Wood construction

The upper floors of the Lake Street building are going up fast. In one morning, a crew of four carpenters frames a whole apartment wall on the deck, nailing studs between plates with pneumatic guns, sheathing it with panels of oriented strand board while it lies flat, and then tipping it up into place and bracing it. A truck arrives with floor trusses made in a factory the day before. Joon-ho, who builds furniture as a hobby, picks up an offcut and reads the stamp on it: a species group, a grade, a mill number, the letters KD for kiln-dried. 'Every piece is graded,' Professor Lindqvist says. 'It is the most sophisticated structural system in North America that people still think of as simple.'

Wood as a building material

Wood is the oldest structural material still in wide use and the most common for houses in the United States. It is strong for its weight, easy to cut and fasten with simple tools, a good insulator compared with steel and concrete, renewable when forests are managed well, and it stores carbon absorbed by the tree as it grew. Its weaknesses are that it burns, that it decays when it stays wet, that insects can attack it, and that it changes size as its moisture content changes.

Wood is a natural material made of long cellulose fibers bound by lignin, running mostly along the trunk. It is therefore much stronger along the grain than across it, a property called anisotropy. A piece of lumber can carry large loads parallel to the grain in compression and tension, but it splits easily along the grain and crushes more easily across it, for example where a joist bears on a narrow plate. Knots, the remains of branches, interrupt the grain and weaken the piece, especially on the tension side of a beam.

Most structural lumber in North America comes from softwoods, conifers such as spruce, pine, fir, hemlock and larch. Species with similar properties are sold together in groups, such as SPF (spruce-pine-fir), Douglas fir-larch and Hem-fir. In the Upper Midwest, SPF from Canada and Southern Pine from the southern states are both common.

Moisture and shrinkage

A living tree is full of water, often more than its own dry weight. After the tree is cut, the free water in the cell cavities dries out first without changing the size of the wood; then the water bound in the cell walls begins to dry, and the wood shrinks. Lumber is usually kiln-dried to about 19% moisture content or less, and in a heated building it continues to dry to 8 to 12%. Shrinkage across the grain is about twenty to forty times greater than along it. A floor joist 9¼ in deep may shrink several millimeters in depth as it dries, while its length hardly changes.

For a designer, this has practical consequences. In a multi-story wood building, the floors and plates, with their grain horizontal, shrink much more than the studs, with their grain vertical. Over four or five stories, the accumulated shrinkage can reach 1 in or more, so pipes, elevator shafts, stairs, brick veneer and concrete cores that do not shrink must be detailed to allow for it. Wet wood also decays: fungi need moisture content above about 20% to grow. Keeping wood dry, in storage, during construction and in service, is the single most important rule of wood durability.

Lumber sizes and grades

Lumber is sold by nominal size, but the actual size is smaller because it is planed smooth after drying. A 2 × 4 is actually 1½ × 3½ inches, its size after drying and planing. Common sizes are listed below.

Each piece is graded visually or by machine for strength and stiffness and stamped with its grade. Common structural grades are Select Structural, No. 1, No. 2 and Stud. Most framing uses No. 2 or better. Machine-stress-rated lumber is tested piece by piece and used where stiffness matters, as in trusses. The span tables in the code give the maximum span for each size, spacing, species group and grade.

Framing systems

Wood framing in North America developed from the heavy timber frames of European settlers, with large posts and beams joined by mortise and tenon, into light frames of many small members nailed together. The balloon frame appeared in Chicago in the 1830s, as cheap machine-cut nails and sawmill lumber became available. In a balloon frame, the studs run continuously from the sill to the roof, two or three stories tall, and the floor joists are nailed to their sides. It was fast to build, but long studs became scarce, and the hollow wall cavities ran from the basement to the attic like chimneys, spreading fire.

By the mid-twentieth century, the balloon frame had been replaced by the platform frame, which is still the standard today. In a platform frame, each floor is built as a platform: joists on a sill or on the walls below, covered by a subfloor. The walls of the next story are framed on the platform, tipped up and nailed down. Each story is a separate box, stacked on the one below. The floor platforms act as fire stops between stories, the studs are short and easy to handle, and the platform gives the crew a safe surface to work on.

The parts of a platform frame have standard names. The sill plate is anchored to the foundation with bolts. The rim joist, or header joist, closes the ends of the floor joists. Joists are typically spaced at 16 in on center, sometimes 12 or 24 in. Walls have a single bottom plate and a doubled top plate, which ties the walls together and carries joists that do not land on a stud. Around openings, a header carries the load over the opening to jack studs, also called trimmers, which are nailed to full-height king studs. Short cripple studs fill above the header and below the window sill.

Roofs are framed with rafters, sloping members from the ridge to the wall, or more often today with prefabricated trusses, triangulated frames of small members joined by toothed steel plates. Trusses span the whole width of a house without interior supports, freeing the plan, and are made in factories to engineered designs. Floors also use prefabricated parts: wood I-joists with a thin web of oriented strand board between solid flanges, and open-web floor trusses through which ducts and pipes can pass.

Sheathing and bracing

Sheathing is the layer of panels nailed to the outside of studs, rafters and joists. The main products are plywood, made of thin veneers glued with the grain of each layer at right angles to the next, and oriented strand board (OSB), made of long strands of wood glued and pressed in layers. Sheathing gives the cladding and roofing something to be fastened to, and it is a structural part of the building: nailed at the edges and in the field, it turns walls into shear walls and floors and roofs into diaphragms that resist wind and earthquake loads. The nailing pattern, nail size and edge distance on the drawings are therefore structural requirements, not suggestions.

Engineered wood and mass timber

Engineered wood products are made by breaking wood into pieces, from veneers to strands, and gluing them back together. Because the defects are spread out, the products are stronger, straighter and more consistent than sawn lumber, and they can be made in sizes that trees cannot provide.

• Laminated veneer lumber (LVL) is made of thin veneers glued with the grain parallel. It is used for beams and headers.
• Parallel strand lumber (PSL) is made of long strands. It is used for heavy beams and columns.
• Glued-laminated timber, or glulam, is made of layers of lumber glued together. It can be made long, deep and curved, for arches, beams and columns.
• Cross-laminated timber (CLT) is made of layers of lumber with each layer at right angles to the next, forming large panels for floors, walls and roofs.
• Nail-laminated and dowel-laminated timber are panels of lumber on edge, fastened with nails or hardwood dowels.

These products have made mass timber construction possible, buildings with large solid timber floors, walls and columns. Mass timber chars on the surface in a fire, and the char layer insulates the wood inside, so a large timber column keeps its strength far longer than its size would suggest. T3 in Minneapolis, completed in 2016, was the first modern tall timber office building in the United States, a seven-story building with nail-laminated timber floors and glulam columns above a concrete base. The 2021 International Building Code introduced three new mass timber construction types, allowing timber buildings up to eighteen stories with full encapsulation. Because wood is light, a mass timber building needs smaller foundations, and much of it is made in factories and assembled quickly on site.

Fasteners and connections

Wood members are joined by nails, screws, bolts, metal connectors and, in traditional and mass timber work, carpentry joints and dowels. Nails are the workhorse of light framing; the code specifies their size and number for every connection. Metal hangers carry joists from beams, hurricane ties connect rafters to walls against uplift, hold-downs anchor the ends of shear walls to the foundation, and straps tie floors together. Connections are where most wood structures fail, by splitting, nail withdrawal or crushing, so they deserve as much attention as the members.

# Simplified check of a floor joist: deflection under live load and bending stress
b, h = 1.5, 9.25             # in, a 2x10
spacing = 16.0               # in on center
L = 13 * 12.0                # in span (13 ft)
E = 1.4e6                    # psi, SPF No. 2 (NDS supplement)
live, dead = 40.0, 10.0      # psf

I = b * h ** 3 / 12          # in4
S = b * h ** 2 / 6           # in3
w_live = live * spacing / 12 / 12               # lb/in along the joist
defl = 5 * w_live * L ** 4 / (384 * E * I)      # in
print(f"Live-load deflection {defl:.2f} in, limit L/360 = {L / 360:.2f} in")

w = (live + dead) * spacing / 12 / 12           # lb/in, allowable stress design
M = w * L ** 2 / 8                              # in-lb
Fb = 875 * 1.1 * 1.15                           # psi: base value x size x repetitive member
print(f"Bending stress {M / S:.0f} psi, allowable about {Fb:.0f} psi")

Live-load deflection 0.25 in, limit L/360 = 0.43 in
Bending stress 790 psi, allowable about 1107 psi

The joist passes both checks with room to spare. In practice a builder or architect would read the span table in the code instead of calculating, but the calculation shows why the tables look as they do: deflection, which depends on the cube of the depth, usually governs floor joists, and a deeper joist is far stiffer than a wider one.

Fire and wood

Wood burns, but how dangerous a wood building is depends on its size, its details and its protection. Light wood frames are protected mainly by gypsum board, which contains chemically bound water that absorbs heat as it is driven off, and by fire stops and fire blocks, solid wood or other materials that close the hidden cavities in walls and floors so that fire and smoke cannot race through them. Sprinklers greatly improve safety in larger wood buildings. Heavy timber and mass timber resist fire by charring, which forms a protective layer at a fairly predictable rate of about 1½ inches per hour, so engineers can size members with extra wood that may char away while the core still carries its load.

Exercise 4: a framing model

This exercise takes about four hours.

• Build a model at 1/2 in = 1 ft of one corner of a platform-framed building: foundation, sill, floor joists, subfloor, one wall with a window, and the start of the next floor.
• Use strip wood or card at the correct scale sizes for each member.
• Label every part: sill, rim joist, joists, subfloor, bottom plate, studs, king and jack studs, header, cripples, top plates.
• Photograph the model from three angles and draw an exploded axonometric of it.
• Deliverable: the model, the photographs and the labeled axonometric.

Self-critique checklist

• Are my members at the actual sizes, not the nominal sizes?
• Does every load above the window reach the foundation through the header and jack studs?
• Have I shown the doubled top plate and its overlap at the corner?
• Is the sheathing shown as a structural layer?