When designing and constructing buildings in today’s built environment, a significant amount of cross-discipline collaboration goes into ensuring a smooth process from start to finish. Beginning with the end-user’s needs and the architect’s vision, a modern building is brought to life with the contributions of many designers, engineers and project managers.

One way this manifests is in fire safety requirements, which follow prescribed requirements for building separation with rated construction intended to prevent the passage of smoke or fire from one part of the building to another. This compartmentation can protect occupants in most parts of a building, even when a dangerous fire has occurred in another area. 

A fire protection engineer can work with an architect to ensure that codes and standards are properly applied, including any possible exceptions to the base requirements. 

For example, if an architect wants to provide windows between those spaces that have a rated separation, the windows must be fire-rated. However, a special application window sprinkler may be used to achieve an equivalent rating. 

Another common design feature, atriums, are often designed to provide a large vertical space connecting multiple floors and can create a bright, airy environment. However, this large vertical space would allow smoke and heat from a fire to move easily from floor to floor, endangering occupants throughout the building unless a barrier between floors is provided. 

Most adopted building codes require atriums to be enclosed with rated walls — with some exceptions. One option is to enclose the atrium with glass and use closely spaced sprinklers to achieve an equivalent rating to a solid fire-rated wall. Often, the rules that apply to using window sprinklers and those that apply to atrium sprinklers can get mixed up, so understanding those nuances is important. 

Other scenarios also exist in the codes and standards that allow the use of closely spaced sprinklers to permit certain common building features, such as stair or escalator openings and across-the-stage openings in a theater. These are often referred to as “water curtains.” Similar rules regarding spacing for these systems have, perhaps, led to additional confusion and misuse of the term across all the scenarios described herein. 

Window sprinklers

Window sprinklers are special application sprinklers per NFPA 13 section 15.2, also described in section 9.3.15 as sprinkler-protected glazing. All window sprinkler manufacturers must meet a specific set of testing requirements to demonstrate their effectiveness in preventing the failure of windows in rated walls. 

These sprinklers include very specific rules on the position and spacing and the window types they are permitted to protect. 

The primary mechanism these sprinklers employ to achieve equivalency to rated glazing is to fully wet the glass pane they are protecting. Any dry spots on the glass, such as in the corners of the window, could result in superheating of that portion of the window in a fire. 

When a fire hose stream is applied in such a scenario, the rapid drop in temperature can cause the window to shatter and create an opening for the fire to travel through, defeating the usefulness of the rated wall. Typical sprinklers discharge in an umbrella-shaped pattern and, thus, would not provide the same cooling effect as an approved window sprinkler; it would not fully wet all corners of the glass. 

Window sprinklers come in a few different configurations to allow for window protection in a variety of configurations, but some strict requirements apply in all cases: 

• There can be no horizontal mullion on the window being protected, as this can disrupt water flow down the glass and create dry spots. 

• The protected windows must be fully vertical and flat; they cannot be sloped or curved. 

• The windows must be completely inoperable, which also means that glass doors are not permitted to be protected with window sprinklers to achieve an equivalency to a fire rating.

• All other manufacturers’ requirements must be followed regarding the distance from the window, the allowable height of the window and the spacing of the sprinklers from one another.

Another less common use for window sprinklers is by employing them in an exposure protection arrangement to mitigate an issue with required fire separation distances between buildings. They are permitted to be used per NFPA 13 section 8.7, as the directional nature of the discharge pattern can be a benefit over using standard sprinklers since you are required to account for only half of the spray being directed at the wall when calculating the flow from the standard spray sprinkler. 

Window sprinklers can be used either with the linkage intact in an automatic means or with the link removed to be used as part of a deluge system. Again, all manufacturer and prescriptive requirements must be met. 

Atrium enclosure 

When an atrium is proposed for a multistory building, separation requirements can be found in the various adopted building codes to mitigate the danger of fire and smoke traveling from floor to floor. The primary requirement is to separate the atrium from each communicating floor with a 2-hour fire barrier. If the building is fully sprinklered, the barrier can be reduced to a 1-hour fire barrier. 

However, enclosing an atrium with glass walls is permitted with the use of closely spaced, standard spray sprinklers. These sprinklers must be spaced 6 feet on center; if walkways are on both sides of the glass, the sprinklers must be installed on both sides as well. If no walkway is on the atrium side of the window, you may eliminate sprinklers on that side. 

Glass doors are permitted in the enclosure as long as they are self- or automatic-closing doors. This allowance for a glass door in the atrium enclosure may be why there is often confusion when using window sprinklers, as it is not permitted to protect doors in a rated wall with a window sprinkler. 

Similar spacing requirements and the fact that we are using sprinklers on glass to provide an equivalency to a rated separation is often where the misunderstanding on the differences between window sprinklers and atrium enclosure stem from. It should be noted very clearly in the building specifications what approach is being used and to reference the code sections that allow it. 

As stated previously, atrium enclosure protection with sprinklers is achieved with standard spray sprinklers. While you could use window sprinklers in these applications, they are more expensive and, thus, not typically chosen when standard sprinklers will suffice. 

Water curtains

Water curtains, while a commonly used term, are not explicitly defined in NFPA 13 or the International Code Council codes. Even though it is not a defined term, it is used in a few locations throughout these codes and standards. 

One such location is in International Building Code (IBC) section 410.2.5, related to a proscenium curtain for the opening at the front of a stage. This code section requires the stage opening to be protected with an actual curtain that is fire-rated, rated horizontal sliding doors or an approved water curtain complying with section 903.3.1.1. 

This section then points you to NFPA 13 section 9.3.13.2, which states that the proscenium opening shall be a deluge system with open sprinklers spaced no more than 6 feet apart. How this is described is potentially one of the origins of the term being applied anywhere we use closely spaced sprinklers.

In IBC section 705.9.2, openings required to be protected comply with the requirements of section 716, with an exception for a fully sprinklered building where the exterior openings are protected by a “water curtain” using automatic sprinklers approved for that use.

We are also allowed to use closely spaced sprinklers along with draft curtains to protect vertical openings in places such as malls with stairs or escalators connecting between floors. This is in both NFPA 13 section 9.3.5 and IBC section 1019.3 Exception 4.

Unless it’s specifically permitted in the building code or as a special application sprinkler that has proven its functionality through testing, building professionals are not allowed to propose the use of sprinklers in a “water curtain” to avoid the required rated wall. 

The use of new technologies and alternative applications of existing technologies, such as water mist to create a water curtain, could potentially be used in a performance-based design, but the engineer of record would have to prove the equivalency and rigorously defend it. 

Guidelines exist in published performance-based design guides and in IBC 104.2 if an ambitious engineer would like to go down that path. In other parts of the world, we do see some examples of these types of technologies used, but the United States tends to be stricter around construction regulations. 

For example, the National Building Code of India is used to govern building construction; this standard permits the use of specific water mist nozzles to create a water curtain to achieve compartmentation in parking structures. 

Even in the United States, there are holdovers from previous codes, such as the New York City Administrative Code, Title 27: Construction and Maintenance, Chapter 1: Building Code, Subchapter 5: Fire Protection Construction Requirements, Article 5: Prevention of Interior Fire Spread, Section 27-339 Fire segregation of occupancies. The language exists from the 1968 NYC Building Code and allows sprinklers to be used in lieu of a rated wall to separate nonresidential kitchens from adjoining dining spaces.

Sprinklers remain one of the most effective active protection systems against the threat of fire, and their use to provide equivalencies to solid fire ratings provides flexibility to fire protection designers. Fire protection engineers play a key role in helping to guide the decisions to ensure sprinklers comply with the guidelines governing their use. 

As people continue to develop new technologies and techniques of applying existing technologies, we may yet see other ways sprinklers can benefit us in the built environment.

Melisa Rodriguez is the vertical business development leader for storage with Johnson Controls. She is a licensed fire protection professional engineer with 20 years of industry experience and is also NICET-certified Level IV in water-based systems layout. Rodriguez is president of the Society of Fire Protection Engineers Minnesota Chapter, vice-chair of the Minnesota Governor’s Council for Fire Prevention and Control, and a member of various fire protection industry committees.