“Timber City,” an exhibit on the second floor of the National Building Museum, lets visitors go hands-on with the new “mass timber” technologies that have made wood construction trendy again. The exhibit closes next week, on September 10.
These new technologies involve gluing small pieces of wood together into larger sheets and beams — like plywood, but on much larger scales. These engineered wood products, particularly sheets of cross-laminated timber (CLT) and beams of glue-laminated timber, have the strength and size to be used in “heavy frame” construction, where substantial beams or walls carry the weight of a building. This is an entirely different approach to building than the light-gauge “stick” construction that's widely used for low-rise buildings.
This isn't the wood construction you've seen before
The difference is underscored by comparing the immense 64-foot and 40-foot long CLT signs announcing Timber City to the cut-away walls of stick construction found in the nearby “House & Home” exhibit. Although both are made from 2” x 4” lumber, the CLT panels' sheer mass gives them entirely different structural capabilities. The possibilities further multiply when wood is used together with steel and concrete, like rebar to add tensile strength or concrete to dampen vibrations.
Mass timber has much to offer throughout the wood's life cycle, as the exhibit illustrates with informative panels, videos, and examples. Growing trees absorb carbon dioxide from the atmosphere — indeed, one reason for wood's sudden popularity is that wood used in buildings takes carbon out of the atmosphere, resulting in a carbon footprint savings of 60-75% compared to other energy-intensive building materials like concrete. Harvesting wood and manufacturing wood products generates income for rural areas, and there are plenty of trees available — 50% more, by volume, in the US today than in 1952.
Building with wood is often faster and easier than with other materials. That's partly because wood is lighter relative to its strength, which results in less material to transport and assemble. It's also easier to work with, whether with hand tools on-site or precision machinery for off-site prefabrication.
GGWash contributor Neil Flanagan points out these are complementary: wooden pieces “can be made very precisely, but unlike steel modular, you can just cut the panels or beams if you need to. So, once on site, the building [can be] erected much faster than an equivalent concrete building, saving on labor and logistics.”
As a comparison, a 220,000-square-foot office building in Minneapolis made from nail-laminated timber was framed in 9.5 weeks during the winter, when cold temperatures make it impossible to pour concrete in Minnesota. A concrete-framed office building in DC built at the same time took more than twice as long to frame.
Wood materials can also improve energy efficiency. Think about the surfaces around a playground on a sunny day: a metal slide gets hot quickly, but a wooden bench remains comfortable.
In some ways, wood is stronger than steel
Surprisingly, that thermal efficiency means that mass timber can be more fire resistant than steel construction (much less stick construction). Sufficiently large wooden beams, like a giant log thrown onto a campfire, singe and char around the edges rather than suddenly bursting into flames.
Flanagan says “this might seem counterintuitive, but ash is a terrible conductor of heat. Therefore, the ash forms a protective layer” around the beam, prolonging its life. Steel, by contrast, conducts heat and loses strength even in lower-temperature fires — which is why new wood buildings conceal the steel bolts that hold their beams together within the wood.
Flanagan also notes that “when fire protection people talk about fire resistance, they don’t mean you can just dust the ash off and put your stuff back — even in a concrete building, that’s not always true. They mean that it will not collapse” during an evacuation. Many steel or concrete buildings have lost structural integrity during a major fire and had to be demolished.
The codes' recognition that fires are inevitable, but deaths are not, has saved many lives: the number of deaths in apartment fires has dropped by half since the early 1980s, even as the number of fires has scarcely declined.
New building codes now allow wood-framed buildings to reach 85 feet in height, almost as high as the 90-foot height limit outside downtown DC. (Portland recently issued a permit for a 148-foot timber building.) An increasing number of commercial and civic buildings are using mass timber: Hines, the developer of CityCenterDC, built the Minneapolis offices and will replicate it in Atlanta and Chicago. Besides the time savings and ecological benefits, many architects and developers say that wood's warmth is popular in loft-like new offices.
You might suspect that this exhibit is a marketing ploy, and indeed, it's sponsored by the US Forest Service and the Softwood Lumber Council. One reason why is because pests like the Western pine beetle have left many US forests struggling with a surplus of small trees. These trees are good fodder for mass timber, just as interest is growing.
Join author Payton Chung for a guided tour of “Timber City” on Friday, September 1, at 2:30 PM, and stay to explore other exhibits like the Hive and Architecture of an Asylum, or Hill Country's Backyard BBQ. The tour is free with museum admission.