Wood Properties and Tree Types

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What is a characteristic of deciduous trees?

They have broad leaves and shed them in the fall

What is the purpose of reducing the stresses in the samples at failure?

To provide a safety factor of approximately 2.5

What is the main composition of wood?

Cellulose

What is the function of the cambium layer?

To produce new fibers that increase the diameter of the tree

What is heartwood composed of?

Dead fibers

What is the purpose of the bark?

To protect the growing cells from insects

What is the purpose of testing small, perfect samples of wood?

To determine the stresses at failure and the modulus of elasticity for each species

What is the safety factor used to reduce the stresses in the samples at failure?

Approximately 2.5

What is the purpose of the National Design Specification for Stress-Grade Lumber and Its Fastenings?

To provide grades established for each species of wood with usable allowable stresses for each grade

What is the standard used for testing small, perfect samples of wood?

ASTM D2555

What is the purpose of the Lumber Classification Standards?

To classify lumber according to appearance, strength, shape, and use

What is the title of the standard that includes basic allowable stresses for clear lumber under long-time service at full design load?

ASTM D245

What is the primary function of the Southern Forest Product Association?

Providing grading rules for pine species in the United States

Which of the following agencies provides grading rules for Sitka spruce and Douglas fir?

The National Lumber Grades Authority

What is the minimum width required for a piece of lumber to be considered a board?

2 in.

What is the term that includes all finished or semi-finished wood shaped with parallel longitudinal surfaces?

Grading Lumber

What is the primary use of hardwood lumber purchased from local mills?

Construction

What is the minimum thickness required for a piece of lumber to be considered Dimension Lumber?

2 in.

What is the primary consideration for grading factory and shop lumber?

Usable and waste ratio

What is the main difference between boards and dimension lumber?

Usage and application

Why is a smaller safety factor justified for repetitive members?

They are assisted by surrounding members

What is the primary characteristic of timbers?

They are used for shoring earthwork or mine tunnels

What is the purpose of stress grading for dimension lumber and timbers?

To assign a grade based on strength

What is the basis for grading boards?

Appearance and defects

What type of defect affects strength and is permitted only in the lower grades?

Shakes

What type of defect is caused by unequal shrinkage during seasoning?

Cup and Crook

Which type of defect is an accumulation of resinous material?

Pitch

What type of defect is a lengthwise separation of the wood caused by seasoning or flattening of cupped lumber?

Checks

What type of defect is a result of bark or missing wood on an edge or corner of a piece of lumber?

Wane

Which type of defect is not permitted in all grades?

Shakes

Study Notes

Wood Characteristics

  • Wood has universal appeal and is generally pleasant to work with, regardless of how it is cut or finished.
  • Wood has a pleasant odor when freshly cut.

Types of Trees

  • Deciduous trees (hardwoods) have broad leaves that shed in the fall.
  • Coniferous trees (softwoods) do not shed their leaves.

Tree Structure

  • Fibers consist of long, narrow, hollow cells that grow outward from the tree, increasing its diameter.
  • The cambium layer is a thin, growing layer between the bark and sapwood.
  • Heartwood is composed of dead fibers and has a darker shade due to resins, gums, and minerals.
  • Pith is a thin vein of soft tissue extending the length of the tree, with no strength.
  • Bark protects the growing cells from insects but has no structural value.

Wood Properties

  • Cellulose is a structurally sound material that makes up all wood species.
  • Allowable stresses for lumber are determined by testing small, perfect samples and reducing the stresses at failure to provide a safety factor of approximately 2.5.
  • Moduli of elasticity are published in ASTM D245.

Lumber Grading

  • The National Design Specification for Stress-Grade Lumber and Its Fastenings includes grades established for each species of wood with usable allowable stresses for each grade.
  • Lumber classification standards have been established for the classification of lumber according to appearance, strength, shape, and use.
  • Grading rules are provided by organizations such as the Southern Forest Product Association and the National Lumber Grades Authority.

Lumber Types

  • Dimension lumber is used for joists, studs, and rafters.
  • Timbers are used infrequently as part of a structure, and are often used for shoring earthwork or mine tunnels.
  • Boards are used for roofs, floors, siding, paneling, and trim.

Factors in Grading Lumber

  • Stains, checks, shakes, cup, crook, wane, splits, pitch, and pockets are all considered in grading lumber.
  • These factors affect appearance and strength, and are permitted to some extent in all grades.

Test your knowledge of wood properties and types of trees, including deciduous and coniferous trees. Learn about the characteristics of wood and how it is classified. Improve your understanding of allowable stresses for lumber and safety factors.

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