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CHAPTER SEVEN

DESIGN OF TIMBER BEAM

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7.1 Introduction

Comparison between timber and concrete/steel

Concrete/Steel

• Manufactured product• Strength can be determine

e.g : Grade 30, 40, 50py 250, 275, 460 N/mm2

• Compressive and tensile strength

Timber

• Natural material• Characteristics: uncontrolled and keeps

changing• Timber axis : elongation, radius, tangent• Timber strength : not constant

- bending strength- tension parallel to grain - compression parallel to grain- shear parallel to grain- compression perpendicular to

grain

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7.1.1 Physical Properties of Timber

TIMBER STRENGTH

Temperature

Moisture Content

Density

Condition of growth

Position of tree

Grain structure

Defects

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Density□ The best single indicator of the properties of a timber and is a major factor determining its

strength

□ Specific gravity (SG) or relative is a measure of timber’s solid substance

□ SG : Ratio of the oven dry weight to the weight of an equal volume of water

□ Basic specific gravity of commercial timber ranges from 0.29 to 0.81 and most, falling between 0.35 – 0.60

□ SG varies considerably between species and within individual pieces of the same species

Temperature

□ Temperature increase, timber strength decrease

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Moisture Content

□ Influenced timber characteristics quality□ Dry timber : high strength□ Fibre Saturation Point (FSP)

- point at which water in cell cavity are dried, but the cell wall still saturated (moisture inside thecell wall only)

- 25% of moisture content□ 19% of FSP are suggested by Malaysian researchers□ Timber moisture strength must less than 19% to achieve high strength□ Moisture content ≥ 19%, moist, use wet stresses in calculation (Table1 and 4 MS 544:Part 2:2001)

< 19%, dry, use dry stresses in calculation (Table2 and 4 MS 544:Part 2:2001)Timber axis – elongation, radius, tangent□ Strength change/different in each three axis

- bending strength, tension parallel to grain, compression parallel to grain, shear parallel to grain,compression perpendicular to grain

Moisture content

Strength and stiffness

19% FSP

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□ Two important characteristics connected to FSP

i. Shrinkage/expansion : Due to nature called internal wood motion that can not been vanish but can be reduce by preservation

ii. Strength Moisture content > FSP; strength and volume are constant

< FSP; strength increase and volume decrease

□ Wood motion is inconspicuous in grain length direction but clear in radius and tangent

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Grain structure□ Grain is the longitudinal direction of the main elements of timber (fibres)□ The angle of the micro fibrils within the timber also influence the strength of the timber, as

with the effects of the grain, if the angle of deviation increase, the strength decrease

Position in Tree□ Wood near tree root is more denser, stronger and harder than at the top of the tree

Condition of Growth□ Environment factor influencing tree growth

- temperature, rainfall, soil and spacing between trees

Defects□ Defects in timber either natural defects or seasoning defects will effect structural strength as

well on stability, durability fixing, and finished appearance of timber

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7.1.2 Construction Stage

□ Timber in dried condition: high strength and durable□ Dimension must be uniform and stable□ In construction design stage use dry stresses□ If moist : i. defect

ii. Wood pest attack – termite, fungus□ Use wet stresses for thick size because it is hard to dry ( B ≥ 100mm )□ Size reduction due to planer

i. nominal size ≤ 50 mm – reduce 5mmii. nominal size > 50 mm & < 300 – reduce 10mm

B

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7.1.3 Classification/Grouping of Timber□ Basically, timber are divided into two groups namely hardwood and softwood. Most tropical wood

inclusive into hardwood□ Forest Research Institute Malaysia (FRIM) has classified hardwood into three subgroup which are

i. Heavy hardwoodii Medium hardwoodiii Light hardwood

□ For strength grouping, timber can be divided into four (4) groups namely A, B, C and DA > B > C > DStrength reduce

Timbe

r Clas

sifica

tion

Heavy Hardwood

Medium Hardwood

Light Hardwood

Softwood

- ρ > 880 kg/m3

- Constructional timber- Balau, Cengal, Merbau,resak, tembusu etc

- ρ = 720 - 880kg/m3

- moderately heavy to heavy construction- Keruing, mengkulang, tualang,kasai,kalat, punah etc

- ρ < 720 kg/m3

- general utility timber- Nyatoh, meranti, rubberwood,gerutu,medang etc

- Damar minyak, podo, sempilor etc

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7.2 Timber Grade□ For each group, grading of timber based on wood defect:

i. Basic - no defectii. Selectiii Standardiv Common

□ It referred to defect that occurs on timberi. Grain slopeii. Curve (not straight)iii Knot

□ Need to be graded because defects can not be avoided

Select < Standard < CommonLess defects

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7.3 Basic and Grade Stresses1. Basic Stresses

Stress which can be sustained safely and permanently by solid timbers that containing no visible strength reducing characteristics.

2. Grade StressesStresses that can be sustained safely and permanently by timber with particular grade trough the process of reduction by strength ratio

□

σ basic = (x – ks)/Fsσ basic = (x – ks)/Fs where; x = averagek = 2.33s = sisihan piawaianFs = safety factor

σ grade = σ basic x strength ratio σ grade = σ basic x strength ratio where; Grade strength ratio (%)Select 80Standard 63Common 50

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3. Permissible StressesPermissible strength is calculated by multiplying the grade stresses with appropriate modification factors (k- factor) to allow for the effects of parameters such as load duration. Load sharing, moisture content and etc.

σ permissble = σ grade x modification factorsσ permissble = σ grade x modification factors

where; modification factors = k1, k2,k3,k4,k5,k6…….k9

(K1 – K9 is for solid timber design)

Note : Table 1, 2 an 4 is for long term loading

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7.3.1 Modification Factors□ Duration of Loading, K1 (Table 5)

Grade stresses are based to structure service duration. Consideration are made for bigger strength. Timber is capable for higher loads for short periods (a few seconds). It will effects timber strength and permissible stress. Factors are given in Table 5 MS 544:Part 2:2001

□ Load-sharing system, K2 (Cl.10)Consist four or more members such as rafter, joist, trusses or wall stud, spaced a maximum of 610mm centre to centre and has adequate provision for the lateral distribution of loads. If this criteria are fulfilled, used

K2 = 1.1 and used MOE mean in Table 4

If member acting alone, used K2 = 1.0 and take MOE minimum

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□ Bearing Stress, K3 (Table 6)At any bearing on the side of timber, the permissible stress in compression perpendicular to the grain is dependent on the length and bearing position.

For bearing < 150mm and located ≥ 75mm from the end of the member, the grade stresses should be multiplied by the modification factor K3 given in Table 6

Modification factor for bearings of any length at end members, and bearing ≥ 150mm in length at any position, take K3 = 1.0

75mm or more

Bearing less than 150mm

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□ Shear at notched ends, K4 (Cl. 11.4)Square corner notch at ends of a flexural member cause a stress concentration. It will reduce the shear and the shear strength should be calculated by using the effective depth,he

(a) for a notch on the top edge

(b) for a notch on the underside

a

hhe

K4 = he/h – [(h-he)/he2]e untuk a < heK4 = he/h – [(h-he)/he

2]e untuk a < he

K4 = 1.0 untuk a > heK4 = 1.0 untuk a > he

hhe

K4 = he/hK4 = he/h

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□ Form factor, K5 (Cl.11.5)Based on the shapes of the solid timber member cross-section where the load acting parallel to one of main axis

□ Depth factor, K6 (Cl. 11.6)The grade bending stresses given in Table 1, 2 and 4, apply to material having depth, h up to 300mm

For beam depth greater than 300mm, the grade bending stresses should be multiply by the depth modification factor K6 where

for solid and glued laminated beams

K5 = 1.18 K5 = 1.41K5 = 1.0

K5 = 0.81 [ (h2 + 92300)/h2 + 56800)]K5 = 0.81 [ (h2 + 92300)/h2 + 56800)]