When you construct hotel lodging for the U.S. Army, not only do the buildings need to be comfortable and well-equipped, they need to be blast resistant to mitigate any potential security risk. It’s a tall order that mass timber is proving able to meet, its performance on par with steel-stud wall construction.
Strong and stable, cross-laminated timber (CLT), used as walls, roofs, and floors in mid-rise buildings and increasingly in taller timber towers, was shown to be remarkably robust in the world’s first blast tests on full-scale structures framed with the renewable construction material.
Tests at Tyndall Air Force Base in Florida, which varied the blast force, material grade, number of plies and other factors, were conducted by WoodWorks in collaboration with Karagozian & Case Inc. (KCI), the University of Maine and the Air Force Civil Engineer Center. Funding is primarily from the Softwood Lumber Board and the U.S. Dept. of Agriculture Forest Products Lab.
The testing program was initiated, in part, by the U.S. Army when Lendlease, its primary developer-builder of hotels and guest lodging beds at bases across the country, was looking to use CLT instead of steel framing because it offered the potential for faster, more efficient construction, according to an article in Engineering News-Record (ENR).
In all tests, three conducted in 2016 and four in 2017, the structures remained intact under significant explosive loading well beyond their design capacity.
Mark Weaver, principal of Karagozian & Case Inc., advises that for blast, CLT is just as good as a properly sized, steel-stud wall. Based on the test results, “mass timber structural systems can effectively resist blast loads in the elastic response range with little noticeable damage,” said Weaver, in the same ENR article.
Lendlease built the testing structures, each of which were exposed to successively larger blasts over a period of seven tests; two of the mock buildings were 27 feet tall with two-foot-tall parapets and window cutouts at 12 feet, and the third was 23 feet tall, with two-foot-tall parapets and window cutouts at 10 feet. Both “buildings” had 15-square-foot footprints.
After exposing the structures to 32, 67, 199 and finally 610 pounds of TNT (some tests also included additional gravity loading), it was concluded that, for blast exposure, CLT tested performance closely matched the model performance. Because CLT panels contain multiple laminated layers, the remaining wood provides some residual strength after blast loading.
The Protective Design Center is reviewing the final reporting this fall and will be issuing formal recommendations on standoff distances and modelling by the end of 2018.
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