Mass timber and fire design

Mass timber is the name for large, solid engineered wood elements: glulam beams and columns, cross-laminated timber (CLT) panels, nail-laminated and dowel-laminated timber (NLT and DLT), and panels made of veneers. Over the last fifteen years it has changed what wood buildings can be, from houses and low-rise apartments to offices, schools and towers of a dozen stories or more. This lesson designs the CLT roof of the training pavilion and then checks the roof for a one-hour fire, because the most common question about mass timber is how it behaves when it burns.

Cross-laminated timber

CLT panels are made of layers of lumber, called laminations, laid side by side, with each layer at right angles to the one below, and glued face to face under pressure. Panels have an odd number of layers, typically three, five or seven, and are made up to about 10 ft wide and 40 to 60 ft long. The product standard is ANSI/APA PRG 320, which defines stress grades such as E1, with machine-rated longitudinal layers, and V2, with visually graded layers. The NDS has included CLT design provisions since its 2015 edition, and the IBC references PRG 320.

Crossing the layers gives the panel two-way strength and stiffness, makes it dimensionally stable in its plane, and lets it act as a slab, a wall or a deep beam. The cost of this is that the cross layers contribute almost nothing to bending in the main direction and are loaded in rolling shear, the weakest mode of wood: fibers rolling over one another across the grain. Rolling shear stiffness is roughly one tenth of the ordinary shear stiffness, so CLT panels deflect noticeably in shear, especially at short spans.

Effective stiffness and strength

Because the cross layers are flexible in shear, the panel does not behave as a solid section. The NDS uses effective properties based on the longitudinal layers, with an effective bending stiffness \((EI)_{eff}\) and shear rigidity \((GA)_{eff}\), and the manufacturer's PRG 320 values give the panel's \((F_bS)_{eff}\). The deflection includes a shear term, and the NDS also applies a reduction to \((EI)_{eff}\) to account for shear deformation in a uniform-load beam check.

For a three-layer panel, the shear analogy reduces to simple forms. With laminations of thickness \(t\), the bending stiffness of the two outer layers about the center is

and the midspan deflection of a simply supported panel under uniform load adds a shear term to the familiar bending term:

Here \(G\) is the shear modulus along the grain, about \(E/16\), and \(G_r\) the rolling shear modulus, about \(G/10\). For a short, deep panel the shear term can be a third of the total; for the pavilion roof it is about a tenth.

The pavilion roof deck

The roof deck is a 4-1/8 in. three-ply E1 panel, laminations 1-3/8 in. thick, with \(E = 1.7 \times 10^6\) psi and \(F_b = 1950\) psi for the longitudinal layers. The panels span 10 ft between glulam beams with the outer layers parallel to the span, and the team designs them conservatively as simple spans even though most panels are continuous over two or three beams. The ASD load is \(25 + 42 = 67\) psf.

# Lesson 4: CLT roof panel and one-hour fire check of the glulam beam (NDS, ASD)
# --- 3-ply CLT, 3 x 1-3/8 in., grade E1, simple span 10 ft between glulam beams, per foot width
t, E, Fb = 1.375, 1.7e6, 1950.0
G, Gr = E / 16, E / 160
L, wD, wS = 120.0, 25.0, 42.0                       # in., psf
b = 12.0
I = 2 * (b * t**3 / 12 + b * t * t**2)
EI = E * I
GA = (2 * t)**2 * b / (t / (2 * G) + t / Gr + t / (2 * G))
w = wD + wS                                          # plf per ft width, D + S
M = w * (L / 12)**2 / 8                              # lb-ft per ft
S_eff = I / (1.5 * t)
Ma = Fb * 1.15 * 0.85 * S_eff / 12                   # CD = 1.15; 0.85 effective-stiffness reduction
dS = 5 * (wS / 12) * L**4 / (384 * EI) + (wS / 12) * L**2 / (8 * GA)
print(f"CLT: EIeff = {EI/1e6:.0f} x 10^6 lb-in2/ft, GAeff = {GA/1e3:.0f} x 10^3 lb/ft")
print(f"     M = {M:.0f} vs {Ma:.0f} lb-ft/ft, snow deflection = {dS:.3f} in., L/360 = {L/360:.2f} in.")

# --- One-hour fire: glulam beam 6-3/4 x 21, 24F-V4, exposed on three sides
bt, dt, t_hr = 6.75, 21.0, 1.0
a_char = 1.5 * t_hr ** 0.813                          # nominal char rate 1.5 in./hr
a_eff = 1.2 * a_char
br, dr = bt - 2 * a_eff, dt - a_eff
Sr = br * dr**2 / 6
M_fire = ((25.0 * 10 + 35.0 * bt * dt / 144) + 42.0 * 10) * 30**2 / 8   # D + S, lb-ft
CV = min(1.0, (21 / 30)**0.1 * (12 / dr)**0.1 * (5.125 / br)**0.1)
Fb_fire = 2.85 * 2400 * CV                           # design-stress-to-member-strength factor 2.85
M_cap = Fb_fire * Sr / 12
print(f"\nFire: effective char depth = {a_eff:.2f} in., residual {br:.2f} x {dr:.2f} in.")
print(f"      M (D + S) = {M_fire/1000:.1f} kip-ft, fire capacity = {M_cap/1000:.1f} kip-ft, ratio {M_fire/M_cap:.2f}")
print(f"      CLT: {a_eff:.2f} in. of 4.125 in. is lost, so only the top lamination ({t} in.) still spans")
S1 = b * t**2 / 6
print(f"      CLT fire: M = {M:.0f} vs {2.85*Fb*S1/12:.0f} lb-ft/ft, ratio {M/(2.85*Fb*S1/12):.2f}")

CLT: EIeff = 115 x 10^6 lb-in2/ft, GAeff = 638 x 10^3 lb/ft
     M = 838 vs 5205 lb-ft/ft, snow deflection = 0.092 in., L/360 = 0.33 in.

Fire: effective char depth = 1.80 in., residual 3.15 x 19.20 in.
      M (D + S) = 79.3 kip-ft, fire capacity = 106.6 kip-ft, ratio 0.74
      CLT: 1.80 in. of 4.125 in. is lost, so only the top lamination (1.375 in.) still spans
      CLT fire: M = 838 vs 1751 lb-ft/ft, ratio 0.48

The panel is far stronger than it needs to be: \(M = 838\) lb·ft per foot against an allowable 5205 lb·ft, and the snow deflection is 0.09 in. against span/360 = 0.33 in. A three-ply panel is the thinnest that is practical, and it is the fire, vibration, acoustic and diaphragm requirements, not ordinary strength, that would push a roof or floor to five plies. Here the roof also acts as the diaphragm that carries wind and earthquake loads to the walls, which CLT does well when the panel joints are screwed with splines.

How wood burns

Wood does not melt or lose strength suddenly in fire. When its surface is heated it decomposes, giving off flammable gases that burn, and leaves a layer of char. Char is a good insulator, so it slows the heating of the wood underneath. Just behind the char line, a thin layer of wood is heated above about 100 °C and has lost much of its strength and stiffness, but the core stays cool and keeps its full properties. A large timber member therefore burns inward at a fairly steady rate and keeps carrying load on its shrinking core. That is why heavy timber has long been accepted as fire-resistant construction, and why a steel beam, which loses half its strength at about 550 °C, often needs more protection than a glulam beam of the same capacity.

NDS Chapter 16 gives the method to calculate the fire resistance of exposed wood members. The char depth after time \(t\) hours is \(a_{char} = \beta_n t^{0.813}\), with a nominal char rate \(\beta_n = 1.5\) in./hr, and the effective char depth, which includes the heated zone that has lost strength, is \(a_{eff} = 1.2a_{char}\), or 1.8 in. for one hour. The remaining section is checked under ASD loads, with the allowable design value multiplied by a design-stress-to-member-strength factor: 2.85 for bending and tension and 2.58 for compression. For CLT, the char depth accounts for lamination fall-off, and glulam beams for a one-hour rating need an extra outer tension lamination.

The CLT deck exposed from below loses 1.8 in. after an hour, all of the bottom lamination and part of the middle one. Only the top layer, which spans in the main direction, remains effective, and it carries the load at a ratio of 0.48. Fall-off of charred laminations at the glue lines can speed up charring for some adhesives, which is why PRG 320 now requires adhesives that pass a test for this behavior.

Connections in fire

Steel connectors exposed to fire heat quickly and conduct heat into the wood around them, so they are the weak point of a mass timber structure. Concealed connections, with knife plates and dowels set deep within the wood and the plate edges covered by wood plugs or char-depth cover, solve the problem. The pavilion's beam-to-column connections in lesson 5 use slotted-in knife plates with the bolt heads recessed, so the wood protects the steel.

NLT, DLT and other panels

Nail-laminated timber, an old system revived, is made of dimension lumber set on edge and nailed face to face; it spans one way and is often built on site or in a simple shop. Dowel-laminated timber uses hardwood dowels instead of nails, so it has no metal and can be machined easily, and the dowels lock in as the softwood around them swells. Both act as one-way decks, and both rely on a structural sheathing layer on top to act as a diaphragm. Panels made of veneer, such as mass plywood, give two-way action like CLT. The choice between them depends on span, appearance, local fabrication and cost; for the pavilion, CLT was chosen because the panels arrive in a few large pieces and give a clean, flat ceiling.

Code limits on mass timber buildings

The 2021 International Building Code added three mass timber construction types: Type IV-A, allowing up to 18 stories with all mass timber protected by noncombustible material; Type IV-B, up to 12 stories with some exposed timber; and Type IV-C, up to 9 stories with the mass timber permitted to be exposed. Heavy timber construction became Type IV-HT. States adopt these provisions on their own schedules, and the designer checks the edition in force in Minnesota for each project. Smaller buildings like the pavilion can be built of Type IV-HT or Type V construction with exposed timber, provided the member sizes and fire ratings required for the building's size and use are met.

Other design issues for mass timber

Mass timber floors are light compared with concrete, so footfall vibration and sound transmission need attention. Typical solutions are a concrete or gypsum topping, resilient layers and careful span selection. Moisture during construction is a real risk: CLT panels left exposed to rain before the roof is closed in can swell, stain and trap water at joints, so the contractor protects the panels and the schedule closes in the building quickly. Shrinkage is small along the panel but not across the thickness, so detailing at stacked floors and at elevator cores, which do not shrink, matters in tall buildings. Finally, mass timber is prefabricated: the panels arrive cut with every opening and connection, which shortens construction time but means that design decisions must be made early.