Structural Timber (Part1)

STDSGN2 Structural Timber Design Chapter 1

Introduction; the particular type of tree from which wood comes is called species. Although

there are thousands of species of trees, most structural wood comes from a few dozen species that are selected for commercial lumber processing.

The two groups of tress used for building purposes are the softwoods and hardwoods. Softwoods are coniferous or cone bearing, whereas hardwoods have broad leaves. The terms softwood and hardwood are not accurate indications of the degree of hardness of the various species of trees. Certain softwoods are as hard as medium-density hardwoods, whereas some species of hardwoods have softer wood than some of the softwoods.

Wood as a Structural Material; for structural applications, wood is most commonly found as either sewn

timbers, lumber, or glued laminated members (glulams). In the interest of economy and to permit using wood more efficiently, increasing amounts of wood to day find their way into manufactured structural materials or members such as 1) plywood, hardboard, chipboard, flakeboard, waferboard and plastic/wood laminates (these are known collectively as wood composites); and 2) manufactured members such as plywood-lumber beams or wood trusses.

Problems in Use of Wood for Structures; for most structural applications, fortunately, wood’s advantages far outweigh

its disadvantages. However, structural designers must learn how to cope up with 1) wood’s variability and 2) its response to environmental conditions.

Variability;- probably the more serious of the problems encountered when using timber. Wood

properties vary from species to species, from one position to another in the tree, from one tree to another grown in the same locality and between trees grown in one locality and those grown in another. Generally, humans have little control over the quality of wood a tree produces, although strides are being made in that direction by means of selective tree farming.

Moisture Content;- the moisture content of wood installed in a structure may change with time,

eventually reaching an equilibrium moisture content that depends on the average relative humidity of the surroundings. However, as the relative humidity within a building may not be constant, the equilibrium moisture content may vary with time. With any change of moisture content, wood will either shrink or swell and warp. Collectively, these size and shape changes are known as dimensional instability. Dimension and shape changes due to moisture change can be reduced or avoided by proper seasoning (drying) and by proper attention to details of design.

Duration of Loading;- causes strength changes-the longer a load remains on a wood member, the

weaker the wood member becomes. Luckily, the structural designer can easily consider this problem and compensate for it in the design procedure. Duration of loading has negligible effect on modulus of elasticity but, because of creep, deflections are time-dependent.

Durability;- finally, under some conditions of wood’s durability will be limited.

Weathering, decay, insects or fire can obviously limit the useful life of a wood structure. Yet this problem, too, can be overcome by selecting the proper kind of wood and by proper design, treatment, construction and maintenance.

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Advantages of Wood as a Structural Material; wood’s principal advantages over the other common structural materials are;

Economy;- a wood structure is frequently less costly to construct than a similar

structure in either structural steel, reinforced concrete, precast concrete or masonry. Obviously, a completely valid cost comparison would have to include such costs as operation, maintenance, insurance and the like over the entire life of the structure. However, it should also be logical from observing structures all around that, even, when all of the factors mentioned are considered, wood is frequently the most economical choice for buildings up to three or four stories high.

Appearance;- wood structures can be either aesthetically pleasing or unattractive,

depending entirely on the care that goes into its design. Early colonial structures provide excellent examples of attractive design and of construction details that ensure good durability despite the fact that wood can decay.

Ease of Working and Reworking;- wood can easily be cut, shaped and finished in the field. Prefabrication at

another location is generally not needed, although prefabrication may be employed as a timesaving or money-saving measure. Existing wood structures can be revised or added to more easily than similar structures of any other material.

Durability;- if proper care is taken in details of design, in maintenance and in selecting

suitable preservative and fire retardant treatments, wood will be durable. If untreated wood is allowed to remain moist in service, however, it will decay. The secret to ensuring durability is four-fold;

use proper species and grades avoid undesirable environmental conditions use proper design details use whatever treatment is necessary and economically feasible

Strength-Weight Ratio of Wood;- when wood is used for all components of a structure, the total weight is

often less than for a structure of other materials. This is particularly true in comparison to reinforced concrete. The strength-weight ratio of wood is advantageous wherever dead load is an appreciable part of the total load.

Wood Axes; because of its internal structure, wood is orthotropic, which means that its

properties (physical and mechanical) differ in the three main mutually perpendicular directions – longitudinal, radial and tangential. Strength and modulus of elasticity vary in the three directions and there are six values of Poisson’s ratio. Shrinkage (or swelling) occurring as wood’s moisture content changes also differs in the three directions; this is what may cause wood to warp as it either dries out or takes in additional moisture.

Material Properties of Wood; mechanical properties of wood, determined from small, nearly perfect wood

specimens (small clear specimens) become the bases for the allowable stresses given by design codes and specifications. Of these properties, those concerning strength and stiffness are the interest of the structural designer. However, the designer is also interested in other properties, though on a lesser degree, such

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as creep, durability, hardness, dimensional stability and thermal insulating qualities. The strength and stiffness properties of wood of most interest in structural design are;

- compressive parallel to the grain- modulus of elasticity parallel to the grain- tensile strength parallel to the grain- compressive strength perpendicular to the grain- modulus of rupture (bending strength)- longitudinal shear strength (horizontal shear)- shear modulus

Specific Gravity: all strength and stiffness properties depend on how much cellulose is present,

so it is expected that heavier woods to be both stronger and stiffer than lighter woods.

Time-Dependent Behaviour of Wood;

Creep;- when a wood structural member is loaded, it deflects immediately. If the load

remains on the member, deflection increases with time. This increase above and beyond the initial deflection is called creep deflection. Creep can occur in any type of member (beam, column or tension member) but is of greatest concern for beams.

Load-Duration Effect;- since the length of time that a load is present on a wood member affects the

ultimate or breaking strength of that member, the designer of wood structures must consider that effect. Otherwise, to ensure a desired level of safety, the designer would have to be overly conservative, designing as though all loads were to be in place forever.

Defects in Lumber; any irregularity in wood that affects its strength or durability is called a

defect. Because of the natural characteristics of the material, several common defects are inherent in wood.

Knot;- a portion of a branch or limb that has been surrounded by subsequent growth

of the tree. There are several types and classification of knots and the strength of a structural member is affected by the size and location of those it may contain. The grading rules for structural lumber are specific concerning the number, sizes and position of knots and their presence is considered when establishing the allowable unit stresses.

Shake;- a separation along the grain, principally between the annual rings. Shakes

reduce the resistance to shear and consequently members subjected to bending are directly affected by their presence. The strength of members in longitudinal compression (columns, posts, etc.) is not greatly affected by shakes.

Check;- a separation along the grain, the greater part of which occurs across the

annual rings. Checks generally arise from the process of seasoning. Like shakes, checks also reduce the resistance to shear.

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Split;- defined as the lengthwise separation of the wood that extends through the

piece from one surface to another.

Decay;- the disintegration of wood substance due to the action of wood-destroying

fungi. Decay is easily recognized, for the wood becomes soft, spongy or crumbly. The growth of fungi is encouraged by air, moisture and a favorable temperature. If air is excluded, for instance, when wood is constantly submerged, fungi cannot exist. Wood is often impregnated with preservatives such as coal tar and creosote to prevent growth of fungi. The development of fungi is also prevented by the application of paint to the wood when it is dry. The extent of decay is generally difficult to determine; therefore any form of decay is usually prohibited in structural grades of wood.

Pitch Pocket;- an opening parallel to the annual rings that contains pitch, either solid or

liquid.

Seasoning of Wood; all green wood contains moisture and the serviceability of wood is improved by

its removal. The process of removing moisture from green wood is known as seasoning; it is accomplished by exposing lumber to the air for an extended period or by heating it in kilns. Whether air-dried or kiln dried, seasoned wood is stiffer, stronger and more durable than green wood. The removal of moisture results in the shrinkage of the fiber cells. This shrinkage causes internal stresses that result in checking and warping, but development of these defects can be controlled to some extent by proper seasoning procedures. The moisture content of wood is defined as the ratio of the weight of water in a specimen to the weight of the oven-dry wood expressed as percentage.

Use Classification of Structural Lumber; because the effects of natural defects on the strength of lumber vary with the

type of loading to which an individual piece is subjected, structural lumber is classified according to its size and use. The four principal classifications are defined as follows;

Dimension;- this consists of rectangular cross sections with nominal dimensions, 2” to 4”

thick and 2” or more wide. This classification is further divided into light framing grades, 2” to 4” wide and joists and planks, 5” and wider.

Beams and Stringers;- rectangular cross sections, 5” or more thick and a width more than 2” greater

than the thickness are graded for strength in bending when loaded on the narrow face.

Posts and Timbers;- square or nearly square cross sections with nominal dimensions, 5” x 5” and

larger are graded primarily for use as posts or columns but adapted to other uses where bending strength is not especially important.

Decking;- this consist of lumber, 2” to 4” thick, 6” and wider, with tongue and groove

edges or grooved for spline on the narrow face. Decking is graded for use with the wide face placed flatwise in contact with supporting members.

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Nominal and Dressed Sizes; an individual piece of structural lumber is designated by its nominal cross-

sectional dimensions. However, after being dressed or surfaced on four sides (S4S), the actual dimensions of this piece decreases.

Grading of Structural Lumber; grading is necessary to identify the quality of lumber, which are established

in relation to strength properties and use classification so that allowable stresses for design can be assigned.

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Structural Timber (Part1)