Method for measuring the resistances produced on parallel and perpendicular veneers in plywood under nail embedment loading
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The resistance of wooden materials against embedment loading is an important property that governs the performance of timber joints with dowel-type fasteners including nails. When plywood is used as a sheet material in a nailed joint, veneers with different grain directions are simultaneously embedded. The resistance produced on individual veneers has to be measured to elucidate the embedment properties of plywood. This study proposes a method for measuring resistance on individual veneers. Embedment behavior was modeled using springs arranged in parallel, and the equations for calculating resistance on individual veneers were derived. Embedment tests were conducted using structural plywood composed of sugi or karamatsu. Mechanical behaviors of individual veneers were revealed as followings. In the initial range of deformation, the slope of the resistance–deformation relationship of the veneer with grain parallel to the load direction is higher than that of the resistance–deformation relationship of the veneer with grain perpendicular to the load direction. The parallel veneer began to yield under a low range of deformation, and the resistance of the perpendicular veneer under a high range of deformation became almost equal to that of the parallel veneer.
KeywordsPlywood Embedment Nail Load–deformation relationship Spring model
Plywood-sheathed structural diaphragms with nailed joints are commonly used in the construction of light wooden frames or other types of structures. Past studies have shown that the in-plane shear resistance performance of the diaphragms is mainly dominated by the shear performance of the nailed joints [1, 2, 3, 4]. The embedment behavior of wood by a nail is an important characteristic that strongly influences the shear performance of the joints. This characteristic has been used to analyze or predict shear performances on the basis of European Yield Theory [5, 6], elastic foundation model [7, 8, 9, 10, 11, 12], and finite element method [13, 14, 15]. Therefore, the embedment behavior of wood and wood-based materials by a nail must be clarified. The embedment behaviors of various types of wooden materials by dowel-type fasteners have been investigated under different loading conditions. For example, Chui and Ni  conducted embedment tests on spruce, maple, and plywood specimens under reversed cyclic load. Jeon and Kong  evaluated the embedment behavior of glulam specimens made of Japanese cedar by ash pegs. Bal  investigated the characteristics of plywood reinforced with glass fiber fabric. Sawata and Sasaki  evaluated the embedment properties of decay-treated solid wood specimen.
When the load is applied to the structural diaphragms with nailed joints, the joints are loaded from many angles. Then, the influence of the grain direction of wood or wood-based material on the embedment behavior is of major concern. Jeong et al.  subjected glulam specimens made of Japanese cedar to embedment tests. They applied load on longitudinal, radial, and tangential directions and obtained the highest load capacity in the longitudinal direction. Bleron et al.  investigated the embedment characteristics of a gonfolo rose specimen with various grain directions. Under embedment load applied perpendicular to the grain direction, lower load at plastic threshold and higher maximum load were observed than the case of parallel.
Plywood is a wood-based panel material produced by alternately laminating veneers with parallel and perpendicular grain directions. The grain direction of each veneer may affect embedment behavior in plywood–solid wood nail joints. On the shear performance of plywood–solid wood nailed joints, the effect of the grain direction of plywood has been investigated under single-shear loading. Stewart  conducted a single-shear test with nailed joint specimens. He prepared two types of plywood joint specimens. In the joint specimes, the plywood was attached to become that the grain direction of surface veneer were parallel or perpendicular to the load direction. The load–deformation curves of the two types of specimens were not considerably different. Harada et al.  conducted the tests with similar types of specimen and reported no large difference in maximum load. However, the deformation at maximum load in the case of plywood with perpendicular grain of the surface veneer was higher than that of parallel grain. Meanwhile, Demirkir and Colakoglu  reported that stiffness was higher when the surface direction of plywood was parallel to the loading direction than when the surface direction of plywood was perpendicular to the loading direction. Hagiwara  reported no difference in shear performance between the parallel and perpendicular grain directions of plywood. However, when the angle between the grain and load directions was 30°, 45° or 60°, resistance performance decreased under a small load range.
The resistance of nailed joints under single-shear loading is determined by various factors, e.g., bending properties of nail, pull out strength of nail from wood, nail-head pull-through resistance. In addition, the combination of plywood embedment and solid wood embedment resistances is also one of the important factors for it; however, the above studies [22, 23, 24, 25] did not ignore the resistance produced by solid wood. The authors posit that only the resistance produced in plywood should be considered to clarify the influence of the grain direction of plywood. Some studies have conducted embedment tests on plywood specimens [18, 26, 27], and Kamada  showed the experimental data of the two grain directions (i.e., the grain direction of surface veneer was parallel or perpendicular to the load direction) using 3-ply plywood made of larch. According to his data, the higher stiffness was observed when the grain direction of surface veneer was parallel to the loading direction, whereas almost same maximum load was observed in the two. In this study, an embedment test was conducted on plywood specimens with grain directions of surface veneers that are parallel and perpendicular to the load direction. Moreover, to understand the resisting mechanism of plywood precisely, the method for calculating the resistance–deformation relationship exhibited by individual parallel and perpendicular veneers was proposed.
Materials and methods
Series of plywood specimens
Grain direction of surface veneer
Number of specimens
Sugi (Japanese cedar)
Karamatsu (Japanese larch)
Results and discussion
Embedment test results
Method for calculating the load–deformation relationships of parallel and perpendicular veneers
Therefore, the relationship between load P and the embedment deformation of the plywood δply [i.e., only the relationship in part (B)] could be calculated by obtaining the bending deformation of the nail δnail.
The relationships between the resistance and deformation of each veneer could be obtained by calculating the resistances under various δply.
Resistance–deformation behavior of each veneer
Comparing the results for parallel and perpendicular veneers (shown as solid and dotted thick lines) reveals that the resistance of the former had a larger slope than the that of the latter during the initial stage of the test. The slope of the parallel veneer began to become small gradually under approximately 0.3 mm of deformation, then kept the resistance after it. The perpendicular veneer increased its resistance until under approximately 2.0 mm of deformation. The two thick lines intersected when deformation reached approximately 2.0 mm (i.e., no difference attributable to the resistance contributed by the grain direction of the veneer was observed under such a large deformation).
Next, the obtained behavior (Fig. 9) was discussed by comparing with past reports on the anisotropic strength properties of plywood. Asano and Tsuzuki  attempted to reveal the compression properties of veneer by conducting compression tests on Lauan plywood specimens. They reported that perpendicular veneer shows low modulus of elasticity and strength. This tendency was experimentally confirmed by Kuwamura , who subjected veneer specimens to an in-plane compression test under steel-plate loading and found that the compression strength of parallel veneer was approximately 7.5 times higher than that of perpendicular veneer. Although the resisting behavior in this study (Fig. 9) revealed the difference between parallel and perpendicular veneers, the difference was not as large as the results of above reports [32, 33]. This result may be attributed to differences in loading conditions, loaded with steel plate or nail. Some researchers have reported [34, 35] that when fastener diameter is sufficiently small, the contribution of grain direction to differences in characteristics of embedment behavior decreases. The diameter of the nail used in this study is small enough to reduce the influence of grain direction. This effect, in turn, may have resulted in the smaller differences than compression results obtained under steel-plate loading.
To analyze the mechanical properties of nailed joint, precisely understanding of the deformation around nail is important. This study conducted the nail embedment test with plywood specimens for comparing the behaviors between different grain direction in surface veneer. Sugi and karamatsu plywood specimens with thicknesses of 12 mm (5-ply) were loaded by a CN75 nail in accordance with the ASTM setup. This test revealed that the grain direction of the surface veneer considerably affects stiffness but negligibly affects maximum load under embedment resistance.
Additionally, the authors tried to calculate the resisting behavior produced on individual veneers. The presented method clarifies the mechanical contribution of veneers with parallel and perpendicular grain to the resisting behavior of plywood under embedment loading by a nail, which helps to understand the deformation of plywood in detail. The proposed method was developed on the basis of a plywood deformation model constructed with springs arranged in parallel. The experimental results showed that under low deformation, the parallel veneer had a higher slope than the perpendicular veneer. The parallel veneer began to yield under approximately 0.3 mm of deformation, whereas the perpendicular veneer began to yield under approximately 2.0 mm of deformation. The deformation behaviors of the two veneers were negligibly different when deformation exceeded 2.0 mm. It is hoped that the results become one of the basic academic knowledge for evaluating the mechanical behavior of nailed joint under loading from many angles.
Future investigations are required to confirm the validity of the proposed method. The proposed method must be applied to plywood specimens with other thicknesses/compositions because this study focused on limited types of plywood specimens. Additionally, the proposed model assumes that the shape of the nail in plywood remains linear and embedment deformation is assumed equal in the five veneers. Under actual loading conditions (e.g., single-shear load), the model should be modified to consider the unequal embedment deformation among veneers in plywood. Therefore, future studies are required to use the presented results for analyzing the mechanical behavior of plywood-sheathed nailed joints.
KO performed the embedment test, and was a major contributor in writing the manuscript. MH was a co-performer of the test. TS and KM contributed to the interpretation of the data. MH, TS, and KM contributed to improve the quality of the manuscript. All authors read and approved the final manuscript.
This study is funded by Research grant #201713 of the Forestry and Forest Products Research Institute.
The authors declare that they have no competing interests.
Availability of data and materials
The test materials, test method, and the data we recorded are shown in the manuscript with uncolored descriptions.
This study was financially supported by Research grant of the Forestry and Forest Products Research Institute (Number: #201713).
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