How connections behave in fire
Where the structure lets go
Section titled “Where the structure lets go”In fire design it is easy to focus only on the members: how big the section is, how much it heats up, how much capacity remains. In practice, though, a structure rarely fails in the middle of a member. The connection gives up first.
There are several reasons, and all of them follow from the thin sheet.
The sheet around the screw
Section titled “The sheet around the screw”The capacity of a thin-walled connection is limited not by the strength of the screw but by the behaviour of the sheet: the screw works in bearing, and the sheet can pull out or tear around the hole. When hot the sheet is softer, so this failure mode appears sooner.
Restraint forces from thermal expansion
Section titled “Restraint forces from thermal expansion”A heated member would elongate. If the structure holds it, the difference turns into force — and that force lands on the connection. In cold design this action does not exist; in fire it can govern.
Twisting
Section titled “Twisting”A heated open section can twist and load the connection in a direction it was not designed for.
What this means in design
Section titled “What this means in design”Connections have to be designed for fire too
Section titled “Connections have to be designed for fire too”Verifying the fire resistance of the members is not enough. Fire design has to extend to the connections, particularly where a break in the load path would cause immediate collapse.
The lining has to protect the connection too
Section titled “The lining has to protect the connection too”The weak point is often exactly where the lining struggles to wrap round: corners, headers, hold-downs, floor-to-wall junctions. If the connection is left unprotected, the rest of the wall is protected in vain.
Hold-downs are critical
Section titled “Hold-downs are critical”Uplift restraint and bracing connections are where the global stability of the structure depends on a few fasteners. These deserve the most thought.
What depends on workmanship
Section titled “What depends on workmanship”Behaviour in fire also rests on installation quality: a badly driven or missing screw may still fit within the safety margin when cold, but not when hot. Connections are therefore checked before lining, not after.
The point
Section titled “The point”In a fire the structure lets go where there is least material: at the connection. Fire thinking has to start from the connection, not from the middle of the member.