Estimation equations for moment resistances of L-type screw corner joints in case goods furniture.

Abstract

The effects of the number of screws and screw sizes on bending moment resistance of L-type furniture comer joints constructed of particleboard (PB) and medium density fiberboard (MDF) with resin surfacing were investigated in this study. Six different sizes of screws which included two diameters (4 and 5 mm) and three lengths (40, 50, and 60 mm) were utilized for constructing the test specimens. Specimens were fastened with only screws. Two, three, and four screws were used in the joints of the specimens that have the same width. Specimens were tested under static compression loads that tended to close the joints and tension loads that tended to open the joints. Results indicated that the ultimate moment resistance was obtained with the MDF specimens when the number of screws was four in the joints. MDF comer joints yielded higher moment resistance than PB comer joints for both compression and tension tests. Test results also showed that a screwed corner joint became stronger as either screw diameter or screw length or number of screws was increased. Screw length was found to have a larger influence on bending moment resistance than screw diameter. Furthermore, the average bending moment resistances of screwed corner joints evaluated in this study in compression and tension could be estimated by means of the developed equations.

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In general for furniture products, three essential construction methods were utilized. Furniture was constructed by the case (panel type) or frame (skeleton) method or by a combination of the two, the complex method. The case construction means that the members of furniture were panels. In the frame construction, the members of furniture were rails instead of panels (Kasal et al. 2006). Case goods construction furniture is widely used in homes and offices.

Cabinet manufacturers design and manufacture ready-to-assemble (RTA) furniture with its components connected by mechanical fasteners only to increase their export of case goods to other countries. Furthermore, lacking a rational strength design method for furniture with low-cost fasteners such as screws and dowels, many manufacturers have turned to proprietary RTA fasteners (Tankut 2006).

RTA furniture is specifically designed and manufactured to be sold in a flat package, which allows consumers to take it home or to the office and assemble it themselves. Shipping furniture unassembled lowers costs by eliminating assembly costs and reducing shipping costs. The savings are then passed, in part, to the consumer in the form of lower purchase prices. RTA furniture evolved from knock-down furniture, which has been on the market for many years. As a result of new materials, new manufacturing techniques, new fastening hardware, and updated styling, modern RTA furniture is a dramatic improvement over knock-down furniture. RTA furniture has gained rapid acceptance and is thought to be the fastest growing segment of the world's furniture market (Pepke 1988).

Today, joints without adhesives are common in case goods furniture construction because their use allows furniture to be shipped in knock-down condition and assembled on site, which greatly reduces shipping costs. Screws are widely utilized to connect the corner joints of these types of cases without glue. The rational design of the screw-connected joints requires information about the moment resistances of these joints in particleboard (PB) and medium density fiberboard (MDF).

Important considerations in the furniture engineering of screwed cases are the arrangement of the number of screws and screw sizes that should be used in joining the sides to bottom and top of the case. Limited information is obtainable concerning the moment resistances of screw-type corner joints in case construction. Available information is generally related to the direct withdrawal resistances of glued screw-type joints (Eckelman 1974, 1975, 1978; Rajak and Eckelman 1993).

The relationship between number of fasteners used and joint strength defined by the "zone of influence" or "zone of failure" of the fastener was investigated (Rajak 1989, Ho 1991, Zhang 1991). This term explains that an individual fastener is supported for a finite distance by the material on either side of it. As a result, when a fastener connection such as a screw in the side of a case fails, it causes a portion of the side wall on either side of it to fracture. Rajak (1989) stated "zones of failure" that extended from 40 to 50 mm on either side of the fastener case furniture corner joints constructed with large screws. Ho (1991) obtained a value of a little more than 40 mm on either side. In the case of dowels in similar material, it was found that the "zone of failure" extended about 40 mm on either side of the dowel (Zhang 1991). According to these studies, the minimum fastener spacing will gave maximum fastener strengths.

When cases were connected with multi-dowel corner joints constructed of PB, Zhang and Eckelman (1993) found that a space of 75 mm between two dowels gives the highest moment resistance per dowel. They also emphasized that edge break-out was the normal mode of failure.

Joints with 457 mm in width were constructed with up to 36 fasteners. Screwed and doweled joints were tested in compression for determining the bending moment resistance. According to results of the tests, the bending moment resistances increase rapidly until the "zones of influence" of the fasteners overlap. There was no increase in the moment resistance of the joints beyond that point (Liu and Eckelman 1998). The variability in edge breaking strength obtained for various wood composites were investigated (Eren and Eckelman 1998). Results indicated that the ultimate bending moment resistance can be obtained in corner joints with certain types of mechanical fasteners such as pan head screws and through bolts.

Zhang et al. (2005) investigated the effects of screw sizes, loading, material type, panel surface condition, and gluing on moment resistances of three-screw L-type comer joints for tension and compression tests. Results showed that surfacing PB with synthetic resin and assembling joints with glue applied to the contact surfaces of the face and butt members significantly obtained better moment resistances than joints constructed of only PB. Furthermore, 5-mm diameter by 50- or 60-mm-long screws were suggested for case furniture construction. The bending moment resistances of corner joints for cases constructed of 32-mm-thick laminated PB and MDF under compression and tension loads were investigated (Tankut 2005). The dowel spacing effects on the bending moment resistance of the comer joints were determined. Results indicated that the maximum moment is obtained in joints when spacing between the dowels is at least 96 mm.

The stability of several screw-based fasteners for connecting 15.87-mm-thick furniture grade PB was investigated. Separate statistical tests were performed for the effects of screw shank diameter, screw thread pitch, screw thread design, and fastener type on edge screw withdrawal resistance, bending moment resistance of comer joints, and lateral edge load of butt-jointed shelf units. All three test results showed that screws on their own performed markedly better than either dowels or screws with PVC anchors. The use of thicker screws or screws with anchor was not recommended for butt-jointed shelving applications using PB. Screws with specialized thread configurations such as beveled edges or alternating thread heights performed well, but were not significantly different from plain-threaded screws of the same dimensions in any of the loading configurations (Park et al. 2006).

Although L-type, screw connected comer joints are commonly employed in the construction of the cabinet furniture, limited information is available on the effects of number of screws, screw sizes (diameter and length), loading type, and board material type on the moment resistance of the joints.

The purpose of this study was to obtain practical information concerning the moment resistance of screwed corner joints that the furniture engineers could use in the strength design of case furniture. The objectives were to:

1. Compare bending moment resistance of the screwed corner joints constructed of different panel materials, namely PB and MDF.

2. Determine the effects of number of screws used in the joints on bending moment resistance of screwed comer joints.

3. Determine the effects of screw sizes (diameter and length) utilized for connecting the specimens on bending moment resistance of the screwed comer joints.

4. Estimate the mean bending moment resistances of screwed corner joints evaluated in this study with developed estimation equations.

Materials and methods

Experimental design

The configuration of the screwed L-type corner joint specimens used in the study is shown in Figure 1. In this study, Zhang and Eckelman's (1993) work was utilized for the preparation of the samples. Each specimen consisted of two principal structural members, a face and butt member. The members jointed together by 2, 3, or 4 screws along the joint area. Placements of the screws in the joint area for the specimens connected with 2, 3, or 4 screws are given in Figures 2(a), 2(b), and 2(c). The face member measured 350 by 158 by 18 mm, the butt member measured 350 by 140 by 18 mm. Specimens were assembled using screws only.

[FIGURE 1 OMITTED]