How are trusses affected by fire?
The open space provides for continuous fire and superheat smoke spread. These conditions weaken the truss and consequently the floor being supported. Once firefighters begin working on top or below the space, it creates a hazard of collapse (Dunn).
Why are trusses weak during a fire?
The higher surface area-to-volume ratio of trusses compared to joists allows trusses to burn more quickly. In addition, the metal gusset plates that hold wood truss components together may fail quickly as fire consumes the wood in which the gusset teeth are shallowly embedded.
How a lightweight wood truss will fail when exposed to a fire?
It is these plates that have been found to contribute to the early collapse potential. As fire penetrates a space containing a lightweight truss system, the metal gusset plates rapidly absorb heat, even prior to direct flame impingement. As the metal heats, wood charring quickly occurs around the teeth of the plate.
How quickly do lightweight wood floor trusses collapse in a typical basement fire?
Floor collapse in six minutes. Engineered wood floor assemblies have the potential to collapse very quickly under well-ventilated fire conditions. When it comes to lightweight construction, there is no margin of safety.
What is the cause of most fire deaths?
The majority of fire-related deaths are caused by smoke inhalation of the toxic gases produced by fires. Actual flames and burns only account for about 30 percent of fire-related deaths and injuries. The majority of fires that kill or injure children are residential fires.
When a truss fails how much area is involved?
Trusses have large surface-area-to-mass ratio that leads to poor performance and early failure under fire conditions. When a truss fails, the collapse area is large (117).
What is the weakest part of a truss?
The weakest point of the planar truss is the top chord, which is in compression. The bottom chord, because it is in tension, resists lateral movememt fairly well.
What causes truss failure?
Damage to the structure from weather, earthquakes, landslides, floods, or fires can cause even the strongest roof trusses to fail. Structural damage or an impact to the building might suddenly overload one or more sections. Accidental damage during renovations can also cause failure due to impact or stress.