The Effect of Moisture Content on the Screw Holding Capacity of Birch and Pine Plywood

Plywood and OSB are used for wooden construction as covering materials. Solid lumber and structural composite lumbers, such as LVL, PSL, and OSL, are used as the main members. Different type of nails, screws, and bolts are used to assemble the members to each other in connection points of the wooden construction. There are many different factors that affect the durability of the connection points. For example, these factors include the species of the tree, the composition of the material, density of material, type of fasteners, and the thickness of the material. In previous studies, the effects of these factors on the fasteners’ holding capacity were studied. Therefore, the effects of moisture content on the screw holding capacity of plywood were investigated at different moisture percentages, i.e., approximately 0, 10, 20, 30, and 40% and at the maximum moisture content. According to the data that were obtained, how the moisture content affects the screw holding capacity of birch and pine plywood were determined.

The appearance of the surface of the plywood is one of the major properties of plywood that is used in the production of furniture and as interior decoration.

tHE  mechanical performance of the plywood is a major property for plywood that is used in the construction of buildings, forming cement, and in some other heavy-duty usages. Fastener holding capacity of the plywood is one of the most important properties of plywood that is used in the construction of buildings, and many researchers have conducted studies to address this issue. For example, Erdil et al. (2002) determined the screw holding capacity of hardwood and softwood plywood, and they reported that the screw holding capacity of plywood can be predicted as functions of the screw diameter, the depth of penetration, and the density of the panel. Herzog et al. (1999) determined nail- withdrawal and nail head-pull through strength for different conditions. But, there are limited studies about the effects of moisture content on the screw holding capacity of plywood. Therefore, in this study, we focused on how the moisture content affects the screw holding capacity of pine and birch plywood.

Results and Discussions Table 1 provides the moisture content, density, and screw holding capacity of pine plywood and birch plywood. When the data in Table 1 were analyzed, it was observed that the moisture content of the test groups were very close to dedicated to moisture content of the groups. In addition, there was no significant difference between the moisture contents of the pine and birch plywood. But, there was a significant difference between the densities of the pine and birch plywood. This result was expected due to the difference in the density of wood from pine trees and birch trees. Concerning this issue, in previous studies, many researchers have reported that the density of plywood was greater than the density of the wood that was used to produce the plywood (Bal and Bektaş, 2014; Çolak et al., 2003; Örs et al., 2002; Özen, 1981). In addition, hardwood plywoods (except okume, poplar, and some other trees) have greater density than softwood plywoods (Bozkurt and Göker, 1986; Aydın and Çolakoğlu, 2008). Density is one of the many important factors that affect the technological properties of solid wood and wood-based panels. There are many factors that affect the density of trees, including the species .Therefore, it was expected that the density of the birch plywood would be greater than that of the pine plywood.

Conclusions In this study, the effects of moisture content on the screw holding capacity of pine plywood and birch plywood were determined. According to data that were obtained, the following results can be inferred; the screw holding capacity of the plywood panels decreased as their moisture content increased, but there were no significant differences between groups E and F. The decrement of screw holding capacity of the birch plywood was more than that of the pine plywood. There are significant polynomial relationships between moisture content and screw holding capacity of pine plywood and birch plywood. In addition, the coefficient of determination of the relationship for birch plywood was greater than that for pine plywood due to the density of the birch plywood.

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