Sources and References

• Canadian Wood Council. Wood Design Manual 2020 (WDM 2020), including CSA O86-19 Engineering Design in Wood. Ottawa: CWC, 2020.
• CSA Group. CSA O86-19: Engineering Design in Wood. Mississauga: Canadian Standards Association, 2019.
• Breyer, Donald E., Kenneth J. Fridley, Kelly E. Cobeen, and David G. Pollock. Design of Wood Structures — ASD/LRFD, 7th ed. New York: McGraw-Hill, 2014.
• Faherty, Keith F., and Thomas G. Williamson, eds. Wood Engineering and Construction Handbook, 3rd ed. New York: McGraw-Hill, 1999.
• Porteous, Jack, and Abdy Kermani. Structural Timber Design to Eurocode 5, 2nd ed. Oxford: Wiley-Blackwell, 2012.
• American Wood Council. National Design Specification (NDS) for Wood Construction, 2018 ed.
• Karacabeyli, Erol, and Brad Douglas, eds. CLT Handbook: Cross-Laminated Timber. FPInnovations and American Wood Council, 2013.

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Chapter 1: Introduction to Wood as an Engineering Material

1.1 The Role of Timber in Structural Engineering

Wood is one of the oldest structural materials, yet it remains central to contemporary construction. A renewable biological resource, timber has experienced a significant resurgence owing to advances in engineered wood products and growing interest in low-carbon construction. Structural timber design encompasses a wide spectrum of applications, from light-frame residential construction to heavy mass timber commercial and institutional buildings that compete with steel and concrete in span and height.

The mechanical properties of wood are fundamentally determined by its cellular anatomy. Wood cells — tracheids in softwoods and fibres in hardwoods — are elongated along the grain direction, imparting anisotropy: stiffness and strength parallel to the grain are several times greater than those perpendicular to the grain. This directional dependence is a defining consideration in all structural timber design.

1.2 Wood Products and Structural Forms

Modern timber construction draws on a range of wood products optimised for different structural applications:

Sawn lumber is cut directly from logs and graded visually or by machine (machine stress-rated, MSR). Species-specific grades determine allowable stresses. Sawn lumber is the primary material in light-frame construction.

Timber refers to sawn members with a cross-sectional dimension of 114 mm or greater in both directions. Timber is used for heavy-post-and-beam construction and as columns, girders, and transfer members.

Glued-laminated timber (glulam) is fabricated by adhesively bonding multiple horizontal laminations, with grain running parallel to the member axis. Glulam achieves strength and stiffness well above those of comparable sawn lumber, allows the production of large and curved members, and reduces variability through averaging of defects across laminations.

Cross-laminated timber (CLT) consists of odd numbers of lumber layers (typically three, five, or seven) bonded with structural adhesive, with alternating layers oriented perpendicular to each other in plan. This cross-lamination confers two-way spanning capacity analogous to reinforced concrete flat-plate systems. CLT panels serve as floors, roofs, and shear walls in mass timber buildings.

Nail-laminated timber (NLT) and dowel-laminated timber (DLT) are panel products formed by assembling sawn lumber planks edgewise and fastening them mechanically (NLT) or with hardwood dowels (DLT). Both are used as floor and roof deck panels.

Structural composite lumber (SCL), including laminated veneer lumber (LVL), parallel strand lumber (PSL), and laminated strand lumber (LSL), redistributes defects and achieves high, consistent engineering properties.

1.3 Limit States Design Framework

Structural timber design in Canada is governed by the limit states design (LSD) philosophy codified in CSA O86 and consistent with the National Building Code of Canada (NBC). The fundamental design requirement is:

\[ \phi R \geq \alpha_D S_D + \alpha_L S_L + \alpha_Q S_Q + \ldots \]

where \(\phi\) is the resistance factor, \(R\) is the specified resistance, and the right-hand side represents factored loads combining dead loads \(S_D\), live loads \(S_L\), and other effects \(S_Q\). Two classes of limit states are distinguished:

• Ultimate limit states (ULS): conditions relating to safety, including strength, stability, and fracture.
• Serviceability limit states (SLS): conditions affecting function and appearance, including deflection, vibration, and long-term creep.

In wood design, serviceability — particularly deflection and vibration of floor systems — is frequently the governing limit state, especially for long-span glulam and CLT floors.

1.4 Mechanical and Engineering Properties

Wood properties are sensitive to several physical factors that must be accounted for in design through modification factors:

Moisture content strongly affects stiffness and strength. Dry service conditions are defined as moisture content below 15% for sawn lumber and below 16% for glulam in CSA O86. The wet service factor \(K_S\) reduces reference strengths and moduli when these thresholds are exceeded.

Load duration modifies wood strength because wood under sustained load creeps and can fail at stresses substantially below short-term test strengths. The load duration factor \(K_D\) ranges from 1.15 for short-duration loads (wind, earthquake) to 0.65 for permanent loads, reflecting that wood sustained at high stress for years can fracture at roughly 60% of its short-term modulus of rupture.

Size and volume effects reflect that larger members have a higher probability of containing a critical defect at a highly stressed location. Volume effect factors \(K_Z\) for tension and bending reduce design values as member volume increases.

System factors \(K_H\) recognise that in a repetitive member system (e.g., joists at 400 mm on centre), loads can redistribute away from a failed member to adjacent members, increasing effective system capacity.

Treatment factors \(K_T\) modify strengths for members that have been preservative- or fire-retardant-treated by processes that may affect wood strength.

The specified strengths \(f\) in CSA O86 are 5th-percentile values derived from testing, divided by test-factor adjustments for load duration and safety. The factored resistance of a member is:

\[ F_r = \phi \cdot f \cdot K_D \cdot K_H \cdot K_{Sb} \cdot K_T \cdot K_Z \cdot A \quad \text{(or } S, \text{ as appropriate)} \]

where the product of modification factors adjusts the specified strength to the conditions of the actual application.

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Chapter 2: Design of Tension Members

2.1 Behaviour in Tension Parallel to Grain

Tension parallel to grain is the highest specified strength in wood. Failure in tension is brittle and abrupt, governed by fracture at defects — knots, checks, and slope of grain — rather than yielding. Because of this, size effects are pronounced: the volume effect factor for tension parallel to grain reduces the apparent strength more severely than for bending or compression.

2.2 Specified Strength Values

CSA O86 tabulates the net section tensile specified strength \(f_t\) for each combination of species, species group, and grade. For sawn lumber, Net-section area accounts for the reduction in cross-section at notches, bolt holes, or split-ring grooves. For glulam, the combinational grade is specified by the combinational designation (e.g., 20f-E) and different values apply to tension laminations and compression laminations.

2.3 Design Procedure

The factored tensile resistance is:

\[ T_r = \phi_t \cdot f_t \cdot K_D \cdot K_H \cdot K_{St} \cdot K_T \cdot K_{Zt} \cdot A_n \]

where \(A_n\) is the net cross-sectional area. The design condition is:

\[ T_f \leq T_r \]

Key considerations in tension member design include:

1. Selecting a grade that provides adequate \(f_t\) under the calculated factored tension demand.
2. Computing net area by subtracting fastener holes. For bolted connections, the hole diameter is typically taken as the bolt diameter plus 2 mm.
3. Verifying that connection length is sufficient to preclude block shear or row tear-out failures at the ends.
4. Checking slenderness: although not subject to buckling in the classical sense, tension members in trusses or bracing systems should be checked for adequate stiffness to avoid vibration issues.

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Chapter 3: Design of Compression Members

3.1 Column Behaviour and Slenderness

Wood columns fail by one of two mechanisms depending on their effective length-to-depth ratio:

• Short columns fail by crushing of wood fibres at the gross section (material failure).
• Slender columns fail by elastic or inelastic lateral buckling (Euler instability or interaction of crushing and buckling).

The slenderness ratio \(C_c = L_e / d\), where \(L_e\) is the effective length (product of \(K \cdot L\) using effective length factors analogous to those for steel) and \(d\) is the least cross-sectional dimension, governs the transition. CSA O86 limits \(C_c\) to 50 for sawn lumber and timber columns in normal service.

3.2 Sawn Lumber and Timber

The factored compressive resistance parallel to grain for sawn lumber and timber is:

\[ P_r = \phi_c \cdot f_c \cdot K_D \cdot K_H \cdot K_{Sc} \cdot K_T \cdot C_c \cdot A \]

The column stability factor \(C_c\) in CSA O86 is expressed as a function of the reference column slenderness ratio and material modulus of elasticity, following a column interaction formula analogous to those in other structural codes. It accounts for eccentricity and initial imperfections implicitly.

3.3 Stud Walls and Built-Up Columns

In platform-frame and balloon-frame construction, studs at 400 mm centres act as a group. The system factor \(K_H\) is applicable when the assembly includes sheathing that provides lateral restraint. Built-up columns — multiple pieces of sawn lumber fastened together — must have adequate fastening to ensure composite action. CSA O86 provides reduced \(K_H\) values for built-up columns relative to single solid columns of equivalent gross area, reflecting incomplete composite action unless fastening satisfies prescribed spacing requirements.

3.4 Glulam Columns

Glulam columns are designed using the same general interaction framework but with glulam-grade specified compressive strengths and moduli. Glulam typically provides higher \(E_{05}\) (5th-percentile modulus of elasticity) than sawn timber, resulting in higher buckling resistance for a given cross-section and effective length. Curved glulam columns introduce additional considerations for radial stress.

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Chapter 4: Design of Bending Members

4.1 Flexural Behaviour and Governing Checks

Bending members — beams, joists, girders, and rafters — are among the most common structural timber elements. Design must address:

1. Bending (flexural) resistance at the extreme fibre.
2. Shear resistance at sections near supports.
3. Bearing (compression perpendicular to grain) resistance at support and load points.
4. Lateral torsional buckling for unbraced beams.
5. Deflection at SLS.

The factored moment resistance of a sawn lumber or timber beam is:

\[ M_r = \phi_b \cdot f_b \cdot K_D \cdot K_H \cdot K_{Sb} \cdot K_T \cdot K_{Zb} \cdot S_x \]

where \(S_x\) is the section modulus about the strong axis and \(K_{Zb}\) is the size factor for bending.

4.2 Lateral Stability of Beams

An unbraced beam loaded at or above its centroid is susceptible to lateral torsional buckling. CSA O86 uses the slenderness ratio:

\[ C_B = \sqrt{\frac{L_{ub} \cdot d}{b^2}} \]

where \(L_{ub}\) is the unsupported length between points of lateral restraint, \(d\) is the depth, and \(b\) is the breadth. For \(C_B\) values below the transition value defined in the standard, full bending resistance is available. Above the transition, a lateral stability factor \(K_L\) reduces the factored resistance. Full lateral bracing, achieved by attaching a deck or roof diaphragm to the compression face, eliminates the lateral stability reduction.

4.3 Shear Design

Wood beams must be checked for longitudinal shear (rolling shear parallel to the grain planes) near supports. The factored shear resistance is:

\[ V_r = \phi_v \cdot f_v \cdot K_D \cdot K_H \cdot K_{Sv} \cdot K_T \cdot \frac{2}{3} A \]

CSA O86 permits the critical section for shear to be taken at a distance \(d\) from the face of the support when the beam bears on full width supports, because loads applied within this zone are transferred to the support by direct compression rather than shear.

Notches at the tension face near supports create stress concentrations and reduce effective shear area; their use is strongly discouraged, and when unavoidable they require separate fracture mechanics-based checks.

4.4 Bearing (Compression Perpendicular to Grain)

At supports and under concentrated loads, compression perpendicular to grain \(f_{cp}\) is frequently limiting, especially in dimension lumber. The factored bearing resistance is:

\[ Q_r = \phi_c \cdot f_{cp} \cdot K_D \cdot K_{Scp} \cdot K_T \cdot A_b \cdot K_B \]

where \(A_b\) is the bearing area and \(K_B\) is a bearing length factor that recognises the stiffening effect of surrounding fibres when bearing length is less than 150 mm and the bearing is not at the end of the member.

4.5 Glulam Beams: Straight, Tapered, and Curved

Glulam beams are produced in straight, single-tapered, double-tapered, and curved profiles. Additional checks arise for non-prismatic and curved geometries:

Tapered beams: Taper introduces a component of shear stress parallel to the taper cut face and a reduction in the volume of the highly stressed zone. CSA O86 and Breyer et al. present interaction equations for combined bending and transverse shear at the taper cut.

Curved beams: Curvature introduces radial stresses perpendicular to the grain. For concave curvature under upward loading (e.g., arches), radial tension perpendicular to grain is generated at the inner lamination face and must not exceed the perpendicular tensile resistance. The radial stress is:

\[ f_r = \frac{3 M}{2 b \cdot d^2} \cdot \frac{d}{R} \]

where \(R\) is the radius of curvature. CSA O86 limits the ratio \(d/R\) to control curvature-induced stress and lamination bending during manufacturing.

4.6 Combined Axial Load and Bending

When axial force coexists with bending — as in wall studs under combined wind pressure and floor gravity load, or in rafter-ties — interaction equations are used. For combined compression and bending:

\[ \left(\frac{P_f}{P_r}\right)^2 + \frac{M_f}{M_r} \leq 1.0 \]

For combined tension and bending, the interaction is linear:

\[ \frac{T_f}{T_r} + \frac{M_f}{M_r} \leq 1.0 \]

Both conditions must be satisfied when the sign of the axial force reverses under different load combinations.

4.7 Serviceability: Deflection and Vibration

Deflection limits under SLS loads are typically \(L/360\) for live load and \(L/240\) for total load for floor members sensitive to cracking of finish materials, following NBC and good practice references. Long-term creep under sustained loads must be accounted for; CSA O86 recommends that deflections for permanent and quasi-permanent loads be multiplied by a creep factor (typically 2.0 for dry service conditions) to obtain total long-term deflection.

Floor vibration in mass timber buildings has become an important design topic. CLT and glulam floors with spans exceeding approximately 8–10 m often require vibration assessment. The recommended approach in the literature is to check the fundamental natural frequency against a minimum threshold (typically \(f_1 > 8\) Hz or 9 Hz for office occupancy) and to limit the unit impulse response to a velocity criterion, recognising that human sensitivity to vibration is greatest between 4 and 8 Hz.

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Chapter 5: Cross-Laminated Timber (CLT) Design

5.1 Mechanics of CLT Panels

CLT panels span primarily in one direction for standard panels but are capable of two-way action when aspect ratios and boundary conditions favour it. The key structural characteristic is the orthogonal layup: alternating parallel and perpendicular lamination layers create a composite section in which the perpendicular layers contribute less flexural stiffness but provide shear transfer between layers.

The effective bending stiffness \((EI)_{eff}\) of a CLT panel is computed using the gamma method or the shear analogy method. The shear analogy decomposes the panel into two fictitious beams: a flexurally rigid but shear-flexible beam representing the CLT composite, and a second beam capturing the rolling shear deformation in perpendicular layers. This method yields both the effective bending stiffness and the effective shear stiffness:

\[ (EI)_{eff} = \sum_{i=1}^{n} E_i \cdot I_i + \sum_{i=1}^{n} E_i \cdot A_i \cdot z_i^2 \]

where \(z_i\) is the distance from the neutral axis to the centroid of lamination \(i\).

The gamma method introduces a connection efficiency factor \(\gamma\) (ranging from 0 to 1) at interfaces between layers to account for partial composite action due to shear deformation of perpendicular layers:

\[ \gamma_i = \frac{1}{1 + \frac{\pi^2 E_i A_i G_{90} t_{90}}{L^2 k_{ser}}} \]

5.2 Rolling Shear

Rolling shear is a shear mode in which wood fibres attempt to roll over each other when shear acts across the growth rings in transverse laminations. The rolling shear modulus \(G_{90}\) for Norway spruce and comparable species is approximately 50–100 MPa, roughly 1/10 of the longitudinal shear modulus. This low value governs shear deformation in the perpendicular layers and is the primary reason CLT panels deflect more than isotropic plate models would predict. CSA O86 uses a reduced effective shear stiffness for CLT to capture this mechanism.

5.3 Axial and Bending Design of CLT Panels

When CLT panels are used as wall elements (vertical load bearing), only the parallel laminations resist compression and bending, while perpendicular layers provide lateral stability to the parallel laminations. The axial capacity per unit width is:

\[ P_r = \phi_c \cdot f_{c,0} \cdot K_D \cdot K_{Sc} \cdot K_T \cdot b \cdot \sum t_{parallel} \]

For bending design of floor panels, the elastic section modulus accounts only for the parallel laminations, discounted for shear lag where applicable.

5.4 Combined Axial and Bending of CLT Panels

Wall panels under combined axial gravity load and lateral (wind or seismic) load require interaction equation checks analogous to those for solid timber columns, using the effective bending stiffness and axial capacity from Sections 5.2 and 5.3. Second-order (P-delta) effects become significant for slender CLT walls in multi-storey mass timber buildings.

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Chapter 6: Design of Connections

6.1 Connection Philosophy in CSA O86

Connections in timber structures must be designed to avoid brittle, sudden failure modes and to achieve adequate ductility, particularly in seismic applications. CSA O86 provides capacity design approaches, calculating the resistance of each fastener type based on the lesser of the fastener’s own bending/yielding capacity and the wood-bearing capacity at the interface.

The European Yield Model (EYM), formalised in Johansen’s yield equations and incorporated into both CSA O86 and Eurocode 5, identifies several failure modes for dowel-type fasteners (nails, screws, bolts, dowels, lag screws):

• Mode I: Wood bearing failure in main or side member.
• Mode II: Pivot of fastener in one member with bearing failure in both members.
• Mode III: Single plastic hinge in fastener with bearing failure in one member.
• Mode IV: Two plastic hinges in fastener — the ductile mode preferred for seismic design.

The yield mode governs based on the ratios of member thickness to fastener diameter and the yield moment of the fastener relative to wood embedment strength.

6.2 Nails and Screws

Nails resist lateral (shear) loads perpendicular to their shank axis. The lateral resistance per nail is calculated from Johansen yield equations, reduced by load angle factors when the nail is loaded at an angle to the grain. Self-tapping structural screws (STS) can resist both lateral and axial (withdrawal) loads and have revolutionised CLT connection detailing.

Withdrawal resistance of nails and screws is sensitive to moisture cycling (the fastener can lose its grip in repeated wet-dry cycles) and to direction of withdrawal relative to grain. Withdrawal from end grain provides much lower resistance than from side grain and is generally avoided for primary structural connections.

6.3 Bolts, Dowels, and Lag Screws

Bolted connections permit larger load transfer per connection group but require close tolerances to avoid load eccentricities. Hole oversize (typically 1–2 mm for clearance) means that bolts are not effective at zero load; slip must be accounted for in connection design when deflection is critical.

Dowels are smooth-shank pins sized to achieve a press fit, eliminating slip at service load levels. They are commonly used in engineered timber joints requiring stiffness (e.g., beam-to-column connections in moment frames).

Lag screws (coach screws) combine the bearing mechanism of a bolt with the withdrawal mechanism of a screw. Their design in CSA O86 treats the shank (smooth portion below the head) as a bolt for shear and the threaded portion for withdrawal.

Group effects reduce the capacity per fastener as the number of fasteners in a row (parallel to the load direction) increases. CSA O86 applies a group reduction factor \(C_G\):

\[ C_G = \frac{1 + (n-1) \cdot \eta}{n} \]

where \(n\) is the number of fasteners in a row and \(\eta\) is an efficiency factor that decreases with increasing member stiffness relative to fastener stiffness.

6.4 Timber Rivets

Timber rivets (also called Laminated Veneer Lumber rivets or glulam rivets) are hardened steel nails of rectangular cross-section driven into pre-drilled holes in steel side plates. They exhibit high load capacity, particularly in tension parallel to grain connections, because their staggered geometry forces wood failure in a pyramidal plug mode. CSA O86 provides extensive tabulations for rivet connections as a function of rivet length, spacing, and number.

6.5 Detailing for Durability and Fire Performance

Connection detailing significantly affects long-term performance. Key durability principles include:

• Avoiding configurations that trap moisture or impede drainage at contact surfaces.
• Using galvanised or stainless steel fasteners in exposed or high-humidity environments.
• Maintaining adequate end distance, edge distance, and spacing to prevent splitting.

For fire resistance, connections are often designed to be protected (recessed or covered) so that charring of the wood member occurs more slowly than at unprotected surfaces. Connection failure is frequently the limiting mode in fire, as fastener temperatures rise before the surrounding wood reaches the charring front. Exposed steel connections may need to be thermally protected by wood covers or intumescent coatings.

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Chapter 7: Fire Safety of Timber Structures

7.1 Charring of Wood

Wood is combustible, but its fire behaviour is predictable. Upon exposure to fire, wood chars at a well-defined char front that advances at an approximately constant rate through the cross-section. Behind the char front, the wood is structurally ineffective; ahead of it, wood retains its ambient-temperature mechanical properties remarkably well because of low thermal conductivity.

The design charring rate \(\beta_n\) in CSA O86 Annex B is 0.7 mm/min for softwood sawn lumber and glulam and 0.65 mm/min for CLT (reflecting the additional resistance of glue lines). After a fire exposure of \(t\) minutes, the char depth is:

\[ a_{char} = \beta_n \cdot t \]

The residual cross-section — the original cross-section minus char on all exposed faces — is then designed to carry factored loads under fire conditions using ambient-temperature resistance factors (or with minor reductions for elevated-temperature effects in the pyrolysis zone immediately behind the char front).

7.2 Fire Resistance Rating Requirements

The NBC specifies minimum fire resistance ratings (FRR) in hours (e.g., 1 h, 1.5 h, 2 h) for structural elements based on occupancy and building construction type. Heavy timber construction (Type IV) is recognised in the NBC as providing acceptable fire performance through massive cross-sections that char slowly, provided members satisfy minimum dimension requirements.

For buildings exceeding the Type IV wood construction limits, fire protection through gypsum board assemblies, sprinkler systems, or oversize member dimensions may be required. The 2015 and 2020 editions of the NBC expanded permissions for combustible construction in tall wood buildings up to 12 storeys, provided comprehensive sprinkler protection and compartmentation requirements are met.

7.3 Component Additive Method

For light-frame assemblies, the component additive method (CAM) in CSA O86 Annex A calculates the total assembly fire resistance as the sum of contributions from each layer:

\[ \text{FRR}_{assembly} = t_{framing} + \sum t_{membrane} \]

Contributions are tabulated for framing members (based on dimension and spacing), sheathing, insulation, and membranes (gypsum board layers). This method reflects the protection afforded by membranes in delaying the onset of charring in the framing members.

7.4 Fire Resistance of Large Cross-Section Members

For large cross-section sawn timber, glulam, and CLT elements, CSA O86 Annex B uses the char depth method. The remaining structural capacity of the reduced cross-section is verified using the full specified strengths (without fire reduction factors for wood), since the timber ahead of the char front remains at ambient temperature during the exposure period. Load combinations for fire design use reduced load factors because the probability of simultaneous maximum occupancy loads and severe fire events is low.

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Chapter 8: Introduction to Lateral Design

8.1 Lateral Load Paths in Wood Buildings

Lateral forces from wind and seismic events are resisted by a hierarchy of structural elements:

1. Diaphragms (horizontal): floors and roofs act as rigid or flexible plates that collect and distribute lateral forces to vertical lateral force-resisting elements.
2. Shear walls: vertical panels (plywood or OSB-sheathed stud walls, or CLT walls) transfer horizontal forces from diaphragms to the foundation.
3. Collectors/drag struts: members parallel to the loading direction that collect forces from the diaphragm and deliver them to shear wall ends.
4. Hold-down anchors and shear anchors: connect shear walls to the floor below or to the foundation, resisting overturning moments and sliding.

The correct identification and detailing of load paths is essential. Discontinuities — offsets, openings, changes in plan geometry — must be bridged with explicit load transfer details.

8.2 Flexible and Rigid Diaphragms

Diaphragm classification as flexible or rigid determines how lateral forces are distributed to shear walls:

• Flexible diaphragms distribute forces in proportion to tributary area (analogous to a simply supported beam spanning between shear walls). This is conservative for the design of individual shear walls in irregular plans.
• Rigid diaphragms distribute forces in proportion to shear wall stiffness, and torsional effects must be considered explicitly when the centre of mass (CM) is offset from the centre of rigidity (CR).

CSA O86 and NBC allow light-frame wood floors with blocking and adequate fastening to be designed as flexible diaphragms. CLT floors with well-detailed connections may be classified as rigid for the purposes of lateral force distribution.

8.3 Torsional Eccentricity

When CM and CR do not coincide, the lateral force \(V\) generates a torsional moment \(M_T = V \cdot e\), where \(e\) is the eccentricity. The NBC requires that a minimum design eccentricity (accidental eccentricity) of 0.1 times the plan dimension perpendicular to the loading direction be added to the calculated eccentricity to account for uncertainty in mass distribution. Each shear wall then resists both a direct shear component and a torsional shear component; walls on the flexible side of the CR see increased demand.

8.4 Shear Wall Design

Light-frame shear walls resist in-plane lateral forces primarily through sheathing-to-framing nailing. The shear capacity per unit length of wall depends on sheathing thickness, panel orientation, and nail size and spacing at panel edges. CSA O86 tables list capacities per metre of wall length for common configurations.

CLT shear walls resist lateral forces through the CLT panel acting in shear (with rolling shear in perpendicular layers governing for multi-storey walls) and through anchorage connections (hold-downs and angle brackets) that resist overturning and sliding. The flexibility of these connections dominates the overall wall stiffness in multi-storey CLT buildings.

8.5 Seismic Design Considerations

Seismic design of timber structures requires attention to ductility and energy dissipation. The NBC assigns ductility-related force modification factors \(R_d\) and over-strength-related factors \(R_o\) to different lateral force-resisting systems. Light-frame shear walls with standard wood-based sheathing are assigned \(R_d = 2.0\) and \(R_o = 1.7\), reflecting moderate ductility achieved through nail slip and yielding.

CLT shear wall systems rely on inelastic deformation in the hold-down and angle bracket connections — the CLT panels themselves remain essentially elastic. Dissipative connections must be detailed with sufficient ductility (typically Mode IV nail or bolt yielding) and must be protected against brittle failure modes (block shear, plug shear, net section failure) through capacity design. Non-dissipative connections and members (gravity columns, beams, foundations) are designed for forces amplified by the over-strength factor to ensure the ductile fuses yield before brittle elements fail.

Diaphragm design under seismic loading requires that the diaphragm be designed for forces that may exceed those delivered by the equivalent static force procedure at each storey, particularly for long, flexible diaphragms spanning between widely spaced shear walls. Blocking, boundary members, and chord splices must be designed to transfer chord forces arising from diaphragm bending in its own plane.

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Chapter 9: Synthesis — Structural System Considerations

9.1 Gravity Design Integration

A complete structural timber design integrates the member-level checks of Chapters 2–5 with a system-level understanding of load paths. Gravity loads travel from cladding and roofing through purlins and rafters to primary beams (glulam or CLT), to columns or walls, and finally to the foundation. At each transfer point, bearing capacity (compression perpendicular to grain) and connection capacity must be verified.

9.2 Mixed Systems and Hybrids

Contemporary timber buildings frequently combine mass timber with light-frame elements and with other structural materials. Podium construction — a multi-storey light-frame or CLT superstructure on a concrete transfer slab and podium structure — is common for urban residential buildings. Interface details at the wood-concrete boundary must address differential settlement, moisture protection, and connection transfer of horizontal and vertical forces.

Post-and-beam glulam frames with CLT floor panels represent another common hybrid, combining the architectural expressiveness of exposed glulam with the floor capacity of CLT. Beam-to-column moment connections are challenging in timber and are typically avoided by using pin-ended beams and achieving lateral stability through shear walls rather than moment frames.

9.3 Sustainability and Life Cycle Considerations

Wood stores atmospheric carbon sequestered during tree growth. Engineered wood products with high utilisation of raw timber resource (CLT, glulam, LVL) extend this benefit by enabling large-scale structural use. Life cycle assessment (LCA) of timber structures consistently shows lower embodied carbon than equivalent steel or reinforced concrete structures, provided the timber originates from sustainably managed forests with verified chain of custody.

Durability details — moisture management, adequate bearing areas, proper connection protection — are critical to ensuring that timber buildings achieve design service lives comparable to concrete and steel construction. Poorly detailed timber structures have historically experienced premature deterioration; modern timber engineering practice draws heavily on the lessons of those failures.