Thread: UL To Study Engineered Wood Trusses & Fire

It will be interesting to see the results of this study.

Underwriters Labs will study stability of trusses

September 13, 2007

Northbrook-based Underwriters Laboratories Inc. said Wednesday it received a $991,900 grant from the Department of Homeland Security to investigate the structural stability of engineered lumber -- especially lightweight wooden trusses -- and evaluate the effectiveness of extinguishing agents used in fighting fires in modern structures.

The International Association of Fire Chiefs, Chicago Fire Department and University of Maryland Fire Protection Department are cooperating in the research, UL said.

Lightweight wooden trusses, made with engineered lumber, are found in 65 percent of new residential and commercial developments, according to the Wood Truss Council of America.

Allowing for faster, more cost-effective construction, evidence has indicated that lightweight wood trusses may become unstable and collapse more quickly during fires than traditional trusses. UL said.

"The research conducted under this grant should shed new light on an issue we've long suspected was causing instability for firefighters and leading to injuries," said Chief Steven Westermann, president of the fire chiefs association.

Earlier research suggested that unprotected lightweight wood truss assemblies can fail within six to 13 minutes of exposure to fire.

Between 1998 and 2003, the National Institute of Occupational Safety and Health attributed 13 firefighter fatalities and nine firefighter injuries to the collapse of buildings built with lightweight wood trusses.

During the same time frame, five fatalities and two injuries are attributed to collapses of buildings with heavy timber, solid-joist lumber truss construction.

"Building and roof collapse are the most dangerous elements of firefighting," said Raymond Orozco, Chicago's fire commissioner. "The move toward more and more lightweight construction means that men and women in the fire service must have the best information available to be able to determine the risk and timing of structural failure."

UL, an independent, not-for-profit product safety certification organization, expects the research project to take a year and expects to report the findings in early 2009.

The International Association of Fire Chiefs represents leaders of more than 1.2 million firefighters internationally.

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Is that what the Department of Homeland Security is for?

I inspected a home for a local bigwig fire official and he was adamantly opposed to trusses for just this reason. That was probably 4-5 years ago.

So, assuming changes are mandated, how would we report this? "Potentially unsafe to firefighters in the event of a roof/attic fire"?

It does raise some interesting quesitons. I am curious what the mode of failure is for an engineered floor joist (e.g., TJI) during a fire. Could it be the thin web fails first either by burning through or losing strength due to high temperatures? Or could it be the glues used to connect the flanges to the web fail at high temperature? Either way the flanges are left uselessly flopping around.

Remember how the World Trade Center towers collapsed - the heat from burning jet fuel lowered the bending strength of the steel floor trusses allowing them to bend and pull away from their end supports at the exterior walls. They then pancaked all the way to the ground.

Originally Posted by Bruce Breedlove

It does raise some interesting quesitons. I am curious what the mode of failure is for an engineered floor joist (e.g., TJI) during a fire. Could it be the thin web fails first either by burning through or losing strength due to high temperatures? Or could it be the glues used to connect the flanges to the web fail at high temperature? Either way the flanges are left uselessly flopping around.

Remember how the World Trade Center towers collapsed - the heat from burning jet fuel lowered the bending strength of the steel floor trusses allowing them to bend and pull away from their end supports at the exterior walls. They then pancaked all the way to the ground.

Bruce:

I vote for the glue, though I don't have any evidence. It seems that some of the roof collapses would be attributable to the use of finger-joint rafters.

Aaron