Structural Design of Timber Structures 1
- 1. CIV 4141 STRUCTURAL TIMBER AND MASONRY DESIGN
- 2. Structural Timber Design • Objective: To introduce the inherent botanical and structural characteristics of timber and to illustrate the process of design for structural timber elements.
- 3. Introduction • Timber is one of the oldest known materials of construction. • The inherent variability of timber, which makes it unique in its structure and mode of growth, results in characteristics and properties which are distinct and more complex than those of other common structural materials such as concrete, steel and brickwork.
- 4. • As with any material, it is necessary for those using it to know its nature and limitations. • With man-made materials, standards of manufacture and design codes readily guide users, specifiers and designers through the limitations of the particular material.
- 5. • Timber is a natural material which is not subjected to pre-use factory processing. • This makes it even more necessary that the limitations of use dictated by its natural form be fully understood. • Timber is considered as a material inexhaustible in supply.
- 6. Sources of Timber • Timber is sourced from mature trees, of whatever type. • The Engineer should have a knowledge of the nature and growth pattern of trees in order to understand the behavior, under a variety of circumstances, of those timber elements used in the construction industry.
- 7. Growth Structure • A tree is considered in the form of its three subsystems: • The roots, • The trunk, • The crown. • Each subsystem has a role to play in the growth pattern of the tree.
- 8. • These roles are quite distinct but equally important in their integrated contribution to the quality of the tree, which in turn directly dictates the quality of the marketable timber and its related cost.
- 9. Figure 1: Growth Structure
- 10. The Root • The function of the root system is two fold: • It absorbs moisture-containing minerals from the soil for transfer via the trunk to the crown. • The spread of the roots through the soil acts as an anchorage, or foundation, which enables the growing tree to withstand wind forces.
- 11. The Trunk • The trunk provides rigidity, mechanical strength and height to maintain the crown at a level above ground at which it functions efficiently. • In addition, the trunk contains and protects the growth cells and provides a two-way transport system for moisture travelling up from the roots and sap travelling down from the crown.
- 12. The Crown • The function of the crown is to provide as large a ‘catchment area’ as possible covered with leaves which contain chlorophyll. • In the leaves the photosynthetic process takes place whereby light is used to break up the carbon dioxide absorbed from the air. • In the resultant process, oxygen is released and the carbon combines with the sap from sugar, cellulose and other carbohydrates on which the growth of the tree depends.
- 13. • Each subsystem is important, it is with the trunk that specifiers and users need to be most concerned since it is within the trunk that the marketable timber exists. • The periphery of the trunk is formed of bark, which comprises of an outer bark and an inner bark (also known as bast).
- 14. • The outer bark is rough in texture and dense enough in consistence to provide a protective coat covering the vital growth and food layers immediately inside it. • The inner bark is soft, moist and spongy and transports the converted sap from the leaves to the growing parts of the tree.
- 15. • Between the inner bark and the actual growing timber is a thin layer of cells called the cambium. • It is here that growth takes place by the splitting of single cells into two cells, each of which grows and splits in a process which continues throughout the growing season, eventually forming a sheath of cells which in cross section appears as a ring, referred to as an ‘annual ring’.
- 16. • These cells, which make-up the wood tissue, or xylem, on the inner side of the cambium, are tubular in shape, with diameters between about 0.02 and 0.50mm, and vary in length from about 1 mm in hardwoods to 6 mm in softwoods.
- 17. • (Hardwood is a type of wood that comes from deciduous trees, which are trees that shed their leaves annually. • Hardwoods are generally denser and harder than softwoods. • Examples of hardwood trees include: Oak (Quercus spp.), Maple (Acer spp.), Walnut (Juglans spp.), Cherry (Prunus spp.), Mahogany (Swietenia spp.))
- 18. • (Softwood is a type of wood that comes from coniferous trees, which are trees that produce cones and have needle-like leaves. • Examples of softwood trees include: Pine (Pinus spp.), Spruce (Picea spp.), Fir (Abies spp.), Cedar (Cedrus spp.), and Cypress (Taxodium spp.))
- 19. • They provide means of conducting and storing food and also provide mechanical support. • In many species of trees, each annual ring appears to have two layers. • The inner layer consists of cells with comparatively large cavities and thin walls. • This cellular structure is due to a more rapid spring growth and is referred to as springwood.
- 20. • (Springwood is a type of wood that is formed during the early part of the growing season, typically in the spring.) • Springwoods are characterized by: • 1. Wide cells 2. Thin walls 3. Less dense)
- 21. • Later in the year, cells grow more slowly and have thicker walls and smaller cavities, resulting in a heavier, harder and stronger material called the summerwood. • The amount of summerwood may vary in different species of tree and as a result of different weather and soil conditions.
- 22. • (Summerwood is a type of wood that is formed during the late part of the growing season, typically in the summer. • It is characterized by 1. Narrow cells, 2. Thicker walls, 3. Denser)
- 23. • This affects the overall density of the timber, which has a direct relationship with the strength of the timber. • Other groups of cells, known as medullary rays, run at right angles to the main cells from the outer layers inward. • These carry food material from the inner bark to the cambium and act as storage tanks for the food material by transporting the excess towards the center of the tree, where it is stored in cells in the inner rings which cease to function as a live part of the tree.
- 24. • This older timber is known as heartwood and is usually darker in colour as well as being drier and harder than the living layer, known as sapwood. • Heartwood is composed of dead tissue, its cells being composed of dead tissue, its cells being completely filled, and its function is the mechanical support of the tree.
- 25. Heartwood • (1. Inner layer: Heartwood is the inner, older layer of wood in a tree trunk. • 2. Dead cells: It is composed of dead, inactive cells that no longer conduct water and nutrients • 3. Darker color: Heartwood is typically darker in color than sapwood • 4. More durable: Heartwood is generally more durable and resistant to decay than sapwood • 5. Less prone to warping: Heartwood is less prone to warping and shrinkage than sapwood.)
- 26. • Sapwood contains more moisture and is not as strong (in the green state) as heartwood, but after seasoning, when both heartwood and sapwood are reduced to the moisture content, the difference in density and strength is very small. • Sapwood is inferior to heartwood in respect of durability, containing starches which may attract insects and provide food for fungal growth.
- 27. Sapwood • (1. Outer layer: Sapwood is the outer, younger layer of wood in a tree trunk. • 2. Living cells: It is composed of living, active cells that conduct water and nutrients. • 3. Lighter color: Sapwood is typically lighter in color than heartwood. • 4. More susceptible to decay: Sapwood is more susceptible to decay and insect damage than heartwood. • 5. More prone to warping: Sapwood is more prone to warping and shrinkage than heartwood.)
- 28. • Sapwood is very permeable and more easily impregnated with preservative and, where the conditions under which the timber must serve are such that treatment with a preservative is essential, it may be beneficial to use sapwood as a deliberate choice.
- 29. Tree Classification • The botanical name for those plants which grow outwardly, acquiring a new sheath of cellular tissue during each growing season is exogens, and this classification can be sub-divided into: • 1. Angiosperms, or Dicotyledons, which have broad leaves shed in the autumn and which are normally classed as hardwoods. • 2. Gymnosperms, or Conifers, which have needle- like leaves, broadly evergreen, and which are generally classed as softwoods.
- 30. • It should be noted that the terms hardwood and softwood in relation to a species of tree do not necessarily indicate relative hardness or density; • balsa, for example, although soft in texture and easily worked, is a hardwood by classification whereas yew, a softwood by classification, has a density six times that of balsa.
- 31. • The ease with which Conifers are grown and their speed of growth make softwoods commercially much cheaper than hardwoods, which have a slower rate of growth.
- 32. • Most of the constructional timber used in building work is of the softwood classification, ideally stress-graded and used in sizes which are suited to the task on hand, for maximum cost benefit.
- 33. Conversion of Timber • Conversion is the process whereby the felled trunk is converted into marketable sizes of timber. • Since the trunk is not of the same diameter over its length, it is not possible to obtain the same amount of marketable timber over the entire length of the trunk.
- 34. Characteristics of Timber • The principal characteristics of timber are: • Strength • Durability • Finished appearance • All of these are derived from natural characteristics present in the growing tree.
- 35. Strength • Strength of timber is affected by the following factors: • Density • Moisture content • Grain structure
- 36. • Density is almost certainly an indication of strength: the more dense the timber the stronger it is.
- 37. • Some other characteristics which influence design and are specific to timber are: • the difference in strength when loads are applied parallel and perpendicular to the grain direction, • the duration of the application of the load, • the method adopted for strength grading of the timber.
- 38. Moisture Content • The behaviour of timber is significantly influenced by the existence and variation of its moisture content. • The moisture content, as determined by oven drying of a test piece, is defined in Annex H of BS 5268 as: • w = 100()/ • where: ( is the mass of the test piece before drying (in g), is the mass of the test piece after drying (in g).
- 39. • Moisture contained in ‘green’ timber is held both within the cells (free water) and within the cell walls (bound water). • The condition in which all free water has been removed but the cell walls are still saturated is known as the fibre saturation point (FSP). • At levels of moisture above the FSP, most physical and mechanical properties remain constant.
- 40. • Variations in moisture content below the FSP cause considerable changes to properties such as: • weight, • strength, • elasticity, • shrinkage and • durability.
- 41. SEASONING • Seasoning of timber is the process of drying timber to remove excess moisture, reducing the risk of decay, warping, cracking, and improved stability. • There are two methods generally used: • 1. Natural Seasoning Methods • 2. Artificial Seasoning Methods
- 42. Natural Seasoning Methods • Air Seasoning: Timber is stacked in a well- ventilated area, allowing it to dry naturally. • Open Shed Seasoning: Timber is stored in an open shed, protected from the elements but still allowing air circulation. • Covered Shed Seasoning: Timber is stored in a covered shed, providing better protection from the elements.
- 43. Artificial Seasoning Methods • 1. Kiln Seasoning: Timber is dried in a controlled environment, using heat and ventilation to speed up the drying process. • 2. Dehumidification Seasoning: Timber is dried using a dehumidification system, which removes moisture from the air. • 3. Vacuum Seasoning: Timber is dried in a vacuum chamber, where the air pressure is reduced to speed up the drying process. • 4. Radio Frequency Seasoning: Timber is dried using radio frequency energy, which heats the wood and speeds up the drying process.
- 44. • The anisotropic nature of timber and differential drying out caused by uneven exposure to drying agents such as wind, sun or applied heat can result in a number of defects such as • twisting, • cupping, • bowing and cracking, as shown in Figure 1.
- 45. Figure1: Distortions due to differential directional shrinkage
- 46. Defects in Timber • In addition to the defects indicated in Figure 1 there are a number of naturally occurring defects in timber. • The most common and familiar of such defects is a knot (see Figure 2). • Normal branch growth originates near the pith of a tree and consequently its base develops new layers of wood each season which develop with the trunk.
- 47. Figure 2: Defects due to knots
- 48. • The presence of knots reduces the strength of timber. • The reduction in strength is primarily due to the distortion of the grain passing around the knots and the large angle between the grain of the knot and the piece of timber in which it is present. • During the seasoning of timber, checks often develop around the location of knots.
- 49. • In a mill when timber is converted from a trunk into suitable commercial sizes, a wane can occur when part of the bark or rounded periphery of the trunk is present in a cut length, as shown in Figure 4. • The effect of a wane is to reduce the cross- sectional area with a resultant reduction in strength.
- 50. Figure 3
- 51. • A shake is produced when fibres separate along the grain: this normally occurs between the growth rings, as shown in Figure 4. • The effect of a shake in the cross-section is to reduce the shear strength of beams; • it does not significantly affect the strength of axially loaded members.
- 52. Grading of Timber • Grading of timber is the process of evaluating and classifying timber into different grades based on its quality, characteristics, and intended use. • The goal of grading is to ensure that timber meets the required standards for strength, durability, and appearance.
- 53. Methods of Grading • 1. Visual Grading: This method involves visually inspecting the timber for defects, such as knots, cracks, and warping. • 2. Machine Grading: This method uses machines to measure the density, moisture content, and other properties of the timber. • 3. X-Ray Grading: This method uses X-ray technology to detect internal defects in the timber.
- 54. • 4. Acoustic Grading: This method uses sound waves to detect defects in the timber. • 5. Grading by Measurement: This method involves measuring the timber’s dimensions, such as length, width, and thickness.
- 55. Grades of Timber • Timber can be graded into several categories, including: • 1. Structural Grade: This grade is used for timber that will be used in load-bearing applications, such as beams and joists. • 2. Appearance Grade: This grade is used for timber that will be used in applications where appearance is important, such as furniture and flooring.
- 56. • 3. Utility Grade: This grade is used for timber that will be used in applications where strength and durability are not critical, such as packaging and pallets. • 4. Industrial Grade: this grade is used for timber that will be used in industrial applications, such as pulp and paper production.
- 57. • Visual defects considered when assessing timber strength include: • location and extent of knots, • slope of grain, • rate of growth, fissures, • wane, • distortions such as bowing, springing, twisting, cupping, resin and bark pockets, and insect damage.