WORKING STRESSES OF MAHOGANY (SWIETENIA MACROPYHLLA KING)

PHILIPPINE WOOD SEASONED FOR 45 DAYS

ANDRES N. MANAOIS JR.

ROJIE V. LACUESTA

MICAHEL VLADIMIR V. BUENAVENTURA

OCTOBER, 2012

Introduction

Philippines is reach of natural resources one of which is timber. Timber is very useful most

especially when building materials like cement, aggregates and the like is not available in a certain area;

thus, it is the last resort in the construction of houses or shelters.

The National Structural Code of the Philippines (NSCP) established by the Association of

Structural Engineers of the Philippines (ASEP) discussed and set standards in the design of timber

structures. ASEP based the codes and standards with the standards of ASTM, etc.

NSCP entails the design procedure in proportion with the obtained strength of Philippine

timbers. Using standardized procedure they determine the working stresses for visually-graded

unseasoned timber for the reason of safe design. Working stresses are bending and tension parallel to

the grain, compression parallel and perpendicular to the grain, shear parallel to grain and modulus of

elasticity.

But sometimes because timbers are located in mountainous area, thus, making it hard for

harvest and availability, it will take a period of time for the lumber to be delivered in the construction

site. Is it safe when the design of timber is based on NSCP’s working stress of unseasoned wood and yet

timbers in the construction site undergo a period of time of season after harvest?

With the above question the researcher was motivated to determine the working stresses of

seasoned Mahogany after 45 days.

Objectives

The main purposed of the study is to determine if Mahogany seasoned 45 days will have a

greater working stress compared to NSCP standards. Specifically it aims to determine the following

working stresses:

1. Bending

2. Shear

3. Compression Perpendicular to the grain

4. Compression Parallel to the grain

5. Tensile

6. Moisture Content

Methods

The specimen was taken at Kiangan, Ifugao and is more or less 20 years old. After harvest the

specimen was seasoned for 45 days (air dried) and was prepared for testing. Testing Equipment use was

the Universal Testing Machine. Moisture Content was also determined.

1. Bending test

Size: 2” x 2” x 30”

2. Shear Parallel to the grain

3. Compression Perpendicular to the grain

Size: 2” x 2” x 8”

4. Compression Parallel to the grain

Size: 2” x 2” x 8”

5. Tensile Test

Size: 1” x 1” x 15”

6. Moisture Content

Moisture Content was determined by weighing the sample and then placing in the oven

for 24 hours. After the drying, weight of sample is again determined.

Results

1. BENDING TEST

Specimen no. 1:

Dimensions: Width: 51 mm

Depth: 52 mm

Length: 763.5 mm

Load(KN) Deflection at Mid-span (mm) Bending Strength at Failure(MPa)

1 2

2 4

3 6

4 8

5 18

6 27

6.18 31 51.30

Specimen no. 2

Dimensions: Width: 54 mm

Depth: 54 mm

Length: 764 mm

Load(KN) Deflection at Mid-span (mm) Bending Strength at Failure

1 3.5

2 5

3 6.5

4 8.5

5 10

6 13

6.90 19 50.22

Specimen no. 3

Dimensions: Width: 51 mm

Depth: 54 mm

Length: 765 mm

Load(KN) Deflection at Mid-span (mm) Bending Strength at Failure

1 2.5

2 5

3 7

4 8.5

5 10.5

6 13.5

7 17

7.14 18.5 55.09

Mahogany Load(KN) Deflection at Mid-span (mm) Bending at Failure (MPa)

Average

Magnitude

6.74 34.25 52.20

2. SHEARING TEST

Specimen No. Sheared Area mm2

(b x d)Load at Failure (KN) Shear Stress (MPa)

1 53 x 53 20.34 7.24

2 52 x 53 17.74 6.44

3 53 x 53 16.60 5.90

Average Magnitude 18.23 6.53

3. COMPRESSION PERPENDICULAR and PARALLEL TO THE GRAIN TEST

Specimen no. Compressive StrengthPerpendicular to the Grain (MPa)

Compressive StrengthParallel to the Grain (MPa)

1 7 24

2 8 31

3 10 27

AverageMagnitude

8.33 27.33

4. TENSILE TEST

Specimen No. Area mm2

(b x d)Load at Failure (KN) Shear Stress (MPa)

1 51 x 52.5 31.86 42

2 54 x 54 21.28 26

3 51 x 54 24.8 32

Average Magnitude 25.98 33.33

5. MOISTURE CONTENT

No. of Specimen 1 2 3

Original Weight, grams 490 488 494

Oven Dry Weight, grams 338 362 354

Mahogany Original Weight Oven Dry Weight Moisture Content

Average Magnitude 490.67 351.33 39.66 = 40%

Findings

Based on the result the following was determined:

1. The bending stress at the failure is 52.20 MPa. The bending stress exceeds the NSCP Standard

which is 16.5, 13.0 and 10.3 MPa for 80%, 63% and 50% stress grade respectively.

2. The shear stress at the failure is 6.53 MPa. The shearing stress exceeds the NSCP Standard

which is 2.71, 2.13 and 1.69 for 80%, 63% and 50% respectively.

3. The compressive stress perpendicular to the grain at failure is 8.33 MPa. The compressive stress

parallel to the grain exceeds the NSCP Standard which is 10.5, 8.24 and 6.54 MPa for 80%, 63%

and 50% respectively.

4. The compressive stress parallel to the grain at failure is 27.3 MPa. The compressive stress

perpendicular to the grain exceeds the NSCP Standard which is 3.83, 3.01 and 2.39 MPa for 80%,

63% and 50% respectively.

5. The tensile stress parallel to the grain at failure is 33.33 MPa. The tensile stress parallel to the

grain exceeds the NSCP Standard which is 16.5, 13.0 and 10.3 MPa for 80%, 63% and 50% stress

grade respectively.

6. The moisture content is 40% of the weight of the specimen.

Conclusion

Based on the findings the following was concluded:

1. That the 45 days-seasoned mahogany is stronger than unseasoned with regards to its working

stresses determined.

2. That the standard set by NSCP is very safe because it was based on unseasoned timber.

3. That the less the moisture contents of the mahogany it becomes stronger until it reaches its

optimum moisture content and optimum strength.

4. That mahogany is strong in bending compare with compression and shearing.