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Photo 3 demonstrates what truss impingement could look like on a thermal imager. Notice how the trusses have disappeared into the heat of the fire coming from the left side. If the roof support system is at or near the same temperature as the super-heated gases of the fire, then the roof system is being aggressively attacked by the fire. A heavy support system, such as post and beam, should endure this heat impingement longer than a light support system, such as open trusses. Notice that the TI does not predict the time to collapse; it merely helps us identify that the countdown to collapse has probably begun.
The thermal imager is not a magic tool; but it can be a very effective tool in helping company officers, as well as incident commanders, make better decisions. As building construction gets lighter in the pursuit of cost reduction, firefighters will have less time to operate in those structures. The advantage of this lightweight construction is that heat transmits quickly to the outside surfaces, allowing us to use a TI to interpret what is happening inside the building. We can also use a TI to verify the integrity of the roofing system above as we advance in a structure.
For more insight on the effects of construction on using a TI for size up, visit the Technology section of Firehouse.com. And keep the Charleston Nine, their families, their friends and their coworkers in your prayers.
September's Firehouse article discusses how a TI can help at a commercial building, especially in size up. The usefulness of a TI in size up can be affected by the type of construction. Online, we'll examine how different types of construction can mask what is happening inside the structure. Type III is address in the print article; the remainder will be addressed here.
This wood frame construction is probably the lightest weight construction. The fact that it is wood probably helps keep us, as firefighters, from getting overly confident inside these buildings. We know they will burn…it's just a matter of time. Because there is usually little mass in the overall construction, the effects of a fire are transmitted rapidly to the exterior walls. As a result, the outside surfaces of the structure heat up quickly in response to where the fire is inside the building.
The heavy timber construction of days past is probably the most durable. The thick timbers, as well as the millions of bricks, have the ability to absorb a tremendous amount of fire and heat. In fact, I once worked in such a building that had sustained significant fire damage in 1944…some of the timbers still had charring on them, but had been painted over. These buildings, frequently mills or old warehouses, will generally hide much of the fire from a TI on the outside. Because the fire must heat so much mass, it generally does not impact that outside surface of the building. As such, a TI will rarely show the advancement of a fire inside a Type IV structure.
Type II and I
These structures tend to be high rises, although they are not exclusively high rises. These types of buildings may be coated in materials that are very reflective of infrared energy, such as polished steel or mirrored glass. As such, reading the effects of a fire can be nearly impossible. Many of these buildings, while heavier than a Type III, are still much lighter than a Type IV. As such, if the exterior shell allows, an active fire may transfer heat to the exterior of the building, allowing a TI to "see" what the heat signature looks like and where the fire may be headed. Keep in mind that the exterior material will probably be the biggest hindrance to reading what is happening inside the building.
Lighter weight construction will be more likely to show the impact and travel of an active fire. The exterior material will impact the "readability" of the heat signature, as certain reflective materials may mask the interior conditions. By bringing your TI with you on fire planning exercises, as well as inspections, you can gain practice evaluating and interpreting thermal images from the various types of construction. As always, when working outside, don't discount the effects of the sun on your thermal image.