Nothing Special   »   [go: up one dir, main page]

Engr. Miriam B. Villanueva

Download as docx, pdf, or txt
Download as docx, pdf, or txt
You are on page 1of 10

Introduction

A. Timber Used for Structural Members

Timber is the woody materials of trees that are suitable for house construction, bridge and ship
building, furniture and cabinet making, carving and engraving for most structures used in agriculture,
commerce and industries. We refer to trees in the forest as standing timber, or to round or square logs as
round and square timber.

Unlike many building materials, wood is not a processed material but organic material generally
used in its natural state. Wood is that fibrous substance which composes the pith and the bark. More
specifically, wood is defined as the lignified water conducting, strengthening and storage tissues of
branches, stem and roots of trees. Technically, wood is known as “xylem”.

Wood is the most common building materials because of the following properties.

1. A strong material
2. Has durability
3. Light in weight
4. Ease of fastening
5. With artistic and natural beauty

The advantages of wood as a building material are:

1. In proportion to weight, wood is stronger than most materials.


2. Wood is easily worked with tools. It can be fabricated into many shapes, sizes and designs.
3. Wood is excellent non-conductor of heat. It has a special value in making dwellings warm in winter
and cool in summer.
4. The grains and appearance is conductive to artistic and architectural design, adding beauty and
attraction to furniture’s and interior finishes.
5. The use of timber connectors in wide trusses and spans, towers and general constructions permit
the use of small wood members.
6. It is abundant in many shapes, sizes and forms and is a renewable resource.
7. Wood does not deteriorate in value if properly handled and protected.
8. It is not readily by changing styles.
9. It has prompt resale value.
10. Neither heat, cold nor climate changes, may seriously affect wood.

B. Physical Properties of Wood

1. Strength is the general term used in reference to the ability of wood to resist stresses and strain.
Different wood varies materially in the following manner:
a. Wood is resistant to compression along the fibers.
b. Stiffness or the ability to resist bending as in floor joists and beams supporting heavy load.
c. Strength in tension or the ability to resist in lengthwise stresses.
d. Shearing strength or the ability of the fibers to resist rupture along or across the grain.

2. Moisture is an important factor in the strength of wood. Thus, to a certain extent, strength increases
with the degree of seasoning of wood.

Engr. Miriam B. Villanueva 1


Note: Knots or other defects in wood also influence strength. The size, character and location of the
knots are of importance. Example; in cross bending strength, knots on the upper surface of the beam
do not detract from the strength as much as on the lower part of the beam.

3. Weight of wood is also important consideration. Heavy woods are generally strong, light wood are
usually weak.
4. Hardness is expressed as resistance to indentation or to the saw or axe across the grain. Hardness is
dependent largely on weight, structural elements of the wood and degree of seasoning. This feature
is important in several methods of utilization such as flooring, furniture, cross ties, handles and many
small wood articles.

5. Cleavability is the resistance of wood to cleavage along the grains. The line of least resistance in
cleavage is along the radius because the wood rays are in this direction.
Note: Wood splits much more easily when wet because moisture softens the fibers and reduces
adhesion across the grain. Straightens in the grain however, determine the ease with which wood
splits.

6. Flexibility and toughness, moisture content of wood influences flexibility to a considerable degree.
Toughness refers to combined strength, shock resistance and pliability of wood.

7. Durability as applied to wood means the ability to resist decay or simply the length of its life under a
given condition. Durability may also include the resistance of wood to the influence of mechanical
wear.

Relationship between Physical, Mechanical Properties and Durability

Absolutely, there is no direct relation between the physical and mechanical properties of wood
and its durability. One example is the weight, strength, stiffness, hardness or toughness does not seem to
have any influence on the durability of wood. Some of these properties, however, may aid in the
prevention of injurious effects of abrasion on mechanical wear.

However, there is an obvious relation between the color of the heartwood and durability. The
darker the heartwood, the more durable is the wood. Many species with light colored heartwood are
perishable. Therefore, the durability of any species depends on certain chemical component present in
the wood such as:

a. Resins of the wood


b. Gums of the wood
c. Tannin and other decay resisting materials

These give a darker discoloration to the heartwood of several species and this phenomenon
explains the relations of color to durability in wood.

8. Color is often means of identification of wood. As mentioned earlier, heartwood is generally much
darker in color than sapwood. Although in some species there is a very little differentiation in color
between the heartwood and sapwood.

Note: All freshly sawed woods are much lighter in color than when exposed to the air or sunshine for
some time. Oxidation turns all wood to darker shade.

C. Classification of Wood

Engr. Miriam B. Villanueva 2


The wood that are classified as good lumber used in building construction are those that grows
larger by the addition of new layer on the outer surface each year identified by botanists as “oxygen’s”

Some trees grow rapidly depending upon the climate, soil, moisture and food elements. Some
grow up to 1/8” to ½” in diameter per year or more. Whereas, some grow at the rate of only ¼” to ½” in
10 to 15 years or more.

1. Mode of Growth
a. Exogenous are those growing trees which are the most preferred for lumbering.
b. Endogenous are the insides growing trees. This kind of tree is less preferred for lumbering because
of the center core of its log which is soft and brittle in character.

2. Density refers to the quality of the mass and volume of the wood such as:
a. Softwood known also as conifers or evergreen.
b. Hardwood which are generally deciduous and has broad leaves.

3. Leaves .The characteristics of the tree when it comes to its leaves are:
a. Needle shaped
b. Broad shaped

4. Shades or Color of wood. The shades or color of the wood varies from either of the following:
a. White
b. Yellow
c. Orange
d. Red
e. Brown
f. Black, etc.

5. Grain. The grain of wood is classified into:


a. Straight grain
b. Cross grain
c. Fine grain
d. Coarse grain

6. Nature of the surface when sewed.


a. Plain
b. Grained
c. Figured or marked

D. Preparation of Wood

Logging is defined as the harvesting of the tree crops which consist of a sequence of operations
such as:

1. Felling the tree


2. Skidding means that pieces of logs are moved to an assembly area, loaded to transport equipment
then carried out of the forest to a sawmill.
3. Bucking which is the process of sawing into smaller pieces after the removal of the branches.

Engr. Miriam B. Villanueva 3


Previously, all felling and bucking operations were done by the use of motor powered chain saw
due to the following advantages:

1. Lower labor cost


2. Bigger production
3. Time elements

Where trees are too large, long or heavy to be moved without cutting, they are bucked into logs
after they are felled. The logs are then dragged or carried to an assembly area. If they are moved on the
ground, they are said to be skidded. If they are carried on a cable, or by a helium filled balloon, they are
yarded.

Lumbering is the term applied to the operation performed in preparing wood for commercial
purposes. It involves logging which is the process of operation of cutting tress, including the hauling and
delivery to the saw mill.

Sawing, historical record shows that saws for cutting logs into lumber originated about 1500 B.C.
The Romans built the first framed saw, a device that holds a long straight saw in the center of an elongated
frame. This saw is still in use in many parts of the world today. The introduction of water powered straight
saw and later the steam powered circular saw was brought by the early Industrial Revolution.
The Band Saw is a continuous steel band running on wheels which was used in the United States
in 1869. The modern sash gang or gang saw is no more than a motor driven version of the old Roman
frame saw using several blades instead of one.

Of these entire log cuttings, the principal one employed for almost the entire history of logging
industry was the circular saw. On the other hand, the band saw being the thinnest and fastest saw was
used in most small and large sawmills. This is due to their speed and narrow kerf (the path cut through a
log). Some sawmills have band saws with teeth on both sides so that the logs maybe cut while passing in
either directions.

The methods and manner of log sawing employed are the following:

1. The plain or bastard sawing


2. The quarter or rift sawing

The plain or bastard sawing is by cutting the logs entirely through the diameter with a parallel
chord tangential to the annual rings.

The quarter or rift sawing is divided into four methods of sawing:

a. Radial method
b. Tangential method
c. Quarter tangential
d. Combined radial and tangential

Notes:

1. Quarter sawed lumber is less affected by moisture changes, less warping and shows special grains
effects.
2. It has better abrasion and wears resistance.
3. Finishes are more uniform.

Engr. Miriam B. Villanueva 4


4. Quarter sawing is more expensive than the plain sawing because the logs are turned frequently
while being cut. Most of the log woods are wasted in the process.
5. Rotating of the log on the carriage in order to obtain cuts of the highest quality is most commonly
practiced in sawing of hardwood lumber.

After the log is cut, the slab and board fall on a conveyor and conveyed to an edger and end
trimmer where the sides and ends of the board are squared. The board is then graded, sorted and finally
stacked for drying and shipment.

Definitions of Terms

1. Surfaced or Dressed Lumber is a planed lumber having at least one smooth side.
2. S2S and S4S are planed or dressed lumber of which the number connotes the number of smooth
sides. S2S means smooth on two sides and S4S on four sides.
3. Slab is a kind of rough lumber which is cut tangent to the annual rings of wood running the full
length of the log and containing at least one flat surface.
4. Timber is a piece of lumber five inches or larger in its smallest dimension.
5. Plank is a wide piece of lumber from 2 to 5 inches thick.
6. Board is a piece of lumber less than 1 - 1/2” thick and at least 4 inches wide.
7. Flitch is a thick piece of lumber.
8. Fine grained when the annual rings are small, the grain of marking which separates the adjacent
rings is said to be fine grained. When large, it is called coarse grained.
9. Straight grained when the direction of the wood fibers are nearly parallel with the sides and edges
of the board, it is said to be straight grained. When the lumber is taken from a crooked tree, it is
classified to be crooked or coarse grained.
10. Lumber is the term applied to wood after it is sawed or sliced into boards, planks, sticks, etc. for
commercial purposes.
11. Rough lumber is the term applied to unplaned or undressed lumber.
12. Silviculture is the process of growing timber crops of better and more valuable species as rapidly
as possible through scientific forestry.

E. Defects in Wood

Irregularities found in wood are considered as defects. The most common defects are:

1. Abnormal growth are identified as follows:

a. Heart shakes these are radial cracks originating at the heart of the logs.
b. Wind shakes or cup shakes are cracks or breaks across the annual rings of timber during its
growth caused by excessive bending of the tree due to wind.
c. Star shakes are composed of several heart shakes which radiate from the center of the log in
a star-like manner.
d. Knots usually occur at the starting point of a limb or branch of the wood.

2. Due to deterioration caused by:

a. Dry rot is the presence of moisture in wood caused by fungi in seasoned wood.
b. Wet rot usually takes place sometime in the growth of the tree caused by water saturation.

F. Seasoning of Lumber

Engr. Miriam B. Villanueva 5


Seasoning means drying of lumber. Trees when fallen or cut down contains moisture in their cell
layers. This moisture has to be expelled thoroughly to preserve the lumber from shrinkage or decay. Water
contained in freshly cut wood would constitute from one third to more than two thirds of its weight. The
removal of much of this moisture is accomplished by seasoning which is also known as conditioning or
curing.

Experiments has proven that timber which are immersed in water immediately after being fallen
and squared is less subject to splitting and decay. Water reduces warping but makes the wood brittle and
less elastic.

Soaking timber into liquid is the method of seasoning practiced and adopted by the ancient
Roman builders. Woods are sometimes steeped in oil of cedar to protect them from worm attack. Salt
water is also used as wood preservative. It makes the wood harder, heavier and durable. However, the
use of salt water on wood intended for use in building construction has a tendency to attract moisture
from the air and moisture is one enemy of wood.

There are two methods being adopted in seasoning of lumber, they are:

1. Natural or Air Seasoning is considered as one of the best methods adopted in seasoning of lumber
although the period involved is relatively longer than the artificial seasoning.
2. Artificial Seasoning under this method, the lumber is stacked inside a drying kiln and then
subjected to steam and hot air under pressure. Artificial seasoning is the process being adopted
for quick drying of the wood, but wood which undergo this process, is considered quite inferior in
quality as compared to those that are dried by natural seasoning. The different artificial seasoning
methods employed are: force air drying, kiln drying and radio frequency dielectric drying.

G. Wood Decays and the Methods of Preservation

Wood does not decay naturally through age nor will it decay if it is kept constantly dry or
continuously submerged in water. The common causes of decay in wood are: alternate moisture and
dryness, fungi and molds, insect and worms, and heat and confined air.
The essential requirements to achieve successful preservation of woods are through good
seasoning and the correct process of preserving wood enumerated as follows:

1. External Processing is accomplished by coating the wood with preservative chemicals that will
penetrate the fibers.
2. Internal Processes is a chemical compound impregnated at a specified pressure to permeate the
wood thoroughly.

The external non pressure process of preserving wood is the application of penetrating tar oils,
carbolineum, spiritine, solignum, etc. it may be applied on the surface of wood either by brush, spray or
by immersion. External preservatives could only be effective if the wood to be treated is absolutely dry
and well seasoned to absorb a sufficient quality of the chemicals. All tar oil products should be applied
hot.

H. Lumber and Related Products

The important wood related products for commercial purposes are:

Engr. Miriam B. Villanueva 6


1. Veneer and Plywood these are veneer slice laid and bonded together with glue or synthetic
resins. The different types of plywood are: soft plywood, hardwood plywood and exterior or
marine plywood.
2. Hardboard or Pressed wood is made from wood chips which are exploded into fibers with high
pressure steam. The lining in the wood itself binds pressed wood together with fillers or artificial
adhesives applied. Pressed wood is equally strong in all directions but brittle in character. Its color
varies from light to dark brown.
3. Particle Boards are manufactured from wood chips, curls, fibers, flakes, strands, shavings, slivers,
etc. which are bound together and pressed into sheets and other molded shapes. Particle board
has equal strength in all directions of a given cross section area. It is not brittle and can resist
warping.

I. Timber as Structural Materials

Unlike many building materials, wood is not a processed material but an organic material generally
used in its natural state. The most important of the numerous factors that influence its strength are
density, natural defects (knots, checks, slope of grain, etc.) and moisture content. Because the effects of
natural defects on the strength of lumber vary with the type of loading to which an individual member is
subjected. Structural timber is classified according to its size and use.

The Three Major Classification of Structural Timber

1. Joists and planks are rectangular cross – sections with nominal dimensions 2 to 4 inches thick and 4
inches or more wide graded primarily for strength in bending edge wise or flat wise.
2. Beams and stringers are rectangular cross – sections with nominal dimensions 5 inches or more thick
and 8 inches or more wide graded for strength in bending when loaded on the narrow face.
3. Posts and columns are square or nearly square cross – sections with nominal dimensions 5 x 5 inches
and longer graded primarily for use as posts and columns but adopted to either uses where bending
strength is not especially important.

J. Steps in Design and Analysis of Timber design

The first step in the design of beams is the computation of the load or loads. The beam will be required
to support. The design of a beam consist firstly the determination of the dimensions of a cross – sections
in which the extreme fiber stress does not exceed the allowable stress for the material used. This is what
we mean when we say the beam is first designed for bending or flexure. Having a determined a cross –
section to meet this requirements, we then investigate it to see that the allowable horizontal shearing
stress is not exceeded. If the shearing stress is found to be excessive, a large section is required.

Next the beam is investigated with respect to deflection to see that the actual deflection will not
exceed the prescribed limit. When a cross – sections that satisfies the requirements has been determined,
the dimensions of the end bearings are determined to provide that the stressed in compression
perpendicular to the grain do not exceed the allowable stresses given in the building code.

1. Design for Bending

The design of a wood beam for strength is bending is accomplished by the use of the flexure formula:

Mc
fb =
I

Engr. Miriam B. Villanueva 7


I
But S =
c
Then,
M
fb =
S

Flexure Stress Diagram


fc

d
2
d N.A.

d
2
ft

6M
The form equation used in design is fb = bd2

Where: M = maximum bending moment, kN – m


Fb = allowable extreme fiber stress , MPa
fb = actual flexure stress in extreme fibers, MPa
S = required section modulus, mm3
d = depth of beam,mm

Note: Beams greater than 300 mm in depth have reduced values for the maximum flexural stress, this is
300 1/9
done by using a size factor, defined as cF = ( ) .
d

Where: cF = size factor


d = depth of beam, mm

2. Check for Horizontal Shearing Stress

A beam has a tendency to fail in shear by the fibers sliding past each other both vertically and
horizontally. The vertical shear strength of wood beams is seldom of concern because the shear resistance
of wood across the grain is much larger that it is parallel to the grain, where the horizontal shear forces
develop.

The horizontal shearing stresses are not uniformly distributed over the cross – section of a beam but
are greatest at the neutral surface. The maximum horizontal unit shearing stress for rectangular sections
3 3V
is 2 times the average vertical unit shearing stress. This is expressed by the formula: fv = 2bd for
rectangular only.

Where: fv = maximum unit horizontal shearing stress, MPa


V = total vertical shear, N
b = width of cross – section, mm
d = depth of the cross – section, mm

3. Design for bearing

For most beams, bearing consists of compression stress perpendicular to the grain. This is done for
beam end bearing or for bearing under concentrated loads where the length of bearing along the beam
V
is 150 mm or more. The actual bearing stress is computed as: vb = A .
b

Where: vb = actual bearing stress, MPa


V = total vertical shear, N

Engr. Miriam B. Villanueva 8


Ab = bearing contact area, MPa
Lb = bearing length, mm

For shearing lengths an increase in allowable stress is delimited if the bearing does not occur
closer than 75 mm from the end of the beam. The increase consists of multiplying the factor equal to:
Lb + 9.5
.
Lb

Lb + 9.5
Modified allowable bearing stress, vb′ = vb x .
Lb

4. Design for deflection

Deflections in wood structures bend to be most critical for rafters and joists, where span – to – depth
ratios are often pushed to the limit. The allowable limit for the deflection of beams in floor construction
1
that supports plastered ceilings or partitions is generally taken to be 360 of the span; for beam that do not
1 1
support plastering top be 240 of the span length and for highway bridges the limit is frequently 200 of the
span.
5 w L4
δact =
384 EI

Where: δ = deflection, mm
L = length of span, m
N
w = uniformly distributed laod,
mm
N
E = modulus of elasticity, mm2
I = moment of inertia, mm4

K. Stresses in Beams

1. Flexural or Bending Stress


Mc
fb = INA
General flexure for any shape of beam section

Where: fb = flexural or bending stress, MPa


M = bending moment, N – mm
c = distance of fiber from the neutral axis, mm
I = centroidal moment of inertia, mm4

a. Solid Rectangular Section


fc

d c
2
d N.A.
d
2

ft

Stress Diagram
6M
fb =
bd2

Where: fb = flexural or bending stress, MPa

Engr. Miriam B. Villanueva 9


M = bending moment, N – mm
c = distance of fiber from the neutral axis, mm
I = centroidal moment of inertia, mm4
Note: Fb = allowable bending stress and fb = actual bending stress

b. For Circular Section


fc

d
c
2
d N.A.
d
2
ft
32M
fb =
лd3
2. Shearing Stress

a. Vertical Shearing Stress


V
A beam seldom fails in vertical shearing stress fv(vertical) = bd

b. Horizontal Shearing stress


VQ
fv = General formula for Horizontal Shearing Stress
Ib

d
2
d N.A.

d
2

b
Note: Horizontal shearing stress (fv) is maximum at the neutral axis

3V
For Solid Rectangular Section fv = 2bd

Where: fv = horizontal shearing stress, MPa


V = vertical shear, N
Q = statical moment of inertia of the section above or below the neutral axis, mm3
I = centroidal moment of inertia, mm4
b = base of section at the neutral axis, mm

Engr. Miriam B. Villanueva 10

You might also like