Monthly Mechanics: Wood

This is Part 3 of a series about building materials. Read Part 1 (Steel) and Part 2 (Reinforced Concrete).

Wood is a convenient material for construction. It literally grows on trees, it’s soft enough to cut quickly with hand tools, and it accepts fasteners easily. That means wood framing has been around a lot longer than steel and concrete – probably as long as humans have built shelters. Wood is still one of the fastest materials to build with, especially with the advent of power tools, and has one of the smallest carbon footprints.

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The convenience of wood construction.

Foresters divide the myriad species of trees into hardwoods (deciduous) and softwoods (coniferous). Hardwoods are denser and best for fine woodworking like cabinetry. It’s softwoods, with their fast growth and straight grain, that are used for high-volume and high-strength applications like framing buildings.

Engineers also care about which wood species is used, but they care even more about the grade. Lumber is either visual graded (i.e. by actual people looking and feeling) or machine graded (i.e. by a mechanical stress test). The straightness of the grain, and the presence of knots (which are really just locations along the original tree trunk where branches grew), has a tremendous effect on the lumber’s structural properties.

If you can find a piece of dimensional lumber made from clear structural redwood, you can enjoy a bending strength of 1750 psi. At the other end of the spectrum is utility-grade eastern hemlock, with a bending strength of 175 psi – an order of magnitude weaker than the redwood. Your typical off-the-shelf dimensional lumber in New England is #2 grade southern pine, with a middle-of-the-road bending strength of 975 psi.

Engineers then adjust these strength values based on the geometry and environment of the structure they’re designing. Things like how big the lumber is, whether it’s braced along its length, whether it’s used on edge or flat, whether it’s repetitive (as in a series of floor joists spaced 16” or 24”), and whether it’s exposed to extreme temperatures or moisture all affect the safe strength to use for design calculations.

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Lumber stamp.

This Old House has a nice explanation here of how to read the stamp on a piece of dimensional lumber. The stamp tells you the species and grade, as well as the moisture content and the mill where it was cut.

For mostly archaic reasons, wood is rarely used for commercial construction in the US, but the tide may be turning. The development of stronger wood products like cross-laminated timbers (CLTs) – layers of lumber bonded together with the grain at right angles, sort of a super-plywood – enables wood construction to go far higher than the four or five stories at which dimensional lumber stick-framing tops out. (CLTs can have a bending strength of 2250 psi or more.) The tallest completed wood building is 18-story Brock Commons in Vancouver, and architects in Tokyo have plans to go as high as 70 stories, a true “plyscraper.”

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Cross-laminated timber fabrication.

In the US, where codes limit wood construction to 85 feet, the tallest wood building is a seven-story office tower in Minneapolis. But with advances in fireproofing, and the strength and stability of the newest wood products, the American Wood Council is lobbying for a change.

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T3 Tower in Minneapolis.

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Tiny Tuesday: Super Wood

Materials scientists at the University of Maryland, College Park, have discovered a method to make wood stronger by making it denser. In this Scientific American article, team member Hu Liangbing claims their product has up to 50 times the strength in compression and 20 times the stiffness (resistance to deformation) than the original wood.

It’s a two-step process. First the wood is boiled and soaked in a chemical solution that removes some of the lignin (a polymer) from the plant cells but retains most of the cellulose (the main source of strength). Then the wood is compressed, crushing the cell walls into the empty spaces, and gently heated to form new molecular bonds. “You have all these nanofibres aligned in the growth direction,” explains Hu. The result is three times as dense as the original wood, moldable into different shapes, and moisture resistant – a quality never before achieved with densified wood.

The chemicals themselves are sodium hydroxide, also known as lye, and sodium sulfite. Neither chemical is difficult or energy-intensive to manufacture. (In fact, lye can be made at home from water and table salt plus a power source.) Both are soluble in water and harmless when diluted or neutralized, though they need to be handled carefully when concentrated as their high alkalinity (pH>9) can cause severe burns. So the process seems OK from an environmental perspective.

Hu’s team is talking about human-scale applications like lightweight furniture and body armor. It would take a huge scale-up in the process to make densified wood an affordable option for homebuilding and other structural uses. In the journal Nature, other materials scientists assert the two-step process isn’t worth the expense compared to techniques like steaming the wood or adding resins, which also improve strength.

All’s Well With an LVL

Chris is renovating a 120-year-old house and wanted to open up the floor plan. Chris hired PERCH to design a pair of beams that could replace the existing loadbearing walls, spanning the 18-foot room width and picking up the existing floor joists. Chris wanted to procure the beams inexpensively and install them easily, with little or no disruption to the ceiling height.

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“Before.”

Right away, using a steel wide-flange or T shape was out. Steel beams are expensive and heavy, and connecting them to the stringers would have required either complex details or temporary removal of the stringers. Ordinary dimensional lumber wasn’t strong enough for such a long span within the confines of the ceiling height, so PERCH looked to laminated veneer lumber (LVLs) for more strength. A pair of 3.5-inch-wide, 9.5-inch-deep LVLs were strong enough to get the job done. Joist hangers allowed the LVLs to share depth with the 2×8 joists, so the lumber only protruded down an inch from the existing finish ceiling.

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The new header beam on the right requires support down to the foundation at the location marked with a red circle.

PERCH also investigated a hybrid solution called a flitch plate: a steel plate sandwiched by two pieces of lumber. In theory the steel increases the assembly’s overall elastic modulus while the lumber keeps the beam light and enables nailed connections. But in the final analysis a flitch plate was too complicated. The beam would need to be assembled first, with through bolts ensuring the wood and steel share loads, and then it would be prohibitively heavy to lift into place.

PERCH developed an installation procedure (which required temporary support of the floor joists) and checked that a column of dimensional lumber built into the existing exterior stud walls would provide sufficient support. Chris followed the procedure and reported that everything went up perfectly.

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“After.”