The impact of ventilation on strip mall fires: Breaking down the research
The UL Firefighter Safety Research Institute releases preliminary analysis of study of coordinated fire attack operations
In April, the UL Firefighter Safety Research Institute released another of its research reports, this time on the effects of ventilation during strip mall fires.
“Exploratory Analysis of the Impact of Ventilation on Strip Mall Fires” reinforces the ventilation considerations discussed by the recent UL FSRI study on the coordination of suppression and ventilation in single-family residential fires.
While the observations cannot yet be deemed conclusive, there are several noteworthy items that remind us of specific hazards apparent in these occupancies and the usual construction characteristics of strip malls.
Setting up the test
The study took place in Fairborn, Ohio, where a 12-unit strip mall, built in 1970, was acquired prior to its demolition.
Seven experimental fires occurred in four separate units to study the effects of both horizontal and vertical ventilation independently as well as in coordination with one another.
Typical of the construction associated with strip malls, the Fairborn site had concrete block and masonry outer walls, with lightweight unprotected open web steel trusses supporting a metal clad roof with foam insulation and a weather-proof rubber membrane (see Figure 1 for an example of such construction). Further, an interior drop ceiling is normally hung by supporting wires and provides an approximately 2-foot open space where electrical wiring, mechanical duct work and fluorescent lighting fixtures are secured to the trusses (see Figure 2.)
The type of fuel utilized was consistent throughout the seven experimental fires and was representative of potential fuels in a mercantile-type occupancy. Group A plastic commodity boxes are typically seen in rack storage testing for automatic sprinklers, are well characterized, and represent a high potential energy fuel. Each corrugated cardboard box is filled with non-expanded polystyrene (plastic) cups, layered and further divided with cardboard partitions. Depending on the size of the unit, the number of carboard boxes varied per experiment to ensure enough fuel was present for the expected duration of the test.
The fire suppression tactics for these experiments was limited, in consideration for firefighter safety, to an exterior attack only. The hoselines were either a 1¾-inch hoseline flowing 160 gpm or a 2½-inch hoseline flowing 260 gpm. Again, the basic experiments did not consider fire control during this phase of the research. Firefighters were also limited to operating outside of collapse zones, away from the awning/canopy, parapet, exterior walls or sidewalk overhangs.
Collapse was a major concern during this research, and two safety officers were utilized.
The primary telltale signs observed prior to a collapse were cracks appearing in the load-bearing walls or those walls beginning to lean outward due to separation from expanding trusses during the fire. One of the seven test fires resulted in roof collapse after the steel trusses, which expanded during the fire, began to cool and contract. The contraction of the steel trusses pulled them off the front load-bearing wall, resulting in a lean-to-style collapse.
Ventilation and flow paths
Test #1 was a control test to study the baseline fire dynamics, so the building was ignited with all doors and windows closed. The fire eventually became oxygen-deprived and while fuel was still present, the fire began to burn itself out.
Subsequent tests started with the front door open, followed by the front door and horizontal ventilation accomplished by breaking the front display windows, or the front door open with vertical ventilation.
Vertical ventilation was accomplished with a series of pre-cut 4x4-foot holes that were remotely activated and incorporated both 32 and 64 square feet of ventilation.
Other tests used various combinations of all three ventilation paths up to the open door with both horizontal and vertical ventilation.
In all cases, the more the ventilation during the fire, the higher the temperatures and pressures of the fire gases due to the increase in the oxygen, and flow paths of both incoming fresh air and exhausting fire gases.
The instrumentation clearly captured the developing flow paths, as most of the fires reached a flashover condition shortly after ventilation began without any coordinated fire suppression efforts. The more ventilation, the more fire growth, as well as higher temperatures and gas pressures. This was true for both horizontal and vertical ventilation.
This research seems to indicate that no additional ventilation of any kind should be used until fire suppression lines are in place ready to start extinguishment. That is also consistent with the results of the recent study on single-family dwellings.
At this time, there are no corresponding recommendations on coordinating suppression tactics for strip malls as there were for single-family dwellings, as this study was focused primarily on the impact of ventilation. There are, however, several considerations that are relevant from these strip mall test fires:
- Crews should perform a thorough size-up of the structure, including trying to ascertain the construction type and direction of the roof trusses to help determine the location of potential failures, such as distortion or expansion of the roof trusses that could cause a roof or wall collapse. Size-up also includes the need for eyes on the roof (preferably from and adjacent unit or aerial ladder) and the rear of the structure. Eyes on the roof can also indicate the presence of HVAC units and signs of an impending collapse. If the fire is located in the rear, the best access may be a rear entry door, limiting the path of interior travel for suppression crews.
- Limit ventilation, both horizontal and vertical, until hoselines are in place and suppression has begun.
- Watch for indicators of an imminent flashover (e.g., increased smoke and heat under pressure from the exhaust flow path).
- Set up collapse zones early into the fire with a sector or safety officer watching for indicators of collapse, such bulging or cracked exterior load bearing walls or sagging roof lines, while establishing safe operating distances from parapets, awnings or sidewalk overhangs.
A word of caution
Over the years, strip malls can change occupancies, perhaps several times. It is not unusual to even see a church or other place of public assembly now occupying what was once retail space. The strip mall could also have been built with a single use occupancy in mind, but now might have been subdivided into several smaller units or stores as illustrated (see Figure 3).
Further, unprotected open web steel trusses with a metal clad roof are not limited to just strip malls. Several buildings in my department’s response area were built in the early-1950s for retail stores on the first floor, basement storage below and tenant apartments on the second floor – all using unprotected open web steel trusses on all levels (see Figure 4). Such construction features like this should always be noted during your department’s fire inspection program and passed along to the suppression crews for preplanning purposes or for premises history.
Safety and SOGs first
As always, chiefs and training officers should review the current UL strip mall study in light of their department’s standard operating guidelines (SOGs).
As funding and further acquired structures become available, it appears that UL’s research on this type of structure will continue and hopefully include best practices on tactics for extinguishment, with keeping firefighter safety as its primary goal.