Kim, Chul-Ki
(Wood Engineering Division, Forest Products Department, National Institute of Forest Science)
,
Kim, Kwang-Mo
(Wood Engineering Division, Forest Products Department, National Institute of Forest Science)
,
Lee, Sang-Joon
(Wood Engineering Division, Forest Products Department, National Institute of Forest Science)
,
Park, Moon-Jae
(Wood Engineering Division, Forest Products Department, National Institute of Forest Science)
4점 휨 시험에서 지간 길이에 따른 낙엽송 제재목의 휨 성능 변화를 알아보기 위하여 연구를 진행하였다. 연구에 사용된 시험편의 크기는 38(너비) ${\times}$ 89(깊이) ${\times}$ 3,600(길이) $mm^3$이며, 평균 기건 밀도와 함수율은 각각 $543.5kg/m^3$, 10.5%이었다. 낙엽송 육안 등급 1등급 248본을 두 그룹으로 나눠, 지간 거리 1,650 mm와 3,000 mm에서 휨 실험을 진행하여 휨 강도와 휨 탄성계수를 도출하였다. 휨 탄성계수는 유의 수준 5%에서 지간 거리에 따라 차이가 없다고 판단된 반면 휨 강도는 차이가 있었으며, 지간에 반비례하였다. 지간 거리 1,650와 3,000 mm에서 휨 강도의 5% 하한치는 각각 28.65와 25.70 MPa로 확인되었다. 지간 거리에 따른 휨 강도 차이는 백분위 수가 증가함에 따라 커지는 것으로 확인되었으며, 이는 와이블 최약 링크 파손 이론에 의한 치수 효과 때문으로 사료된다. 따라서 지간 대 깊이 비가 15 이상으로만 제한되어 있는 목구조용 실대재 휨 시험법(KS F 2150)에 치수 효과를 고려할 수 있는 방법이 포함되어야 할 것으로 판단된다. 이를 통해 다양한 치수의 제재목에서 얻어지는 강도를 설계 값에 반영할 수 있을 것으로 기대된다.
4점 휨 시험에서 지간 길이에 따른 낙엽송 제재목의 휨 성능 변화를 알아보기 위하여 연구를 진행하였다. 연구에 사용된 시험편의 크기는 38(너비) ${\times}$ 89(깊이) ${\times}$ 3,600(길이) $mm^3$이며, 평균 기건 밀도와 함수율은 각각 $543.5kg/m^3$, 10.5%이었다. 낙엽송 육안 등급 1등급 248본을 두 그룹으로 나눠, 지간 거리 1,650 mm와 3,000 mm에서 휨 실험을 진행하여 휨 강도와 휨 탄성계수를 도출하였다. 휨 탄성계수는 유의 수준 5%에서 지간 거리에 따라 차이가 없다고 판단된 반면 휨 강도는 차이가 있었으며, 지간에 반비례하였다. 지간 거리 1,650와 3,000 mm에서 휨 강도의 5% 하한치는 각각 28.65와 25.70 MPa로 확인되었다. 지간 거리에 따른 휨 강도 차이는 백분위 수가 증가함에 따라 커지는 것으로 확인되었으며, 이는 와이블 최약 링크 파손 이론에 의한 치수 효과 때문으로 사료된다. 따라서 지간 대 깊이 비가 15 이상으로만 제한되어 있는 목구조용 실대재 휨 시험법(KS F 2150)에 치수 효과를 고려할 수 있는 방법이 포함되어야 할 것으로 판단된다. 이를 통해 다양한 치수의 제재목에서 얻어지는 강도를 설계 값에 반영할 수 있을 것으로 기대된다.
This study was conducted to confirm an effect of span length on bending properties of larch dimensional lumber in the four point bending test. The size of specimen in this study was 38 (width) ${\times}$ 89 (depth) ${\times}$ 3,600 (length) $mm^3$, and average air-dr...
This study was conducted to confirm an effect of span length on bending properties of larch dimensional lumber in the four point bending test. The size of specimen in this study was 38 (width) ${\times}$ 89 (depth) ${\times}$ 3,600 (length) $mm^3$, and average air-dry density and moisture content of the specimens was $543.5kg/m^3$ and 10.5%, respectively. Visually graded No. 1 dimensional lumbers of 248 were divided by two groups to compare modulus of rupture (MOR) and modulus of elasticity (MOE). One group was tested in the four point bending test with span length of 1,650 mm, and other was tested with span length of 3,000 mm. While MOE was not different according to span length in 5% significance level, MOR was different in accordance with span lengths and was in inverse proportion to change of span length. Fifth percentiles of MOR in span length of 1,650 and 3,000 mm were 28.65 and 25.70 MPa, respectively. It was confirmed that the difference between MORs in each case increased as normalized rank increased. This is because of size effect in Weibull weakest link failure theory. Therefore, KS F 2150, in which there is only regulation about span to depth ratio of 15 or more, is needed to be revised to contain a method considering size effect for MOR. From the method, various results of bending test with different size of lumber could be used to determine design value of lumber.
This study was conducted to confirm an effect of span length on bending properties of larch dimensional lumber in the four point bending test. The size of specimen in this study was 38 (width) ${\times}$ 89 (depth) ${\times}$ 3,600 (length) $mm^3$, and average air-dry density and moisture content of the specimens was $543.5kg/m^3$ and 10.5%, respectively. Visually graded No. 1 dimensional lumbers of 248 were divided by two groups to compare modulus of rupture (MOR) and modulus of elasticity (MOE). One group was tested in the four point bending test with span length of 1,650 mm, and other was tested with span length of 3,000 mm. While MOE was not different according to span length in 5% significance level, MOR was different in accordance with span lengths and was in inverse proportion to change of span length. Fifth percentiles of MOR in span length of 1,650 and 3,000 mm were 28.65 and 25.70 MPa, respectively. It was confirmed that the difference between MORs in each case increased as normalized rank increased. This is because of size effect in Weibull weakest link failure theory. Therefore, KS F 2150, in which there is only regulation about span to depth ratio of 15 or more, is needed to be revised to contain a method considering size effect for MOR. From the method, various results of bending test with different size of lumber could be used to determine design value of lumber.
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문제 정의
Considering that KS F 2150 proposes that over 15 of the span length-depth ratio, span length can be freely set, there may be some questions with regard to the aforementioned research results. Thus, this study examined the effect of the span length-depth ratio on the bending properties of visually graded Korean pine lumber.
제안 방법
The study compared the bending standard allowable stress calculated by multiplying 0.475 to the 5% lower limit of the bending strength based on span length and the design allowable stress, which was produced by multiplying the size coefficient 1.5, proposed by KS F 2162, to the standard allowable stress of the grade No. 1 larch, proposed by the dimension and quality standards of lumber by the National Institute of Forest Science. The standard allowable stress at 1,650 mm and 3,000 mm was 13.
The study conducted a bending test on 38 × 89 mm lumber to determine the effect of span length on the bending performance of structural lumber.
The study conducted the four-point bending test using a universal material testing machine (MTS, U.S.), aiming to examine the impact of span length on the bending performance using two types of span length (1,650 and 3,000 mm). Here, the span length-depth ratio was 18.
To examine the bending strength change of visually graded No. 1 lumber by span length, the bending strength was calculated by using the maximum load acquired from the bending test. Shown in Table 2 are the statistical results of the bending strength by span length.
While the bending test is in progress, the rotary variable differential transformer (Kyowa, Japan) was used to measure the displacement of the central axis between the span length. Using the measured load and displacement values, the study acquired the modulus of elasticity, including the bending strength and shear deformation of the narrow dimensional section. The defects affecting the grade were placed arbitrarily between the load span length regardless of the tensile and compressive sections.
대상 데이터
Kiln dried 2 × 4 Korean pine lumber (38 (width) × 89 (depth) × 3,600 (length) mm3 ) was purchased from the Jungbu Lumber Distribution Center (Yeoju, Gyeonggi) at National Forestry Cooperatives Federation. The number of the purchased lumbers was 550, and 248 visually graded No. 1 specimens were used in the test. The visual grading was performed by the dimension and quality standard for lumber products by the National Institute of Forest Science, which is shown in Table 1.
The test used Korean pine (Lartix kaempferi (Lamb.) Carriere), which is abundant in Korea and is preferred as structural use, as the specimen. Kiln dried 2 × 4 Korean pine lumber (38 (width) × 89 (depth) × 3,600 (length) mm3 ) was purchased from the Jungbu Lumber Distribution Center (Yeoju, Gyeonggi) at National Forestry Cooperatives Federation.
데이터처리
To verify if the bending performance changes by span length, t-test was conducted on the bending strength and the modulus of elasticity at 5% of significance level. Before starting the t-test, F-test was conducted at 5% of significance level for verifying the significance difference in bending performance based on each span length. While the modulus of elasticity uses the average value as the representative value, the bending strength uses the standard allowable stress based on the 5% of the lower limit as the representative value.
To verify if the bending performance changes by span length, t-test was conducted on the bending strength and the modulus of elasticity at 5% of significance level. Before starting the t-test, F-test was conducted at 5% of significance level for verifying the significance difference in bending performance based on each span length.
이론/모형
Thus, to determine the change of the bending strength by span length, the bending standard allowable stress by condition was determined and compared. To determine the standard allowable stress, 5% lower limit was calculated by the normal, lognormal, and 2-parameter Weibull distributions as the parametric methods. The probability density function of each distribution can be defined in the following.
후속연구
(1994) reported that when the span length-depth ratio was small and the ratio of the modulus of elasticity to the modulus of stiffness, including shear deformation, was large, the span length-depth ratio made a huge effect on the modulus of elasticity including shear deformation. However, it was also reported that the ratio of the modulus of elasticity and the modulus of stiffness including shear deformation would change, and therefore, it is believed that additional research would be necessary on the change of the modulus of elasticity by span length on the bending lumber.
, 2013), Oh (2014), which compared the value from literature to the modulus of size in length through the regression analysis of the log value in strength and the log value in depth in bending strength, is the only research on size effect. Thus, it is determined that additional research on size effect is necessary. Particularly, it is believed that it is possible to include an equation that can consider size effect as in ASTM D1990 or to make revisions in which to include an equation to consider size effect as well as limiting the span length-to-depth ratio in KS F 2150.
참고문헌 (18)
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Korean Standard. 2004. Adjustment factors applied to allowable stresses of structural timber and glued laminated timber: KS F 2162. Korean Standards Association, Seoul, Korea
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Madsen, B. 1992. Structural behavior of timber, Timber Engineering Ltd,
Ministry of Land, Infrastructure and Transport 2017. Statistics on building permisstion and commencement works,
Notification. 2016. Specifications and quality standards of lumber: 2016-8. National Forest Research Institute, Seoul, Korea
Oh, S.C. 2014. Estimation of Depth Effect on the Bending Strength of Domestic Japanese Larch Structural Lumber using Weibull Weakest Link Theory. Journal of the Korean Wood Science and Technology 42(2): 112-118.
Pang, S.-J., Lee, J.-J., Oh, J.-K. 2013. Evaluation of Allowable Bending Stress of Dimension Lumber; Confidence Levels and Size-adjustment. Journal of the Korean Wood Science and Technology 41(5): 432-439.
Park, C.-Y., Pang, S.-J., Park, J.-S., Kim, K.-M., Park, M.-J., Lee, J.-J. 2010. Study of the distribution properties and LRFD code conversion in Japanese larch. Journal of the Korean Wood Science and Technology 38(2): 94-100.
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