Prepared by: Engr. John Le Carre A. Enerlan, CIT University, CEA-CE_1401 Source: Notes of Engr. Larry C. Almonte

CE 523 - TIMBER DESIGN

Characteristic of Wood Trees are either deciduous, having broad leaves and usually shedding them in the fall, or

coniferous having needles and cones containing seeds. In the wood industry the deciduous trees are called hardwoods and the coniferous are called softwoods. Hardwoods are generally harder than softwoods, but there are many exceptions. The oaks, for instance, are very hard, whereas the basswood, classified among the hardwoods, is extremely soft. On the other hand, the Southern yellow pine and Douglas fir, although classified as softwoods, are, in realty, two of the hardest woods.

The trees used for lumber are exogenous, that is, they increase in size by a growth of new wood on the outer surface under the bark. The cross section of a tree trunk reveals the layers of new wood that are formed annually. These layers, called annual rings, are frequently composed of light and dark layers. The light ring, being grown in the spring of the year is called the springwood, while the darker ring called the summerwood. Thus, the number of annual rings at the base of a tree indicates the age of the tree.

Wood consists of long, narrow, hollow cells called fibers. Scientifically, wood is called xylem. New fibers grow at the outside of the tree, increasing the diameter layer by layer. The thin, growing layer is called the cambium layer. The sapwood is within the cambium layer, and its fibers are active in the life processes of the tree, but they do not grow. Sapwood is the band of rings at the outer edge of the trunk. It carries the sap from the roots to the leaves. As the tree ages, the sapwood gradually changes to heartwood and new sapwood is formed. The heartwood at the center of the trunk consists of dead fibers. The heartwood usually darker in color than the sapwood because of the resin, gums and minerals it contains. At the center is a thin vein of soft tissue called pith extending the length of the tree having no strength. Outside the cambium layer is the bark, which protects the growing cells from the insects but is useless as a structural material.

The fibers of all wood specie consist of a structurally sound material called cellulose (approximately 70% of the volume) cemented with lignin. Paper is made of the cellulose from wood.

The structure of trees consists of longitudinal bundles of wood fibers or cells. Smaller bands of fibers, called medullary or wood rays, radiate from the center of the tree trunk and serve to bind the structure together.

Defects in Lumber:

Workability – is the ability of a fresh (plastic) concrete mix to fill the form/ mold properly with the desired work (vibration) and without reducing the concrete’s quality.

Any irregularity in wood that affects it strength and durability is a defect. The most common defects:

1. Check – a crack or lengthwise separation of the wood across the annual rings. It generally arises from the process of seasoning. Seasoning is the process of reducing the moisture until a suitable moisture level is reached.

2. Shake – a separation along the grain principally between the annual rings. Both check and shake reduces the resistance of wood to shear.

3. Split – separation of the wood due to the tearing apart of wood cells. 4. Knot – a portion of a branch or limb incorporated in the body of a tree. The strength

of a member is affected by the location and the size of the knots it contains.

Prepared by: Engr. John Le Carre A. Enerlan, CIT University, CEA-CE_1401 Source: Notes of Engr. Larry C. Almonte

5. Wane – the term applied to bark, or absence of wood or bark, on the edge or corner of a piece of sawed wood. In specifying, wane maybe avoided by requiring material that is square edge.

6. Pitch Pocket – an opening parallel to the annual rings containing pitch, either solid or liquid.

7. Cup and Crook – distortions caused by unequal shrinkage during seasoning.

Decay – the disintegration of wood substance due to the action of wood-destroying fungi. The growth of fungi is encouraged by air, moisture, and a favorable temperature.

Green wood – are wood which containing substantial moisture.

The four major enemies of wood:

1. Insects (Termites) 2. Marine Borders 3. Fungi 4. Fire

Classification of Lumber: Lumber – the product of the saw and planning mill not further manufactured than by sawing, re-sawing and passing lengthwise through a standard planning machine, crosscut to length and matched. Those pieces 1 ½ inches or less in thickness and 2 inches or more in width are boards. Pieces at least 2 inches thick and less than 5 inches or more wide are dimension lumber. Pieces 5 inches or more in thickness and width are timbers. The three classification of softwood lumber:

1. Yard Lumber - Comprises the material less than 5 inches thick used for general building

purposes. It include boards and siding less than 2 inches thick, flooring laths, shingles, pickets, finishing material, planks less than 4 inches, and joist 4 inches or less thick. Studding and the common sizes of joist and rafters are included in this classification.

2. Structural Material - Includes lumber (except joist and planks) 5 inches or more in thickness and

width. They are generally referred to as timbers. Under the classification of structural material: A. Joist and Planks – 2 inches to 4 inches thick or more wide B. Beams and Stringers – 5 inches or more thick and 8 inches or more wide C. Post and Timber – 5 inches x 5 inches and larger

3. Factory and Shop Lumber - Includes factory plank graded for door, sash and other cuttings 1 ¼ inches or

more thick and 5 inches or more wide. It is used for general millwork and other industrial commodities.

Lumber is finished in one of several ways. Rough lumber remains as sawed on all four sides with no further finishing. Dressed lumber or surfaced lumber is planned or surfaced on at least one face. It is designated as S1S if surfaced on one edge. The abbreviations S2S, S1E, S1S2E and S4S are used for other combinations of sides and edges that are surfaced. Worked lumber is dressed and also worked to provide tongue and groove or shiplap joints and/or to change the cross section in some other way.

Quantities of lumber are measured and sold by the by the foot board measure (fbm). One board foot is a quantity one foot square by one 1 inch thick. It is equal to a volume of 144 cubic inches.

Prepared by: Engr. John Le Carre A. Enerlan, CIT University, CEA-CE_1401 Source: Notes of Engr. Larry C. Almonte

EXAMPLE:

If lumber costs Php 20.00 per board foot, how much will a 5 lengths - 2” x 4” x 6’ cost you?

SOLUTION:

Volume of Lumber = b x h x L x number of lengths

= (2 inch)(4 inch)(6 ft) x (ଵଶ ୧୬ୡ୦ୣୱଵ ୲

) x 5 lengths

= 2 880 in3

Convert Volume of Lumber to board foot = 2 880 in3 x ( ଵ ୠୢ.୲ ଵସସ ୡ୳ୠ୧ୡ ୧୬ୡ୦ୣୱ

)

Convert Volume of Lumber to board foot = 20 bd.ft

Cost of lumber = Volume of lumber in board foot x unit price of lumber

Cost of lumber = 20 bd.ft x Php 20.00/bd.ft

Cost of lumber = Php 400.00

Types of Problems

1. Design – given the load, determine the size 2. Investigation – given the size, determine the load

Criteria for Design:

1. Safety 2. Economy 3. Practicability/ Aesthetics

I. Flexural Members

- Structural members subjected to transverse loads. They include floor joist, ceiling joist, stringers, girders, purlins, girts, etc.

Modes of Failure:

1. Bending Stress 2. Horizontal Shear Stress 3. Deflection