Module 1: Introduction to Timber Engineering

Module 1: Introduction to Timber Engineering

Timber is one of the oldest and most versatile construction materials. Unlike manufactured materials such as steel or concrete, wood is a naturally grown, organic material. This biological origin gives it unique physical and mechanical properties that must be fundamentally understood before any structural design can occur.

Properties of Philippine Wood

The structural behavior of wood is vastly different from isotropic materials like steel (which has the same strength in all directions). Wood's strength is highly directional.

Wood as an Orthotropic Material

Wood is composed of longitudinal cellular fibers (like a bundle of straws). Because of how a tree grows, wood exhibits different, independent mechanical properties in three mutually perpendicular axes:

Longitudinal (parallel to the grain): The strongest direction, aligning with the vertical growth of the tree. Wood has high tensile and compressive strength in this axis.
Radial (perpendicular to the grain): Crossing the growth rings in a radial direction. This axis runs outwards from the center of the tree.
Tangential (tangent to the growth rings): Typically the weakest direction. Shrinkage is also most pronounced in this direction during drying.

Consequently, allowable stresses and design capacities depend highly on the direction of the applied load relative to the grain orientation. Wood subjected to shear forces parallel to the grain is much weaker than when subjected to forces perpendicular to the grain.

Hygroscopicity

Wood acts like a sponge; it constantly absorbs or releases moisture from the surrounding environment to achieve an Equilibrium Moisture Content (EMC). This hygroscopic nature causes wood to shrink and swell, significantly affecting its strength, stiffness, and dimensional stability.

Anisotropic Shrinkage and Swelling

Because of its orthotropic nature, wood does not shrink or swell uniformly in all directions when its moisture content changes below the Fiber Saturation Point (FSP, approx. 25-30% MC).

Longitudinal Shrinkage: Shrinkage parallel to the grain is virtually negligible (typically 0.1% to 0.2% from green to oven-dry).
Radial Shrinkage: Shrinkage across the growth rings is moderate (typically 3% to 6%).
Tangential Shrinkage: Shrinkage tangent to the growth rings is the most severe (typically 6% to 12%), roughly twice as much as radial shrinkage.

This differential (anisotropic) shrinkage causes warping, twisting, cupping, and the formation of checks as the wood dries, fundamentally altering its structural geometry.

Key Takeaways

Wood is orthotropic, meaning its properties differ along its longitudinal, radial, and tangential axes.
Longitudinal Axis: Parallel to the grain. Wood is incredibly strong and stiff in tension and compression along this axis.
Radial Axis: Perpendicular to the grain, radiating outward from the pith to the bark.
Tangential Axis: Perpendicular to the grain, tangent to the growth rings. Wood is generally weakest and shrinks most along this axis.
Load capacity is highly dependent on the direction of applied forces relative to the grain.
Hygroscopicity affects wood strength, requiring adjustment based on moisture content.

Lumber Grading & Species Grouping

The Philippines is home to diverse wood species. For structural design, the NSCP groups these species based on their mechanical strength, natural durability, and density. This grouping simplifies the selection of reference allowable stresses.

NSCP Species Grouping

Philippine wood species are typically classified into structural groups. Understanding the group is critical for retrieving correct reference design values.

High Strength (Group I): Yakal, Guijo, Ipil, Narra, Molave. These dense, highly durable hardwoods are prioritized for heavy-duty structural members like main girders, columns, and critical load-bearing framing.
Medium Strength (Group II): Apitong, Tanguile, Red Lauan, White Lauan. Commonly referred to as Philippine Mahogany, these species are moderately strong and widely used for general structural framing, roof trusses, and flooring.
Low Strength (Group III/IV): Tangile, Almon, Bagtikan. These woods are generally softer, lighter, and possess lower strength. They are suitable for light framing, architectural partitions, paneling, and non-structural interior applications.

Key Takeaways

Philippine wood species are grouped by the NSCP according to their mechanical strength.
Group I encompasses high-strength, dense hardwoods (e.g., Yakal), while Group II features medium-strength woods (e.g., Apitong).

Visual vs. Machine Stress Grading

Grading Methods

Visual Grading: The traditional and most common method in the Philippines. A trained grader inspects the physical appearance of the lumber, evaluating the size and location of defects (knots, checks, slope of grain) to manually assign a grade and predict its structural capacity. It is conservative and qualitative.
Machine Stress Grading (MSG): An automated, quantitative method where the lumber is passed through a machine that continuously measures its stiffness (Modulus of Elasticity) by applying bending loads. It provides more accurate, reliable, and often higher design values than visual grading because it directly measures mechanical behavior rather than just observing surface defects.

Growth Characteristics and Defects

Beyond specific gravity and moisture content, a piece of lumber's structural capacity is heavily influenced by naturally occurring growth characteristics and physical defects. The NSCP accounts for these through visual or machine grading, but understanding their effects is crucial for a timber engineer.

Knots: Portions of a branch that have become incorporated in the body of a tree. Knots interrupt the direction of the grain, creating severe stress concentrations and significantly reducing tensile strength and flexural capacity.
Shakes: Separations along the grain, typically occurring between the rings of annual growth. They severely reduce horizontal shear resistance.
Checks and Splits: Separations of the wood fibers across the annual rings (checks) or extending completely through the piece (splits). They usually occur during the drying process due to uneven shrinkage and also degrade shear capacity.
Wane: Bark or lack of wood on the edge or corner of a piece of lumber. It reduces the effective cross-sectional area and bearing capacity.
Slope of Grain: Any deviation of the wood fibers from a line parallel to the edge of the lumber. Even slight slopes of grain can drastically lower the allowable tensile and compressive strengths parallel to the grain.

Physical Defects and Structural Grading

Wood is a natural material and inherently contains physical defects that disrupt the uniform flow of stresses, drastically reducing its load-bearing capacity. The process of structural grading essentially evaluates the size and frequency of these defects to assign safe, reliable working stresses.

Common Structural Defects in Timber

Knots: Formed where branches grew from the main trunk. A knot forces the longitudinal wood fibers (grain) to deviate sharply around it. This deviation creates massive localized stress concentrations and drastically reduces tensile and bending strength. Knots located on the tension edge of a bending member are particularly critical.
Checks and Shakes: A check is a separation of the wood fibers across the growth rings, typically caused by uneven drying (shrinkage stresses). A shake is a separation along the growth rings (between rings), often occurring while the tree is still standing. Both defects severely compromise the member's horizontal shear strength by reducing the continuous shear area along the neutral axis.
Slope of Grain: Wood strength is highly sensitive to the angle of the grain relative to the longitudinal axis of the member. Even a slight deviation (e.g., a 1-in-10 slope) can significantly reduce both tensile and compressive strength compared to perfectly straight-grained lumber.
Wane: The presence of bark or a lack of wood on the edge or corner of a piece of lumber. It geometrically reduces the cross-sectional area, directly lowering bending and compression capacity.

NSCP Provisions & Adjustment Factors

The NSCP utilizes the Allowable Stress Design (ASD) philosophy for timber. The tabulated base (reference) design values for a given wood species are derived under idealized, standardized conditions (typically dry use, normal load duration). Because real-world structural applications vary widely, these reference values must be modified using adjustment factors.

Allowable Stress Design Equation

The fundamental requirement in timber design is that the actual induced stress must not exceed the adjusted allowable stress (

F'

). The adjusted allowable stress is calculated by multiplying the reference design value (

F

) by all applicable adjustment factors.

$$
F' = F \\times (C_D \\cdot C_M \\cdot C_t \\cdot C_F \\cdot C_r \\dots)
$$

Factor Application

Not all adjustment factors apply to every stress state. For example, the Size Factor (

C_F

) primarily affects bending (

F_b

), tension (

F_t

), and compression parallel to grain (

F_c

), but typically does not apply to shear (

F_v

) or compression perpendicular to grain (

F_{c\perp}

Common Adjustment Factors

Checklist

Load Duration Factor ( $C_D$ ): Wood exhibits an incredible ability to support higher loads for short periods. This is a unique characteristic differentiating timber from steel or concrete. A timber member under a long-term permanent load (like dead load) will eventually fail at a stress level lower than its short-term capacity. This factor adjusts allowable stresses upward for short-duration loads and downward for permanent loads, based on the shortest duration load in a combination.
- Permanent Dead Load: $C_D = 0.90$
- Normal (10-year) Floor Live Load: $C_D = 1.00$
- Two-month Snow Load: $C_D = 1.15$
- Seven-day Roof Live Load: $C_D = 1.25$
- Wind or Earthquake (10 minutes): $C_D = 1.60$
- Impact Load (Seconds): $C_D = 2.00$
Wet Service Factor ( $C_M$ ): Moisture significantly weakens wood. Tabulated values assume a Moisture Content (MC) $\le 19\%$ . If the member will be used in service conditions where MC exceeds 19% (e.g., outdoors, unseasoned wood), a reduction factor ( $C_M \lt 1.0$ ) must be applied.
Temperature Factor ( $C_t$ ): Sustained elevated temperatures degrade wood strength. If the wood will experience sustained temperatures exceeding $38^\circ\text{C}$ ( $100^\circ\text{F}$ ), a reduction factor is required.
Size Factor ( $C_F$ ): Used for visually graded dimension lumber and timbers. As the depth of a bending member or the size of a tension/compression member increases, the statistical probability of encountering a strength-reducing defect (like a knot or slope of grain) increases. Thus, allowable stresses decrease as member size increases.
Volume Factor ( $C_V$ ): Specifically applies to Glulam bending members. It accounts for the statistical effect of volume on the member's strength, reducing the allowable bending stress as the volume increases.
Flat Use Factor ( $C_{fu}$ ): Applies when dimension lumber is loaded flatwise (on its wide face) instead of edgewise. Because the depth is significantly smaller, the probability of a defect at the extreme fiber is lower, so the allowable bending stress increases ( $C_{fu} \gt 1.0$ ).
Incising Factor ( $C_i$ ): Applies to wood that has been mechanically incised (punctured with small cuts) to allow preservative treatments to penetrate deeper into refractory species. Incising reduces the structural strength, requiring a reduction factor ( $C_i \lt 1.0$ ).
Repetitive Member Factor ( $C_r$ ): Applies to bending members (like joists, rafters, or studs). When three or more parallel members are spaced closely together (typically $\le 610$ mm or 24 inches on center) and joined by load-distributing elements (like sheathing), they share loads and exhibit greater systemic reliability. For such systems, bending stress is increased by $15\%$ ( $C_r = 1.15$ ).

Key Takeaways

Reference design values from NSCP must be adjusted for actual conditions using modification factors.
Load duration ( $C_D$ ) can significantly increase allowable stresses for short-term loading conditions like wind or seismic.
Environmental and service conditions (e.g., wet service, temperature) usually reduce the allowable capacity.
The Volume Factor ( $C_V$ ) applies uniquely to Glulam, whereas the Size Factor ( $C_F$ ) applies to solid sawn lumber. Both reduce strength with increasing member size.

Engineered Wood Products

While sawn lumber is fundamental, modern timber engineering increasingly relies on engineered wood products (EWPs). These materials are manufactured by binding together wood strands, veneers, lumber, or other forms of wood fiber with adhesives to form larger, stronger, and more structurally reliable composite materials.

Types of Engineered Wood Products

Glued Laminated Timber (Glulam): Composed of multiple layers of dimensional lumber bonded together with moisture-resistant structural adhesives. Glulam allows for the creation of massive, long-span beams and curved arches that would be impossible with solid sawn timber. It distributes natural defects (like knots) throughout the member, resulting in higher and more reliable design stresses.
Cross-Laminated Timber (CLT): A structural panel product made by gluing layers of solid-sawn lumber perpendicular to adjacent layers. This alternating grain direction provides excellent dimensional stability, high in-plane and out-of-plane strength, and allows CLT to be used as load-bearing walls, floors, and roofs in mass timber buildings.
Laminated Veneer Lumber (LVL): Manufactured by bonding thin wood veneers together under heat and pressure, with the grain of all veneers running parallel to the longitudinal axis. LVL is exceptionally strong, straight, and dimensionally stable, commonly used for high-strength headers, beams, and edge-forming material.

Wood Preservation and Treatments

To increase the natural durability of timber against decay fungi, marine borers, and wood-destroying insects (like termites), structural wood is often chemically treated.

Treatment Methods and Structural Impact

Preservative Treatments: Chemicals like CCA (Chromated Copper Arsenate), ACQ, or Creosote are forced deep into the wood cells using pressure cylinders. While these protect the wood, the waterborne treatment process often requires redrying the wood, and the high pressure/temperature can slightly reduce the mechanical strength (accounted for in the Incising Factor, $C_i$ ).
Fire-Retardant Treatments (FRT): Chemicals applied to reduce surface flame spread. Because FRT chemicals are often hygroscopic and the treatment involves high temperatures, structural design values for FRT wood are typically reduced by 10% to 20% compared to untreated wood.

Specific Gravity and Moisture Content

The physical properties of timber, such as density and moisture, drastically affect structural performance.

G

Density as a Strength Indicator

The structural strength and stiffness of solid sawn lumber, glulam, and structural composite lumber are strongly correlated with their specific gravity (

G

) and overall density.

Specific Gravity: The ratio of the weight of the wood material (oven-dry weight) to the weight of an equal volume of water. Because wood cell wall material is essentially the same across all species (specific gravity $\\approx 1.5$ ), the macroscopic specific gravity essentially dictates how much solid wood material exists in a given volume versus hollow air space (lumens).
Strength Correlation: Denser woods (higher $G$ ) like Yakal have thicker cell walls, resulting in significantly higher bending capacity, compressive strength, and crucially, greater fastener-holding capacity (dowel bearing strength).

Moisture Content (MC) and Density

Wood is a hygroscopic material, meaning it absorbs and releases moisture to reach equilibrium with its environment (Equilibrium Moisture Content, EMC).
The Fiber Saturation Point (FSP) is approximately 25-30% MC. Below the FSP, changes in moisture cause shrinkage or swelling and significantly alter strength.

Fire Resistance of Timber

Despite being combustible, heavy timber (and mass timber products like CLT) possess excellent fire resistance. The outer layer chars at a predictable rate (typically 0.6 mm/min), insulating the inner core and allowing the member to retain its structural integrity far longer than unprotected steel in a fire event.

Key Takeaways

Engineered wood products (EWPs) like Glulam, CLT, and LVL overcome natural defects found in solid sawn lumber.
They offer superior strength, dimensional stability, and the ability to span longer distances or act as mass timber elements.
Timber structural design heavily relies on physical and mechanical properties specific to the wood species (e.g., Yakal, Guijo, Apitong).
Wood is an orthotropic material; its strength characteristics vary dramatically depending on the load direction relative to the grain.
The NSCP provides essential reference design values that must be modified using specific adjustment factors to determine the adjusted allowable stress ( $F'$ ).
Key adjustment factors include Load Duration ( $C_D$ ), Wet Service ( $C_M$ ), Temperature ( $C_t$ ), Size ( $C_F$ ), and Repetitive Member ( $C_r$ ). Proper application is critical for safe design.

PreviousARCHE 4: Steel and Timber Design - Theory & Concepts

NextModule 1: Introduction to Timber Engineering - Examples & Applications

Properties of Philippine Wood

Wood as an Orthotropic Material

Hygroscopicity

Anisotropic Shrinkage and Swelling

Lumber Grading & Species Grouping

NSCP Species Grouping

Visual vs. Machine Stress Grading

Grading Methods

Growth Characteristics and Defects

Physical Defects and Structural Grading

Common Structural Defects in Timber

NSCP Provisions & Adjustment Factors

Allowable Stress Design Equation

Factor Application

Common Adjustment Factors

Checklist

Engineered Wood Products

Types of Engineered Wood Products

Wood Preservation and Treatments

Treatment Methods and Structural Impact

Specific Gravity and Moisture Content

Specific Gravity (GGG)

Density as a Strength Indicator

Moisture Content (MC) and Density

Fire Resistance of Timber

Specific Gravity ( $G$ )