Acoustic tomography. Ever heard of it?
This is a nondestructive imaging technique to determine what’s going on inside a tree. According to “Evaluation of Acoustic Tomography for Tree Decay Detection,” internal decay is an ever-present possibility which seriously compromises lumber production. For every 100 million board feet of timber harvested annually in the U.S., interior decay is estimated to destroy about 30 million board feet.
Acoustic tomography can help maximize lumber production and it is also used in urban forestry to assess the health of trees. Now it has a new and exciting use.
Whole Tree Architecture and Construction is partnering on research with the US Forest Products Lab using acoustic tomography as the one of the steps in determining the strength of forked tree limbs for construction.
The test is being conducted on ash wood because Wisconsin is about to feel the full force of the Emerald Ash Borer invasion , which will jeopardize our state’s 757 million ash trees. We will soon be looking at millions of dead ash trees, and it would be good to have something constructive to do with them. Whole Trees uses and promotes the use of unmilled timber — particularly from waste trees, so this is a good opportunity to get some solid data on the strength of these millions of branching timbers.
Whole Trees is working with acoustic tomography equipment they rented from Bruce Allison, a prominent arborist in the area who has worked with many landmark trees. The same equipment was recently used to assess the health of the grand old trees on the UW-Madison Union Terrace.
The first step of the process was identifying, peeling and harvesting the test trees, most of which came from LaCrosse and Vernon counties, which are under quarantine now that Emerald Ash Borer has been identified there. Each tree had to be thoroughly inspected by the Wisconsin Department of Agriculture, Trade and Consumer Protection before it could be moved to Madison for testing.
Next the trees have been kiln dried. A living tree is positively sloshing! When we were peeling trees for our house last summer, you could actually get splashed sometimes as the bark pulled away. According to Marc Joyal, kiln manager at the Forest Products Lab, a living tree can be up to 70% water. Lumber is considered ready to use at somewhere between 14-19% water. Marc said kiln drying whole timbers is new for the lab, and they are perfecting their technique as they work on this project.
The circumference is measured just below the fork, and 7 nails are placed at equal distance around the tree. Then the nails are attached to equipment and each nail is tapped with a special tool while the computer calculates the speed at which each other sensor picks up that signal. The process is repeated 12 inches below the fork, and the computer produces an image that indicates the soundness of the wood.
“The whole point is to figure out what is going on inside the wood,” explained Kysa Heinitz of Whole Trees. “ Then we will correlate it with a test of the strength of the branched timbers. This will provide the numbers we can use to test in the field and be sure which trees are strong enough.”
When trees grow branches, bark tissue at the joint can get turned in under growing wood. That’s called included bark, and can keep on growing for a while, then ultimately decay to create a weak spot.
After the tests are done, Whole Trees hopes that they will be able to tell which joints are strong enough for construction projects. “We would like to have a really simple test we can do in the field,” says Della Hansmann of Whole Trees.
It’s cool to see this non-conventional building process getting the kind of scientific evaluation that can influence future green building projects.and even building codes. I’ll be writing more next week when the trees are tested to the breaking point.