Figure 1. Limiting hazard posed by smoke layer in an atrium.

The design and testing requirements for smoke management systems for covered malls and atria are

included in NFPA 92B1. While covered malls and atria may appear to be different spaces for the purpose of

limiting the hazard posed by smoke, they pose similar problems. Both covered malls and atria have large

undivided spaces, relatively small amounts of fuel, and often have tall ceiling heights.

About 15 years ago, the design basis for covered malls and atria changed from providing a number of ach to

providing the exhaust necessary (if at all) to limiting the hazard posed by smoke. In the model building

codes in the United States, the default design requirement stipulates that the smoke layer cannot descend

within 6 ft of the highest walking level that is part of the exiting path2. This requirement is depicted in Figure

1. The current hazard-limiting design necessitates that the design process consider the size of the design

fire and conduct calculations to determine the characteristics of the smoke management system3. The focus

of most smoke management system designs consists of the exhaust and makeup air supply capacities and

their arrangement. Because acceptance test procedures need to be consistent with the design basis, the

test procedures also had to change.

This paper describes frequent errors in design approaches and procedures for acceptance tests for smoke

management systems in covered malls and atria that have been related to the author.

Design Pitfalls 

Figure 2. Smoke exhaust rate requirement.

As with any fire protection system, the design of a smoke management system needs to be commensurate

with the potential hazard posed by a fire. Consequently, selection of the design fire(s) is the essential first

step of the design. The selection of candidate design fires seeks to identify reasonable scenarios that

provide the greatest demand on the smoke management system (i.e., they represent “worst case”

situations). The demand may be expressed in terms of the smoke exhaust capacity required, temperature of

operating equipment, or maintaining a particular level of visibility or other smoke condition.

Optimistic Design Fire 

The design fire is specified in terms of the fuel packages and their location in the covered mall or atrium.

One of the principal parameters used to describe fuel packages is their expected heat release rate.

Experimental data on heat release rates for many fuel packages is available in several references4,5,6 (Figure

2).

In an attempt to reduce the required smoke exhaust capacity, it’s very tempting for designers to suggest

that the maximum heat release rate expected for a fire in an atrium will be very small and the atrium will be

a wide-open, pristine area with very few combustibles. However, does such a small heat release rate

consider the presence of holiday decorations or weekend trade shows? Designing the smoke management

system to react only to a small fire could significantly limit the use of the space. Reportedly, an optimistic

selection of the design fire prevented a new arena from hosting a major trade show given that the

commodities associated with the trade show had heat release rates that were greater than the design fire.

Doubling the heat release rate of the design fire only results in a 25% increase in the smoke exhaust

capacity required. Being pessimistic when selecting the design fire may greatly expand the flexibility of the

facility and permit it to be utilized in a fashion consistent with the vision of the owner. Revising the smoke

management system to expand its capabilities after construction can be cost-prohibitive.

Does High Capacity Exhaust Produce Plugholing? 

Figure 3. Diagram of plugholing.

The smoke exhaust rate needed to arrest the descent of the smoke layer is indicated in Figure 3. For large

clear heights, the required smoke exhaust can be very substantial.

When providing large exhaust capacities, the quantity of exhaust to be included needs to be determined. If a

lesser number of high capacity fans are selected as a means to provide the total exhaust required, a check

needs to be provided to assess whether the “strength” of the fans is likely to create a hole in the smoke

layer, as indicated in Figure 4. This phenomenon is referred to as “plugholing,” a phrase that originated in

British research literature. This phenomenon is similar to water draining from the bathtub. As the tub is near

empty, a gurgling noise can be heard coming from the drain as air mixes with the water.

In general, plugholing occurs more easily as the temperature and depth of the smoke layer becomes

smaller. As a result of a hole being created in the smoke layer, the exhaust fans will be removing air from

below the smoke layer as well as the smoke. If plugholing occurs, the full capacity of the fans is not being

utilized to remove smoke alone, thereby decreasing the effectiveness of the exhaust fans, leading to a

descent of the smoke layer.

How Is Makeup Air Provided? 

In order for the exhaust fans to be effective, makeup air needs to be provided below the design smoke layer

position. In terms of the scientific principles affecting the performance of the smoke management system,

the makeup air can be provided by any means, including natural ventilation (opening exterior doors) and

mechanical supply fans.

Applicable codes may require a particular proportion of the makeup air to be provided mechanically. In

general, relying on the leakage of exterior building components is not sufficient to satisfy the amount of

makeup air supply required. The stipulation of makeup air provided is that it be limited to a velocity of 200

ft/min or less at the location of the design fire7.

Just as importantly, recent research has indicated the need to distribute makeup air supply points around

the perimeter of the atrium or covered mall and not to concentrate the points only on one side. Computer

modeling of situations where the makeup air is asymmetrically provided has indicated that such a design

may deflect the flame to one side, thereby appreciably increasing the amount of smoke production, which in

turn results in a lower smoke layer position. For this particular example, the smoke layer depth changes

from 20 ft to 39 ft solely due to the change in the makeup air supply arrangement.

Acceptance Testing 

Acceptance tests of smoke management systems in covered malls and atria are conducted using a

variety of techniques, ranging from rational approaches to those that are arbitrary. The most

important consideration in developing a plan for an acceptance test for a smoke management system

is that the test should seek to confirm that the performance parameters of the system are being

achieved.

Criteria For Acceptance Test 

Figure 5. Minimum exhaust capacity to cause plugholing.

One of the most significant flaws with the performance of acceptance tests involves the failure to

stipulate acceptance criteria prior to conducting the test. This is analogous to playing a game without

first deciding if more points wins or loses. Imagine the ensuing debates on a golf course or football

field if the rules weren’t established prior to the game. Acceptance criteria should relate to the design

basis of the system, e.g., exhaust rates and makeup air supply rates.

Visible Smoke Tests? 

The next most significant issue involving acceptance tests of these systems for some installations is

what appears to be an automatic requirement of using visible smoke. While the use of visible smoke

can be valuable in some applications, in other cases it can be used for the wrong reasons, perhaps

giving a false sense of security or an inappropriate view of performance. Consequently, a particular

engineering purpose for such a test needs to be identified prior to conducting tests with visible smoke,

at least to determine appropriate acceptance criteria, if not to establish a relevant protocol.

Sometimes, the visible smoke tests are rationalized in terms of needing to simulate conditions during a

fire. However, this is the only type of fire protection system for which such a demand is made.

Simulated fire tests are not required of other active fire protection systems such as sprinklers or

detectors, nor are they required for passive fire protection systems such as firewalls.

Some acceptance tests involve “filling the space” with smoke from smoke bombs and seeing how long

it takes to clear the area or to observe smoke movement patterns. In the case of a “time-to-clear”

measurement, smoke management systems in covered malls and atria are not designed to clear the

space of smoke. As such, why should they be tested in that manner?

In contrast, needing to observe smoke movement patterns with the operating smoke management

system is an appropriate concern. However, does the entire space need to be filled in order to conduct

that assessment? Usually, the smoke movement patterns of interest are located at specific points,

(near doorways, store front openings, etc.). Smoke movement patterns near openings can be observed

in much simpler ways than filling the entire space with smoke. Small smoke sources can be used to

produce visible smoke near the opening in order to observe the direction of smoke flow near that

opening. Lightweight paper or air speed and direction measurements can also be used to indicate

direction of travel.

Visible smoke tests may also be mandated because of a lack of confidence in the calculations or to

confirm the appropriateness of assumptions. This is a reasonable concern, given the relatively

contemporary nature of the computational techniques and the limited validation efforts for the

application of these methods in the innovative designs that are often seen in covered malls and atria.

If the design fire is located away from the enclosing walls, where the smoke plume can rise to the

upper portion of the space without encountering significant obstructions such as balconies or where

the horizontal cross-section atrium gets narrower with height, one of the axisymmetric plume

equations (equations 6.2.1.1b or 6.2.1.1c in NFPA 92B) have probably been applied to establish the

design requirements. These particular equations were developed over 30 years ago, and their origin

traces back to the mid-1950s. Their predictive capability has been examined numerous times,

including one application in an arena reported by Dillon8. As such, there’s little need to conduct a

visible smoke test to confirm their accuracy. Doing such would be similar to confirming friction loss

calculations provided by the Hazen-Williams equation in fire sprinkler piping, which is unheard of.

If a visible smoke test is being conducted to assess whether the smoke management system is able to

arrest the descent of the smoke layer for other situations, the test scenario should seek to capture the

same smoke production and movement mechanisms as in an actual fire. As such, cold smoke from

smoke bombs should be avoided, as the smoke does not have the buoyancy of hot smoke and thus will

move quite differently. In addition, the smoke production characteristics of smoke bombs are opposite

to those of typical fires, where the smoke production rate of the smoke bombs decreases with time.

Heating the smoke should be done with extreme caution. Use of heating sources should be just

sufficient to provide enough buoyancy for the smoke to reach the ceiling. Further, test engineers

should confirm (prior to the test) that the heat output of any heating sources is appreciably less than

that needed to cause any damage to the facility. Controls and extinguishment means need to be

available to terminate the test promptly if necessary. The composition of the smoke bombs is also

known to be carcinogenic, so anyone exposed to the smoke should wear self-contained breathing

apparatus.

Finally, the movement of visible smoke will be influenced by the combination of forces affecting air

movement in buildings that are present during the time of the test. As such, the performance observed

during the test also will be dependent on the conditions present. If the conditions change, the

performance may also change. Unless the presence of such forces is acknowledged and taken into

account, a false sense of security may be created as a result of a successful visible smoke test on the

test day when the conditions served to aid the performance of the system.