Modern Timber Structures: Daily Educational Insight — Trunk Composition & Utilization

This installment explores the processing and utilization of each specific section of a tree trunk. For details regarding features such as knots—which are referenced herein—please refer to our previous posts.

Traditionally, the dimensions of structural timber components were limited by the size of the parent tree. Typically, the largest sawn timber components currently available feature cross-sections measuring up to 0.30 meters and lengths reaching up to 6.0 meters. During the process of converting trees into lumber, characteristics that reduce strength—most commonly knots and sloping grain—are identified; these characteristics determine the specific types of timber products for which the material is suitable (Figure 2).

**Radial Face (Quarter-sawn)**
A cut surface sawn along the direction of the tree's growth—passing through the pith (center) and perpendicular to the annual growth rings—is known as a radial face.

**Tangential Face (Flat-sawn)**
A cut surface sawn along the direction of the tree's growth—but *not* passing through the pith—is known as a tangential face. On a tangential face, the annual growth rings typically exhibit a distinctive "V-shaped" grain pattern.

**Bark:** Removing the bark from timber is crucial for its efficient utilization; any timber destined for slicing or processing machinery must be completely free of bark. However, bark itself can be repurposed for use as fuel or as a soil mulch.

**Sapwood:** The outer section of the timber contains the fewest knots. This knot-free wood is typically processed into boards or planks ranging in thickness from one to three inches.

**Heartwood:** Located closer to the center of the timber, this section features a higher density of knots and is therefore less suitable for use as standard boards. Thicker planks, as well as square or rectangular beams, are typically sawn from this section. The central core of the timber is primarily utilized for structural beams; this material possesses sufficient inherent strength to withstand the structural compromises that knots might otherwise cause. Knots are most prevalent in this central region because it corresponds to the area where the tree's lower branches were shed or removed during its early years of growth, leaving behind knots that were subsequently encased as the tree continued to grow outward.

🪵 **Composition of the Tree Trunk:**
(1) **Bark**
The bark serves as the tree's protective outer layer and also functions to transport nutrients during the tree's active growth periods.
(2) **Cambium**
Located between the inner layer of the bark and the xylem (wood tissue), the cambium constitutes the tree's primary growth tissue. (3) Xylem
The xylem is situated between the pith and the cambium, comprising both sapwood and heartwood. Possessing a solid structure, it serves as a primary material for construction and similar applications. The sapwood is located closer to the bark and is lighter in color, while the heartwood is situated closer to the pith and is darker in color.
(4) Pith
The pith is tube-shaped and runs longitudinally through the very center of the trunk and branches. It consists of thin-walled cells formed during the tree's early stages of growth; its texture is soft, making it susceptible to decay.
(5) Medullary Rays
Medullary rays are vertical fibers that radiate outward from the pith, extending across the annual rings. They are visible in both the cross-sections and longitudinal sections of the trunk. In certain tree species—such as oak, elm, and ash—the medullary rays are coarse and prominent; in others—such as conifers, catalpa, and birch—they are finer and less conspicuous.