LUMBER USED IN CIVIL ENGINEERING CONSTRUCTION PROJECTS

Design values for lumber are contained in grading rules established by the National Lumber Grades Authority (Canadian), Northeastern Lumber Manufacturers Association, Northern Softwood Lumber Bureau, Redwood Inspection Service, Southern Pine Inspection Bureau, West Coast Lumber Inspection Bureau, and Western Wood Products Association.

The rules and the design values in them have been approved by the Board of Review of the American Lumber Standards Committee. They also have been certified for conformance with U.S. Department of Commerce Voluntary Product Standard PS 20-94 (American Softwood Lumber Standard).

In addition, design values for visually graded lumber may be established in accordance with ASTM D1990, ‘‘Standard Practice for Establishing Allowable Properties for Visually-Graded Dimensional Lumber from In-Grade Tests of Full- Size Specimens.’’

Design values for visually graded timbers, decking, and some species and grades of dimension lumber are based on provisions of ‘‘Establishing Structural Grades and Related Allowable Properties for Visually Graded Lumber,’’ ASTM D245.

ASTM D245 also specifies adjustments to be made in the strength properties of small clear specimens of wood, as determined in accordance with ‘‘Establishing Clear Wood Strength Values,’’ ASTM D2555, to obtain design values applicable to normal conditions of service.

The adjustments account for the effects of knots, slope of grain, splits, checks, size, duration of load, moisture content, and other influencing factors. Lumber structures designed with working stresses derived from D245 procedures and standard design criteria have a long history of satisfactory performance.

Design values for machine stress-rated (MSR) lumber and machine-evaluated lumber (MEL) are based on nondestructive tests of individual wood pieces. Certain visual-grade requirements also apply to such lumber.

The stress rating system used for MSR lumber and MEL is checked regularly by the responsible grading agency for conformance with established certification and quality-control procedures.

ORGANISMS THAT DEGRADE WOOD USED IN CIVIL ENGINEERING CONSTRUCTION BASIC ENGINEERING TUTORIALS

ORGANISMS THAT DEGRADE WOOD USED IN CIVIL ENGINEERING CONSTRUCTION
What Are The Organism That Degrade Wood Used In Civil Engineering Construction?


Wood can experience degradation due to attack of fungi, bacteria, insects, or marine organisms.

Fungi
Most forms of decay and sap stains are the result of fungal growth. Fungi need four essential conditions to exist: food, proper range of temperature, moisture, and oxygen.

Fungi feed on either the cell structure or the cell contents of woody plants, depending on the fungus type. The temperature range conducive for fungal growth is from 5°C to 40°C (40°F to 100°F). Moisture content above the fiber saturation point is required for fungal growth. Fungi are plants and, as such, require oxygen for respiration.

Fungi attack produces stains and/or decay damage. To protect against fungal attack, one of the four essential conditions for growth needs be removed. The most effective protection measure is to keep the wood dry by correct placement during storage and in the structure.

Fungi growth can also be prevented by treating the wood fibers with chemical poisons through a pressure treatment process.Construction procedures that limit decay in buildings include the following:

1. Building with dry lumber that is free of incipient decay and excessive amounts of stains and molds
2. Using designs that keep the wood components dry
3. Using a heartwood from decay-resistant species or pressure-treated wood in sections exposed to above-ground decay hazards
4. Using pressure-treated wood for components in contact with the ground.

Insects
Beetles and termites are the most common wood-attacking insects. Several types of beetles, such as bark beetles, attack and destroy wood. Storage of the logs in water or a water spray prevents the parent beetle from boring.

Quick drying or early removal of the bark also prevents beetle attack. Damage can be prevented by proper cutting practices and dipping or spraying with an appropriate chemical solution.

Termites are one of the most destructive insect that attacks wood. The annual damage attributed to termites exceeds losses due to fires. Termites enter structures through wood that is close to the ground and is poorly ventilated or wet.

Prevention is partially achieved by using pressure-treated wood and otherwise prohibiting insect entry into areas of unprotected wood through the use of screening, sill plates, and sealing compounds.

Marine Organisms
Damage by marine boring organisms in the United States and surrounding oceans is principally caused by shipworms, pholads, Limnoria, and Sphaeroma. These organisms are almost totally confined to salt or brackish waters.

Bacteria
Bacteria cause “wet wood” and “black heartwood” in living trees and a general degradation of lumber. Wet wood is a water-soaked condition that occupies the stem centers of living trees and is most common in poplar, willows, and elms.

Black heartwood has characteristics similar to those of wet wood, in addition to causing the center of the stem to turn dark brown or black. Bacterial growth is sometimes fostered by prolonged storage in contact with soils.

This type of bacteria activity produces a softening of the outer wood layers, which results in excessive shrinkage when redried. Bacterial attack does not pose a significant problem to common structural wood species.

COMMERCIAL GRADE OF WOODS USED IN CIVIL ENGINEERING CONSTRUCTION BASIC AND TUTORIALS

COMMERCIAL GRADE OF WOODS USED IN CIVIL ENGINEERING CONSTRUCTION BASIC INFORMATION
What Are The Different Commercial Grade Of Woods?

Lumber is graded by the various associations of lumber manufacturers having jurisdiction over various species. Two principal sets of grading rules are employed: (1) for softwoods, and (2) for hardwoods.

Softwoods. 
Softwood lumber is classified as dry, moisture content 19% or less; and green, moisture content above 19%. According to the American Softwood Lumber Standard, softwoods are classified according to use as:

Yard Lumber. Lumber of grades, sizes, and patterns generally intended for ordinary construction and general building purposes.

Structural Lumber. Lumber 2 in or more nominal thickness and width for use where working stresses are required.

Factory and Shop Lumber. Lumber produced or selected primarily for manufacturing purposes.

Softwoods are classified according to extent of manufacture as:


Rough Lumber. Lumber that has not been dressed (surfaced) but has been sawed, edged, and trimmed.

Dressed (Surfaced) Lumber. Lumber that has been dressed by a planning machine (for the purpose of attaining smoothness of surface and uniformity of size) on one side (S1S), two sides (S2S), one edge (S1E), two edges (S2E), or a combination of sides and edges (S1S1E, S1S2, S2S1E, S4S).

Worked Lumber. Lumber that, in addition to being dressed, has been matched, shiplapped or patterned:

Matched Lumber. Lumber that has been worked with a tongue on one edge of each piece and a groove on the opposite edge.


Shiplapped Lumber. Lumber that has been worked or rabbeted on both edges, to permit formation of a close-lapped joint.

Patterned Lumber. Lumber that is shaped to a pattern or to a molded form.

Softwoods are also classified according to nominal size:
Boards. Lumber less than 2 in in nominal thickness and 2 in or more in nominal width. Boards less than 6 in in nominal width may be classified as strips.

Dimension. Lumber from 2 in to, but not including, 5 in in nominal thickness, and 2 in or more in nominal width. Dimension may be classified as framing, joists, planks, rafters, studs, small timbers, etc.

Timbers. Lumber 5 in or more nominally in least dimension. Timber may be classified as beams, stringers, posts, caps, sills, girders, purlins, etc.

Actual sizes of lumber are less than the nominal sizes, because of shrinkage and dressing. In general, dimensions of dry boards, dimension lumber, and timber less than 2 in wide or thick are 1⁄4 in less than nominal; from 2 to 7 in wide or thick, 1⁄2 in less, and above 6 in wide or thick, 3⁄4 in less.

Green-lumber less than 2 in wide or thick is 1⁄32 in more than dry; from 2 to 4 in wide or thick, 1⁄16 in more, 5 and 6 in wide or thick, 1⁄8 in more, and 8 in or above in width and thickness, 1⁄4 in more than dry lumber.

There are exceptions, however. Yard lumber is classified on the basis of quality as:

Appearance. Lumber is good appearance and finishing qualities, often called select.

Suitable for natural finishes
Practically clear
Generally clear and of high quality
Suitable for paint finishes
Adapted to high-quality paint finishes
Intermediate between high-finishing grades and common grades, and partaking somewhat of the nature of both

Common. Lumber suitable for general construction and utility purposes, often given various commercial designations.

For standard construction use
Suitable for better-type construction purposes
Well adapted for good standard construction
Designed for low-cost temporary construction
For less exacting purposes
Low quality, but usable

Structural lumber is assigned modulus of elasticity values and working stresses in bending, compression parallel to grain, compression perpendicular to grain, and horizontal shear in accordance with ASTM procedures.

These values take into account such factors as sizes and locations of knots, slope of grain, wane, and shakes or checks, as well as such other pertinent features as rate of growth and proportions of summerwood.

Factory and shop lumber is graded with reference to its use for doors and sash, or on the basis of characteristics affecting its use for general cut-up purposes, or on the basis of size of cutting. The grade of factory and shop lumber is determined by the percentage of the area of each board or plank available in cuttings of specis determined from the poor face, although the quality of both sides of each cutting must be considered.

Hardwoods.
Because of the great diversity of applications for hardwood both in and outside the construction industry, hardwood grading rules are based on the proportion of a given piece that can be cut into smaller pieces of material clear on one or both sides and not less than a specified size.

Grade classifications are therefore based on the amount of clear usable lumber in a piece. Special grading rules of interest in the construction industry cover hardwood interior trim and moldings, in which one face must be practically free of imperfections and in which Grade A may further limit the amount of sapwood as well as stain.

Hardwood dimension rules, in addition, cover clears, which must be clear both faces; clear one face; paint quality, which can be covered with pain; core, which must be sound on both faces and suitable for cores of glued-up panels; and sound, which is a general-utility grade.

Hardwood flooring is graded under two separate sets of rules: (1) for maple, birch, and beech; and (2) for red and white oak and pecan. In both sets of rules, color and quality classifications range from top-quality to the lower utility grades.

Oak may be further subclassified as quarter-sawed and plain-sawed. In all grades, top-quality material must be uniformed in color, whereas other grades place no limitation on color.

Shingles are graded under special rules, usually into three classes: Number 1, 2, and 3. Number 1 must be all edge grain and strictly clear, containing no sapwood. Numbers 2 and 3 must be clear to a distance far enough away from the butt to be well covered by the next course of shingles.

TYPES OF WOOD BUILDING FRAMING SYSTEMS BASIC AND TUTORIALS

BUILDING FRAME SYSTEM THAT USES WOOD BASIC INFORMATION
What Are The Different Building Framing System Using Wood?


There are various types of framing systems that can be used in wood buildings. The most common type of wood-frame construction uses a system of horizontal diaphragms and vertical shearwalls to resist lateral forces, and specifically with the design of this basic type of building.

At one time building codes classified a shearwall building as a box system, which was a good physical description of the way in which the structure resists lateral forces. However, building codes have dropped this terminology, and most wood-frame shearwall buildings are now classified as bearing wall systems.

It is felt that the designer should first have a firm understanding of the behavior of basic shearwall buildings and the design procedures that are applied to them. With a background of this nature, the designer can acquire from currently available sources the design techniques for other systems.

The basic bearing wall system can be constructed entirely from wood components. See Fig. 1.1. Here the roof, floors, and walls use wood framing.

In addition to buildings that use only wood components, other common types of construction make use of wood components in combination with some other type or types of structural material. Perhaps the most common mix of structural materials is in buildings that use wood roof and floor systems and concrete tiltup or masonry (concrete block or brick) shearwalls.

See Fig. 1.2. This type of construction is common, especially in one-story commercial and industrial buildings. This construction is economical for small buildings, but its economy improves as the size of the building increases.

Trained crews can erect large areas of panelized roof systems in short periods of time. See Fig. 1.3.

Design procedures for the wood components used in buildings with concrete or masonry walls are also illustrated throughout this book. The connections between wood and concrete or masonry elements are particularly important and are treated in considerable detail.

Wind and seismic (earthquake) are the two lateral forces that are normally taken into account in the design of a building. In recent years, design for lateral forces has become a significant portion of the design effort.

The reason for this is an increased awareness of the effects of lateral forces. In addition, the building codes have substantially revised the design requirements for both wind and seismic forces. These changes are the result of extensive research in wind engineering and earthquake-resistant design.

WOOD CHEMICAL COMPOSITION BASICS AND TUTORIALS

CHEMICAL COMPOSITION OF WOOD BASIC INFORMATION
What Are The Chemical Composition Of Wood?

Wood is composed of cellulose, lignin, hemicellulose, extractives, and ash-producing minerals. Cellulose accounts for approximately 50 percent of the wood substance by weight (USDA-FS, 1999).

The exact percent is species dependent. It is a linear polymer (aliphatic carbon compound) having a high molecular weight. The main building block of cellulose is sugar: glucose.

As the tree grows, linear cellulose molecules arrange themselves into highly ordered strands, called fibrils.

These ordered strands form the large structural elements that compose the cell walls of wood fibers. Lignin accounts for 23% to 33% of softwood and 16% to 25% of hardwood by weight.

Lignin is mostly an intercellular material. Chemically, lignin is an intractable, insoluble, material that is loosely bonded to the cellulose. Lignin is basically the glue that holds the tubular cells together.

The longitudinal shear strength of wood is limited by the strength of the lignin bounds.


Hemicelluloses are polymeric units made from sugar molecules. Hemicellulose is different from cellulose in that it has several sugars tied up in its cellular structure.

Hardwood contains 20% to 30% hemicellulose and softwood averages 15% to 20%. The main sugar units in hardwood and softwood are xylose and monnose, respectively.

The extractives compose 5% to 30% of the wood substance. Included in this group are tannins and other polyphenolics, coloring matters, essential oils, fats, resins, waxes, gums, starches, and simple metabolic intermediates.

These materials can be removed with simple inert neutral solvents, such as water, alcohol, acetone, and benzene. The amount contained in an individual tree depends on the species, growth conditions, and time of year the tree is harvested.

The ash-forming materials account for 0.1% to 3.0% of the wood material and include calcium, potassium, phosphate, and silica.

GLUED-LAMINATED STRUCTURAL MEMBERS BASICS AND TUTORIALS

GLUED-LAMINATED STRUCTURAL MEMBERS BASIC INFORMATION
What Are Glued-Laminated Structural Members?

The gluing together of multiple laminations of standard 2-in.-nominal-thickness lumber has been used for many years to produce large beams and girders. This is really the only option for using sawn wood for large members that are beyond the feasible range of size for single sawn pieces.

However, there are other reasons for using the laminated beam that include the following:

Higher Strength. Lumber used for laminating consists of a moisture content described as kiln dried. This is the opposite end of the quality range from the green wood condition ordinarily assumed for solid-sawn members.

This, plus the minimizing effect of flaws due to lamination, permits use of stresses for flexure and shear that are much higher than those allowed for single-piece members. The result is that much smaller sections can often be used, which helps to offset the usually higher cost of the laminated products.

Better Dimensional Stability. This refers to the tendency for wood to warp, split, shrink, and so on. Both the use of the kiln-dried materials and the laminating process itself tend to create a very stable product. This is often a major consideration where shape change can adversely affect the building construction.

Shape Variability. Lamination permits the production of curved, tapered, and other special profile forms for beams. Cambering as compensation for service load deflection, sloping for roof drainage, and other useful custom profiling can be done with relative ease. This is otherwise possible only with a truss or a built-up section.

Laminated beams have seen wide use for many years and industry wide standards are well established. Cross-sectional sizes are derived from the number of laminations and the size of the individual pieces used. Thus depths are multiples of 1.5 in. and widths are slightly less than the lumber size as a result of the finishing of the product.

Minor misalignments and the unavoidable sloppiness of the gluing process result in an unattractive surface. Finishing of the sides of beams may simply consist of smoothing them off, although various special surface textures can also be created.

Investigation and design of glued-laminated timber members are done primarily with the procedures explained for solid-sawn beams as described in Section 4.4. Criteria for design is provided in most building codes, in the National Design Specification (NDS) (Ref. 3), and in the literature provided by manufacturers and suppliers of the products.

These elements are manufactured products and are mostly not able to be transported great distances, so information about them should be obtained from local suppliers. Individual elements of glued-laminated timber can be custom profiled to produce a wide variety of shapes for structures.

For very large elements this is not a problem, but for smaller structures the curvature limits of 2-in.-nominal-thickness lumber may be critical. Manufacturers of laminated products usually produce the arch and gabled elements as standard forms.

Structural design of the products is usually done by the manufacturer’s engineers. Form limits, size range, connection details, and other considerations for these products should be investigated with individual manufacturers.

Custom shapes can be produced, such as those with double curvature. Many imaginative structures have been designed using the form variation potential of this process.

Columns may be produced with 1.5-in. laminations, presenting the same advantages as those described for beams: higher strength and dimensional stability being most critical. It is also possible to produce glued-laminated columns of greater length than that obtainable with solid-sawn members.

In general, laminated columns are used less frequently than beams or girders and are mostly chosen only when a special shape is desired or when some of the inherent limitations of other options are restrictive.

ALLOWABLE STRESS FOR LUMBER USED IN CONSTRUCTION BASICS AND TUTORIALS

ALLOWABLE STRESS FOR LUMBER USED IN CONSTRUCTION BASIC INFORMATION
What Is The Allowable Stress For Lumber Used In Construction?


The National Design Specification for Wood Construction (NDS) (AF&PA, 1997) makes comprehensive recommendations for engineered uses of stress-graded lumber. Stress values for all commercially available species groups and grades of lumber produced in the U.S. are tabulated in the NDS.

The moduli of elasticity for all species groups and grades are also included in these tables. These tabulated values of stresses and moduli of elasticity are called base design values. They are modified by applying adjustment factors to give allowable stresses for the graded lumber.

The adjustment factors reduce (or in some cases increase) the base design stress values to account for specific conditions of use that affect the behavior of the lumber. A list of these adjustment factors and a discussion of their use follows.

Load Duration — CD
The stress level that wood will safely sustain is inversely proportional to the duration that the stress is applied. That is, stress applied for a very short time (e.g., an impact load) can have a higher value than stress applied for a longer duration and still be safely carried by a wood member.

This characteristic of wood is accounted for in determining allowable stresses by using a load duration factor, Cduration factor varies from 20 for an impact load (duration equal to one second) to 0.9 for a permanent load (duration longer than 10 years).

ACI Committee 347 recommends that for concrete formwork, a load duration factor appropriate for a load of 7 days should be used. This corresponds to a value for CD of 1.25. ACI Committee 347 says this load duration factor should only be applied to concrete forms intended for limited reuse.

No precise definition of limited reuse is given by the ACI committee, but the no increase for duration of load should be used for concrete forms designed to be reused a high number of cycles.

Moisture — CM
Wood is affected by moisture content higher than about 19%. Higher moisture content significantly softens the wood fibers and makes it less stiff and less able to carry stresses. The reduction in allowable strength depends on the type of stress (e.g., shear stress is affected less than perpendicular to grain compressive stress) and the grade of the lumber.

Size — CF
Research on lumber allowable stresses has shown that as cross-sectional size increases, allowable stresses are reduced. A size factor, CF, is used to increase base design values for different sizes of lumber.

Repetitive Members — Cr
The NDS allows bending stresses to be increased for beams that share their loads with other beams. The increased allowable stress is referred to as a repetitive member stress.

For a beam to qualify as a repetitive member, it must be one of at least three members spaced no further apart than two feet and joined by a load-distributing element such as plywood sheathing.

When these three requirements are met, the allowable bending stress can be increased by 15%. This corresponds to a value for Cr of 1.15. Repetitive member stresses may be appropriate for some formwork components.

Because the intent of allowing increased stress for repetitive flexural members is to take advantage of the load sharing provided by continuity, gang panels assembled securely by bolting or nailing and intended for multiple reuse would seem to qualify for this increase.

ACI Committee 347 specifies that they should not be used where the bending stresses have already been increased by 25% for short duration loads.

Perpendicular to Grain Compression — Cb
Allowable perpendicular to grain bearing stress at the ends of a beam may be adjusted for length of bearing according to: lb is the length of bearing parallel to grain.

Horizontal Shear Constant — CH
Shear stress in lumber beams used as components of concrete forms is usually highest at the ends of the members. For beams having limited end defects (e.g., splits, checks, cracks), the values of allowable shear stress can be increased. This is done by using a shear constant CH that depends on the size of end defects and varies from 1 to 2.

Temperature — CT
Sustained high temperatures adversely affect some properties of wood. It is unusual for concrete forms to be exposed to temperatures high enough to require the use of a temperature adjustment factor. For temperatures in excess of 100°F, the stresses and moduli should be adjusted using CT.

Stablity — CP
Like all columns, wood shores will safely carry axial loads in inverse proportion to their effective slenderness. The more slender a wood shore is, the less load it will support because of the increased influence of buckling. Prior to the 1997 edition of the NDS, wood columns were divided into three categories Cb = (1b + .375) lb (short, intermediate, and long) according to their slenderness.

Allowable stresses and loads were then found using three different formulas — one for each category. Beginning with the 1997 NDS, allowable loads for all wood columns are found using a stability adjustment factor, CP, that reduces the base stress to account for the buckling tendency of the column.

It is no longer necessary to divide wood shores into three categories to find allowable loads.

DEFECTS IN LUMBER (WOOD) USED IN CIVIL ENGINEERING CONSTRUCTION BASICS AND TUTORIALS

LUMBER DEFECTS (WOOD) - BASIC INFORMATION
What Are the Different Types of Lumber Defects?


Lumber may include defects that affect either its appearance, its mechanical properties, or both. These defects can have many causes, such as natural growth of the wood, wood diseases, animal parasites, too rapid seasoning, or faulty processing. Common defect types are shown in Figure 10.11.

Knots are branch bases that have become incorporated into the wood of the tree trunk or another limb. Knots degrade the mechanical properties of lumber, affecting the tensile and flexural strengths.

Shakes are lengthwise separations in the wood occurring between annual rings. They develop prior to cutting the lumber and could be due to heavy winds.

Wane is bark or other soft material left on the edge of the board or absence of material.

Sap Streak is a heavy accumulation of sap in the fibers of the wood, which produces a distinctive streak in color.

Reaction Wood is abnormally woody tissue that forms in crooked stems or limbs. Reaction wood causes the pith to be off center from the neutral axis of the tree. It creates internal stresses which can cause warping and longitudinal cracking.

Pitch Pockets are well-defined openings between annual rings that contain free resin. Normally, only Douglas fir, pines, spruces, and western larches have pitch pockets.

Bark Pockets are small patches of bark embedded in the wood. These pockets form as a result of an injury to the tree, causing death to a small area of the cambium. The surrounding tree continues to grow, eventually covering the dead area with a new cambium layer.

Checks are ruptures in wood along the grain that develop during seasoning. They can occur on the surface or end of a board. Surface checking results from the differential shrinkage between radial and tangential directions and is confined mostly to planer surfaces. Cracks due to end checking normally follow the grain and result in end splitting.

Splits are lengthwise separations of the wood caused by either mishandling or seasoning.

Warp is a distortion of wood from the desired true plane (see Figure 10.10). The four major types of warp are bow, crook, cup, and twist. Bow is a longitudinal curvature from end to end. Crook is the longitudinal curvature side to side.  Both of these defects result from differential longitudinal shrinkage.

Cup is the rolling of both edges up or down. Twist is the lifting of one corner out of the plane of the other three. Warp results from differential shrinkage, differential drying due to the production environment, or from the release of internal tree stress.

Raised, Loosened, or Fuzzy Grain may occur during cutting and dressing
of lumber.


Chipped or Torn Grain occurs when pieces of wood are scooped out of the
board surface or chipped away by the action of the cutting and planing tools.
Machine Burn is an area that has been darkened by overheating during cutting.