AMHERST, Mass. – Research assistant Conrado Araujo punches a key, mighty electromechanical motors surge, and a giant steel arm begins to bear down on a long wooden beam. In this airy lab in a four-story building made of similar beams, the crucial question is when the wood will break.
Araujo’s computer traces the load: 16,000 pounds, 18,000 pounds, 21,000 pounds. Finally, under the weight of nearly six cars, the wood surrenders with a sharp crack.
“Impressive,” Araujo says.
This is no ordinary block being tested at the University of Massachusetts, Amherst. It is “engineered wood,” composed of about 20 boards of eastern hemlock glued together in perpendicular layers to make a thick, extraordinarily strong slab 12 inches wide and 6½ inches high. The potential of such cross-laminated timber — also called “mass timber,” for massive — is exciting builders, city planners, architects and environmentalists around the world.
Builders see it as a way to construct midrise structures faster and cheaper. City planners see a fast track that could help reduce housing shortages. Architects love its light weight and look. And some environmentalists tout its ability to lock up carbon to combat climate change.
“We call it a win, win, win, win,” said Robert Perschel of the New England Forestry Foundation.
Advocates envision a radical shift in construction, with wood buildings of seven to 18 stories sprouting wholesale in cities, drastically reducing the cement and steel that generate tons of greenhouse gases. By using wood, proponents argue, the carbon stored in it during tree growth is retained within floors and walls.
Not all environmentalists are on board, though. The Sierra Club contends that nearly two-thirds of trees’ carbon is lost to the atmosphere when forests are cut and milled, and replanting young trees does not always offset that loss.
“It’s a drastic simplification to say that wood stores CO2 and thus it’s a perfect material,” said mechanical engineer Jeremy Gregory, an MIT scientist who runs a research group supported by the concrete industry.
Single-family houses and small buildings in the United States have long used “stick” construction — framing with wood. But vast numbers of midrise buildings are made of concrete. Producing the cement in concrete typically requires high temperatures, cooking limestone in a process that billows greenhouse gases. Workers who then mix the product must wait for it to dry, which can add months to construction site schedules.
By contrast, mass timber can be prefabricated and quickly fitted together with special fasteners as if it were an Ikea bookshelf. Less time is needed, as well as fewer workers.