Timber Engineering: Durability, Products & Treatments

Assessment Structure and Deadlines

The course has three separate assessment streams, each handled by different instructors.

1. Concrete Module (handled by other staff)
   • One laboratory report + one assignment, worth 10\% of the total grade.
   • Communication of lab dates and hand-in deadlines comes directly from the concrete staff.

2. Wood & Steel Module (current lecturer)
   • Online test worth 10\% of the course (multiple-choice format).
   • Test opens the preceding Friday and must be submitted by 13\,\text{August}\,23{:}59 (Wednesday night).

3. Remaining Topics
   • Quizzes and a semester exam managed by Enrique.
   • A separate assessment in Doug’s transport section—dates already on the master schedule.

Students repeatedly ask for clarification even though the slide appears at every lecture; the lecturer urges everyone to internalise these dates to avoid last-minute confusion.

Moisture Fundamentals (Yesterday’s Review)

The class previously examined how water interacts with timber.

• Swelling & Shrinkage: Wood is hygroscopic—cell walls absorb/lose moisture.
• Equilibrium Moisture Content (EMC): The moisture level where wood is neither gaining nor losing water relative to ambient humidity.
• Measurement Techniques
– Oven-drying method (reference standard).
– Electrical-resistance meters (demonstrated in class).

Importance: Regardless of material (concrete formwork, hybrid systems, etc.), professionals inevitably deal with timber moisture—knowledge is essential.

Engineered Wood Products (EWP) Overview

Engineered wood solves size limits, removes defects, and offers shape flexibility compared with plain sawn lumber.

Sawn Timber Basics

• Tree stem diameter limits sections to roughly 450\,\text{mm}.
• Market is \approx 90\% softwood (radiata pine dominates NZ, small pockets of Douglas-fir).
• Nominal dimensions trace back to Imperial sizing (e.g.
4\times2 in → 50\times100\,\text{mm}; actual size 45\times90\,\text{mm}).

Glued-Laminated Timber (Glulam)

• Produced for decades; NZ companies include Techlam, Red Stag, TimberLab.
• Laminations finger-jointed, glued, and preservative-treated.
• Non-homogeneous lay-ups let designers place high-grade laminations at tension/compression faces with lower-grade core stock, increasing beam capacity.

I-Joists / I-Beams

• Hybrid product: solid or LVL flanges + OSB/plywood web.
• Optimised second moment of area: material concentrated at top/bottom where bending stresses peak; web transfers shear.
• Common on truck beds—spans larger/loads higher than sawn joists can handle.

Laminated Veneer Lumber (LVL)

• Two big NZ manufacturers: Nelson Pine, Carter Holt Harvey.
• Thin rotary-peeled veneers glued in parallel orientation; strength more uniform than sawn timber.
• Veneer optimisation raises grade and reduces defects.

Poles / Rounds

• Simplest utilisation: de-bark, shave, treat, use whole log.
• Applications: retaining walls, lookout platforms, marine piles.
• Green hue = CCA treatment (chromated copper arsenate) providing multi-decade soil contact resistance.
• End-grain vulnerability addressed by metallic caps to block UV and moisture ingress and to curb splitting.

Cross-Laminated Timber (CLT)

• First commercialised in Austria, 1994; introduced to NZ 2012 (small Nelson plant) then large Red Stag facility in Rotorua (15–16 m long × 3 m wide panels).
• Structure: orthogonal layers (like oversized plywood) using 1-component polyurethane adhesive that cures with wood moisture.
• Factory-CNC-routed openings for ducts, windows, etc.
• Density \approx400\,\text{kg m}^{-3} vs concrete \approx2400\,\text{kg m}^{-3} ⇒ transport savings, lighter cranes, reduced foundation loads.
• Best suited to multiresidential (3–7 storeys) or industrial sheds; single-family houses usually stick-framed.
• Sustainability & prefabrication appeal amid Auckland densification.
• Ongoing research: bridge decks, adhesive fire performance.

Durability Threats to Timber

Major degradation agents:

1. Insects & marine borers (termites, Teredo worm).

2. Fungi (needs >20\% MC).

3. Moisture cycling.

4. Ultraviolet light (surface greying, cracking).

5. Bacteria.

6. Mechanical abrasion & fastener loosening.

7. Chemicals (e.g., corrosive pool atmospheres).

Good vs Poor Detailing

• Detailing outranks chemistry—the right geometry keeps timber dry, ventilated, and shaded.
• Heritage Asian temples survive centuries via clever eaves, sacrificial layers, and off-ground support.

Examples:
• Metal cap over pile top blocks end-grain uptake; absence leads to checking and decay migration (wet \approx24\% MC vs adjacent \approx16\%).
• 1990s NZ ‘leaky-building’ crisis: stucco over untreated framing, zero eaves, no cavity → cracks → water ingress → framing MC >20\%, black rot. Surveyors now probe with moisture meters to diagnose (>24\% = decay risk).
• Balconies: design independent structural support so exterior decay cannot propagate into main frame; allows higher hazard treatment outside only.

Chemical Preservation & Hazard Classes

NZ/AU building codes specify minimum treatment by exposure class.

Chemical Notes:
• CCA (chromate-copper-arsenate): inexpensive & effective; green tint from copper. Concerns: arsenic is carcinogenic, disposal issues (cannot burn, risk of leachate). Being phased out in EU, US, AU but prevalent in NZ.
• Emerging substitutes: MCQ (micronised copper quaternary), ACQ (alkaline copper quaternary). Less toxic yet data gaps (bonding, long-term durability) remain.
• Fire toxicity: boron and other salts can release harmful fumes when timber burns—critical in multi-storey designs.

Fasteners & Corrosion

Treatment chemicals + environment dictate connector material:

Exposure flowchart hierarchy
\text{Stainless Steel} > \text{Hot-Dip Galvanised} > \text{Standard Zinc-Coated}

• Geothermal or sea-spray, fully exposed → stainless mandatory.
• Exterior but away from coast, sheltered → hot-dip galvanised acceptable (e.g., veranda beams).
• Interior closed cavities (roof, wall) → standard zinc-coated fine.

Torsional strength limits screw length in stainless; designers must check availability.

Protective Detailing Toolbox

1. Rain-screens & ventilated cavities create sacrificial claddings; airflow dries both layers.

2. Sloping horizontal members to shed water; or add sacrificial cap.

3. Elevate members off ground on concrete plinths or steel shoes (prevents splash-back).

4. Flashings at joints Redirect water away from end-grain or connection interfaces.

5. Paint vs Stain
   • Stain: lower upfront cost, reapply yearly/bi-yearly.
   • Paint: primer + intermediate + topcoat, higher cost, longer interval.

6. Off-site prefabrication (CLT, Glulam): precision CNC, factory drying, fewer cuts exposed on site.

Advantages of Timber Lightness

• Density ratio: 400/2400 \approx 1/6 that of concrete.
• Implications: smaller cranes, lower transport cost, reduced foundation design, quicker erection.

Hybrid Construction Trends

• Combination of steel, concrete, and engineered timber is increasing.
• Example: glue-lam columns with steel beams and aluminium rain screens in covered bridges.

Sustainability & Circular Economy Considerations

• 50-year service life mandated by NZ Building Code (Section B2 durability).
• End-of-life disposal: CCA and other heavy-metal treatments impede recycling and energy recovery; research underway on safer alternatives and recycling methods.

Ethical & Practical Implications

• Balancing low cost (CCA) vs worker/ environmental safety (arsenic).
• Importance of robust detailing to avert billion-dollar failures (leaky-building saga).
• Prefabrication reduces site waste and labour—social and economic sustainability.

Upcoming Topics

The next lecture will cover fire performance in timber and allow for additional Q&A. Unlike last week, it WILL run as a standard lecture session.