A Benign-Looking Fireground Can Be a Deadly Fireground Two National Institute for Occupational Safety and Health (NIOSH) fatality investigation reports, 98-FO7 and F2004-14, involve firefighters advancing into obscured-visibility, low-heat conditions. In both cases, as they began their advance...
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At seven minutes, the average temperature within the warehouse was lower than the temperature at three minutes. At seven minutes, this fireground does not look particularly intimidating (note the light, lazy smoke). Again, most of you would agree that a coordinated offensive operation would be feasible. Some seasoned fire officers might be thinking backdraft, but where are the signs? There's no puffing or sucking smoke, there's no pressure, the smoke is light colored and the temperature is low. So what's the problem, right?
If you are the fire officer who arrives and is sizing-up the seven-minute photo, you would have no idea that this fire-growth evolution has just occurred; you would have no idea how much time has elapsed; and you would have no idea which side of the fire-growth curve you are contemplating.
This sudden upward trajectory of fire growth was caused by opening the front door; opening this horizontal "damper" provided sufficient oxygen for the fire-growth curve to swing upward again. Had the door remained closed, the fire would have continued to smolder and conditions inside would have remained static. On the no-value, decay-side of the fire-growth curve, sometimes the best thing to do is nothing. Take advantage of the time provided by the static fire condition because there is no value and there is no hurry. The fire-growth clock has stopped. Establish your command post, summon and assemble additional resources, and draft your action plan. If you decide to open a damper (ventilate horizontally), make sure that everything is set up and ready to go and ensure that everybody knows what to do. On the back side of the fire-growth curve, vertical ventilation is a valid ventilation method; however, don't forget the roof structure was exposed to a hostile fire before you got there (look again at the four-minute photo).
As pointed out by NIST and the Phoenix Fire Department, no accelerants or petrochemical-based fireload were present inside the warehouse. Had petrochemical-based fireload (plastics, flammable liquids, foam rubber, nylon carpeting, etc.) been present, the average Btu output would have doubled and the impact to the triangular gravity resistance system more severe.
As mentioned earlier, the primary focus of NIST was to gather data related to building collapse. The focus of this article is fire behavior and enhancing size-up. It was sobering for me to discover that I could arrive at a building fire, view all four sides of the structure, nail my size-up of smoke conditions (color, pressure, etc.), confirm by thermal imager that the temperature is relatively low and be 100% wrong in determining value and declaring the operational mode as offensive. I could be 100% wrong if I failed to determine which side of the fire-growth curve I was on.
How does a master craftsman fire officer determine which side of the fire-growth curve is being observed? The chart at the top of page 79 depicts cues you can use.
Of course, the easiest point on the fire-growth curve to size-up is when the fire is free-burning (at the top of the curve). Because both sides of the curve can look and feel the same, the challenge is to identify whether you're on the growth side or the decay side of the curve. Unfortunately, admiring a fire through the windshield will not reveal which side of the curve you are observing.
Watch the clock on the warehouse video. Notice that the fire looked "better" at seven minutes than it did at four minutes. (Recall that until the front door glass was removed, the fire was static.) If you were to arrive at the warehouse fire and you were looking at the seven-minute conditions, you would think you have arrived early and that there could be value. You would even get suckered into believing that occupant survivability is high.
What you would not know is that fire growth stopped at the five-minute mark and that the fire is static. So long as nobody horizontally ventilates the warehouse, the fire will remain static, as fire growth is impossible because of the lack of oxygen. Three minutes after the horizontal "damper" (the glass door) at the front of the warehouse was opened, the temperature started to climb at a thermocouple array 75 feet away. Thirteen minutes after ignition, thermocouple data transmission was lost due to structural failure on the interior.