Reading a Commercial Structure With a Thermal Imager

As the fire service brotherhood, and even the nation, still comes to grips with the tragedy, the disaster in Charleston, SC, reminded me how dangerous commercial fires can be. The buildings are large, the fire loads can be massive, and the construction...


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As the fire service brotherhood, and even the nation, still comes to grips with the tragedy, the disaster in Charleston, SC, reminded me how dangerous commercial fires can be. The buildings are large, the fire loads can be massive, and the construction usually works against us. This month's article examines how to read commercial fires with your thermal imager. This is not, in any way, an assessment, of the tactics used that fateful day in June; frankly, I know only what has been reported in the media, and could not even pretend to offer insight into the events of that horrible day.

This column, rather, is a refresher to firefighters everywhere on using a TI to evaluate the inside and outside of a commercial building.

Reading the Outside

We often forget that the thermal imager can give us information prior to committing firefighters inside a building. The amount of information, and the importance of the information, will vary greatly depending on a number of factors. Building construction is probably the most vital factor, as it determines how much heat is retained by the building.

Most new commercial construction follows the same pattern across the U.S.: cinderblock or concrete slabs are stacked around the perimeter for walls; holes are left for windows and doors; metal web trusses are strung across the top of the walls and a flat deck is installed on top. There may be regional variations, but most new commercial construction is a lightweight masonry wall system with a lightweight metal roofing system. In other words, it's common Type III construction.

The lightweight nature of this construction is a boon to builders; it is easy to erect and the materials are relatively inexpensive. These buildings are a mixed blessing to firefighters: they cannot absorb as much heat as heavier construction, which means heat escapes to a thermal imager. Since they cannot absorb as much heat, they are at greater risk for collapse after a fire.

Photo 1 is a thermal image of a Type III building on fire, where the right occupancy is a carpet store and the left is a restaurant. The image demonstrates how quickly the fire "tells its story" to the thermal imager. This image was taken approximately 10 minutes after the initial alarm went out. The thermal image shows dramatically how the fire has spread across the roof line, indicating that the fire has spread underneath the roof across most of the carpet store. The windows show heavy heat build up, as does the door.

The thermal image tells us that this fire is advanced and building up heat rapidly. Knowing that the roof is likely support by metal trusses, and that there is no life risk in the store, an officer may re-evaluate interior attack plans in this situation.

Reading the Inside

In these commercial buildings, one of our biggest concerns as firefighters is the stability of the truss roof system. Again, trusses are a boon to builders as they create low-cost, lightweight methods of spanning larger areas and bearing more weight. However, trusses are a bust for firefighters. Once the truss is damaged by heat, the strength of the truss decreases exponentially, eventually leading to a collapse.

When firefighters operate underneath a truss roof, they need to be keenly aware of how far the fire has advanced in the truss system. Once the truss is exposed to the fire, firefighters know that the clock has started ticking, and time is running out underneath the truss. Photo 2 shows a truss roof under normal conditions. A close examination shows that the left portion of the roof is supported by open metal web trusses; the right side is supported by a more traditional metal post and beam system. In high heat conditions, we would expect the right side to endure longer than the left, as the lighter weight truss system will be damaged by heat faster.

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