LONGE TERM DIFLECTION OF

COCONUT RAFTERS M.R.M.KIYAS 10/158

CONTENTS

INTRODUCTION

SCOPE, OBJECTIVE

LITERATURE REVIEW

COCONUT RAFTERS PRODUCTION

PROPERTIES OF COCONUT RAFTERS

METHODOLOGY

WORK SHEDULE

INTRODUCTION • There are various types of roof structures in the industry, which are concrete,

steel, timber and etc.

• In Sri Lanka the most of residential buildings are of timber roof structure. Purlin, rafter, reaper and joist the major parts of timber roof structure.

• conventionally in Sri Lanka people use various type of timber for rafter preparation however mostly use the coconut timber, because the availability of coconut timber in Sri Lanka is higher than the any other timber and prices of coconut rafters are also lower than the others.

• Also the usage and handling of coconut timber is also very easy than other timbers available.

• Even though coconut timber is conventionally used for roof structure the major problem of this is the long term deflection often referred as creep. Thus, I define my scope to increase the strength of the timber using metal sheet.

• I assume that when I apply the metal sheet with the timber I can improve the strength of the timber and in my objective I try to test the properties of coconut timber.

• I got some properties of coconut timbers I try to test my self because I’m try to test for our country climate and coconut timbers. I assume that the properties of coconut timber will be helpful in testing of deflection when loaded.

When I apply the metal I consider some configuration of the metal sheet.

They are;

1. Can drive nail.

2. The metal sheet should be get the proper connection with the rafter

3. The sheet should be economical

4. Metal sheet should not corrode.

I check the aluminums and Amano sheet for the metal sheet, I analyze these two selected the Amano sheet because it is satisfy the entire properties said above.

Scope

Strengthening of coconut rafter and joists using the metal sheet.

That is when we apply the metal sheet the deflection will be controlled and strength also will be increase.

Objective

1. Determine the deflection of the rafter with metal sheet and without metal seat and compare each other.

2. Determine the properties of coconut timber.

LITERATURE SURVEYCoconut has been cultivated by man for 4000 years. The main produce of coconut

is copra, which is converted into oil.

The coconut tree bears fruit until approximately 70 years of age.

Coconut timber has two advantages that can make it aviable timber substitute for certain end uses. It has low raw material cost and a green image being a plantation by-product.

There are many houses and buildings in Sri Lanka with roof structures built of coconut woods that are still in use after 100 years. Nowadays in Sri Lanka, more than 395,000ha are under coconut palms. Kurunegala, Chilaw.

The colour tones are range from golden to near ebony, with dark brown flecks. There are colour divisions which can be related to timber density as dark brown tones, medium brown tones and light golden tones.

The seasoned, high density product colour ranges from medium to dark brown with prominent, darker, quill-like streaks.

 Coconut timber (Rafter) production

The actual operation of sawing coconut trunks is difficult, this is probably mainly due to extremely hard sclerenchyma fibers and the parenchyma tissue disintegrates into a fine abrasive which is not easily removed from the cut and which causes frictional heat increase.

In Sri Lanka, processing of coconut timber traditionally involves the use of an axe and edges to obtain nearly rectangular cross section. Four or six pieces can be obtained from each annular potion.

Traditional method of cutting and cutting using machine

Properties of coconut timber 01. Physical properties

The coconut palm has a smooth, slender stem that grows to a height of about 25 meters with an average diameter of 300 mm.

The hardest, densest part of the wood is found on the outer perimeter of the trunk, which gives the palm its strength, while the wood’s high silica content gives the palm elasticity.

Towards the center of the trunk, the wood gets relatively softer.

Physical properties depend on

• Density

• Moisture content

• Shrinkage

Density

A: Center (very light/faint grains), Low density.

B: Towards the outer ring (light grains), medium density

 

C: Outer ring (deep grains), High density or hard wood

Moisture content• Wood has very good water transportation properties because it needs water to

grow. After the tree is cut down and sawn in to timber many of these water transportation properties remain.

• Wood is a hygroscopic material which means that it absorbs and desorbs moisture from the surrounding air.

• The moisture content in wood is therefore dependent on the relative humidity of the surrounding air.

• Moisture in wood can either be found as moisture in the cell wall or as free water inside the lumens.

• The moisture content is influenced by various factors, amongst others by the ambient temperature and the relative humidity of the air.

Describes the variation of initial moisture content for different densities of coconut wood

 

200 400 600 800

390

130

258

172

86

Through center of stem Through radial direction

Density / (kg/m3)

Initial mois-ture /%

10

20

30

90

80

70

60

50

40

Shrinkage

• The tangential shrinkage as coconut wood is greater than the radial shrinkage and longitudinal shrinkage of coconut wood is negligible.

• The variation of shrinkage with oven dry density content and initial moisture content are given below in figures in radial and tangential directions of coconut wood.

Shrinkage with Oven dry density content

200 400 600 8000

1

2

3

4

5

6

7

3.37

4.17

4.97

5.77

3.52

4.32

5.12

5.92

Radial Tangential

Shrinkage %

Oven dry density (kg/m3)

Shrinkage with Initial moisture content

5.14

4.42

3.7

2.98

5.42

4.62

3.82

3.02

Radial Tangential

Shrinkage/%

Initial moisture con-tent/ (%) 20 40 60

80

 Physical properties of coconut wood

 

Physical properties (units)

 

Range =low–high density wood

Density - basic (kg/m³) 100 – 1020

Density - air dry (kg/m³) 200 – 1170

Density for flooring products (Janka hardness

>7kN) (kg/m³)

>700

Specific gravity 0.26 - 0.59

Shrinkage: tangential, green to dry (%) 3.0 - 6.0

Shrinkage: radial, green to dry (%) 2.7 - 7.4

Unit shrinkage: tangential 0.05–0.42 | high density: 0.32–0.38

Unit shrinkage: radial 0.05–0.34 | high density: 0.24–0.3

Engineering properties of coconut timber

Density

range

Moisture

content / (%)

Static

Bending

Stress at

proportional

Limit/(Mpa)

Compression

parallel to grain

Compression

perpendicular to

grain

Modulus of

Elasticity/

(MPa)

Maximum

Crushing

Strength

/(MPa)

Stress at

proportional

limit /(MPa)

Impact

Bending /

(N)

High 57 51.6 7988 49 8.3 20.2

12 61.7 9747 57 9 20.1

107 30.4 5151 31 2.8 18.3

Medium 12 38.4 5282 38 3.4 10.1

240 13.1 2287 15 1.3 8.4

Low 12 15.4 2914 19 1.7 9

Mechanical properties with density range of coconut timber

 

Mechanical properties (units)

 

Range =low–high density fiber

 

Modulus of elasticity: dry (GPa)

 

2–25 | high density:11.4

Modulus of rupture: dry (GPa) 28–205 | high density: 104

Maximum crushing strength: dry

(MPa)

19–57 | high density: 40

Janka hardness : dry (kN) 0.7 – 23.9

 

Chemical properties

(units)

 

Range =low–high density fibre

Inorganic pure ash (%) 0.75 (0.25–2.4)

Silica (%) 0.07 (0.01–0.2)

Lignin (%) 25.1

Holocellulose (%) 66.7

Pentosans (%) 22.9

Starch (%) 4.3–4.6 , (>6 months old; starch reduces with

age)

Chemical properties with density range of coconut timber

 Applications of coconut wood

• Coconut wood, it has many applications as both a structural and interior design materials such as in building and bridge construction, but also for tools, toys and other items of daily necessity.

• High density timber is suitable for general structural purposes such as pillars, trusses, rafting, furniture, window and door frames

• Medium density timber can be used for walls, ceiling joists and horizontal studs.

• Low density is used in non-load bearing applications like wood paneling, internal trim and ceiling, as well as home wears.

• But low density material never use for construction.

• As a rule, experts suggest that coconut wood with density below 400kg/m3 should not be used as structural framing material but only in the internal parts of a building such as ceilings and wall linings in the form of boards and wall shingles.

• “In Sri Lanka, where coconut wood is becoming widely used in house construction, a coconut wood resource of 95 million senile trees would give 14.25 million cubic meters of sawn wood for economic utilization”

• Based on this estimate, 50 coconut palms are sufficient to build one housing unit. Hence, with 360 million senile coconut palms, a total of about 7.2 million housing units could be built.

Durability• Untreated coconut wood had limited natural durability, suggesting it cannot be

used in weather exposed conditions. However, above ground durability greater than 10 years has been observed for higher density wood.

• Particularly when the middle 1/3 of stem is used for roof structure, a preservative such as so weignum. But for bottom 1/3 of stem treatment is not needed. Untreated coconut wood is not resistant to termite attack.

• So, timber exposed to weathering should always be treated. The wood must be dried to a wood moisture content corresponding to the climate in which it is to be used.

• According to exposure classes and situations, coconut wood’s life time could be varied. Natural durability above ground is identified as approximately 7 years for high density coconut wood.

PROPERTIES OF COCONUT TIMBERAvailability of coconut timber

In Sri Lanka extensively I have coconut trees but mostly I have in a coconut triangle area that are Kurunagal, Kandy and Colombo and in the coastal areas of the North and East of the country.• Coconut plantation area : 395000 ha• No of trees : 6.3million• Trees available per year : 315000• Replanting cycle : 20yrs

METHODOLOGY

• Generally, coconut timber is not used as a load carrying member or structural member in construction industry due to low or (unreliable) tensile strength and its flexible behavior allow high deflection.

• Our objective is to find the possibility of deflection control and tensile strength improvement method so I proposed composite model with thin metal sheet to achieve our objective.

Comparative material properties

Cost estimation of the project

 

Details Unit QuantityUnit

rate(Rs)Total (Rs)

1Sample collection for property test(coconut

timber sample)Feet 5 42 210

2

Nails and metal sheet

sample(aluminium+Amanosheet) Item 1 360 360

3 Rafter for creep test(4"x2") Feet 5 500 2500

4 Amano sheet(1.5 feet) Feet 3 110 330

5 Nails g 250 125 125

6Transport

Item 1 2800 2800

   Total amount       6325

Work schedule for my report

weak

Task 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Properties testing of

coconut timber

Sample collection for

creep test

modelling

Mid semester

preparation

Record the deflection

reading

Comparison (our

project is effective or

not and economic

analysis)

Final project report

Final preparation

Thank you