Daily Science Sharing on Modern Timber Structures - Trunk Composition and Utilization

This issue introduces the processing and utilization of each part of the tree trunk. For information on knots and other related topics, please refer to previous issues.

Traditionally, the size of structural timber components is limited by the size of the parent tree. Typically, the largest available sawn timber component has a cross-sectional dimension of 0.30 meters and a length of 6.0 meters. During the processing of trees into timber, characteristics that reduce strength (most commonly knots and grain patterns) are observed, which determine the types of timber products suitable for them (Figure 2).

Radial Section: A section cut along the tree's growth direction, through the pith, and perpendicular to the growth rings is called a radial section.

Tangular Section: A section cut along the tree's growth direction but not through the pith is called a tangential section. Growth rings on a tangential section exhibit a "V" shaped pattern.

Bark: Removing the bark from timber is crucial for maximizing timber utilization; any timber put into a slicer must be bark-free. However, bark can be used as fuel and soil mulch.

Sapwood: The outer part of the timber with the fewest knots. Knot-free timber is typically made into boards or thick planks ranging from one to three inches in thickness.

Heartwood: Located near the center of the wood, knots are more abundant, making this section less suitable for board production. Thicker planks and square or rectangular beams are usually sawn from this part. The center of the wood is primarily used for structural beams; this section is strong enough not to be weakened by knots. Knots are most common here because they are the areas where branches were removed early in the tree, and the knots are covered as the tree grows outward.

🪵Trunk Composition:

(1) Bark: The bark is the tree's protective layer and also transports nutrients during growth.

(2) Cambium: The cambium, located between the inner bark and the xylem, is the tree's growth tissue.

(3) Xylem: The xylem, located between the pith and the cambium, includes sapwood and heartwood. Its solid structure makes it a primary material for construction, etc. Sapwood is closer to the bark and lighter in color; heartwood is closer to the pith and darker in color. (4) Pith: The pith is tubular in shape, running longitudinally through the center of the entire trunk and branches. It is composed of thin cells formed early in the tree's growth, has a soft texture, and is easily decayed.

(5) Medullary Lines: Medullary lines are vertical fibers radiating outward from the pith, running across the annual rings. They can be seen on both cross-sections and longitudinal sections of the trunk. The medullary lines are thick and prominent in some tree species, such as oak, elm, and ash; while in others, they are thinner and less prominent, such as conifers, catalpa, and birch.