Wood and Timber - Properties, Grading & Testing

Wood and Timber

$Learning Objectives$

Explain the biological structure of wood and its engineering idealization as an orthotropic material.
Calculate moisture content and explain fiber saturation, equilibrium moisture, shrinkage, swelling, and dimensional stability.
Relate species, density, grade, defects, moisture, load duration, and grain direction to strength and stiffness.
Explain moisture, bending, compression, tension, shear, grading, and connection-related testing concepts.
Evaluate decay, insect, fire, treatment, and service-exposure risks without universal threshold claims.
Distinguish solid-sawn lumber from glulam, LVL, plywood, OSB, CLT, and other engineered products.

$Wood as a natural engineering material$

Wood is a cellular composite with substantial natural variability. Properties depend on species, growth characteristics, density, moisture, grain direction, defects, temperature, load duration, and processing. It is anisotropic in the broad sense and is commonly idealized as orthotropic along longitudinal, radial, and tangential material axes.

$Macroscopic anatomy and material axes$

Sapwood and heartwood: differ in biological function, permeability, and sometimes natural durability.
Growth rings, rays, vessels, and tracheids: influence transport, shrinkage, appearance, and mechanical response.
Longitudinal (L): parallel to grain/tree axis and normally the strongest/stiffest direction.
Radial (R): perpendicular to growth rings from pith toward bark.
Tangential (T): tangent to growth rings and commonly the direction of greatest drying shrinkage.

$Anisotropic and orthotropic are compatible descriptions$

Natural wood is anisotropic because properties depend on direction. Engineering analysis often approximates that directional behavior using three mutually perpendicular orthotropic axes, L-R-T.

$Interactive wood-behavior simulation$

Use the simulation to compare moisture, grain direction, and wood-property concepts. Product design values still depend on species, grade, size, moisture designation, and governing timber standard.

Wood Moisture Measurement & Directional Movement

First calculate direct oven-dry-basis moisture content from observed masses, then separately explore a simplified directional movement index below an illustrative fiber-saturation reference.

Laboratory/test simulatorThe mass-loss calculation is direct; the movement model remains conceptual and species/product dependent.

Relevant standards map

ASTM D4442· MethodASTM D143· MethodASTM D198· Method

Practice controls sampling/specimen preparation where applicable → test method defines measurement → specification/code defines required performance → project documents define the controlling acceptance basis. Do not infer acceptance from a standard designation alone.

Laboratory evidence chain

1. Sample / lot represented
2. Specimen identity and condition
3. Apparatus and verification status
4. Procedure and method-critical controls
5. Raw readings / observations
6. Checked calculation
7. Validity and deviation review
8. Engineering interpretation
9. Specification / code comparison
10. Traceable report and disposition

Direct moisture-content observations

Mass before oven drying112 g

Oven-dry mass100 g

Initial moisture content for movement model30 %Final moisture content for movement model12 %Illustrative FSP30 %Tangential FSP→OD movement index8.0 % of FSP referenceRadial FSP→OD movement index4.0 % of FSP reference

Direct oven-dry-basis moisture content

12.0%

MC=\frac{m_{wet}-m_{dry}}{m_{dry}}100\\%

Formal testing still requires the method-defined drying endpoint, apparatus, specimen handling, and reporting precision.

Modeled response

shrinkage

I_i=S_{i,FSP\to OD}\frac{MC_f^*-MC_i^*}{FSP}

Tangential movement index

-4.80%

Radial movement index

-2.40%

Clear-wood measurements, structural grade values, moisture-meter readings, and dimensional-movement data are different evidence streams. Structural design values require the applicable species/grade/product basis rather than a small-clear or generic movement value.

$Moisture Content (MC)$

Mass of water in wood expressed relative to oven-dry wood mass under the specified test procedure.

Wood Moisture Content

Oven-dry-basis wood moisture content from wet and oven-dry masses.

MC=\frac{m_{wet}-m_{dry}}{m_{dry}}\times100\%

Variables

Symbol	Description	Unit
$MC$	Moisture content	%
$m_{wet}$	Mass before oven drying	g or kg
$m_{dry}$	Oven-dry mass	g or kg

$Interactive wood-moisture simulation$

Use the simulation to compare moisture states and their effect on wood behavior. Electrical moisture meters are indirect measurements and can require species, temperature, treatment, and depth corrections.

Wood Moisture & Fiber-Saturation Concept

Explore the teaching approximation that most ordinary dimensional change occurs below a fiber-saturation region. FSP is not one universal 30% value.

Moisture content20%Illustrative FSP30%Illustrative total directional shrinkage, FSP→OD8.0%

Modeled shrinkage from FSP state

2.67%

Below the selected FSP, the model scales directional shrinkage linearly for teaching. Actual shrinkage is species-, direction-, history-, and test-dependent.

$Fiber Saturation Point (FSP)$

Approximate moisture condition at which cell walls remain saturated with bound water while little or no free water remains in cell cavities. A value near 30% is often used for teaching, but actual FSP depends on species, temperature, and definition.

$Equilibrium Moisture Content (EMC)$

Moisture condition toward which wood tends when exposed long enough to a given temperature and relative humidity.

$Dimensional stability and shrinkage$

Most ordinary drying shrinkage occurs as bound-water content changes below the fiber-saturation region. Tangential shrinkage commonly exceeds radial shrinkage, while longitudinal shrinkage is generally much smaller. Differential movement can cause cupping, warping, checks, splits, joint opening, and finish damage when drying is nonuniform or restraint is high.

$Interactive shrinkage simulation$

Use the simulation to visualize directional shrinkage. Actual dimensional movement should use product/species data and the expected service moisture range.

Illustrative Directional Wood Shrinkage

This synthetic curve illustrates the common trend that ordinary moisture-related dimensional change occurs primarily below a fiber-saturation region and differs by material direction. It is not species design data.

Loading chart...

Actual FSP and total shrinkage vary with species, direction, temperature, extraction/history and test definition. Longitudinal movement is small for ordinary clear straight-grained wood but is not universally zero.

$Direction-dependent mechanical behavior$

Tension parallel to grain: high clear-wood capacity but sensitive to defects and connection details.
Tension perpendicular to grain: weak and brittle; splitting can occur near notches, curved members, connections, and restrained shrinkage zones.
Compression parallel to grain: important for axial members and end-grain loading.
Compression perpendicular to grain: localized bearing/crushing deformation frequently governs supports and connections.
Bending: combines tension, compression, shear, size, defects, and load-duration effects.
Shear parallel to grain: important near supports, notches, and short/deep members.

Elastic Flexural Stress

Beam-theory relationship for bending stress at distance c from the neutral axis under elastic assumptions.

f_b=\frac{Mc}{I}

Variables

Symbol	Description	Unit
$f_b$	Flexural stress	MPa
$M$	Bending moment	N·mm
$c$	Distance from neutral axis	mm
$I$	Second moment of area	mm⁴

$Interactive timber-flexure simulation$

Use the simulation to study bending response and grain-direction effects. Structural design values are not equal to small-clear-specimen strength and must use the timber code/product grade data.

Timber Beam Stress Illustration

Use elementary beam mechanics to visualize stress demand for a rectangular section. The simulator no longer assigns generic species “allowable stresses” or declares a structural pass/fail.

Span4 mWidth100 mmDepth200 mmMidspan point load5 kN

Max moment

5.00 kN·m

M_{max}=PL/4

Max shear

2.50 kN

V_{max}=P/2

Elastic extreme-fiber stress

7.50 MPa

f_b=6M/(bh^2)

Rectangular max shear stress

0.188 MPa

\tau_{max}=1.5V/(bh)

Materials-course boundary: do not compare these demands with generic species values. Structural timber resistance requires the actual species/species group, grade, size, moisture/service conditions, load duration, adjustment factors, defects, stability, connections, and governing design code.

$Structural Grading$

Process of assigning lumber to a grade using visual or machine-evaluated characteristics correlated with structural performance. Grade, species/species group, dimensions, moisture designation, and applicable adjustments are integral to usable design properties.

$Strength-reducing characteristics and defects$

Knots and associated grain deviation.
Slope of grain.
Checks, splits, shakes, and wane.
Bow, crook, cup, twist, and dimensional nonconformance.
Decay, insect attack, and mechanical damage.
Moisture condition inappropriate for the product or service environment.

$Clear-wood tests are not structural grade values$

Small clear specimens describe fundamental material behavior without many natural defects. Structural-lumber design values account for grade, size, species grouping, moisture, duration, reliability, and other adjustments; clear-wood strength should not be used directly as an allowable structural stress.

$Wood testing and qualification matrix$

Property/evidence	Typical method concept	Critical controls
Oven-dry moisture	Direct mass-loss method	Drying endpoint and accurate mass measurement
Moisture meter	Electrical estimate/correlation	Species, temperature, depth, treatment chemistry, meter verification
Small-clear properties	Directional mechanical tests	Grain direction, dimensions, loading rate, conditioning
Structural lumber	Product/grade qualification	Size, grade, species, conditioning, statistical population
Fasteners/connections	Connection-specific tests	Density, moisture, edge/end distance, fastener type, loading direction
Preservative treatment	Retention/penetration requirements	Treatability, use category, retention, penetration, field cuts

$Moisture, fungi, and insect durability$

Wood-decay fungi require favorable moisture, temperature, oxygen, and nutrient conditions. A statement such as “decay begins above 20% MC” is a useful risk heuristic, not a universal biological switch. Duration of wetting, species, temperature, detailing, treatment, and local biological exposure govern risk.

$Wood durability hierarchy$

Exclude bulk water through roofs, flashing, drainage, capillary breaks, and maintained envelopes.
Permit drying where intermittent wetting can occur.
Separate untreated wood from soil or persistent wetting unless specifically designed for the exposure.
Select naturally durable or properly treated products where required.
Control termite/insect risk using suitable materials, barriers, treatment, site practices, and inspection.
Identify and correct concealed moisture sources rather than treating visible biological growth alone.

$Preservatives are exposure- and regulation-specific products$

Historic and modern preservative chemicals have application restrictions that vary by product and jurisdiction. Specify treatment by the applicable use category, product standard, environmental regulation, retention, penetration, and project requirement rather than treating different preservatives as interchangeable.

$Treatment quality concepts$

Retention: amount of preservative retained per unit volume under the treatment specification.
Penetration: depth and distribution of preservative within required zones.
Incising/preparation: can improve penetration for selected difficult-to-treat species.
Field treatment: cut ends, holes, and notches may require an approved field-applied preservative system.
Fastener compatibility: treatment chemistry can influence corrosion of connectors and fasteners.

$Engineered wood products$

Glulam: adhesively bonded structural laminations with controlled lamination grades/orientation.
LVL: parallel veneer laminations forming a structural composite.
Plywood: cross-laminated veneers providing panel stability and directional strength.
OSB: oriented strands bonded into structural panels.
CLT: large cross-laminated panels with product-specific layup, adhesive, connection, moisture, and fire provisions.
I-joists and structural composite lumber: optimized products requiring manufacturer/product-specific design properties and installation rules.

$Fire behavior of timber$

Wood is combustible, while large timber members can develop a char layer that slows heating of the remaining section. Fire resistance depends on product, species, geometry, char behavior, adhesive, encapsulation, connections, load, and required rating. Protected steel and mass timber fail by different mechanisms and must each be evaluated as complete systems.

$Field acceptance and diagnosis$

High moisture: confirm the project/service target using the required measurement method and condition, dry/condition or reject as specified.
Missing grade stamp/product ID: quarantine until traceability or approved qualification is established.
Major knots, grain deviation, splits, or wane: evaluate against the grading rules rather than estimating capacity visually.
Mold or staining: identify the moisture source and investigate whether decay or section loss is present.
Suspected decay/insects: investigate extent, residual section, moisture, and structural significance.
Field cuts in treated timber: apply required approved field treatment and maintain water-shedding details.

Key Takeaways

Wood is anisotropic and commonly idealized as orthotropic along longitudinal, radial, and tangential axes.
Moisture controls dimensions, strength, durability, and connection behavior; FSP is an approximate transition concept rather than one universal number.
Structural timber properties depend on species, grade, moisture, size, duration, defects, and load direction.
Durable timber construction starts with moisture control and exposure-appropriate treatment.
Engineered wood products require product-specific manufacturing, design, connection, moisture, and fire provisions.

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