8/2/2019 1_2_3_2012
1/81
8/2/2019 1_2_3_2012
2/81
Outline Presentation
WOOD AS A STRUCTURAL MATERIAL
1.1. Introduction 1.2. Wood structure
1.2.1. Anatomy of wood 1.2.2 Methods of conversion 1.3. Natural defects 1.4. Wood preservation 1.5. Fire retardants 1.6. Wood axes 1.7. Lumber grading
1.8. Timber constructions development 1.8.1. Timber frames for houses
1.8.2. Timber frames for bridges 1.8.3. Great timber structures
8/2/2019 1_2_3_2012
3/81
WHY TIMB ER AS A BUILDING MATERIAL ?Since ancient times, wood and stone have been important
construction materials.
ADVANTAGES: The simplicity with which it can be worked by hand or
by machine.
The tooling costs are relatively low compared with
competitive materials. Wood is ideal if it is necessary to erect an individual
structure for a particular purpose but it is equallysuitable for small batch or mass production.
Wood remains the cheapest of all structural materials Its excellent thermal insulation.
The unique aesthetic properties of finished wood.
Its high strength to weight ratio:
8/2/2019 1_2_3_2012
4/81
Wood is strong with outstanding rigidity in bendingand strength in compression.
Wood has exceptional stability in the longitudinal
direction, even when subjected to fluctuatingmoisture content .
Wood is free from corrosion .
The variability between woods of different species
may appear to be a disadvantage to theunintelligent user but it is, in fact, a distinctadvantage, as different species have differentproperties and there is almost always a suitable
wood for a particular purpose.
8/2/2019 1_2_3_2012
5/81
However, there is one feature of wood that isunique amongst all structural materials:
it is a CROPthat can be obtained whereas its competitors
such as stone, brick, metal and plastics areall derived from exhaustible resources.This feature is alone sufficient to ensure thatwood will continue to be used as a structural
material virtually forever.
8/2/2019 1_2_3_2012
6/81
WOOD STRUCTURE
As a plant a tree consists of acrown of branches with leaves,generally supported on a singlemain stem known as the trunk (or
bole) which connects the crown tothe roots in the ground
Anatomy of wood
8/2/2019 1_2_3_2012
7/81
Longitudinal axis
Radial axis
Tangential axis
WOOD AXES
8/2/2019 1_2_3_2012
8/81
Heartwood is the olderwood in the central portion ofa tree, which has ceasedparticipating actively in thephysiology of tree life.
Sapwood is the newerwood, which usually appearsas a lighter coloured band
immediately within the bark,extending inward from a fewtoo many annual rings,depending upon species.
8/2/2019 1_2_3_2012
9/81
8/2/2019 1_2_3_2012
10/81
Woods are commonly divided into softwoods (thecone-bearing plants that are conifers) andhardwoods (the broad-leaved plants meaning
dicotyledonae and monocotyledonae).
Typical softwood speciesare the pines (trad.- pin),firs (trad.- brad), spruces (trad.- molid), and
redwoods (trad.- soiuri de conifere), while typicalhardwood speciesinclude the oaks (trad.- stejar),maples (trad.- artar), beeches (trad.- fag) andbirches (trad.- mesteacan).
8/2/2019 1_2_3_2012
11/81
Methods of conversion
The first level of wood for construction is the log (trad.- bustean,barna). The logs are converted into sawn wood (trad.-cherestea, lemn ecarisat) by means of conversion saws.
TRUNK is barked (peeled) LOG is converting SAWN WOOD
There are different ways to cut the log to produce timber.The
manner in which the log is sawn is usually considered to berelatively unimportant. It is not so, because the manner caninfluence the behaviour of the sawn wood.
8/2/2019 1_2_3_2012
12/81
Methods of conversion The simplest technique is to make a large number of parallel
cuts, a method known as through-and-through, flat sawn or backsawn. The outer boards are largely cut in the tangential plane whilethe middle board is in the radial plane, the angle between the annualrings and the surface of the board progressively varying through theintermediate boards.
Another manner is quarter-sawn boards. It is particularlysuitable for use as flooring as they do not suffer the cupping that is acharacteristic of tangential or outer flat sawn boards.
An alternative method of conversion known as billet sawn is tomake three through-and-through cuts to provide two flat sawnboards from the centre of the log. These will naturally include any
heart defects, which can then be removed when the boards are re-sawn. The remaining wood consists essentially of two half logsoften known as wainscot billets. These billets are then turned on totheir flat face and re-sawn to give a number of boards.
8/2/2019 1_2_3_2012
13/81
(a) throughand throughsawn
(b) quarter cut (c) billetsawn
Methods of log conversion(sawn wood)
end grain(transversesection)
flat sawnplain sawnslash sawn
(tangentialsurface)
rift sawnquarter sawn
(radialsurface)
8/2/2019 1_2_3_2012
14/81
Wood products and their sizes1. timber slat (strip)
2. boards (lumber) (trad.-
cherestea, scandura).
3. sawn timber collar beams(trad.- grinzi).
4. thick plank (trad.- scanduragroasa, dulapi).
5. sawn timber columns and
beams
mm40h
mm60b
mm40h
mm80b
mm100hmm40
mm80b
mm100hmm40
mm100b
mm100h
mm100b
b = width
h = thickness(height, depth)
b = width
h = thickness
(height, depth)
8/2/2019 1_2_3_2012
15/81
THE TIMBER PRODUCTS
- planed square edged board
- planed tongued and grooved board
- planed tongued and grooved with V joint board
(match boarding)- plain weather-board
- rebated weather-board
- boards
- ship-lap weather-board
- sawn timber columns
log
- sawn timber beams - floorboards- doors
- door frames- door stops- architrave- skirting
- panelling
- windows- window frames
- surrounds and faces
- joinery (millwork)
- large-boards and cladding
8/2/2019 1_2_3_2012
16/81
W OOD DEFECTSMainly degradation of wood can be grouped into two broad
categories: biological deterioration from fungal decay orinsect attack, and mechanical deterioration.
Wood has various natural defects, which can influence thestrength and thereby arrive at a value, which isacceptable for these defects.
Defects may be classified as natural defects, chemical
defects, conversion defects and seasoning defects.
All the defects may degrade wood, with the degree ofdegradation being reflected in varying degrees of loss inmechanical properties.
8/2/2019 1_2_3_2012
17/81
W OOD DEFECTS
Seasoning defects. These defects are bowing, springing,twisting and cupping. Seasoning defects are directly relatedto the movement that occurs in timber due to changes inmoisture content. All such defects have an effect on
structural strength as well as on fixing, stability, durabilityand finished appearance.
Chemical defects may occur in particular instances whentimber is used in unsuitable positions or in association withother materials. Most woods are slightly acidic and produceacetic acid if stored in damp conditions. Timber such as oakcontains tannin, which corrode metals. Gums and resinsadversely affect working properties and ability to take glueand surface finishes, while silica in some hardwoods bluntstools.
8/2/2019 1_2_3_2012
18/81
A knot is the part of a branch, which became enclosed in a growing tree.
knot (local disturbance of grain)
NATURAL DEFECTS:Knots, Grain defects (trad. defecte de fibra), Annual ring width,Fissures and cracks, Fungal decay
Grain defects are the measure of the deviation of thefibres from the longitudinal axis of the piece.
Annual ring width can be critical in respect of strength in that excess
width of such rings can reduce the density of the timber.
growth rings
Knots
Grain defects
Annual ring width
Mechanical deterioration
8/2/2019 1_2_3_2012
19/81
Fissures and cracks
A fissure is any separation of fibres in a longitudinal plane and includes checks,shakes and splits. Their existence reduces the cross-sectional area,resisting shear and bending stress.
Fungal decay
Wormholes are permitted to
a slight extent provided thatthere is no active infestation.Wood wasp holes are notpermitted. Decayed woodshould not be accepted.
Biological deterioration
8/2/2019 1_2_3_2012
20/81
WOOD PRESERVATION The following preservatives are recognised in the standards: Preservative oils: Creosote Creosote-coal-tar solutions Creosote-petroleum solutions Oil-borne preservatives: Pentachlorophenol Copper naphthenate Water-borne preservatives: Chromated zinc chloride (CZC) Fluor chrome arsenate phenol (FCAP)
Tanalith (Wolman salts) Celcure Chemonite Greensalt (Eradlith) Boliden salts
8/2/2019 1_2_3_2012
21/81
Fire retardants Recently fire-retardant resin treatments have
been developed.
The alternative is the impregnation of the
wood with fire retardant salts. The fire endurance rating R, in minutes, is:
ZbGR 54.2 in [min.]where: -Z = factor dependent on load applied and member type. It has values
between 1 and 1.5;
-b = width dimension of cross section of beam or of larger dimension
of a column before exposure to fire;
- G = beam or column cross-sectional factor.
8/2/2019 1_2_3_2012
22/81
WOOD AXES
8/2/2019 1_2_3_2012
23/81
L = longitudinal axis
R = radial axis
T = tangential axis
L = longitudinal axisT = transverse axis
Wood is considered to be orthotropic, havingunique and independent properties in thedirection of three perpendicular axes
WOOD AXES
8/2/2019 1_2_3_2012
24/81
SAWN WOOD (TIMBER) GRADINGGrading is the process of classifying timber according to quality for a particular use.
Quality
class
Load type and destination
ITimber elements are subjected to tension and bending(Truss girders, beams and wood dowels)
II a) Timber elements are subjected to compression and
bending
b) Timber elements are subjected to tension and tension +bending where effective stresses are 70% of allowable
wood strengths
III Secondary timber elements (Roof covering)
8/2/2019 1_2_3_2012
25/81
SAWN WOOD (TIMBER) GRADING
Structural designers are interested in strength and stiffness, somodern grading rules provide for what is sometimes called stress grading.
The two methods used for stress grading are:visual grading;machine grading.
The minimum requirements for visual grading standards have beenlaid down in the European Code EN-518 Structural timber Grading
Requirements for visual grading standards. Requirements for machinegrading can be found in EN-519 Structural timber GradingRequirements for machine strength graded timber and gradingmachines.
Guidance on the use of timber in building and civil engineeringstructures is given also by the Romanian code SR-EN 1995-1/2004.
8/2/2019 1_2_3_2012
26/81
To know where we shallgo, we need to know
where the craft hasstarted.
HISTORY OF TIMBER STRUCTURES
8/2/2019 1_2_3_2012
27/81
1.8.1. Timberframes for houses
Primitive structures
Long tent with ridge purlinRound tent
8/2/2019 1_2_3_2012
28/81
Early wood structure
Wood structure
8/2/2019 1_2_3_2012
29/81
Timber house frame
8/2/2019 1_2_3_2012
30/81
Log home
8/2/2019 1_2_3_2012
31/81
Timber frame home
8/2/2019 1_2_3_2012
32/81
Timber frame structure of the Middle Ages
8/2/2019 1_2_3_2012
33/81
Old English style timber frame
8/2/2019 1_2_3_2012
34/81
Typical American timberframe house
8/2/2019 1_2_3_2012
35/81
1.8.2. Timberframes for bridges
Natural pedestrian bridges
8/2/2019 1_2_3_2012
36/81
Primitive timber bridges
8/2/2019 1_2_3_2012
37/81
The Drobeta-Turnu Severin bridge designed
by Apolodor and built by Romans
8/2/2019 1_2_3_2012
38/81
Different timber bridge structures
8/2/2019 1_2_3_2012
39/81
TRADITIONAL ROM ANIAN TIMBERHOUSES
8/2/2019 1_2_3_2012
40/81
TRADITIONAL ROM ANIAN TIMBERHOUSES
8/2/2019 1_2_3_2012
41/81
LOG HOUSE
TRADITIONAL ROM ANIAN TIMBERHOUSES
8/2/2019 1_2_3_2012
42/81
TRADITIONAL ROM ANIAN TIMBERHOUSES
8/2/2019 1_2_3_2012
43/81
8/2/2019 1_2_3_2012
44/81
Silverthorne, Colorado
8/2/2019 1_2_3_2012
45/81
8/2/2019 1_2_3_2012
46/81
8/2/2019 1_2_3_2012
47/81
8/2/2019 1_2_3_2012
48/81
8/2/2019 1_2_3_2012
49/81
Picnic Pavilion
8/2/2019 1_2_3_2012
50/81
SUMM ER HOUSES
8/2/2019 1_2_3_2012
51/81
1.8.3. Great timber structures
Bulk storage building built by Bunnings Limited
for Texada Mines Pty. Ltd, - 41 m span
8/2/2019 1_2_3_2012
52/81
39. 6 m span truss roof aircraft hangars
8/2/2019 1_2_3_2012
53/81
31.7 m span nail jointed arched store
and workshop buildings
8/2/2019 1_2_3_2012
54/81
Wood Research and Development, LLC1760 SW 3rd Street
Corvallis, OR 97333, U.S.A.
8/2/2019 1_2_3_2012
55/81
Outline Presentation
1. INTRODUCTION2. PHYSICAL PROPERTIES
Hardness and toughness Thermal properties
Electrical properties Acoustical properties Density and specific gravity Moisture content
3. MECHANICAL PROPERTIES Stiffness properties Strength properties
4. STRENGTH CLASSES
8/2/2019 1_2_3_2012
56/81
5. INFLUENCE OF VARIOUS FACTORS ONWOOD PROPERTIES
Density
Moisture content
Knots
Fibre and ring orientation
Temperature
Duration of load Chemicals and decay
1. INTRODUCTION
8/2/2019 1_2_3_2012
57/81
The arrangement of fibres in wood suggests that wood may have differentcharacteristics in the various directions within itself. Specifically wood isconsidered to be orthotropic, having unique and independent properties in thedirection of three mutually perpendicular axes.
The mechanical properties of wood used in design process of a buildingelement are usually referred to the following axes: longitudinal axis andtransverse axis. The transverse axis is used instead of tangential or radial axesbecause the variability of the same property about them is less and of minorimportance in timber element design.
The strength of wood is highly dependent upon direction tensile strengthvalues in longitudinal:radial:tangential directions on average are in the ratio of20:1.5:1.
L = longitudinal axis
R = radial axis
T = tangential axis
L = longitudinal axisT = transverse axis
8/2/2019 1_2_3_2012
58/81
Properties of wood a key for civil engineers to
use wood as a building materials
Physical properties Density. Moisture content. Hardness andToughness. Electrical properties. Acoustical
properties. Thermal properties. Behaviour in
fire. Resistance to corrosion and environmentalfactors.
Mechanical properties Strength properties. Elastic properties. Fatiguestrength. Fracture toughness.
Manufacturing
properties
Ability to be shaped by machines. Ability to be
joined by adhesives.
Economic properties Processing cost. Availability.
Aesthetic properties Appearance. Texture and ability to accept specialfinishes.
8/2/2019 1_2_3_2012
59/81
2. PHYSICAL PROPERTIES Hardness and toughness
Sawn-wood is frequently described in terms of itshardness and toughness, but these are terms thatare difficult to define. Sometimes wood is said to bevery tough because it is difficult to saw or plane, or it
has good resistance to abrasion, splitting or shockloads.
The ability to resist excessive shock is probably the property
that can best be described as toughness.
Hardness is the ability of wood to resist penetration
Thermal properties
8/2/2019 1_2_3_2012
60/81
Specific heat is the term used to describe the amount of heat energy that is required toraise the unit mass of the material through one degree of temperature. The specific heatof wood is comparatively high, four times as high as that of copper, but this relates tothe mass of material.
The thermal conductivity of wood is approximately 0.4% of that of steel and 0.05% of thatof copper. The thermal conductivity varies approximately in proportion to density .
Thermal conductivity in the longitudinal direction (L) is 2.25 to 2.75 times the value givenfor the (T) or (R) directions.
The average Longitudinal coefficient of thermal expansionL =3.6 x 10-6 /oC
Radial and tangential coefficients of thermal expansion- for softwoods:
T= 1.8 x 45 x specific gravity x 10-6 [/oC]R= 1.8 x 31 x specific gravity x 10-6 [/oC]- for hardwoods:
T= 1.8 x 32 x specific gravity x 10-6 [/oC]R= 1.8 x 32 x specific gravity x 10-6 [/oC]
p pTemperature affects both dimensional stability and strength of wood. Woodexpands as its temperature increases, as do other construction materials. Itscoefficients of expansion vary with direction, being largest radially andtangentially, and least longitudinally. Wood is a good insulator, that is, it has a
high resistance to heat flow.
El t i l ti
8/2/2019 1_2_3_2012
61/81
Electrical properties
Wood at a low moisture content is normally classifiedas an electric insulator, or dielectric, rather than as aconductor
Tangential and radial resistance exceed longitudinalresistance for wood.
Wood density, moisture effects and temperature haveeffects on resistance of wood to electrical current.
The direct current properties of materials aremeasured by resistivity or by its reciprocal,conductivity.
8/2/2019 1_2_3_2012
62/81
Acoustical properties
Sound insulating values are related to the soundtransmission. The reduction of sound in itstransmission through a material is dependentupon the weight of the material. Since wood has alower density than many structural materials, itseffectiveness in blocking transmitted sound is nothigh.
Sound absorption coefficient for a material isused to determine the total magnitude of theabsorption property of the material.
i d ifi i
8/2/2019 1_2_3_2012
63/81
Density and specific gravity
Specific gravity (G) or relative density is theweight of a substance to that of an equal volumeof water.
Density is the mass per unit volume normally
expressed as kg/m3. Basic specific gravity isdefined as:
waterdisplacedofmass
massdry
V
m
Ggww
g
00
where w is the density of water and
Vg is the green wood volume.
Th d it i ht it l f i f d i ti l l
8/2/2019 1_2_3_2012
64/81
The density or weight per unit volume of a piece of wood is a particularly
important property. Density [kg/m3], wherem is the mass of timber [kg] and
Vis its volume [m3] is defined as:
Wood substance has a density of about 1500 kg/m3.
Wood itself consists of a mixture of wood substance and spaces,
therefore the amount of wood substance per unit volume decidesthe dry density, which can vary in common species from about 300kg/m3 to 800 kg/m3.
Wood is considered to have moderate density if its dry density liesbetween about 360 and 500 kg/m3, so that woods below this rangeare light woods and those above are heavy woods.
V
m
Density at a moisture content [%] is expressed, related to volumetricshrinkage V, as:
VV .
.
.V
.m
V
m
0101
0101
0101
01010
0
0
Moisture content
8/2/2019 1_2_3_2012
65/81
100
weightdry
weightdryweightoriginal
Moisture content, MC or, is the weight of water in thewood expressed as a percentage of the weight
of the oven-dry wood
During seasoning, most of the water in the cell cavities is lost,leaving a condition known as the fibre-saturation point (FSP).
Changes in dimensions tend to be linear with moisture in therange of 5 to 20% moisture content. In this range movementscan be calculated from:
where: - h1 and h2are the dimensions at moisture 1 and 2;- is the coefficient of swelling (positive) or shrinkage(negative).
1212100
1 hh
8/2/2019 1_2_3_2012
66/81
[N/mm2]
MC
20 30 4010
Fibre-saturation point
[%]
Variation of strength versus moisture content
The graph shows that the fibre-saturation point occurs at around 25-30% and 25% is generally accepted as being a norm in sawn lumberand timber strength assessment. Between the fibre-saturation point
and zero moisture content, wood shrinks as it loses moisture andswells as it absorbs moisture. Above the fibre-saturation point, thereis no dimensional change with variation in moisture content.
8/2/2019 1_2_3_2012
67/81
3. MECHANICAL PROPERTIES
(behavior of wood under applied forces)
MECHANICAL PROPERTIES
8/2/2019 1_2_3_2012
68/81
MECHANICAL PROPERTIES
The strength and stiffness properties of most interest instructural design are:
compressive strength parallel to the grain; compressive strength perpendicular to the grain; tensile strength parallel to the grain;
bending strength; shear strength;
modulus of elasticity parallel to the grain; shear modulus.
L = longitudinal axis
R = radial axis
T = tangential axis
L = longitudinal axisT = transverse axis
8/2/2019 1_2_3_2012
69/81
Stiffness properties
The modulus of elasticity, also called Youngs modulus, usually used in
the design process is taken as a longitudinal modulus, EL. Data for ERand ET are not extensive and usually they are not presented asallowable properties for species. However, where a transversemodulus, ET(or E), is essential in design, an approximation often usedis 0.06 times the longitudinal value.
GLR, GLTand GRTdenote the three moduli of rigidity, or shear moduli, in
the (LR), (LT) and (RT) planes respectively.
The six Poisson's ratios are denoted by LR, RL, LT, TL, RT andTR.
8/2/2019 1_2_3_2012
70/81
Prop.
Wood
Modulus of elasticity [N/mm2] Shear modulus
[N/mm2]GRTEL (E//) ET (E)
Softwood 10,00011,300 300 500
Hardwood 11,50014,300 600 1000
Romanian codes present the design values of:- elasticity modulus in longitudinal (parallel) direction, EL- elasticity modulus in transverse (perpend.) direction, ET- shear modulus GRT for softwood and hardwood.
Strength class (charactersic values) system established in
SR EN 338 St t l ti b St th l (EC5)
8/2/2019 1_2_3_2012
71/81
SR-EN 338 Structural timber Strength classes (EC5)
C14 C16 C18 C22 C24 C27 C30 C35 C40
[N/mm2
]fm,k 14 16 18 22 24 27 30 35 40
ft,0,k 8 10 11 13 14 16 18 21 24
ft,90,k 0.3 0.3 0.3 0.3 0.4 0.4 0.4 0.4 0.4
fc,0,k 16 17 18 20 21 22 23 25 26
fc,90,k 4.3 4.6 4.8 5.1 5.3 5.6 5.7 6.0 6.3
fv,k 1.7 1.8 2.0 2.4 2.5 2.8 3.0 3.4 3.8
[kN/mm2]
E0,mean 7 8 9 10 11 12 12 13 14
E0,05 4.7 5.4 6.0 6.7 7.4 8.0 8.0 8.7 9.4
E90,mean 0.23 0.27 0.30 0.33 0.37 0.40 0.40 0.43 0.47
Gmean 0.44 0.50 0.56 0.63 0.69 0.75 0.75 0.81 0.88
[kg/m3]
k 290 310 320 340 350 370 380 400 420
D30 D35 D40 D50 D60 D70
8/2/2019 1_2_3_2012
72/81
m = bending;
t = tension;
c = compression;
v = shear;f = strength
k = characteristic;
0 = parallel to the grain;
90 = perpendicular to the grain.
[N/mm2]
fm,k 30 35 40 50 60 70
ft,0,k 18 21 24 30 36 42
ft,90,k 0.6 0.6 0.6 0.6 0.7 0.9
fc,0,k 23 25 26 29 32 34
fc,90,k 8.0 8.4 8.8 9.7 10.5 13.5
fv,k 3.0 3.4 3.8 4.6 5.3 6.0
[kN/mm2
]E0,mean 10 10 11 14 17 20
E0,05 8.0 8.7 9.4 11.8 14.3 16.8
E90,mean 0.64 0.69 0.75 0.93 1.13 1.33
Gmean 0.60 0.65 0.70 0.88 1.06 1.25
[kg/m3]
k 530 560 590 650 700 900
It ranges from the weakest grade ofsoftwood, C14, to the highest grade
of hardwood, D70, currently usedin Europe.
Experimental data show that all-important characteristic strength
8/2/2019 1_2_3_2012
73/81
and stiffness properties can be approximated from either bending
strength, modulus of elasticity or density. These relationships,
according to EC5, are:
45.0
,,0,5 kmkc ff
8.0
,, 2.0 kmkv ff
kktf 001.0
,90,
16
,0 mean
meanEG
kmkt ff ,,0, 6.0
kkcf 015.0,90,
meanEE ,005.0 67.0
30
,0
,90
meanmean
EE
5 INFLUENCE OF VARIOUS FACTORS ON WOOD
8/2/2019 1_2_3_2012
74/81
5. INFLUENCE OF VARIOUS FACTORS ON WOOD
PROPERTIES
Density ()The relation between mechanical properties and
specific gravity has the form:
where: - S= the value of any particular mechanical property
- G= specific gravity- K, n= constants depending on the particular property beingconsidered.
nKGS
Moisture content
8/2/2019 1_2_3_2012
75/81
Moisture content
Mechanical properties increase with decrease in moisture content. Most
clear wood mechanical properties obey the following relation in the vicinityof 20oC:
2
21
2
21
MCMG
MCMC
MG
MC
MCMC P
PPP
- PMG= value of property for all moisture contents greaterthan moisture content MG(slightly below fibre saturation point),at which property changes due to drying are first observed.
8/2/2019 1_2_3_2012
76/81
In timber design the influence of moisture is taken intoconsideration by assigning timber structures to
service classes. The European code EC5 and the Romanian anexes
define this modification factor, mui.
Code gives the following values (subscript idefines theload type):
- 1.00 for all types of loads and the first service classof the timber construction;
- 0.90 for all types of loads and the second service
class of the timber construction;- 0.70 0.90 for the third service class of the timber
construction and different loads.
8/2/2019 1_2_3_2012
77/81
Knots
Influence of a knot on the mechanical propertiesof a product varies depending upon the size,location, and type of stress that is applied to the
member
8/2/2019 1_2_3_2012
78/81
Fibre and ring orientation
The influence of fibre direction on mechanical properties
can be approximated by Hankinson's formula:
nn QP
PQN
cossin
where:
- N= the property at an angle ;- = the angle between property direction and direction
parallel to the grain;-Q= the property across the grain;- P= the property parallel to the grain;- n= empirically determined constant.
Immediate effect of temperature on
8/2/2019 1_2_3_2012
79/81
Immediate effect of temperature on
strength properties
Temperature
0 +20 +100-100[oC]
+200-200
100
200
Property[percent of value at 20oC]
Duration of load m generally called working condition coefficient or modification factor
8/2/2019 1_2_3_2012
80/81
mdi, generally called working condition coefficientor modificationfactor
Type of
load
Load duration
classSymbol m
di
softwood hardwood
Static bendingShear
Permanent load mdimdf
0.55 0.60
Long term
variable load
0.65 0.70
Short term
variable load
1.00 1.00
Compression Permanent load mdc0.80 0.85
Long termvariable load
0.85 0.90
Short term
variable load
1.00 1.00
Tension Permanent load mdt0.90 0.95
Long term
variable load
0.95 1.00
Short term
variable load
1.00 1.00
Elasticity
modulus
Permanent load mdE1.00 1.00
Long term
variable load
1.00 1.00
Short termvariable load
1.00 1.00
Chemicals and decay
8/2/2019 1_2_3_2012
81/81
Chemicals and decay
Chemicals may degrade wood, the degree ofdegradation being reflected in varying degrees of loss inmechanical properties. The effect of chemicals onmechanical properties is highly dependent upon the
specific type of chemicals. Wood-destroying fungi seriously reduce strength.
One measure of the progress of decay is the amount ofweight loss as a result of fungal attack.
Insects may destroy most of a piece of wood, frequentlywithout external evidence of the damage.
Recommended