Sticks and stones may break your bones, but they’re also great building materials. And the Department of Defense is eyeing a return to stick-based construction in some places where it currently uses concrete and similar materials. Fire and blast tests have already gone well, and the Army is working with universities to test its performance against ballistic weapons.
It’s all thanks to a new material that all the cool architects are talking about: cross-laminated timber. The footnotes version on this stuff is that it’s timber assembled in layers, and each layer is placed at 90 degrees from the previous one.
So, think of a Jenga tower, but with lots of glue so the blocks don’t slide apart. Believe it or not, this actually creates a super-strong structure, so strong that architects are certain they can make skyscrapers with the stuff, though buildings of about five stories are the norm right now and the tallest completed so far is 14 stories.
Believe it or not, this is a passing fire test. Cross-laminated timber passed the test for fire resistance, but organizers were a little disappointed that it never self-extinguished. It was hoped that as the wood charred, which greatly reduces its flammability, the flame would run out of fuel.
But the Pentagon isn’t eyeing the material for tall office structures, or at least not exclusively for that. They allowed the Forest Products Laboratory, part of the U.S. Department of Agriculture, to test CLT structures against blasts. Yeah, they want to know how the buildings will do against bombs.
The FPL has already tested the material when set on fire, when exposed to extreme moisture, and when shaken as it would in an earthquake. The wood did great in the earlier tests, but the military didn’t want to adopt new materials that would get destroyed the first time a big, bad wolf tried to blow it up.
The blast tests were done in 2016 and 2017 at Tyndall Air Force Base. This was before the hurricane wiped out many of the base’s structures (which were not CLT).
That blast looks stronger than the Big Bad Wolf, but somehow, the stick-buildings are still standing.
(Air Force Civil Engineering Center AFCEC, Tyndall Air Force Base)
The wood performed well during the tests, flexing and twisting in some cases but—in most of the tests—surviving the blasts. The panels did rupture during the final test, a test designed to overwhelm the timbers and push them well beyond their design limits. But even then, the buildings were safe to enter and walk through.
Now, Georgia Tech in Atlanta is working on a ballistics test with the Army at Aberdeen Proving Grounds in Maryland. The tests are slated to include additional blast testing as well. So, yeah, the Army wants to figure out whether it makes tactical and strategic sense to have wood buildings and structures, even in some places where it might currently use concrete.
All-in-all, CLT is a promising material for the military, and it’s achieved a lot of acceptance in the civilian world. It’s much better for the environment than concrete, which releases CO2 both in production and construction, and steel, which is energy intensive to mine, smelt, forge, and ship.
Timber, in contrast, actually removes carbon from the atmosphere as it’s created and grown, and it’s very lightweight, so it doesn’t cost as much fuel to move the material.
Currently, though, the material is quite expensive to purchase as there are only a few manufacturers making it. Prices are expected to come down over the next couple of decades. An ambitious plan for a 7-story building is slated for completion in 2041, partially because building right now would require that the builders buy up all available CLT, making the project cost as much as double what normal construction would.