IBC in atriums connecting three or more levels, underground buildings, malls, stages, and smoke-protected assembly occupancies. 

Summary

For the vast majority of fire-resistance-rated construction, the table below represents the relationship between passive fire protection, sprinkler systems, and smoke control.

Table 1: Sprinkler and smoke control systems may be used to reduce or eliminate passive fire resistance. When systems are illustrated to allow reduction each application carries its own set of conditions which should be researched. Courtesy: ccrd

Passive and active fire protection share a combined history that has been shaped by the response to historical losses via code development, research, and technological improvements. The science of fire is still in its adolescence; therefore, a critical appreciation for the history, impetus, and intent of requirements helps all involved build safe and cost-effective buildings.

Donohue is a senior fire protection engineer at ccrd with 17 years of experience facilitating creative designs through a progressive mastery of building, fire, and life safety codes and standards.



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