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.


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.


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.”


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.


T3 Tower in Minneapolis.


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