Which of the following is a primary cause of structural fire collapse?

The primary cause of structural fire collapse is heat weakening structural elements. When a fire occurs, the high temperatures can significantly weaken materials such as steel, wood, and concrete. For example, steel begins to lose its structural integrity at around 1,000°F, and its strength can be reduced by as much as 50% at temperatures above this level. Similarly, wood can become charred and lose its load-bearing capacity when exposed to fire, leading to a potential collapse.

Heat can affect connections and joints, leading to structural failure, and can result in a change in material properties. As the fire continues, structural elements become compromised, increasing the risk of collapse, especially if the fire burns for an extended period or if the building is not built to withstand such heat.

Other factors like low humidity and the presence of flammable liquids may contribute to fire intensity and spread but do not directly cause structural collapse. Excessive water application can lead to additional structural stresses or unintended water damage, but it is the weakening of materials due to extreme heat that is the most direct cause of a collapse during a fire.