Bending strength and modulus of elasticity of BC coastal timbers

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1 Summary Bending strength and modulus of elasticity of BC coastal timbers Yue Chen Graduate Student Frank Lam Professor J. David Barrett Professor Emeritus Department of Wood Science, University of British Columbia 2424 Main Mall, Vancouver, B.C. Canada, V6T 1Z4 A testing program has been conducted to investigate the bending strength and modulus of elasticity of British Columbia (BC) coastal Douglas-fir and Hem-Fir timber. Old-growth (fine grain) and secondgrowth (coarse grain) Douglas-fir and Hem-Fir timber was sampled in two sizes, i.e. 105 mm 210 mm and 105 mm 305 mm. The bending strength and modulus of elasticity were measured in a proof loading test which was conducted in accordance with the requirements of ASTM D 4761 and Japanese test protocol requirements. The proof load stress was selected to break approximately 15 percent of each sample. The characteristic mean modulus of elasticity and the characteristic 5 th percentile of bending strength were derived as a function of member size, species and grade. The modulus of elasticity and bending strength were adjusted to a standard moisture content of 15 percent during data analysis according to ASTM D and a Linear Surface Model moisture adjustment method respectively. Introduction Timber is the most ancient and complex organic structural material in earth and widely used for buildings, ships, furniture and instruments. The efficient use of timber in structures requires accurate material property information for development of refined design procedures, which will benefit the construction industry, especially in North America where timber is widely used for residential and light office and industrial buildings. Canada is the world s largest exporter of softwood timber products. Studies on engineering properties of timber material attract more and more attention from researchers, designers and code officials (Lam et al., 2001, 2005). British Columbia (BC) produces much of the softwood timber shipped internationally. Therefore, it is important to maximize the market value of BC coastal structural and appearance grade wood products. The design properties of large cross-section members graded to the National Lumber Grades Authority (NLGA) grading rule for Dimension Lumber and Beam and Stringer grades published in the Canadian and US building codes have not been confirmed with fullsize tests. Recent studies conducted by the Coast Forest Products Association suggest that the design properties for BC coastal Douglas-fir and Hem-Fir (a commercial species combination including two species: western hemlock and amabilis fir), maybe significantly under-rated. The objective of this test program is to investigate the bending strength and modulus of elasticity of 105 mm 210 mm and 105 mm 305 mm BC coastal Douglas-fir and Hem-Fir. Materials and Methods Sampling

2 Test specimens were sampled from production of three BC coastal mills in order to provide a representative sample of the BC coast resource. The materials were sampled green and then kiln dried using commercial kiln schedules to a target moisture content (MC) of approximately 15 percent. Following drying the lumber was shipped to the Timber Mechanics Laboratory at the University of British Columbia for grading and testing. Grading Visual grading is the most common type of grading performed on lumber. It is done based on both appearance and strength factors. The timber was graded by a senior grading supervisor from the Canadian Mill Services Association based on the NLGA Standard Grading Rules. Each piece was graded to both the NLGA Joist and Plank grade rule and the NLGA Beam and Stringer grade rule. The grade controlling defect and the maximum strength reducing defect (MSRD) were identified and recorded for each specimen. Specimen dimensions and moisture meter readings were taken at the time of testing using a Delmhorst 2-pin moisture meter. The dimensions and moisture meter readings were taken at ¼, ½ and ¾ of the member length. All the information was recorded. Testing Bending tests (Fig. 1) were conducted to establish structural properties of lumber in accordance with the requirements of ASTM D 4761 (Standard Test Methods for Mechanical Properties of Lumber and Wood-Base Structural Materials) and Japanese test protocol requirements summarized as follows: 1/3 point loading conditions 18 to 1 span to depth ratio Tension edge of the bending specimen was selected at random MSRD located randomly within the total test span Tests conducted at a loading rate to cause failure in 1 to 5 minutes The total neutral axis deflection at mid span was measured using a full-span yoke. The total applied load and mid-span beam deflection were monitored and recorded by a computer system. The data was used to calculate the maximum applied stress and the long-span MOE (E L ) for each specimen based on the actual dimensions recorded at the time of test. The member deflection was also measured using the machine deflection measured by the stroke of the load actuator. The MOE was determined over a load range of 0 to N (4000 lb). Following the MOE measurement each member was loaded to a proof load level that was expected to break approximately 15 percent of the sample. The target proof load levels are shown in Table 1. Mid-span deflection measurement device h/2 d h Location in Frame Load Points Fig. 1 Third-point bending test configuration.

3 Table 1. Target proof loads for Douglas-fir and Hem-Fir specimens. Douglas-fir Hem-Fir 105 mm 210 mm 105 mm 305 mm 105 mm 210 mm 105 mm 305 mm Select Structural N (9000 lb) N (9500 lb) N (11000 lb) N (11500 lb) & No.1 No N (8000 lb) N (8000 lb) N (10000 lb) N (10500 lb) Results and Analysis Cumulative probability distributions of Modulus of Elasticity (MOE) and Modulus of Rupture (MOR) of Douglas-fir and Hem-Fir are shown in Fig. 2. Since both of the two grades, i.e., Joist & Plank (J&P) and Beam & Stringer (B&S), are in very similar, only the J&P grade is presented here. The MOE results are adjusted to 15 percent moisture content according to procedures specified in ASTM D , Standard Practice for Establishing Allowable Properties for Visually-Graded Dimension Lumber from In-Grade Tests of Full-Size Specimens (Annual Book of ASTM Standards, 2004). The MOR is adjusted to 15 percent moisture content using the linear surface model (Barrett and Lau, 1994). It is clear that the Select Structural grade presents higher mechanical properties as compared with grades No. 1 and No. 2. In some cases, grade No. 2 is stronger than No. 1 even though the latter is supposed to have a higher value according to the grading rule. The characteristics properties of modulus of elasticity and bending strength are shown in Table Douglas-fir 105mm x 210mm (J&P) (a) Douglas-fir 105mm x 305mm (J&P) (b) Hem-Fir 105mm x 210mm (J&P) (c) Hem-Fir 105mm x 305mm (J&P) (d)

4 Douglas-fir 105mm x 210 mm (J&P) (e) Douglas-fir 105mm x 305mm (J&P) 0 (f) Hem-Fir 105mm x 210mm (J&P) (g) Fig. 2 Cumulative Probability Distributions of MOE and MOR Hem-Fir 105mm x 305mm (J&P) Table 2. Summary statistics NLGA Grades (MC=15%). HF DF 105 mm 210 mm 105 mm 305 mm 105 mm 210 mm 105 mm 305 mm SS SS SS SS Mean E (Gpa) Tol. Limit R5 Mean E (Gpa) Tol. Limit R ( Psi) ( Psi) ( Psi) ( Psi) (3526 Psi) \ 17 (1519 Psi) \ 38 (4362 Psi) (3887 Psi) (3500 Psi) (3393 Psi) No.2 No.2 No.2 No ( Psi) ( Psi) ( Psi) ( Psi) (1916 Psi) (2031 Psi) \ \ (3738 Psi) (2489 Psi) (2206 Psi) (2210 Psi) (h)

5 Conclusions This paper presents the results of the evaluation of MOE and bending strength of the 105 mm 210 mm and 105 mm 305 mm BC coastal Douglas-fir and Hem-Fir. The testing was conducted using a proof loading methodology that provides the complete distribution of MOE and the lower tail of the bending strength distribution. NLGA Joist & Plank and Beam & Stringer grading rules are employed in the study. The data developed in this study will be used to make technical submission to national building code committees for review of the design properties currently assigned to coastal Douglas-fir and Hem-Fir. The code submissions will be prepared for the Canadian Code for Engineering Design in Wood, the American Lumber Standards Committee and the Japanese Ministry of Lands Infrastructure and Transport. The code submission will include consideration of size factors appropriate for 105mm (4- inch) thick material and design properties appropriate for allowable stress design codes and Limit States Design codes. Acknowledgement Gratitude is extended to the Coast Forest and Lumber Association, Canada for providing funding for the project. References [1] Lam F., Barrett J.D., and Nakajima S Engineering properties of Hem-Fir used in Japanese post and beam housing. Forest Products Journal, Vol. 51, No. 10, pp [2] Lam F., Barrett J.D., and Nakajima S Influence of knot area ratio on the bending strength of Canadian Douglas fir timber used in Japanese post and beam housing. Journal of Wood Science, Vol. 51, No. 1, pp [3] American Society for Testing and Materials Annual Book of ASTM Standards. Section 4, Construction, Vol , Wood. ASTM, West Conshohocken, PA. [4] Barrett J.D., and Lau W Canadian Lumber Properties. E.D. Jones, ed. Canadian Wood Council, Ottawa, Canada.