The "Anatomy & Physiology" Of the Structural Fireground

Past installments of this series introduced the strategic classification of building construction. Ordered strategically, based on perceived fire resistance, the five basic types of building construction were listed as follows: Type I — Fire Resistive...


Past installments of this series introduced the strategic classification of building construction. Ordered strategically, based on perceived fire resistance, the five basic types of building construction were listed as follows: Type I — Fire Resistive Type II — Non-Combustible Type IV...


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Try this: You enter the concrete tilt-up big-box store blindfolded. While you're standing in the middle of the store, the blindfold is removed. You look up at the roof structure and see a metal roof deck supported by lightweight steel bar-joist trusses. What type of building construction is the big-box store? You don't even need to see the walls to make this determination: it would be strategically classified as Type II, Non-Combustible. Without seeing the exterior walls, you would know that they are made of a non-combustible material such as concrete or masonry.

Just as with Type V, Wood Frame, you will find Ordinary Construction that is conventional and Ordinary Construction that is lightweight. You will find conventional (legacy), unreinforced masonry buildings and you will find lightweight (contemporary), reinforced masonry buildings. You can find conventional panelized roof systems and you can find lightweight panelized roof systems. Example: timber purlins (conventional) vs. metal plate connected or metal tube web truss purlins (lightweight). It is not uncommon to find Ordinary Construction that feature a lightweight roof, yet has a conventionally joisted floor; next door you may find Ordinary Construction that features both a lightweight roof and a lightweight floor. It's impossible to make this important determination when viewed through the windshield.

These days, a through-the-windshield size-up of building construction can be a crapshoot. For example, because it doesn't convey the type of building construction, declaring "large concrete tilt-up" has no strategic value. Concrete tilt-up is not a type of building construction; it is either Type II or Type III. The only way to determine the type of construction is to step inside and view the roof structure (or consult a pre-plan).

Recall from part one of this series (April 2009) that the basic structural hierarchy of a building is comprised of four components: columns, girders, purlins and joists. (In panelized roof systems, joists are sometimes referred to as sub-purlins). No matter if the roof is supported by giant heavy timbers or pee-wee trusses, the most important component of the structural hierarchy is the column (and a load-bearing wall that serves the same purpose as a column). Although all buildings feature a structural hierarchy, not all buildings feature the same structural hierarchy components and materials. For example one building may feature columns in its hierarchy while the building next door may have replaced the columns with steel rods in tension. An interesting example of this is the San Diego, CA, Fire Museum.

From the street (through the windshield). the humble fire museum building would be classified as Type III, Ordinary, and appears strategically benign. After stepping inside the two-story portion of the building and studying the floor/ceiling system, the museum becomes a no-offensive building. The gravity-resistance system supporting the upper floor is downright scary. For a building like the San Diego Fire Museum to make the strategic leap from benign to scary, it has evolved following a typical progression:

  1. The original two-story portion of the building was around 25 feet wide (from a two-story B-wall to the two-story D-wall).
  2. To expand the occupancy left or right, you would remove the unpenetrated first-floor load-bearing masonry wall (in this case, the floor-one B-wall of the two-story occupancy).
  3. This was done by transferring the load of the floor above to timber girders and columns that replaced the load-bearing masonry wall.
  4. Later, to make more room to sell stuff (or, in this case, to accommodate larger fire apparatus), many of the timber columns had to be removed.
  5. After the timber columns were cut and removed, a short stub remained near the ceiling at the bottom of the girders.
  6. To transfer the load formerly delivered to the columns, cold-drawn steel rods were installed; the steel rods transfer the load from above to adjacent girders.
  7. The steel rods pull upward on each column stub; this upward pull is achieved by tightening turnbuckles that spice the steel rods. Thus, even though the rods are in tension, the upward pull generates compression within the column stubs.
  8. Traveling through the girders that transfer their load to the columns the load of the second story is delivered to the foundation as compression.