Firehouse July 2004

Firehouse Magazine, July 2004
Building Construction
By Jonathan Bastian

Good firefighters understand building construction. They may not be tradesmen, but they know basic construction features and terminology. A knowledge of building construction helps a firefighter understand where the fire has been, where it may be going, and how much time he has to do something about it. Frequently, firefighters have to guess at construction features because they are hidden by smoke, darkness or the building itself. The thermal imager, however, can help.

Essentials

Remember that thermal imagers do not “see through” building construction. However, the TI will display certain building features because of surface temperature differences. Consider four key points for evaluating construction:

In districts with mixed construction, the TI can help identify true brick construction from brick veneer on a wood frame (see Image 1). Remember that the mass of the construction type and the insulation of the building determine how quickly the TI detects the effects of the fire.
Because the outside temperature is usually different from the inside temperature, the TI will “see” studs in exterior walls. Generally, studs will not be visible on interior walls unless there is a heat source (such as a fire behind the wall) emphasizing the framing.
In buildings with exposed trusses, the visibility and temperature of the trusses are critical pieces of information that can be gained only with the TI. Trusses that are clearly visible allow firefighters to verify structural integrity. However, trusses that disappear into a heat cloud or assume the same temperature as a high-heat environment are deadly. If firefighters confirm truss construction, but cannot see all of the truss system with the TI, they should consider the trusses compromised (see Image 2).
Do not forget that being able to see structural features does not make them stronger or more stable. The TI can help confirm the type of construction as well as its integrity. It cannot change the construction or integrity. Firefighters must train to make proper, safe decisions with the information provided by the TI.

Image 1: At this fully involved residential fire, the TI shows the difference between a true brick wall and a veneer. The left portion of the structure is the original true brick wall. The right portion, showing a lighter gray, is wood frame with brick veneer. Photos courtesy of Bullard.

Image 2: This image simulates high heat to the left of the firefighter, indicated by the white cloud. While most of the truss system is visible, the trusses are not visible to the left. The image indicates severe heat attacking the trusses, with the trusses as hot as their surroundings.

Practice Makes Perfect

Evaluating construction is only part of the skill; determining “normal” and “abnormal” is a critical component as well. To practice this:

Use the imager on inside and outside investigations or planning sessions. Get a sense for how different construction types present in various conditions.
Use the TI to view attic and ceiling supports under normal conditions, as well as after fires. See how the heat affects the appearance of the structure.
Practice “reading” construction while driving around the still district. Especially with newer members, have firefighters anticipate construction methods based on exterior appearance and occupancy.
Walk through new construction structures at various stages of completion. Evaluate the building visually and thermally (with the TI) compare the information.

Table Talk

Now that summer is here, some drills are better in air conditioning. While sitting at the firehouse table, members can discuss:

General building construction features.
The risks and benefits of different types of construction under fire conditions.
The effects of weather or the sun when evaluating a structure with a TI.
The potential appearance of different roofing systems and different types of truss systems on a thermal imager.

On Scene

Evaluating structural features improves firefighting effectiveness, as well as firefighter safety. Frequently, firefighters find that the type of construction they were expecting is not what they face once they begin suppression efforts. Image 3 shows the ceiling and roof system of a manufacturing plant. Examine the image and decide which side of the building is at less risk of catastrophic failure and why. Are there potential risks above the fire company?

Image 3: This thermal image from a manufacturing facility shows important details about firefighter safety and structural integrity.

Final Report

First, the image shows several wires and conduit lines running across the ceiling system. If these fall to the floor, they can become entanglement hazards to firefighters. Secondly, the image appears to show a very high ceiling, something firefighters may have expected based on the exterior construction. Close inspection shows that there are two different types of roof support.

The left 1/3 of the image shows open-web metal trusses, while the right 2/3 of the image shows standard post and beam construction. This implies that there are greater open spans to the left than to the right. The right side may have supporting posts or load-bearing walls in the immediate vicinity. Under heavy fire conditions, a truss system is more likely to fail than the post and beam system. A fire company recognizing the right part of the building as more secure would stay closer to solid supports in the event of a collapse.