Módulo 2: Materiales de construcción - Madera | es - 680 - 47029
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Module 1: Módulo 2: Materiales de construcción

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Lumber Sources and Uses
Throughout the ages, people have used wood for a variety of purposes: fuel, building materials, weapons, and transportation. It is important to distinguish lumber from wood.

Lumber refers to the board, timbers, etc., produced from sawmills, whereas wood refers to the materials itself, which comes from many species of trees.

Lumber Sources and Uses
Wood has several advantages:
• It is easily worked.
• It has durability and beauty.
• It has great ability to absorb shocks from sudden loads.
• It is free from rust and corrosion, comparatively light in weight, and adaptable to a countless variety of purposes.

General Classifications of Lumber
Lumber falls into one of two general classifications: hardwood or softwood.

Hardwoods come from deciduous (leaf-bearing) trees such as oak and maple.
Softwood comes from coniferous (cone-bearing) trees such as pine and fir. (The term softwood does not mean it is not strong or durable. On the contrary, some softwoods are harder than certain hardwoods.)

Lumber Defects
Before we discuss lumber grading, which is to a large extent based on the amount and types of defects in the lumber, it is necessary to understand the various kinds of defects.

Lumber defects (e.g., knots) can occur naturally. Defects can also occur during the cutting process. Further defects can occur while the lumber is drying, or because of improper storage or handling.

Defects in lumber can affect the lumber’s appearance, strength, and usability. In most common uses, certain amounts of defects are permitted.

Unless it is severely damaged, some use can be found for just about any piece of lumber.
That is why lumber is graded and why there are many grades.

While some lumber defects occur naturally, most defects occur either during the manufacturing and drying process or as a result of improper storage and handling.

Click on the button to find out more:
Lumber Defects: Moisture and Warping
Trees contain a large amount of moisture. For example, a newly cut 10’ length 2 x 10 lumber can contain more than four gallons of water. Because of its high moisture content, freshly cut (green) lumber cannot be used until it is dried. If green lumber is used, it can cause cracked ceilings or floors, squeaky floors, sticking doors, and other problems.


The Importance of Avoiding Lumber Defects
Unless the lumber, plywood, and other materials are properly stacked and protected, they can be seriously damaged.

• The lumber is exposed to the elements. Sun, wind, rain, and snow can create serious problems.
• What would have been good lumber may now be crooked, warped, or cupped.
• Some lumber will crack when strain is placed against it; there is a good possibility of splits.
• It will cost the contractor money to replace the materials.
• The time involved in getting additional material delivered to the job site could affect the schedule.
• There is a safety hazard associated with using damaged materials.
• Additional time required in working with defective materials is costly.

Lumber Grading
Grading is based on the number, size, and type of defects in the lumber, which determine its load-carrying capacity. The more the defects, the weaker the wood.

Lumber is graded based on its strength, stiffness, and appearance. The highest grades have very few defects; the lowest grades may have knots, splits, and other problems.

Softwood lumber is graded in three major categories: boards, dimension lumber, and timbers.

These categories are further divided into specific types of lumber and then classified by appearance and strength.

Click on the terms to see a breakdown of each category:
BOARDS
Boards, sheathing, and form lumber
DIMENSION LUMBER
Light framing, studs, structural light framing, and structural joists and planks

TIMBERS
Beams and stringers

Lumber Grading Terms
Grading Terms:

Grading involves using five basic size classifications. These classifications are as follows:

• Boards (BD)
• Light framing (L.F.)
• Joists and planks (J&P)
• Beams and stringers (B&S)
• Posts and timbers (P&T)

Other Terms Used in Lumber Grading
Classification establishes several stress-quality grades.

Some other lumber terms used in grading include:

• Nominal size
• Dressed (actual) size
• Dressed lumber
• Dimension lumber
• Matched lumber
• Patterned lumber

• Rough lumber
• Stress-grade lumber
• Surfaced lumber
• Framing lumber
• Finish lumber
• Select lumber

Lumber Trimming
Trimming is the act of crosscutting a piece of lumber to a given length.

• Double-end-trimmed (DET) lumber is trimmed reasonably square by a saw on both ends.

• Precision-end-trimmed (PET) lumber is trimmed square and smooth on both ends.

• Square-end-trimmed lumber is trimmed square.





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