Timber Design

TIMBER DESIGN1. Discuss the following terms:

a.) Cellulose Cellulose is the substance that makes up most of a plant's cell walls. Since it is made by all

plants, it is probably the most abundant organic compound on Earth. It is a long chain of linked sugar molecules that gives wood its remarkable strength. It is the main component of plant cell walls, and the basic building block for many textiles and for paper. Cotton is the purest natural form of cellulose. In the laboratory, ashless filter paper is a source of nearly pure cellulose.

b.) Lignin Lignin is an organic substance binding the cells, fibres and vessels which constitute wood and

the lignified elements of plants, as in straw. It is a polyphenolic polymer that strengthens and waterproofs the cell wall of specialized plant cell types. After cellulose, it is the most abundant renewable carbon source on Earth. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity and do not rot easily. Chemically lignins are cross-linked phenol polymers.

c.) Wood Wood is a hard, fibrous structural tissue found in the stems and roots of trees and other

woody plants. It has been used for thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers (which are strong in tension) embedded in a matrix of lignin which resists compression.

d.) Lignification Lignification is part of the normal differentiation program and functioning of specific cell types,

but can also be triggered as a response to various biotic and abiotic stresses in cells that would not otherwise be lignifying. It is the process of tissues becoming wood.

e.) Pores A tube-like series of water-conducting cells (with bordered pits) which are axially joined by

perforation plates in the cell end walls. The vessels in cross-sections are called pores.

f.) Cambium Outside the sapwood, close to the surface, is the cambium, a thin layer of living cells. These

cells manufacture the wood as they grow. The cambium is covered by a protective layer of bark.

g.) Moisture Content Water content or moisture content is the quantity of water contained in a material, such as

soil (called soil moisture), rock, ceramics, fruit, or wood. Moisture content can be expressed on wet or dry basis.

h.) EMC (Equilibrium Moisture Content) The equilibrium moisture content (EMC) is the moisture content at which the wood is neither

gaining nor losing moisture; this however, is a dynamic equilibrium and changes with relative

humidity and temperature. The moisture content of wood below the fibre saturation point is a function of both relative humidity and temperature of surrounding air.

i.) Orthotropic Orthotropic material is one that has different material properties or strengths in different

orthogonal directions (such as glass-reinforced plastic, or wood).

j.) Board Foot Board foot is the unit of measure for the volume of rough lumber (before drying and planing

with no adjustments) or planed/surfaced lumber. It is the volume of a one-foot length of a board one foot wide and one inch thick.

2. Differentiate and illustrate/draw the following:

a. Hardwoods and Softwoods Hardwood: wood of an angiospermous tree (such as oak, red cedar, or maple) that is heavy,

durable, and hard Softwood: wood of a coniferous tree (such as fir, hoop pine, or pine) that is soft and easy to

cut Regardless of weight or hardness, hardwoods are technically defined as those woods

having vessels (pores), while softwoodsare defined as those not having vessels.

b. Heartwood and Sapwood Heartwood: It is made up of dead cells that no longer serve any purpose except to support the

tree. Heartwood is the central, supporting pillar of the tree. Although dead, it will not decay or lose strength while the outer layers are intact.

Sapwood: In the growing tree, these are the living cells that conduct water, plant sugars, and mineral salts from the roots to the crown. It is often the zone of lighter coloured wood outside the heartwood, adjacent to the bark.

c. Earlywood and Latewood Seasonal growth patterns produce growth rings in some species.

Earlywood: Formed at the start of the growth cycle, lighter in colour and has lower density than latewood.

Latewood: Formed late in a growth cycle, darker in colour and has denser than earlywood.

d. Annual Rings ad Medullary Rings Annual rings: In climates where temperature limits the growing season of a tree, each annual

increment of growth usually is readily distinguishable. It consists of an earlywood and latewood bands.

Medullary rings: Medullary rays are ribbon-like bundles of cells arranged in a radial direction in the tree, that is, they are perpendicular to the annual rings, running from the center (pith) toward the bark.

e. Nominal and Actual Sizes Nominal size: Rough lumber / Nominal size is the commercial designation of width and depth.

It is also the lumber that has been sawed but has not had its four longitudinal surfaces finished saw marks show on each surface.

Actual size: Dressed lumber / Actual size is the net size/dimension of the timber that has been dressed or planned smooth, and is sometimes that. A lumber whose surfaces have been finished (by planning or other means).

f. Allowable and Actual Stress Allowable stress: Also called safe working stress; it is the stress obtained or determined by a

series of tests/experiments. Actual stress: Actual stress is the stress induced by the applied loads on the structural

member which has to be computed by the designer.

3. Enumerate the following:

a. Types of failure under compression parallel to graino Crushingo Wedge Splittingo Shearingo Splittingo Crushing and Splittingo Brooming or End Rolling

b. Factors affecting the strength and stiffness of wood Timbers with similar strength properties are grouped for structural design purposes. All

timbers are therefore classified into strength groups with a seven-class grouping (S1 to S7) for unseasoned timber and an eight-class grouping (SD1 to SD8) for seasoned timber.

c. Strength-reducing defects in timber

d. Drying or seasoning of timber Drying timber to a moisture content range appropriate to the conditions and purposes for

which it is to be used.o Air Drying >> the drying of timber by exposing it to the air. The technique of air drying consists

mainly of making a stack of sawn timber (with the layers of boards separated by stickers) on raised foundations, in a clean, cool, dry and shady place. Rate of drying largely depends on climatic conditions, and on the air movement (exposure to the wind).

o Kiln Drying >> the process of artificial or 'oven' drying consists basically of introducing heat. This may be directly, using natural gas and/or electricity or indirectly, through steam-heated heat exchangers, although solar energy is also possible.

e. Grading of lumber Stress grading is a system for grouping timber in relation to a set of design properties so

that design capabilities can be matched with end-use. A stress grade has an associated suite of design properties including allowable bending stress and characteristic short duration, average modulus of elasticity parallel to the grain. A stress grade of, for example, F8 indicates that for such a grade of material, the characteristic design stress in bending is approximately 25 megapascals (MPa). Visual stress grading is done in accordance with AS 2082-2000 'Visually stress-graded hardwood for structural purposes.'

f. Types of chemical treatment Timber treatment prevents deterioration by insect or fungal attack and preserves the

quality of the timber.

g. Reasons why a factor of safety is required

h. Structural loading Structural loads or actions are forces, deformations, or accelerations applied to a

structure or its components.o Dead Loads >> permanent or stationary loads which are transferred to structure throughout

the life span. Dead load is primarily due to self-weight of structural members, permanent partition walls, fixed permanent equipments and weight of different materials.

o Imposed Loads or Live Loads >> either movable or moving loads without any acceleration or impact. There are assumed to be produced by the intended use or occupancy of the building including weights of movable partitions or furniture etc. The floor slabs have to be designed to carry either uniformly distributed loads or concentrated loads whichever produce greater stresses in the part under consideration. Since it is unlikely that any one particular time all floors will not be simultaneously carrying maximum loading, the code permits some reduction in imposed loads in designing columns, load bearing walls, piers supports and foundations.

o Impact Loads >> caused by vibration or impact or acceleration. Thus, impact load is equal to imposed load incremented by some percentage called impact factor or impact allowance depending upon the intensity of impact.

o Wind Load >> primarily horizontal load caused by the movement of air relative to earth. Wind load is required to be considered in design especially when the heath of the building exceeds two times the dimensions transverse to the exposed wind surface.

o Earthquake Loads >> horizontal loads caused by the earthquake and shall be computed in accordance with IS 1893. For monolithic reinforced concrete structures located in the seismic zone 2, and 3 without more than 5 storey high and importance factor less than 1, the seismic forces are not critical.

i. Types of beams according to supportso Simply supported >> a beam supported on the ends which are free to rotate and have no

moment resistance.o Fixed >> a beam supported on both ends and restrained from rotation.o Over hanging >> a simple beam extending beyond its support on one end.o Double overhanging >> a simple beam with both ends extending beyond its supports on both

ends.o Continuous >> a beam extending over more than two supports.o Cantilever >> a projecting beam fixed only at one end.o Trussed >> a beam strengthened by adding a cable or rod to form a truss