Signs of structural failure
The first indicator of possible structural failure is the type of wall. The material of the wall may provide clues, as well as the structural significance. A free-standing wall has a higher collapse risk than a non-bearing wall.
Other indicators include:
After an interior collapse, the risk of an exterior collapse, and therefore the need for a collapse zone, rises significantly.
A wall may collapse in three general manners.
90° angle collapse
This is the most common type of structural collapse. It is similar to a falling tree. The wall falls straight out and the top hits the ground at a distance equal to the height of the wall.
This type of collapse generally occurs with a masonry wall. It collapses like a curtain dropping from the top, creating a pile of debris at the base of the wall.
A wall leaning inward may not necessarily fall inward. The lower or upper portion may slide or "kick" outward.
Establishing a collapse zone
Collapse zones are traditionally established by a commanding officer. The collapse zone itself should be as wide as the structure and as tall, plus half the height. The reason for this increase in height is that the worst-case scenario (and the most common), a 90° Angle Collapse, must be assumed. A collapse zone should be established with barricade tape and should be enforced if necessary.
A collapse zone can easily limit the access of hose lines. Hose lines should operate outside the zone, on the sides. A secured (and unattended) deluge nozzle can also be put into operation, with caution. An aerial fire apparatus may be required. In this case, the collapse zone should be considered a three-dimensional arc, and aerial ladders may not operate in that arc.