Building Collapse Operations - Part 2

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...

Type III Construction: Ordinary - These structures use load bearing walls of masonry construction, while the roof and the floor can be made of wood. Older buildings of this design can be either balloon frame or braced frame construction on the interior.

Type III Inherent Hazards - Similar to Type II structures, older buildings may also have a parapet wall at the roof line, but with a twist: there may also be a cornice, a decorative extension of the roof boards to give the appearance of an overhang from below. The cornice may also stand alone without the presence of a parapet wall. But, the cornice may be as old as the building and be unstable due to its age and its exposure to the elements. Also, these structures that span over 25 feet will have an additional steel I-beam support called a channel rail installed in the walls to aid in supporting the compressive load of the floors. Braced walls are another sign that the wall has been compromised. Spreader plates, in the shape of stars, plates, or channel sections, are used on the outside of masonry walls to provide some stability to the structure. These plates are connected with the use of unprotected steel or steel cable. Generally, spreader plates that were installed when the building was designed will be in some symmetrical fashion; but beware the spreader plates that are irregular in placement, they will signal a severe problem.

Type IV Construction: Heavy Timber - These structures resemble the construction methods as described in Type III buildings, but the difference is in the size of the materials used in construction. Exterior walls are brick or other type of masonry, but are often doubled in width, compared to similar Type III construction. The interior wood members are larger as well, with the columns being a minimum of eight inches by eight inches in diameter and joists must be at least six inches by 10 inches in diameter. These buildings used to house commercial occupancies, but can now be found as condominiums, churches and museums. The large materials used in their construction make these structures the second-most resistant to collapse.

Type IV Inherent Hazards - The larger surface-to-mass ratios of their structural components make these buildings more resistant to collapse. However, years of neglect, shoddy renovations, and weak connection points can cause complete failure of these buildings. The floor systems are of the self-releasing type, made up of fire-cut beams. The ends of the beams are cut at an angle and sit in a pocket in the wall. The principle here is that if the floor were to fail, the joists would not lever the exterior masonry walls, causing them to break and collapse. However, these fire-cut joists limit the amount of operational time that suppression crews have during a structural fire. Furthermore, once the floors fail, these massive walls become free-standing and can fail.

Additional hazards in these structures include the heavy timber truss. These trusses can be parallel chord or bowstring. Many times they are hidden behind a parapet wall, but identifying these hazards is vital. These trusses can span up to 20 feet on center, so if one section fails, it will open up a hole in the roof that spans 40 feet wide. These trusses incorporate the use of hip rafters, which connect the last trusses to the front and rear walls of the structure. When the roof fails, these rafters transfer the roof load to the front and rear walls, pushing them out violently (see Photo 4). Anyone that is in this collapse zone is in immediate danger.

Type V Construction: Wood Frame - These structures contain walls, floors, roofs, and other structural members are made up entirely of wood. Wood is the primary load bearing material within the structure. The size of the wood board will vary, dependent on its use and type of construction. These structures can be made up in one of the following methods:

Braced frame: Also referred to as "post and beam" or "post and girt," these buildings are made up of vertical posts and horizontal beams/girts. They are held in place using a "mortise and tenon" connection, pinned together with a pin called a trunnel.