The "Anatomy & Physiology" of the Structural Fireground - Part 1

Mark Emery explains why a competent fire officer must understand building construction.

Part 1 - Why a Competent Fire Officer Must Understand Building Construction Building construction is the anatomy and physiology of the structural fireground. Just as the human body must resist the assault of gravity and time, so must a building resist the assault of gravity and time. Just as...

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In his book Collapse of Burning Buildings, Vincent Dunn mentions the "structural hierarchy" of a building. It is this hierarchy of structural elements that engineers and architects must address when designing a building to resist dead load and live load. The basic structural hierarchy includes columns, girders, purlins and joists. Girders, purlins and joists can be conventional or lightweight. This structural hierarchy should be a consideration during pre-incident planning, during size-up and during a meaningful pre-overhaul safety survey.

Command-O-Quiz: Should a member of the structural hierarchy fail, which would be the most catastrophic:

A. Truss girder

B. Truss purlin

C. Wood I-joist

D. Column

Answer: D.

Discussion: In order for dead load and live load to reach the earth as compression, girders, purlins and joists must deliver their loads to columns and bearing walls. Thus, should a column or bearing wall fail, the structural hierarchy will follow. Column causes girder to fail, girder causes purlins to fail, purlins cause joists to fail, failure of joists causes floor to fail.

Note the truss girders and truss purlins in photos 1 and 2. They rank third in the structural importance hierarchy. Because they are the most catastrophic should they fail, columns and bearing walls are always number one in the structural hierarchy. Number two are girders, number three are the purlins and a distant number four are the joists.

Structural Maladies

Human anatomy and physiology can be weakened by gravity, time, genetic flaws, trauma and a variety of maladies; likewise, a building can be weakened by gravity, time, flaws, bad engineering, rot, rust, weather, damage, modifications - and the most devastating of structural maladies: fire.

Construction methods and materials react to fire differently. For example, wood will burn and steel will not burn. However, given enough heat, time and gravity, both will fail. A good rule of thumb is that wood will char one-eighth of an inch for each five minutes of direct flame exposure. Smoke will not char wood; charring requires direct flame impingement. Although steel will not char nor contribute fuel for burning, steel can fail when heated. As previously mentioned, structural steel (columns and girders) will elongate when heated to 1,000°F; structural steel will fail when heated to around 1,300°F. At 1,500°F structural steel is unable to support its own mass (it gets too soft). Cold-drawn steel (cables, steel rods and rebar) will fail when heated to 800°F.

What's the difference between structural and cold-drawn steel? Structural steel is liquefied and poured into a mold, quickly cooled with water, and retains the shape of the mold. Thus steel is a thermo (heat)-plastic (permanent change in shape) material: Heat it, change its shape, cool it and it retains the new shape. Cold-drawn steel is also heated, but not to a pourable liquid state. The steel is heated until soft and then is pressed, stretched and rolled through machines and then quickly cooled to retain its desired shape. Cold-drawn steel is used to resist tensile loads; structural steel is used to resist both compression and tension. Note: "Hot-rolled" steel is not used to resist loads in the structural hierarchy of a building. Sheetmetal is an example of a hot-rolled steel product. The sheetmetal wall enclosing a warehouse is not load bearing.

Call to Action

Study and understand the engineering principles of building construction. Study and understand the various materials and methods used to assemble structural systems. Know and understand the strategic characteristics that differentiate the five types of building construction.

The best resource to obtain this knowledge and understanding is the legacy of the late, great Francis Brannigan and his text Building Construction for the Fire Service. (I prefer my signed copy of his third edition.) His book should be the centerpiece of a fire station library and the foundation for fire officer strategic development. Dust should never collect on an informed strategist's personal copy of Brannigan's book.

Another excellent resource is Collapse of Burning Buildings by retired FDNY Deputy Chief Vincent Dunn. And although it is not directed toward the fire service, another informative reference is Fundamentals of Building Construction: Materials and Methods by Edward Allen (now in its fourth edition).

Next: The Strategic Classification of Building Construction - How to Strategically Classify Each of the Five Types of Building Construction