Fire Damage Assessment
Question:
How do you evaluate whether a metal plate connected truss is still usable after exposure to fire? Are there any recommended tests?
Answer:
There is no one way to evaluate trusses after exposure to fire. All steel connections conduct heat into the wood at some point in a fire. Truss plate connections, being steel, do as well. However, they also reflect heat for a period of time during a fire, which protects the wood below these connections. Once the plate gets hot enough, it conducts heat, contributing to the charring of the wood below the plate and, presumably, around the truss plate teeth. Eventually, charring becomes significant enough that the truss plate loses its holding power and fails. When the char becomes great enough, the load on the truss plate connection causes the wood member to pull away from the truss plate. The fire will not cause the plate to pull or curl away from the joint. It is the load on the wood members that would cause this action.
Under ASTM E119 fire test exposures, wood ignites in approximately two minutes. Charring then proceeds at a rate of approximately 1/30 in. per minute for the next eight minutes. Thereafter, the char layer has an insulating effect and the rate decreases to 1/40 in. per minute. With this information, you can often calculate the approximate time that the truss will fail under standard ASTM E119 fire exposures.
Mass and surface area are the most significant factors in determining the fire endurance performance of steel. Heavy, thicker plates will have greater resistance than lighter, thinner ones.
The first steps in assessing the truss damage caused by a fire is to inspect the plates and the wood. How long were the trusses in contact with the fire? Is there significant char of the wood members? Are the plates discolored? Are the plates well embedded into the wood members?
For more information on this topic, see the fire section of SBCA’s website.