Wood buildings provide an array of economic and environmental benefits. Interest in capitalizing on those benefits by constructing mid- to high-rise buildings using cross-laminated timber (CLT) is growing. CLT is made from layers of dried lumber boards stacked in alternating direction at 90-degree angles, glued and pressed to form solid panels. These panels have exceptional strength and stability and can be used as walls, roofs, and floors. Additionally, calculations have shown that a seven-inch floor made of CLT has a fire resistance of two hours.
In order for wood structures to rise above six stories without special building official permission, changes to the International Building Code are needed. It’s a tall order, but researchers from the Forest Service’s Forest Products Laboratory (FPL) recently completed a series of fire tests that will address concerns about fire performance of wood buildings and help take them to new heights.
In cooperation with the American Wood Council, the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF), and the Forest Service’s State and Private Forestry, FPL researchers recently recreated five fire scenarios in a two-story, full-scale test building constructed using CLT. The results were promising.
“These tests demonstrate that it is possible to build a CLT building that’s fire resistant, even with exposed CLT,” said Sam Zelinka, project leader for the Building and Fire Sciences unit at FPL.
The test building, constructed at the ATF’s Fire Research Laboratory in Beltsville, Maryland, consisted of two identical, furnished one-bedroom apartments. The scenarios tested various arrangements of exposed and unexposed CLT with open doors between living and sleeping areas. Additionally, automatic sprinkler system effectiveness was evaluated.
- Test 1: a mass timber structure fully protected with gypsum wall board was subjected to a large furnishings and contents fire. The test was terminated after three hours without significant charring on the protected wood surfaces of the structure.
- Test 2: approximately 30 percent of the CLT ceiling area in the living room and bedroom were left exposed. The test was terminated after four hours, providing additional time to determine if there would be any significant fire contribution from the exposed CLT. Notably, once the furnishings and contents had been consumed by the fire, the exposed CLT essentially self-extinguished due to the formation of char that protected the underlying wood.
- Test 3: parallel CLT walls were left exposed, one in the living room and one in the bedroom. Similar to Test 2, once the apartment furnishings and contents had been consumed by the fire, during which a protective surface of char formed on the CLT, the mass timber surfaces essentially self-extinguished).
- Test 4 and 5: examined the effects of sprinkler protection. For both tests, all mass timber surfaces in the living room and bedroom were left exposed. Test 4 demonstrated that under normal operating conditions, a single sprinkler easily contained the fire. For Test 5, the fire was allowed to grow in the compartment for 23 minutes before water was supplied to the sprinklers which quickly controlled the fire.
According to Laura Hasburgh, FPL fire protection engineer, the final test was the most exciting because no one knew what to expect with fully exposed CLT walls and ceilings. No tests of its kind had been performed at this scale before, and it turned out to be quite impressive.
“The fire was allowed to burn for 23 minutes before the sprinkler system was activated,” Hasburgh explained. Temperatures soared to around 700 degrees Celsius, but once the sprinklers were activated, cooled to approximately 50 degrees Celsius in a matter of minutes.
Hasburgh added that tests two and three were also remarkable because even with exposed CLT, the fire naturally decayed.
The tests ranged from just eight minutes (when sprinklers were immediately activated) to four hours in length, and data was collected once per second at 500 points throughout the structure. Once the data is analyzed, results will be published in an FPL report and presented to the International Code Council Ad-hoc Committee on Tall Wood Buildings.
“This testing is critical to understanding the performance of timber buildings under fire conditions so the industry can continue to address the safety of the occupants and responding fire fighters,” said Sean DeCrane, chair of the Fire Test Work Group and Industry Relations Manager for Building and Life Safety Technologies at Underwriters Laboratories.
Results from these tests will not only help inform building codes, but also provide useful information for property insurance groups, contribute to more accurate fire behavior modelling, and lead to safer firefighting in CLT buildings.
For more test photos, visit this Forest Service Flickr album.