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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The 2001 warehouse burn-test was performed by NIST in conjunction with the Phoenix Fire Department. A description of test procedures and test data was published in May 2003 and is available at www.fire.gov/collapse/index.htm. The website offers video of the event and a 66-page final report. This article will focus on the first test conducted in the front portion of the warehouse.
The front portion of the single-story, ordinary-constructed warehouse (masonry walls and wood roof) measured 50 feet wide by 90 feet long (4,500 square feet). The roof was supported by conventional wood trusses (the cords were two-inch-by-12-inch lumber). Each triangular truss spanned the 50-foot building width and 15 feet separated each truss. The bottom cord of each truss was 10 feet above the floor and the peak of the roof was 18 feet above the floor. The fireload consisted of four stacks of 10 wood pallets.
The video and test data are more than interesting, for they reveal a tremendous "nugget" that all fire officers and training divisions should study and incorporate into their fireground size-up and development of a fireground strategy. The strategic nugget is revealed within 10 minutes of ignition.
Three thermocouple arrays recorded temperature at 12 points between the floor and the 18-foot roof peak; carbon monoxide was monitored one inch above the floor and three feet above the floor at two locations. The ignition source for the test was newspaper and an electronically controlled book of matches. No petrochemical fireload or accelerants was used.
The Strategic Nugget
OK, let's get to it -- see if you can find the strategic nugget in the following data:
- At three minutes, 15 feet from the front door, the floor temperature was 196 degrees Fahrenheit; three feet above the floor the temperature was 351F. (The fire was getting traction for the climb to flashover.)
- At four minutes, 15 feet from the front door, the floor temperature was 892F; three feet above the floor the temperature was 619F. (The fire was feasting on fuel and oxygen.)
- At five minutes, 15 feet from the front door, the floor temperature was 457F; three feet above the floor the average temperature was 367F. (Lack of oxygen has interrupted the feast.)
- At nine minutes, 15 feet from the front door, the floor temperature at the front was 230F; three feet above the floor the temperature was 243F. (The fire was suffocating.)
As listed above, the numbers probably don't reveal much to you; however, this data becomes significant when supported by photos and viewed on a rudimentary fire-growth curve. Notice that the left side of the curve represents fire growth, evident by the steady rise in temperature as the curve climbs toward free-burning and flashover; the right side of the curve represents temperature decline and fire decay. The steady temperature rise is due to abundant fuel and oxygen; the temperature free-fall is due entirely to lack of oxygen. The decline side of the curve occurs for one of two reasons: the fire runs out of fuel or the fire runs out of oxygen. (A third reason could be fire department intervention.) During the NIST warehouse burn, plenty of fuel and residual heat remained; the fire simply ran out of oxygen.
The Unseen Evolution
What follows is the evolution of a fire within the 4,500-square-foot conventional structure in Phoenix. Pay close attention to time, interior temperature and exterior conditions during each progression of the fire. Pay particular attention to the fact that a no-value, high-risk, defensive fire can minutes later appear to be an acceptable-risk offensive fire. When a fire department arrives on the no-value side of the fire-growth curve and declares the mode offensive, firefighters will enter and advance toward a smoldering ambush. By opening the door and advancing, the trap has been set. More horizontal ventilation (breaking windows) will exacerbate the problem. Time, distance and horizontal ventilation become the enemy.
At the five-minute mark, after four minutes of increasing temperature and decreasing visibility, the temperature suddenly dropped to 477F -- a 50% decrease between four minutes and five minutes. In case you missed it, a nugget has just been revealed. Viewed through a vehicle windshield, a fire officer would have no idea which side of the fire-growth curve is being considered. In fact, one minute after agreeing that the four-minute photo was a defensive fire, many of you would agree that a coordinated offensive operation looks feasible (if your size-up is based solely on conditions shown in the five-minute photo). Don't take the bait -- although exterior conditions have improved, value within the building has not improved. Conditions will appear to "improve" as the minutes pass.