STRUCTURAL STEELWORK & TIMBER DESIGN ECS328
MOHD AZUAN TUKIAR BKBA 2.12 012-5149452
[email protected] F A C UL T Y O F CI V I L E N G IN E E RI N G , UI T M P E N AN G
2. STRUCTURAL TIMBER BEAM DESIGN
1. INTRODUCTION TO STEEL DESIGN
TIMBER DESIGN
3. STRUCTURAL TIMBER COLUMN DESIGN
2. STRUCTURAL TIMBER BEAM DESIGN
1. INTRODUCTION TO STEEL DESIGN
TIMBER DESIGN
3. STRUCTURAL TIMBER COLUMN DESIGN
1. Kn Know ow di diff ffer eren entt typ types es of of defe defect ct in in timber (CO2‐PO3)
LEARNING OUTCOME
2. Kn Know ow phy physi sica call prop proper erti ties es th that at will will affect strength of timber (CO2‐PO3)
3. Unde Unders rsta tand nd th the e desi design gn con conce cept pt in structural timber design (CO2‐PO3)
4. Und Unders ersta tand nd the the defin definiti ition on of of stress stress tha thatt used used in structural timber design (CO2‐PO3)
Timber from well managed forests is one of the most sustainable resources available and it is one of the oldest known materials used in construction It has a very high strength to weight ratio, is capable of transferring both tension and compression forces, and is naturally suitable as a flexural member. Timber is a material that is used for a variety of structural forms such as beams, columns, trusses, girders, and is also used in building systems such as piles, deck members, railway sleepers and in formwork for concrete.
Hōryū-ji pagoda Nara, Japan- 7th century Height: 32.5 m One of the oldest timber buildings in the world
Kizhi Pogost, Karelia, Russia 18th Century
Claremont Hotel Oakland, California, USA – 1906 – Timber frame11 stories Height: 37 m Tower: 49 m & 279 rooms
ABUNDANT RESOURCES
ALLOWED DIFFERENT TYPES OF FINISHES
EASE OF PROCESSING
RENEWABLE RESOURCES
OF TIMBER
LOW THERMAL CONDUCTIVITY
DURABLE (IF TREATED PROPERLY)
NONCORROSIVE
AESTETHIC VALUE
Grain defects Wane
Knot
NATURAL DEFECTS
Fungal decay
Shake
CONVERSION DEFECTS
DEFECTS IN TIMBER
SEASONING DEFECTS CHEMICAL DEFECTS
Annual ring width Cracks & fissures (a long, narrow crack)
NATURAL DEFECTS Occur during the growing period a.
CRACKS & FISSURES (a long, narrow crack): may occur in various parts of the tree & may indicate the presence of decay or beginning of decay
b.
KNOT: due to branch growth of the timber, cause decrease in physical properties, such as tensile & compressive stress reduction in strength due to: I. distortion of grain passing around the knots II. large angle between grain of the knot & grain of the timber
c.
GRAIN DEFECTS: grain is general direction of the arrangement of fibers in wood, can occur in the form of twisted ‐grain, cross‐grain, flat‐grain & spiral‐grain
d.
FUNGAL DECAY : may occur in growing mature timber or even in recently converted timber, good practice to reject such timber
e.
ANNUAL RING WIDTH: can be critical in strength aspect, as excess width can reduce density of timber, thus reduce strength
f.
SHAKE: fibers separate along the grain, normally occurs between growth rings, it reduce shear strength, but not significantly affect strength of axially loaded members
CONVERSION DEFECTS Conversion Defects - Due to unsound practice in the use of milling techniques or undue economic attempt to use every possible piece of timber converted from trunk a. Wane : Can occur when part of the bark or rounded periphery of the trunk is present in a cut length, it reduce cross sectional area, thus reduce strength
CHEMICAL DEFECTS a. may occur when timber is used in unsuitable positions or in association b. some timbers may contain chemicals which corrode metals c. gums (a viscous secretion of some trees and shrubs that hardens on drying but is soluble in water) & resins (a sticky flammable organic substance exuded by some trees and other plants) can inhibit working properties of timber & interfere with the ability to take adhesives
SEASONING DEFECTS a. directly related to movement occurs in timber due to changes in moisture content b. have effect on structural strength, fixing, stability, durability & finished appearance
S T C E F E D N O I S R E N O C & L A R U T A N
S T C E F E D N I N O S A E S
SLOPE OF GRAIN 1. grain is the longitudinal direction of the main elements of the timber 2. in some cases, the grain in cut section is not parallel to the longitudinal axis. This variation may due to: a. poor cutting of the timber b. irregular growth of the tree 3. It has lesser consequence if timber is axially loaded, but has significant drop in bending resistance 4. Increase in grain sloping will result in strength decrease
DENSITY 1. best single indicator of the properties of a timber & is a major factor determining its strength 2. since water volume varies with the moisture content of the timber, specific gravity of timber is expressed at a certain moisture content
KNOTS H T G N E R T S G N I T C E F F A T A T S E I T R E P O R P L A C I S Y H P
1. knots influence the mechanical properties of timber due to: a. effect of the local cross grain which they produce b. checking which may develop in & around them upon drying 2. the influence on strength depends on size, location, shape & type of stress 3. timber with knots is stronger in compression rather than tension, so it is always recommended to place the edge with knots on the compression side 4. practically, knots have no effect on stiffness 5. in short or intermediate column, strength decrease as size of knots increase 6. in long (slender) column, knots do not have significant effect as stiffness is the controlling factor
MOISTURE CONTENT H T G N E R T S G N I T C E F F A T A T S E I T R E P O R P L A C I S Y H P
1.
2.
Influence the behavior of timber Moisture contained in ‘green’ timber is held both within the cells (free water) & within the cells walls (bound water). When a ree wa er has been removed but cells walls are still saturated, it is known as fiber saturation point (FSP). Moisture Content
To control Drying of Timber, Seasoning Method is applied a. Air Seasoning - Timber is stacked & layered with air-space in open-sided sheds to promote natural drying Advantage : inexpensive Disadvantage : space constraint b. Kiln Drying - Timber is dried out in a heated, ventilated & humidified oven Advantage : controlled environment quicker process Disadvantage : requires specialist equipment expensive energy input
Most physical & mechanical properties remain constant
No change in strength
FSP Properties such as weight, strength, elasticity strength & durability will change considerably
Strength increase
PRINCIPAL AXES OF SYMMETRY Force 'A' is applied in the same direction as fibers (parallel to the grain) and is termed the longitudinal direction. Forces 'B' and 'C' are applied across the fibers (perpendicular to the grain) Force 'B' is applied in the tangential direction. Force 'C' is applied in the radial direction. The strongest direction is A. the Longitudinal direction i.e. towards the end grain. It is unlikely to use this except for columns, pit props, etc. For all construction purposes no difference is placed on the tangential or radial faces when calculated strength and deflection of timber. However more attention should be placed on knots, sloping grain, shakes and other timber defects which have more affect on the strength of a board. Therefore the difference between the force directions shown at B and C are negligible in square section timber.
Unlike concrete & steel design which use limit state design concept, timber design is based on permissible stress design concept
Elastic theory is used to analyze structures under various loading condition to give the worst design case Some terms that need to be understood are: Basic stress ‐ the stress which can timber containing no strength reducing characteristics
Grade stress ‐ the stress which can safely be permanently sustained by timber of a particular grade
Dry stress ‐ a stress applicable to timber having moisture content not exceeding 19% Permissible stress ‐ the stress which can safely be sustained by a structural component under particular condition of service & loading. It is equal to grade stress multiplied by appropriate modification factor
Green stress ‐ a stress applicable to timber having moisture content exceeding 19% Strength ratio ‐ the ratio of grade stress to basic stress timber section are chosen so that permissible stresses are not exceeded at any point of the structure
Timber obtained from mills may have defects which reduce the strength, so basic stress cannot be used directly in design calculation
Grade stresses are for timbers of a particular grade. Grade stresses are derived from basic stresses of individual species & governed by the effect of visible gross features of defect such as knots, sloping grain, fissures, etc.
Malaysian Grading Rules (MGR) classify timber into three grades: (1) Select Structural (2) Standard Structural & (3) Common Building (MS 544: PART 2 : 2001 Appendix C (pg. 49))
This grading are done by timber graders using (1) visual grading & (2) machine grading
GRADE STRESS
GRADE STRESS
GRADE STRESS
DURATION OF LOADING, K1
LOAD SHARING FACTOR, K2
1. If the number in one row > 4 and spacing of the members <600 mm .: K2 = 1.1 and use average E 2. Otherwise use K2 = 1.0 and use E minimum
LENGTH & POSITION OF BEARING, K3
Clause 11.2, Table 6, Figure 1
NOTCH AT END OF MEMBERS, K4
NOTCH AT END OF MEMBERS, K4
FORM FACTOR, K5
DEPTH FACTOR, K6
MOD. FACTOR FOR COMPRESSION, K8
MOD. FACTOR FOR COMPRESSION, K8
