Building construction has evolved over time. Although the overall rationale for a structure—to provide shelter—has remained the same, the way we build buildings has changed as new technologies and techniques emerge.
From a firefighting perspective, it is important to realize that the built environment that you respond to in your community is actually an aging collection of different types of buildings, incorporating a myriad of construction methods, materials, and design concepts, along with the hazards associated with each one.
In essence, you must become the local “historian of building construction” to understand how the buildings you protect—and the people in them—will behave under fire conditions.
One thing that hasn’t changed, however, is the laws of physics.
As a fire fighter, you must have a working knowledge of how the forces of nature and humans cause buildings to collapse.
Anticipating the reaction of a particular building to these forces is essential to your safety.
It is important, therefore, that you have an understanding of the basic concepts of construction.
Despite the fact that learning from fire-ground experience is critical, you can’t rely solely on this to keep you safe.
From experience, some fire officers have learned to make useful, but limited, judgments regarding the loss of structural stability in a fire.
Many of the commonly taught indicators, such as sagging floors, strange noises, and the like, may be cases of “too little, too late.”
In addition, experience with one type of building element, such as solid sawn wood joists, is not valid for trusses or wooden I-beams.
Relying on experience alone is not sufficient for today’s fire fighters; they must be aware of the theories and principles involved in building construction.
We will begin our study by looking at the greatest of all fire fighter enemies—gravity.
The force of gravity is the eternal enemy of every building.
Twenty-four hours a day, seven days a week, gravity exerts a force on the building.
Buildings may appear to be just quietly sitting on their foundations, but, in fact, they are under great stresses.
Although this may not be apparent as you look at the structure, the stresses are there nonetheless.
By one means or another, in assembling a structure, the builder has defied gravity. Not all builders recognize this fact.
The gravity resistance system in a building consists of all the structural elements and the connections that support and transfer the loads—that is, the forces attributable to gravity and other sources of stress on the structure.
Fire weakens and destroys the structural elements and/or the connections in a building, and places loads on some structural elements that cannot handle them, which can cause a collapse. Once the collapse starts, the results are unpredictable.
Some buildings are designed to be fire resistive. This means that to some degree they will resist fire-caused collapse.
The majority of buildings are non-fire-resistive, and thus may easily collapse as a result of assault by a significant fire on the gravity resistance system.