Definição e significado de lumber

The examples and perspective in this article may not represent a worldwide view of the subject. Please improve this article and discuss the issue on the talk page. (December 2010)

  Timber in storage for later processing at a sawmill

  Wood cut from Victorian Eucalyptus regnans

Lumber (also known as timber) is wood in any of its stages from felling to readiness for use as structural material for construction, or wood pulp for paper production.

Lumber is supplied either rough or finished. Besides pulpwood, rough lumber is the raw material for furniture-making and other items requiring additional cutting and shaping. It is available in many species, usually hardwoods.^{[citation needed]} Finished lumber is supplied in standard sizes, mostly for the construction industry, primarily softwood from coniferous species including pine, fir and spruce (collectively known as Spruce-pine-fir), cedar, and hemlock, but also some hardwood, for high-grade flooring.

  Terminology

In the United Kingdom and other Commonwealth Countries such as Australia and New Zealand, timber is a term used for sawn wood products, such as floor boards, whereas generally in the United States and Canada, it refers to felled trees, later milled into boards referred to as lumber. "Timber" is also used there to describe sawn lumber not less than 5 inches (127 mm) in its smallest dimension.^[1] An example of the latter is the often partially finished lumber used in timber-frame construction.

In the United Kingdom the word lumber has several other meanings, including unused or unwanted items.

  Dimensional lumber

  Example of 2×6.

Dimensional lumber is a term used for lumber that is finished/planed and cut to standardized width and depth specified in inches. Examples of common sizes are 2×4 (also two-by-four and other variants, such as four-by-two in the UK, Australia, New Zealand), 2×6 (pictured), and 4×4. The length of a board is usually specified separately from the width and depth. It is thus possible to find 2×4s that are four, eight, or 12 feet in length. In the United States and Canada the standard lengths of lumber are 6, 8, 10, 12, 14, 16, 18, 20, 22, and 24 feet. For wall framing, "stud," or "precut" sizes are available, and commonly used. For an eight, nine, or ten foot ceiling height, studs are available in 92 5/8 inches, 104 5/8 inches, and 116 5/8 inches. (Because the term "stud" is used inconsistently when referring to length, care should be taken to always specify the exact, actual length required.) 

Solid dimensional lumber typically is only available up to lengths of 24 ft. Engineered wood products, manufactured by binding the strands, particles, fibers, or veneers of wood, together with adhesives, to form composite materials, offer more flexibility and greater structural strength than typical wood building materials.^[2]

Pre-cut studs save a framer a lot of time as they are pre-cut by the manufacturer to be used in 8 ft, 9 ft & 10 ft ceiling applications, which means they have removed a few inches of the piece to allow for the sill plate and the double top plate with no additional sizing necessary.

In the Americas, two-bys (2×4s, 2×6s, 2×8s, 2×10s, and 2×12s), along with the 4×4, are common lumber sizes used in modern construction. They are the basic building block for such common structures as balloon-frame or platform-frame housing. Dimensional lumber made from softwood is typically used for construction, while hardwood boards are more commonly used for making cabinets or furniture.

Lumber's nominal dimensions are given in terms of green (not dried), rough (unfinished) dimensions. The finished size is smaller, as a result of drying (which shrinks the wood), and planing to smooth the wood. However, the difference between "nominal" and "finished" lumber size can vary. So various standards have specified the difference between nominal size, and finished size, of lumber.

Early standards called for green rough lumber to be of full nominal dimension when dry, but the requirements have diminished over time. For example, in 1910, a typical finished 1-inch- (25 mm) board was ¹³⁄₁₆ in (21 mm). In 1928, that was reduced by 4%, and yet again by 4% in 1956. In 1961, at a meeting in Scottsdale, Arizona, the Committee on Grade Simplification and Standardization agreed to what is now the current U.S. standard: in part, the dressed size of a 1 inch (nominal) board was fixed at ³⁄₄ inch; while the dressed size of 2 inch (nominal) lumber was reduced from 1 ⁵⁄₈ inch to the now standard 1 ¹⁄₂ inch.^[3]

  Grades and standards

Individual pieces of lumber exhibit a wide range in quality and appearance with respect to knots, slope of grain, shakes and other natural characteristics. Therefore, they vary considerably in strength, utility and value.

The move to set national standards for lumber in the United States began with publication of the American Lumber Standard in 1924, which set specifications for lumber dimensions, grade, and moisture content; it also developed inspection and accreditation programs. These standards have changed over the years to meet the changing needs of manufacturers and distributors, with the goal of keeping lumber competitive with other construction products. Current standards are set by the American Lumber Standard Committee, appointed by the Secretary of Commerce.^[4]

Design values for most species and grades of visually graded structural products are determined in accordance with ASTM standards, which consider the effect of strength reducing characteristics, load duration, safety and other influencing factors. The applicable standards are based on results of tests conducted in cooperation with the USDA Forest Products Laboratory. Design Values for Wood Construction, which is a supplement to the ANSI/AF&PA National Design Specification® for Wood Construction, provides these lumber design values, which are recognized by the model building codes. A summary of the six published design values—including bending (Fb), shear parallel to grain (Fv), compression perpendicular to grain (Fc-perp), compression parallel to grain (Fc), tension parallel to grain (Ft), and modulus of elasticity (E and Emin) can be found in Structural Properties and Performance^[5] published by WoodWorks.

Canada has grading rules that maintain a standard among mills manufacturing similar woods to assure customers of uniform quality. Grades standardize the quality of lumber at different levels and are based on moisture content, size and manufacture at the time of grading, shipping and unloading by the buyer. The National Lumber Grades Authority (NLGA)^[6] is responsible for writing, interpreting and maintaining Canadian lumber grading rules and standards. The Canadian Lumber Standards Accreditation Board (CLSAB)^[7] monitors the quality of Canada's lumber grading and identification system.

Attempts to maintain lumber quality over time have been challenged by historical changes in the timber resources of the United States—from the slow-growing virgin forests common over a century ago to the fast-growing plantations now common in today's commercial forests. Resulting declines in lumber quality have been of concern to both the lumber industry and consumers and have caused increased use of alternative construction products^[8][9]

Machine stress-rated and machine-evaluated lumber is readily available for end-uses where high strength is critical, such as truss rafters, laminating stock, I-beams and web joints. Machine grading measures a characteristic such as stiffness or density that correlates with the structural properties of interest, such as bending strength. The result is a more precise understanding of the strength of each piece of lumber than is possible with visually graded lumber, which allows designers to use full-design strength and avoid overbuilding.^[10]

In Europe, strength grading of sawn softwood is done according to EN-14081-1/2/3/4 and sorted into 9 classes; In increasing strength these are: C14, C16, C18, С22, С24, С27, С30, С35 and С40^[11]

• C14 Used for Scaffolding or Formwork
• C24 General construction
• C30 Prefab Rooftrusses and where design requires somewhat stronger joists than C24 can offer
• C40 Usually seen in Glulam

  Hardwoods

In North America, sizes for dimensional lumber made from hardwoods varies from the sizes for softwoods. Boards are usually supplied in random widths and lengths of a specified thickness, and sold by the board-foot (144 cubic inches or 2,360 cubic centimetres, ¹⁄₁₂th of 1 cubic foot or 0.028 cubic metres. This does not apply in all countries; for example, in Australia many boards are sold to timber yards in packs with a common profile (dimensions) but not necessarily consisting of the same length boards.

Also in North America, hardwood lumber is commonly sold in a "quarter" system when referring to thickness. 4/4 (four quarters) refers to a 1-inch-thick (25 mm) board, 8/4 (eight quarters) is a 2-inch-thick (51 mm) board, etc. This system is not usually used for softwood lumber, although softwood decking is sometimes sold as 5/4 (actually one inch thick).

Hardwoods cut for furniture are cut in the fall and winter, after the sap has stopped running in the trees. If hardwoods are cut in the spring or summer the sap ruins the natural color of the timber and decreases the value of the timber for furniture.

  Engineered lumber

Engineered lumber is lumber created by a manufacturer and designed for a certain structural purpose. The main categories of engineered lumber are:^[12]

1. Laminated Veneer Lumber (LVL) – LVL comes in 1 ³⁄₄ inch thicknesses with depths such as 9 ¹⁄₂, 11 ⁷⁄₈, 14, 16, 18, or 24 inches, and are often doubled or tripled up. They function as beams to provide support over large spans, such as removed support walls and garage door openings, places where dimensional lumber isn't sufficient, and also in areas where a heavy load is bearing from a floor, wall or roof above on a somewhat short span where dimensional lumber isn't practical. This type of lumber cannot be altered by holes or notches anywhere within the span or at the ends, as it compromises the integrity of the beam, but nails can be driven into it wherever necessary to anchor the beam or to add hangers for I-joists or dimensional lumber joists that terminate at an LVL beam.
2. Wood I-Joists – Sometimes called "TJI","Trus Joists" or "BCI", all of which are brands of wood I-joists, they are used for floor joists on upper floors and also in first floor conventional foundation construction on piers as opposed to slab floor construction. They are engineered for long spans and are doubled up in places where a wall will be aligned over them, and sometimes tripled where heavy roof-loaded support walls are placed above them. They consist of a top and bottom chord/flange made from dimensional lumber with a webbing in-between made from oriented strand board (OSB). The webbing can be removed up to certain sizes/shapes according to the manufacturer's or engineer's specifications, but for small holes, wood I-joists come with "knockouts", which are perforated, pre-cut areas where holes can be made easily, typically without engineering approval. When large holes are needed, they can typically be made in the webbing only and only in the center third of the span; the top and bottom chords cannot be cut. Sizes and shapes of the hole, and typically the placing of a hole itself, must be approved by an engineer prior to the cutting of the hole and in many areas, a sheet showing the calculations made by the engineer must be provided to the building inspection authorities before the hole will be approved. Some I-joists are made with W-style webbing like a truss to eliminate cutting and allow ductwork to pass through.
   

     Freshly cut logs showing sap running from beneath bark
3. Finger-Jointed Lumber – Solid dimensional lumber lengths typically are limited to lengths of 22 to 24 feet, but can be made longer by the technique of "finger-jointing" lumber by using small solid pieces, usually 18 to 24 inches long, and joining them together using finger joints and glue to produce lengths that can be up to 36 feet long in 2×6 size. Finger-jointing also is predominant in precut wall studs. It is also an affordable alternative for non-structural hardwood that will be painted (staining would leave the finger-joints visible). Care must be taken during construction to avoid nailing directly into a glued joint as stud breakage can occur.
4. Glu-lam Beams – Created from 2×4 or 2×6 stock by gluing the faces together to create beams such as 4×12 or 6×16. By gluing multiple, common sized pieces of lumber together, they act as one larger piece of lumber - thus eliminating the need to harvest larger, older trees for the same size beam.
5. Manufactured Trusses – Trusses are used in home construction as a pre-fabricated replacement for roof rafters and ceiling joists (stick-framing). It is seen as an easier installation and a better solution for supporting roofs as opposed to the use of dimensional lumber's struts and purlins as bracing. In the southern USA and other parts, stick-framing with dimensional lumber roof support is still predominant. The main drawback of trusses are reduced attic space, time required for engineering and ordering, and a cost higher than the dimensional lumber needed if the same project were conventionally framed. The advantages are significantly reduced labor costs (installation is faster than conventional framing), consistency, and overall schedule savings.

  Timber piles

In the US pilings are mainly cut from Southern Yellow Pine (SYP) and Douglas Fir (DF). Treated Piling are available in CCA retentions of .60, .80, and 2.50 pcf (pounds per cubic foot) if treatment is required.

  Defects in lumber

Defects occurring in lumber are grouped into the following five divisions:

  Conversion

During the process of converting timber to commercial form the following defects may occur:

1. Chip mark: this defect is indicated by the marks or signs placed by chips on the finished surface of timber
2. Diagonal grain: improper sawing of timber
3. Torn grain: when a small depression is made on the finished surface due to falling of some tool
4. Wane: presence of original rounded surface in the finished product

  Defects due to fungi

Fungi attack timber when these conditions are all present:

1. The timber moisture content is above 25% on a dry-weight basis
2. The environment is warm enough
3. Air is present

Wood with less than 25% moisture (dry weight basis) can remain free of decay for centuries. Similarly, wood submerged in water may not be attacked by fungi if the amount of oxygen is inadequate.

Fungi timber defects:

1. Blue stain
2. Brown rot
3. Dry rot
4. Heart rot
5. Sap stain
6. Wet rot
7. White rot

  Insects

Following are the insects which are usually responsible for the decay of timber:

1. Beetles
2. Marine borers (Barnea similis)
3. Termites
4. Carpenter ants

  Natural forces

There are two main natural forces responsible for causing defects in timber: abnormal growth and rupture of tissues.

  Seasoning

Defects due to seasoning are the number one cause for splinters and slivers.

  Durability and service life

Under proper conditions, wood provides excellent, lasting performance. However, it also faces several potential threats to service life, including fungal activity and insect damage—which can be avoided in numerous ways. Section 2304.11 of the International Building Code (IBC) addresses protection against decay and termites. This section provides requirements for non-residential construction applications, such as wood used above ground (e.g., for framing, decks, stairs, etc.), as well as other applications.

There are four recommended methods to protect wood-frame structures against durability hazards and thus provide maximum service life for the building. All require proper design and construction:

1. Control moisture using design techniques to avoid decay. 2. Provide effective control of termites and other insects. 3. Use durable materials such as pressure treated or naturally durable species of wood where appropriate. 4. Provide quality assurance during design and construction and throughout the building’s service life using appropriate maintenance practices.

  Moisture control

Wood is a hygroscopic material, which means it naturally absorbs and releases water to balance its internal moisture content with the surrounding environment. The moisture content of wood is measured by the weight of water as a percentage of the oven-dry weight of the wood fiber. The key to controlling decay is to control moisture. Once decay fungi are established, the minimum moisture content for decay to propagate is 22 to 24 percent, so building experts recommend 19 percent as the maximum safe moisture content for untreated wood in service. Water by itself does not harm the wood, but rather, wood with consistently high moisture content enables fungal organisms to grow.

The primary objective when addressing moisture loads is to keep water from entering the building envelope in the first place, and to balance the moisture content within the building itself. Moisture control by means of accepted design and construction details is a simple and practical method of protecting a wood-frame building against decay. Finally, for applications with a high risk of staying wet, designers should specify durable materials such as naturally decay-resistant species or wood that’s been treated with preservatives. Cladding, shingles, sill plates and exposed timbers or glulam beams are examples of potential applications for treated wood.

  Controlling termites and other insects

For buildings in termite zones, basic protection practices addressed in current building codes include (but are not limited to) the following:

• Grade the building site away from the foundation to provide proper drainage. • Cover exposed ground in any crawl spaces with 6-mil polyethylene film and maintain at least 12 to 18 inches of clearance between the ground and the bottom of framing members above (12 inches to beams or girders, 18 inches to joists or plank flooring members). • Support post columns by concrete piers so there’s at least six inches of clear space between the wood and exposed earth. • Install wood framing and sheathing in exterior walls at least eight inches above exposed earth; locate siding at least six inches from the finished grade. • Where appropriate and desired, ventilate crawl spaces according to local building codes. • Remove building material scraps from the job site before backfilling. If termites are found, eliminate their nests. • If allowed by local regulation, treat the soil around the foundation with an approved termiticide to provide protection against subterranean termites.

  Preservatives

  Special fastners should be used with treated lumber because of the corrosive chemicals used in its preservation process

To avoid decay and termite infestation, it is important to separate untreated wood from the ground and other sources of moisture. These separations are required by many building codes and are considered necessary to maintain wood elements in permanent structures at a safe moisture content for decay protection. When it is not possible to separate wood from the sources of moisture, designers often rely on preservative-treated wood.^[13]

Wood can be treated with a preservative that improves service life under severe conditions without altering its basic characteristics. It can also be pressure-impregnated with fire-retardant chemicals that improve its performance in a fire.^[14] One of the early treatments to fireproof lumber which retard fires was developed in 1936 by Protexol Corporation in which lumber is heavily treated with salt.^[15] Wood does not deteriorate just because it gets wet. When wood breaks down, it is because an organism is eating it as food. Preservatives work by making the food source inedible to these organisms. Properly preservative-treated wood can have 5 to 10 times the service life of untreated wood. Preserved wood is used most often for railroad ties, utility poles, marine piles, decks, fences and other outdoor applications. Various treatment methods and types of chemicals are available, depending on the attributes required in the particular application and the level of protection needed.^[16]

There are two basic methods of treating: with and without pressure. Non-pressure methods are the application of preservative by brushing, spraying or dipping the piece to be treated. Deeper, more thorough penetration is achieved by driving the preservative into the wood cells with pressure. Various combinations of pressure and vacuum are used to force adequate levels of chemical into the wood. Pressure-treating preservatives consist of chemicals carried in a solvent. Chromated copper arsenate (CCA), once the most commonly used wood preservative in North America began being phased out of most residential applications in 2004. Replacing it are amine copper quat (ACQ) and copper azole (CA).

All wood preservatives used in the U.S. and Canada are registered and regularly re-examined for safety by the U.S. Environmental Protection Agency and Health Canada's Pest Management and Regulatory Agency, respectively.^[17]

  Timber framing

This unreferenced section requires citations to ensure verifiability.

Timber framing is a style of construction which uses heavier framing elements than modern stick framing, which uses dimensional lumber. The timbers originally were tree boles squared with a broadaxe or adze and joined together with joinery without nails. A modern imitation with sawn timbers is growing in popularity in the United States.

  Residual wood

The conversion from coal to biomass power is a growing trend in the United States.^[18]

The U.S. and Canadian governments both support an increased role for energy derived from biomass, which are organic materials available on a renewable basis and include residues and/or byproducts of the logging, sawmilling and papermaking processes. In particular, they view it as a way to lower greenhouse gas emissions by reducing consumption of oil and gas while supporting the growth of forestry, agriculture and rural economies. Studies by the U.S. government have found the country’s combined forest and agriculture land resources have the power to sustainably supply more than one-third of its current petroleum consumption.^[19]

Three potentially large sources of forest biomass currently not being used in abundance are harvesting residues, particularly those left at the roadside, thinning treatments done in conjunction with efforts to reduce forest fire hazards (mostly in the U.S.), and salvage and recovery of beetle-killed timber (mostly in Canada).

Biomass is already an important source of energy for the North American forest products industry. It is common for companies to have cogeneration facilities, also known as combined heat and power, which convert some of the biomass that results from wood and paper manufacturing to electrical and thermal energy in the form of steam. The electricity is used to, among other things, dry lumber and supply heat to the dryers used in paper-making.

  Remanufactured lumber

Remanufactured Lumber refers to secondary or tertiary processing/cutting of previously milled lumber. The term specifically refers to lumber cut for industrial or wood packaging use. Lumber is cut by ripsaw or resaw to create dimensions that are not usually processed by a primary sawmill.

Resawing is the process of splitting 1 inch through 12 inch hardwood or softwood lumber into two or more thinner pieces of full length boards. For example, splitting a ten foot 2x4 into two ten foot 1x4s is considered resawing.

In addition to resawing lumber, remanufactured lumber can be ripped on a ripsaw using single or multiple blades. Ripping is the process of splitting 1" through 12" hardwood or softwood lumber into two or more narrower pieces of full length boards. For example, splitting a ten foot 2x4 into two ten foot 2x2s is considered ripping.^[20]

  Environmental benefits of lumber

Green building minimizes the impact or "environmental footprint" of a building. Wood is a major building material that is renewable and uses the sun’s energy to renew itself in a continuous sustainable cycle.^[21] Studies show manufacturing wood uses less energy and results in less air and water pollution than steel and concrete.

  See also

  References

  Further reading

• Sathre, R; O'Conner, J (2010). A Synthesis of Research on Wood Products and Greenhouse Gas Impacts (2 ed.). FPInnovations. ISBN 978-0-86488-546-3. http://www.forintek.ca/public/pdf/Public_Information/technical_rpt/TR19%20Complete%20Pub-web.pdf. 

  External links

Look up lumber or timber in Wiktionary, the free dictionary.

• National Hardwood Lumber Association (Rules for Grading Hardwood Lumber - Inspector Training School)
• Timber Development Association of NSW - Australia
• TRADA: Timber Research And Development Association
• The Forest Products Laboratory. US main wood products research lab. Madison, WI (E)
• WCTE, World Conference on Timber Engineering　June 20–24, 2010, Riva del Garda, Trentino, Italy
• Canadian Wood Council