Strength Performance of Full-Size Structural Timber of Dryobalanops Species of Sarawak, Malaysia

Transcription

1 Strength Performance of Full-Size Structural Timber of Dryobalanops Species of Sarawak, Malaysia Alik Duju Senior Researcher TRTTC, Sarawak Forestry Corporation Kota Sentosa, 9325 Kuching, Sarawak, Malaysia & Badorul Hisham Abu Bakar Senior Lecturer Engineering Campus, University of Science Malaysia Seri Ampangan, 143 Nibong Tebal, Pulau Pinang, Malaysia Summary A total of s ix Dryobalanops species viz., beccarii, fusca, oblongifolia, lanceolata, rappa and sumatrensis found in Sarawak, Malaysia were used in this study. The objective of the study was to evaluate the strength performance of full size structural timber of the species. Samples with nominal dimensions of 5 mm by mm were used for the tests. Strength values were determined based on British Standards namely BS 582:1979 and BS 373:1957. Universal testing machine and Horizontal tensile testing machine were used to determine the strength values. It was found that beccarii was the strongest in terms of bending strength (MOR) followed by sumatrensis, lanceolata, oblongifolia, fusca and the weakest was rappa. The mean values of MOR of the timber species was MPa and MPa at green and air-dried conditions, respectively. This showed an increase of 17.14% in their strength. With respect to the tensile strength parallel to grain (TS), beccarii also yielded the highest values followed by oblongifolia, lanceolata, sumatrensis, fusca and the lowest was rappa. The mean values of TS were 62.1 MPa and 71.5 MPa, respectively. An increase of 15.14% in strength was observed. The rates of change in MOR and true modulus of elasticity values for every percent changes of moisture below fibre-saturation point were 1.5% and.36%, respectively. The corresponding values for TS were 1.32% and modulus of elasticity in tension was.96%. The strength ratios of MOR for almost defect-free full size structural and small clear specimens were.75 and.77 under both testing conditions, respectively. 1. Introduction Sawn timber is widely being used as construction materials in the housing and construction industry particularly in Sarawak, Malaysia. Mamit (1987) identified 12 timber species commonly used in this industry. One of the timbers identified is from Dryobalanops species. For years, strength properties of timbers were obtained from testing of small defect free specimens. These strength values are being used to classify timber species to timber groups, which are used to derive the grade stresses. However, the assumptions of stress ratio used to derive the allowable unit stress could not work effectively for every timber species. It is well known that using the results from full size structural timber was considered to be more reliable to allocate the design stress as to eliminate the risk of stress ratio assumptions. In addition, the values will reflect more on the actual strength of timber in use. So far, there is lack of information on strength properties on full size structural timber hence engineers, architect, designers and builders tend to use other materials such as concrete and metal for building and construction. The proper and effective utilization of timber as building and construction materials very much depend on the experience and understand ing the technical data and also structural behavior with

2 regards to each particular timber species and species group. Therefore, the objectives of the study are to obtain the physical and mechanical strength information on full size structural properties of the timber species. 2.1 Materials and Testing Methods Materials and sampling methods A total of six Dryobalanops species found in Sarawak, Malaysia were obtained and used in this study. The timber species viz., beccarii, fusca, oblongifolia, lanceolata, rappa and sumatrensis. The species were selected and identified at the felling sites while re-confirmation to the species level was done at the laboratory. The trees selected for sampling should be in a good form and with minimum of 45 mm dbhob. The selected trees were obtained from two types of forests namely Mixed Dipterocarp Forest and Peat Swamp Forest. Sampling of specimens was made from bottom to top portion of the tree. The log was then ripped into half through the pith to obtain the flitches. The flitches were subsequently further ripped across the diameter and converted to the sample size of 5 mm by mm by 2 mm and 5 mm by mm by 3 mm for static bending and tensile test, respectively. The timber samples were then divided equally into two batches namely for green and airdried conditions. The total number of samples for each species and types of test was for both conditions. Under the air-dried condition, the samples were seasoned from green to air-dry under normal atmospheric condition until the moisture content of the samples reached the equilibrium moisture content. Once the samples reaching the air-dry conditioned, the samples were planed and machined to the required sizes. Prior to strength tests, samples were conditioned at the temperature of 2±3 C Testing Methods The procedure of testing was adopted according to the British Standard BS 582:1979. Static bending tests were carried out with the Universal testing machine of having a maximum loading capacity of 2 kn. The two-point loading system was applied on 18 mm span in which the loading direction was performed perpendicular to the longitudinal axis of the samples. The distance between the supporting and the loading points and the distance between each loading point was mm respectively. Hence, the apparent and true modulus of elasticity (E app and E tru ) and modulus of rupture (MOR) were obtained. Tensile test was carried out with the Horizontal tensile machine of having a maximum loading capacity of kn. The total span of 2 mm was used and the elongation in mm gauge length at mid span of the samples was recorded and measured using personal computer. The load direction was parallel to the direction of the fibres of the test samples. Hence, tensile strength (TS) and modulus of elasticity in tension parallel to grain (E ts ) were obtained. The small clear specimens were obtained from full size structural specimens. The size of test specimen was 2 mm by 2 mm by 3 mm. Centre-point loading was applied to the specimen with the span of 28 mm. The procedure of testing was adopted according to the British Standard BS 373:1957. Universal testing machine of having loading capacity of 5 kn was used to determine the modulus of elasticity (MOE sc ) and modulus of rupture (MOR sc ). 2.2 Moisture Content and Density Determination The specimens used for sampling of moisture content were free from defects such as knots, resin pockets and other natural defects. The moisture content was determined by the oven-dry method in which the specimens were oven-dried at 13±2 C until a constant weight was obtained. For basic

3 density, the water displacement method was used to get the initial volume and weight was measured by electronic balance. Hence, the basic density of test specimens was calculated from the ratio of ovendried weight to green volume at the time of test (at air-dried condition). 3 Results and Discussion 3.1 Strength Properties Six timber species of Dryobalanops were tested for physical and mechanical properties. Physically, the timbers were so similar that it was impossible to differentiate and identify the species individually using naked eye. It could be identified correctly to species level by using microscopic examination on micro- structures of the timber. For commercial and utilization purposes, the timbers were grouped under Dryobalanops species. Hence, the physical and mechanical strength data of species were pooled for further statistical analysis. The mechanical strength of the timber were evaluated both at green and air-dried conditions. The main test results viz., the mean values with coefficient of variation in parenthesis of E app, E tru, MOR, E ts, TS, density and moisture content for full size structural and also MOE sc and MOR sc for small clear specimens both for green and air-dried conditions as shown in Table 1. A total of 2 pieces of timber samples comprising all the six species. Tab 1 Result on the physical and mechanical properties of Dryobalanops species Timber species beccarii fusca oblongifolia lanceolata rappa sumatrensis Dryobalanops species Con n E app (GPa) G AD G AD G AD 17.9 G AD 19.3 G 15.1 AD G 18. AD G AD (12.4) (14.8) E tru (GPa) (11.3) (14.7) STRUCTURAL MOR E ts TS (MPa) (GPa) (MPa) (17.2) (11.3) (19.2) (16.6) (12.1) (15.2) D (g/cm 3 ) (7.6).77 (7.7) MC (%) (24.4) (7.4) SMALL CLEAR MORsc MOEsc (MPa) (GPa) (14.6) (11.9) (11.5) (1.2) It was indicated that the strength values of E app were lower compared to the E tru and this was due to the effect of shear deformation that occurred during the testing of the specimens. It was calculated that the apparent modulus of elasticity was 8.18% and 5.17% lower than true modulus of elasticity tested at green and air-dried conditions, respectively. The mean values of bending strength for full size structural specimens of beccarii was 74.2 MPa, fusca was MPa, D oblongifolia was Mpa, lanceolata was 69.3 MPa, rappa and sumatrensis was MPa at green condition. Under the air-dried condition, the bending strength of beccarii was 9.5 MPa, fusca was MPa, oblongifolia was MPa, lanceolata was 81.8 MPa, rappa 71.2 MPa and sumatrensis was MPa. As far as bending strength was concerned, beccarii was the strongest followed by sumatrensis, lanceolata, oblongifolia, fusca and rappa was the weakest.

4 The overall mean for the bending strength of the species was MPa with 17.2% coefficient of variation and MPa with 16.6% coefficient of variation tested at green and air-dried conditions, respectively. This showed an increase of 17.14% in term of their strength. In term of their tensile strength, beccarii exhibited the highest strength of MPa followed by D oblongifolia with MPa, lanceolata with MPa, sumatrensis with 61.7 MPa, fusca with MPa and the lowest rappa with MPa. The overall mean value of tensile strength of the species was 62.1 MPa with coefficient of varia tion of 19.2% tested at green condition. At air-dried condition, beccarii gave the highest tensile values of MPa and was followed by oblongifolia with MPa, lanceolata with 71.2 MPa, sumatrensis with 7.18 MPa, fusca with 68.3 MPa and rappa with MPa in the descending order. The overall mean of tensile at air-dried condition was 71.5 MPa with coefficient of variation of 15.2%. An increase of 15.14% strength was observed. The overall mean values of bending strength using small clear sample were MPa with coefficient of variation of 14.6% and MPa with coefficient of variation of 11.5% at green and air-dried conditions, respectively. The overall mean values of modulus of elasticity of defect-free samples were GPa with coefficient of variation of 11.9% and 16.1 GPa with coefficient of variation of 1.2% at both conditions, respectively. These data however could not directly be used in timber design, as a lot of factors such as duration of loading, size and shape of member and safety factor have to be considered. In addition, the data from small clear specimens did not reflect the actual strength of full size structural components of timber. 3.2 Density and Moisture Content Based on the density, beccarii was the heaviest followed by sumatresis, lanceolata, oblongifolia, rappa. and fusca was the lightest amongst them. The overall mean for basic density and air-dried density of timber species were.67 g/cm 3 and.77 g/cm 3 respectively. Under its density the timber could be classified under Medium Hardwood as specified under the Malaysian Grading Rules. The mean of moisture content of the species for green condition ranged from 42.36% to 65.25% with their overall mean values of 54.5% with 24.4% coefficient of variation. This range of moisture content was considered as green condition as the moisture content had far exceeded the fibre saturation point. Alik and Nakai (1997b) in their study showed that above fibre saturation point, the strength values appeared to be constant as the moisture content increased. In this case, within the green condition, there was no affect on the strength of timber with an increase or loss of moisture content. The mean value moisture content of air-dried sample ranged from 15.8% to 16.12% and their overall mean values were 15.56% with 7.4% coefficient of variation. Below the fibre saturation point, it would that be very much affected the timber strength. It was observed that there was a high variation in moisture content in green compared to air-dried timber and this could lead to the conclusion that drying would result in a more uniform distribution of moisture content inside the wood. 3.3 Effect of Moisture Content on Strength Properties As indicated in the study conducted by Alik and Nakai (1997b) that the fibre saturation point was 27% for rappa. Hence, the value of 27% was used for reference point of fibre saturation point of the species. Thus, the rate of change for bending strength and modulus of elasticity were 1.5% and.36% respectively for a decrease of 1% of moisture content below fibre saturation point. It was indicated using small clear specimen that the rate of change for modulus of rupture and modulus of elasticity were 3.3% and 1.55% respectively for every 1% changes in moisture content below fibre saturation point. The values indicated that there was a little increased both for modulus of elasticity and modulus of rupture for full size structural compared to the small clear specimens. The possible reason that

5 contributed to the trend was that structural size sample contained a lot of wood defects particularly knots both at flatwise and edgewise orientations of timber sample. Another possible reason was the inherent characteristics of the timber itself and the wide moisture gradient in the bigger timber samples. The rate of change for tensile strength and modulus of elasticity in tension were 1.32% and.96% respectively for a decrease of 1% of moisture content. Figures 1 and 2 showed the normalized ranks of bending and tensile strength in green and air-dried conditions, respectively. The figures indicated that almost parallel increase both for bending and tensile strength starting from the 1 th to the 9 th percentile values Modulus of Ruptrure (MPa) 8 Green Air-dried Percentile Fig 1 Normalized frequency of bending strength at green and air-dried conditions 12 Tensile Strength (MPa) 8 2 Green Air-dried 2 8 Percentile Fig 2 Normalized frequency of tensile strength at green and air-dried conditions It was calculated that the values of modulus of rupture at the 5 th percentile were MPa and 58.1 MPa at green and air-dried conditions respectively. This showed that there were 18.78% different between green and air-dried at the 5 th percentile values. The values for the tensile strength at the 5 th percentile were MPa and MPa at both conditions respectively. This showed an increase of 26.12%.

6 3.4 Relationship between Small Clear and Full Size Structural Specimens in Bending The coefficient of correlation between small clear and full size structural specimens of bending strength tested both at green and air-dried conditions as shown in Figures 3 and 4, respectively. It was indicated that their correlation was.56 and.55 Modulus of Rupture (Small ) (MPa) y =.66x r =.56** Modulus of Rupture (Structural) (MPa) Fig 3 Relationship between small clear and full size structural specimens in bending strength at green condition 18 1 Modulus of Rupture (Small) (MPa) y =.x r =.55** Modulus of Rupture (Structural) (MPa) Fig 4 Relationship between small clear and full size structural specimens in bending strength at air-dried condition The results obviously showed that a weak correlation was found between small clear and structural size timber in term of their modulus of rupture. The conclusion that could be deduced from this study was that the strength values obtained from small clear wood specimens did not work well to be used to correlate the strength of full size structural timber. The possible reason was that structural size samples contain wood defects that could be the major factor affecting their correlation. The best way to express their relationship is through the correction factors through strength ratio. It was calculated that the ratios of almost defect-free structural size and small clear specimens of the species were.75 and.77 at green and air-dried conditions, respectively. The finding was consistent with the study conducted by Alik and Nakai (1997a) using Dipterocarp species.

7 4. Conclusions Based on the results of this study, the findings as follows: i) beccarii was found to be the strongest and rappa the weakest in term of bending and tensile strength. ii) It was observed that their timber strength increased 17.14% for bending and 15.14% for tensile respectively from green to air-dried condition. iii) The rate of change found to be 1.5% and 1.32% in bending and tensile strength, respectively for every 1% changes of moisture below fibre saturation point. iv) Based on their density, the timber species could be classified under medium heavy hardwood. v) The bending strength ratio was.75 and.77 with respect to almost defect free full size structural timber and defect free small clear specimens vi) Weak correlation were observed between the strength of small clear and full size structural specimens 5. Acknowledgements The authors wish to express their sincere gratitude and appreciation to Associate Professor Dr. Mohd Ariff Jamaludin of University of Technology Mara (UiTM) and Dr. Takashi Nakai of ex-staff of FFPRI, Tsukuba, Japan for their suggestion and comments. The authors like to acknowledge to Andrew Nyorik Nibu, Nungah Liang, Aini Siri and Yeo Hui Choo of Sarawak Forestry Corporation for assisting preparation and assisting in sample testing. 6. References [1] Alik,, and Nakai, T. 1997a. Preliminary Study for Structural Grading Based on Full Size Bending Test of Resak durian and Keruing utap of Sarawak. Proceedings of the International Tropical Wood Conference, Kuala Lumpur. June 17-2, pp [2] Alik,, and Nakai, T. 1997b. Effect of Moisture Content on Bending Properties of Wood Proceedings of the TRTTC/JICA Research Seminar 97. Kuching. pp [3] Anon., Methods of Testing Small Clear Specimens of Timber. British Standard Institution. BS 373 : pp. [4] Anon., Methods of Test for Determination of Certain Physical and Mechanical Properties of Timber in Structural Sizes. British Standard Institution. BS 582 : pp. [5] Anon., The Malaysian Grading Rules for Sawn Hardwood Timber Edition. Ministry of Primary Industry. Malaysia. 19 pp. [6] Mamit, J Utilisation of Timber for Housing Construction in Sarawak. TRTTC Technical Report No. TR/11. Forest Department Sarawak. 3 pp.