Mass timber

For the tenth class the students travel to a mass timber plant outside the city, the same one they visited briefly in Structures I. This time they follow a single floor panel from start to finish. Kiln-dried boards are graded by machine, finger-jointed into long lamellas and planed; glue is spread in thin lines; the lamellas are laid in layers, each layer at right angles to the one below, and pressed overnight. A computer-controlled router then cuts the finished panel to size, with openings for a stair and grooves for wiring, before it is wrapped and labeled for a truck. 'Every one of these panels knows exactly where it is going,' says the plant's engineer. 'On site it takes about twenty minutes to fly one in and screw it down.' Szymańska adds her own lesson for the day: 'Mass timber is not just wood that is big. It is a different way of building, closer to assembling furniture than pouring a floor.'

What mass timber is

Mass timber is a family of large, solid engineered wood products used for floors, roofs, walls, beams and columns. Glued-laminated timber, or glulam, met in Structures I, is made of laminations glued with their grain parallel, and is used mainly for beams and columns. Cross-laminated timber, or CLT, is made of layers of lumber glued with alternating grain directions, usually three, five or seven layers, into panels up to about 10 ft wide, 60 ft long and 12 in or more thick. Nail-laminated timber, NLT, is boards stood on edge and nailed together; dowel-laminated timber, DLT, uses hardwood dowels instead of nails or glue. Mass plywood panels and laminated veneer lumber are made from veneers.

Cross-lamination is what makes CLT a panel rather than a plank. Wood is strong and stiff along its grain and weak across it, and it shrinks and swells across the grain with moisture. In CLT, the cross layers lock the panel against shrinkage, give it strength in both directions and allow it to span two ways where it is supported on more than two sides. The outer layers usually run in the main span direction, because they are farthest from the center and contribute most to bending stiffness. The cross layers contribute little to bending in the main direction and are also weak in a particular kind of shear, called rolling shear, in which the fibers roll over each other; that makes CLT's effective stiffness somewhat lower than a solid beam of the same thickness would have.

Systems

Mass timber buildings are built in a few typical ways. In a post-and-beam frame, glulam columns on a grid support glulam beams, and CLT, NLT or DLT panels span between the beams. The grid is often about 20 to 26 ft one way and 10 to 20 ft the other, set by the panel spans and the beam depths. In a point-supported or flat plate system, CLT panels rest directly on columns, sometimes with steel or glulam capitals, as in the student residence Brock Commons at the University of British Columbia, completed in 2017 with eighteen stories: its CLT floors sit on glulam columns about 9 by 13 ft apart, around two concrete cores. In a panel or platform system, CLT walls carry CLT floors, like a giant house of cards, suited to housing with regular party walls.

Mass timber is often combined with other materials. Concrete or steel cores for stairs and elevators provide most of the lateral resistance in taller buildings; concrete podiums carry timber floors above shops or parking; concrete toppings over CLT add mass for acoustics and vibration; and steel beams or connectors help with long spans and transfers. The hub's wing, in Structures I, has a glulam frame with CLT floors and a concrete topping, braced by steel rods. The Greenway Homes common room has a CLT ceiling, and Wren is testing a mass timber version of the north bar for this lesson's exercise.

Spans, stiffness and vibration

Wood is light for its stiffness, which is one of its great virtues and the source of its main design challenge. A CLT floor that is strong enough is often too flexible: it deflects too much under load or, more often, it vibrates noticeably when people walk on it. Floor vibration is not dangerous, but a floor that bounces is perceived as poor quality, and occupants rarely forgive it. Vibration design for wood floors aims to keep the floor's natural frequency high, typically above about 8 to 10 hertz, so that footsteps cannot excite it, and its stiffness under a point load high. Designers use the span formulas in the timber codes, manufacturer tables and, for long or unusual floors, dynamic analysis.

The script below makes a simple estimate for a five-layer CLT panel 6 7/8 in thick. It calculates the bending stiffness from the three long layers, ignoring the cross layers and the shear flexibility of the panel, which makes it slightly optimistic, then checks the live load deflection and the first natural frequency for three spans, using a target of about 9 hertz.

# Five-layer CLT floor panel: simplified stiffness, deflection, vibration and char (ft)
import math
t, E = 1.375, 1700000.0                            # layer thickness (in) and modulus of the long layers (psi)
b = 12                                    # a strip one unit wide (in)
I_layer = b * t ** 3 / 12
# long layers are 1, 3 and 5; the cross layers are ignored (a simplification that slightly overrates stiffness)
EI = E * (3 * I_layer + 2 * b * t * (2 * t) ** 2) / 144
print(f"Panel {5 * t:.3f} in thick: EI about {EI:,.0f} lb ft2 per unit width")
D, Lv = 50, 40                          # dead and live load (psf)
for span in (15, 18, 21):
    d_live = 5 * Lv * span ** 4 / (384 * EI) * 12
    limit = span * 12 / 360
    f1 = math.pi / (2 * span ** 2) * math.sqrt(EI  / (D / 32.2))
    print(f"span {span} ft: live deflection {d_live:.2f} in (limit {limit:.2f}), first frequency {f1:.1f} Hz"
          f" -> {'OK' if d_live <= limit and f1 >= 9 else 'too flexible or too lively'}")
for hours in (1, 2):
    char = 1.8 * hours + 0.3           # effective char about 1.8 in per hour plus a zero-strength layer
    print(f"{hours}-hour fire: char about {char:.1f} in from below, leaving {5 * t - char:.1f} in of the panel")

Panel 6.875 in thick: EI about 3,038,293 lb ft2 per unit width
span 15 ft: live deflection 0.10 in (limit 0.50), first frequency 9.8 Hz -> OK
span 18 ft: live deflection 0.22 in (limit 0.60), first frequency 6.8 Hz -> too flexible or too lively
span 21 ft: live deflection 0.40 in (limit 0.70), first frequency 5.0 Hz -> too flexible or too lively
1-hour fire: char about 2.1 in from below, leaving 4.8 in of the panel
2-hour fire: char about 3.9 in from below, leaving 3.0 in of the panel

The panel easily meets the deflection limit at all three spans, but the frequency check tells a different story. At 15 ft the floor's frequency is close to 10 hertz and it should feel solid; at 18 ft it drops below 7 hertz and would feel lively. Ingrid finds this the most useful lesson of the day: in timber, vibration, not strength, usually sets the span. The cures are to shorten the span, which means more beams; to use a thicker panel; to add a concrete topping, which adds mass and some stiffness; or to make the panel continuous over supports. Manufacturers' tables and the timber design standards, the NDS with its CLT provisions and the manufacturer's evaluation reports, give tested spans that account for the cross layers, the shear flexibility and the connections.

Fire

Large timber members behave well in fire. Wood burns, but as its surface chars, the char layer insulates the wood beneath, which stays cool and strong. The char advances at a fairly steady rate, about 1.5 in per hour nominally, with codes using an effective char depth of about 1.8 in for one hour, including the heated layer below the char. A member that is sized with enough extra wood to lose this layer and still carry its load has a fire resistance rating without any protection. Codes treat mass timber differently from light wood framing for this reason.

Taller mass timber buildings are also protected with gypsum board, a strategy called encapsulation. The 2021 International Building Code introduced three types of mass timber construction, IV-A, IV-B and IV-C, allowing buildings up to eighteen, twelve and nine stories, with different amounts of the timber left exposed. The Ascent tower in Milwaukee, completed in 2022 with twenty-five stories, is among the tallest timber hybrid buildings in the world. In every case, sprinklers are required, connections must be protected as well as members, and concealed spaces must be limited so that fire cannot spread unseen.

Connections, moisture and movement

Connections are the heart of mass timber design, as they are of steel. Glulam beams sit on steel hangers, knife plates slotted into the wood and pinned with dowels, or proprietary aluminum or steel connectors; CLT panels are screwed together with long self-tapping screws through splines or half-lap joints, and fixed to beams and walls with angle brackets and screws. Self-tapping screws, which can be more than 16 in long and inclined to engage the wood in tension, have transformed timber connections in the past twenty years. Connections must allow for wood's movement: wood shrinks across the grain as it dries, roughly 1/32 in per 4 in of width for a drop in moisture content of about four points, so tall buildings must account for the cumulative shortening of floors and the settlement of beams on columns.

Moisture is the other enemy. Wood must be kept dry during construction, because CLT and glulam that soak up rain can swell, check, stain and grow mold, and water trapped under a topping or a membrane can rot a floor. Contractors protect panels with temporary membranes, sequence the roof early, and measure moisture before closing in. In service, the wood must be kept away from ground moisture and leaks, with good envelope detailing, and protected where it is exposed outdoors.

Why mass timber

The appeal of mass timber is partly environmental. Wood grown in sustainably managed forests stores carbon absorbed from the air, and its manufacture uses much less fossil energy than cement or steel, so mass timber buildings typically have lower embodied carbon in their structures than concrete or steel equivalents, though the full accounting depends on forestry practices and what happens to the wood at the end of the building's life. Prefabrication shortens construction: mass timber floors go up quickly, with fewer workers and less noise, which suits tight urban sites. And exposed wood creates warm, calm interiors. Its challenges are cost, which is often higher than light wood framing though competitive with concrete; acoustics, because light floors need toppings and resilient layers; vibration; and the need for early coordination, because once a panel is cut in the factory, changing it on site is difficult.

Exercise 10: mass timber

• Lay out a glulam and CLT structural grid for one floor of your studio project, with panel directions and beam lines.
• Check the panel spans against a manufacturer's table and adjust the grid or the panel thickness.
• Draw a connection between a glulam beam and column, and show how it is protected from fire.
• List the acoustic layers above and below a CLT floor between two homes.

Self-check

• Do I know why vibration often sets the span of timber floors?
• Can I explain why large timber members resist fire?
• Have I planned for shrinkage and for keeping the wood dry during construction?