Metal Connector Plates In Fires
Question:
I am thinking of using wood trusses for the roofing/ceiling structural systems on some houses I shall build. I remember, though, an engineer/volunteer fireman commenting back in 1989 that the connector plates are prone to expand and pop off, early on in a fire, causing catastrophic structural failure. Was this the case, and if so, has this problem been corrected?
Answer:
No, this is not the case now, nor was it ever the case! Metal connector plates are not prone to expand or pop off early on in a fire.
All steel connections conduct heat into the wood at some point in a fire. Truss plate connections, being steel, are no exception. 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. It is certainly not at the start of the fire.
The truss industry has been battling misperceptions from the fire service on metal plate connected wood truss performance for years. For more information on this topic, see the fire section of SBCA’s website.