This month we will continue our discussion regarding building collapse with a look at the different types of building construction, and some specific collapse inherent hazards that are associated with each type. We will also examine different types of collapse patterns that result from the failure of the building's "gravity resistance system," and the problems that each present to the rescuer.
One of the most significant areas of proficiency for all firefighters is the methods of building construction. While it is imperative to understand construction for fire spread and structural behavior, it is just as important to be able to identify potential problems within structures before the fire begins to attack the structure. From a rescuer's standpoint, it will become necessary to push, pull, cut, breach, lift, or tunnel through the materials that make up the collapsed structure. Knowing what materials were used in the construction process of these buildings will better prepare the rescuer to defeat the materials (see Photo 1).
That being said, let's take a look at the five types of building construction, and some of the inherent problems that each present to the rescuer:
Type I Construction: Fire Resistive - These structures are made from non-combustible materials that do not add to the fire load of the structure. They are usually massive steel components that are encased in concrete, that provide a higher fire rating on the components. There may be cast-in-place concrete, or the slabs may be poured off-site and hoisted into place. These structures are considered to be the most resistant to collapse.
Type I: Inherent Hazards - Concrete takes approximately 28 days to fully cure to a point that collapse is no longer a threat. However, the pace of construction moves much faster than that. There have been cases where collapses have occurred when the formwork has failed when the compressive load from the upper floors have over taxed the pre-cured floor (see Photo 2). Secondly, when the slabs are laid into place, they are connected in a variety of methods; some of these connection points have been the result of a catastrophic failure of the structure. Inspection of these types of structures may reveal deteriorating concrete, spalling that is deep enough to expose support rods and cracks that can lead to a collapse.
Type II Construction: Non/limited Combustible - These structures are similar to Type I structures, with one major difference: The structural steel is encased with a light fire-resistive coating such as sheetrock or a sprayed-on coating. These structures employ a lightweight parallel steel bar joist system that holds up a corrugated steel roof deck. Due to the fact that much of the structural steel can be exposed to the properties of fire, these types of structures are considered to be the least resistant to collapse.
Type II Inherent Hazards - The classification of "limited" combustible comes from the materials that make up the roof covering, which, when involved in fire, can lead to a rapid failure of the roof. This type of structure is commonly used for commercial occupancies that require a large open space, such as strip malls. These malls have a large amount of windows in the front wall and that opening is spanned with a steel lintel that will support a free-standing parapet wall. If this lintel is compromised in any fashion, it can result in a complete failure of the parapet wall. Furthermore, there are larger buildings of this type that employ a tilt-slab construction method, which employs a pre-cast wall slab that is tilted into place (see Photo 3). Each slab is braced with steel rods or rebar and insulated at the sides with each other, using a foam insulator strip and caulking. Anchor plates are then used to attach the roof steel bar joists to the exterior walls, tying the integrity of the walls and the roof into each other. The result will be a catastrophic failure of either a large portion or all of the structure.