Editor's note: This article finishes summarizing a small portion of the 79-page Chapter 2, "Principles of Construction," of the 667-page third edition of Building Construction For The Fire Service, by Francis L. Brannigan. Part 1 was published in Firehouse® in February 1996, part 2 in July...
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The vulnerable point is the connection most susceptible to fire. This may be floors away from the suspended structure. The steel tension rod connection of a failing wooden girder on a lower floor, to a steel beam supported on the walls, may be hidden in the cockloft (the space between the top floor ceiling and roof). The cockloft fire may cause the connection to fail, dropping the rod and its load. Thus, a cockloft fire might cause a lower floor collapse. Know your buildings!
SELF RELEASING FLOORS
Many codes require that wood joists in masonry walls be fire-cut. The end of the joist is cut off at an angle to permit the joist to fall out of the wall without damaging the wall.
The removal of wood lessens the inherent resistance of the joist to fire and can precipitate collapse. Wood joists often sag over time. Sometimes the joists are turned over to provide the desired "upward camber." In such cases, there can be very little wood bearing on the wall. This practice should be forbidden.
When the size of the building requires interior columns, buildings with timber interiors, are often built with self-releasing floors. Floor girders are set on brackets attached to columns. A wood cleat or steel dog iron (similar to a big staple) is used to provide minimal stability. Such a floor can be expected to release sooner than if it were tightly connected. Some designers rejected this idea and required tight connections.
Know your buildings! A building with fire-cut or self-releasing floors is designed to collapse. It is the duty of the incident commander and subordinate fire officers to see to it that firefighters are not under the structure when the designed collapse occurs.
OVERHANGING AND DROP-IN BEAMS
Structural design is often intended to be as economical as possible. The economy may be in material or in the work of erection. Consider a space three bays wide. There are two masonry walls and two lines of columns supporting girders. Sometimes it is more economical to let the two outermost beams overhang the girders by two or three feet. The gap is then closed by a beam dropped in and nailed to the overhanging ends of the outer beams. As a result, the drop-in beams are supported only by the nailing. They have no support underneath.
This is not a new practice. I have seen it in hundred-year-old buildings. Take every opportunity to examine buildings being repaired or renovated to "undress the building" to see the hidden hazards.
Long wooden beams are not readily available for building needs. Shorter lengths are often spliced together with metal connectors to produce the desired length.
The resultant beam will carry its design load but the connectors may fall out when heated sufficiently, causing collapse. In some buildings, these connectors may have been made to look decorative. Take a second look! Some years ago, a jai alai arena in Daytona Beach, FL, was destroyed by fire. The roof was supported on laminated wood arches. The owners of the building had been convinced they had a sturdy heavy timber building. Pictures of the fire clearly showed that the arches had fallen apart at the connections.
These examples are but a few of the many connection failures which can occur, possibly catastrophically in a fire. Know your buildings. Trace the load from origin to foundation.
Ultimately, all loads are delivered to the ground through the foundation. The nature of the ground and the weight of the structure determines the foundation. Foundations can range from simple footings to grade beams or a foundation under the entire wall to foundations which literally float the building on poor soil. In some cases, wood or concrete piles are driven either to bedrock or until the accumulated friction stops the pile. Almost all foundations today are of concrete. In some locations, decay-treated wood is used for small houses.
Illustration by Christopher J. Brannigan
Most codes require a "fire cut" when wood joists are fitted into a masonry wall, to let the floor collapse without pulling down the walls. Notice how much wood is removed. This often leaves very little of the beam resting on the wall, thus making the beam liable to early collapse. Sometimes, when a building is being rehabilitated, sagging floor beams are reused by turning them over. This can leave very little wood resting on the wall. It is fair to characterize a building with self-releasing floors as "designed to collapse."