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Moment capacity of oriented strandboard gusset-plate joints for upholstered furniture. Part 2: Fatigue load.

Abstract

In order to obtain the ratios of static-to-fatigue moment capacity, the fatigue performance of T-shaped, end-to-side gusset-plate joints made of oriented strandboard (OSB) was investigated. A total of 108 stapled and glued-stapled joints with gusset-plates of different lengths (6, 8, and 10 in) were subjected to one-side cyclic stepped bending loads. Test results showed that assemblies with OSB gusset-plates would fail within 25,000 cycles when a stepped load level exceeded 63 percent of their static moment capacity. The passing static-to-fatigue ratio averaged 2.1 with the COV of 12 percent. In the stapled joints, the higher ratios were associated with the staple withdrawal as dominating failure mode. In the glued-stapled joints, lower ratios were associated with in-plane shear and the higher ratios with the rupture of the OSB panels.

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The furniture manufacturing, including upholstered furniture, is a dynamic industry with many opportunities to diversify their products using various materials and designs. The use of panel products, such as plywood and oriented strandboard (OSB), to substitute solid hardwood in upholstered furniture frames, is gaining popularity. Gusset-plates are used to join back rail to back post and side rail to back post and other highly stressed joints, which are difficult to reinforce by other means (Zhang et al. 2001). For OSB to access the upholstered furniture industry, technical data on the performance of connections made with OSB must be provided to ensure that it is well-suited for such application. This is the second paper in a series that deals with this topic (Wang et al. 2007), which will focus on the fatigue performance of joints made with OSB framing members and gusset-plates assembled with staples with and without glue.

In engineering, the term of fatigue is defined as the progressive damage that occurs in materials subjected to cyclic loading (USDA 1999). Contrary to common belief, fatigue is the main cause of failure in wood furniture; therefore, special attention must be paid to the fatigue resistance of wood frames (Eckelman and Zhang 1995), which is controlled, in principle, by the fatigue life of the critical joints connecting main frame members. The furniture performance test standards such as General Service Administration (GSA) test regimen FNAE-80-214 A (GSA 1998), requires information on the joint fatigue performance, particularly fatigue failure load and fatigue life, i.e., the number of load cycles survived until failure. The standard performance test regime is based on a zero-to-maximum nonreversed cyclic stepped fatigue load method rather than static or constant amplitude cycling load method (Eckelman 1988b).

Multi-cycle fatigue tests are expensive as they require specialized equipment and considerable testing time in comparison with static tests. Therefore, it would be useful to correlate the static and fatigue performance to characterize various types of joints in the future. Several previous studies were focused on correlating the static and fatigue moment resistance of wood joints with various fasteners. Zhang et al. (2003) investigated the fatigue life of T-shaped, end-to-side assemblies using two-pin dowel joints by subjecting them to one-side constant and stepped cyclic bending loading. A mathematical representation was developed to correlate the applied moment to the number of cycles to failure. Zhang and Quin (2006) studied the bending fatigue life of metal-plate-connected joints in furniture grade pine plywood subjected to one-side cyclic stepped bending loads. They reported that there was a strong relationship between the static moment capacity and the load level causing failure in a fatigue test. The passing fatigue moment level was 46 percent of the static moment capacity.

No information has been found in literature on the fatigue resistance of gusset-plate joints made of OSB. The main objective of this study was to evaluate the fatigue resistance of OSB gusset-plate joints and determine the ratio of the static moment capacities to their fatigue moment capacities for design purposes. The second objective was to gather the information on the failure modes of such assemblies under stepped fatigue loads.

Materials and methods

Each test specimen consisted of post and rail made of structural 23/32-in (18-mm) OSB joined with a pair of 7/16-in (11-mm) OSB gusset-plates symmetrically attached on both sides of the joint with staples with or without glue. Basic material properties, fabrication procedures, and configuration details of the joints tested statically are reported in Wang et al. (2007). For fatigue tests, the panel materials from the same batch were used and the joints of the same configurations were constructed using the same procedures with the exception of the length of the rail and the gusset-plates. The rail length was increased from 16 in (406 mm) to 23 in (584 mm) to accommodate the capacity of the pneumatic cylinders used in the test set-up. The gusset-plates used in fatigue tests were 6, 8, and 10 in (152,203, and 254 mm) long, because the static tests (Wang et al. 2007) showed that shorter and longer gusset-plates were not efficient in these joints. Columns 1 through 7 of Tables I and 2 provide the information on the tested configurations including the number of replicates and the average static moment capacity as reported by Wang et al. (2007).

Joint assemblies were subjected to two loading schedules, representing 1) the backrest frame and 2) seat load foundation test for a 72-in-long three-seat sofa frame without middle upright on the back (Fig. 1). To determine the loads for the first schedule (Table 3), it was assumed that three equidistant point loads shown in Figure la were applied horizontally at the top back rail connected to the top ends of two back posts. The magnitude of these loads at each load step is shown in column 1 (Table 3). These forces deliver loads acting on each of two back posts with the magnitude equal to half of the total load as is shown in column 2 (Table 3). Assuming that the height of the back post in a real sofa is 28 in, the loads produce moment couples shown in column 3 (Table 3). To create the same moments during the tests with the arm of 21 in, the forces shown in column 4 (Table 3) were applied to the rail. Similarly, for the second schedule (Table 4), three equidistant point loads applied vertically to the bottom back rail were assumed as shown in Figure lb and column I (Table 4). Therefore, the end post was subjected to a moment couple shown in column 2 (Table 4), under the load shown in column 3 applied at the 20-in arm.

[FIGURE 1 OMITTED]

The fatigue tests were conducted using pneumatic cylinders in a specially designed supporting frame illustrated in Figure 2, which allowed testing nine specimens simultaneously. In both backrest frame and seat load foundation tests, 25,000 load cycles were applied at a rate of 20 cycles/min at each load level according to the schedules shown in Tables 3 and 4, respectively (GSA 1998). After 25,000 cycles, the load was increased to the next level and load cycling continued. Limit switches were installed on each cylinder to stop the test of a specimen that suffered major damage. When backrest frame joint assemblies passed all levels in the main load schedule the tests continued on to the extended load schedule shown in Table 3 until all the specimens failed. The highest load level sustained by a specimen for 25,000 cycles without failure was used to calculate the "passed" moment. The number of cycles sustained by the specimen at the next load level was included into the cumulative number of cycles, and the load level at failure was used to calculate the "failed" moment. Failure modes were determined for each specimen.

[FIGURE 2 OMITTED]

Results and discussion

Test results of backrest frame and seat foundation joints are summarized in Tables 1 and 2, respectively. Columns 8 to 10 show the average values and coefficients of variation of the passed and failed moments and the cumulative number of cycles to failure. An analysis of variance (ANOVA) general linear model procedure was performed for different sizes of gusset-plates with or without glue applications on the fatigue passed load, fatigue failed load, fatigue passed moment, fatigue failed moment, and fatigue cumulative No. of cycles to failure. Tukey's multiple tests were also performed for the classification of the average fatigue cumulative No. of cycles to failure (column 10). The average values of passed and failed moments and the corresponding average static moment capacities (column 7) were used to calculate, respectively, the passed and failed static-to-fatigue moment capacity ratios shown in columns 11 and 12. Higher static-to-fatigue ratios indicate a larger gap between the fatigue moment capacity of the joint and its capacity determined in static tests; i.e., for a given fatigue resistance, the joint would require a higher static strength. The last column in Tables 1 and 2 lists all observed failure modes and their relative frequency within the group.

As was expected from the static tests, the glued-stapled joints (Table 1) showed significantly higher failure loads and fatigue life in comparison with the unglued stapled joints. Unglued stapled joints with 6-in and 8-in gusset-plates demonstrated no significant difference of fatigue failure loads. Therefore, the static-to-fatigue ratio of the joints with 6-in gusset-plates appeared to be lower considering their lower static strength. The stapled joints with 10-in gusset-plates demonstrated statistically higher fatigue resistance and, accordingly, higher fatigue life; however, their static-to-fatigue ratios were similar to those with 8-in gusset-plates. Analysis of the failure modes suggests contribution of OSB panel failures generally associated with the lower static-to-fatigue ratios, while the higher ratios were associated with withdrawal of staples.

Similar correlations were found for the glued-stapled joints. The joints with 6-in gusset-plates with statistically lower fatigue capacity and fatigue life demonstrated lower static-to-fatigue capacity ratios; whereas the joints with larger gusset-plates (8 in and 10 in) showed no significant difference of fatigue performance with higher static-to-fatigue ratios for the 10-in plates. Analysis of failure modes shows that approximately half of the glued joints with 6-in and 8-in gusset-plates failed from in-plane shear, while all glued joints with 10-in plates failed in rapture of one of the joint members, indicating that in the latter case the strength of the glued area exceeded the strength of the joint members.

It can be noted in Table 2 that because of larger load increments in the seat load foundation test schedule, there were fewer differences observed between pass-fail loads of joints with different gusset-plates, i.e., the load schedule was less sensitive to the differences in the joint configurations. Therefore, in spite of a slightly longer fatigue life, the static-to-fatigue ratios of the joints with larger gusset-plates turned out to be higher for both unglued and glued joint assemblies because of their higher static resistance.

Static to fatigue moment capacity ratio

The ratio of the static to failed fatigue moment varied from 1.34 to 1.89 for stapled joints and from 1.43 to 1.78 for the glued-stapled joints. The corresponding ratio for the passed fatigue moment varied from 1.78 to 2.42 and from 1.65 to 2.24 for the stapled and glued-stapled joints, respectively, with the lower values obtained for the joints with smaller gusset-plates. The average ratio of static moment to fatigue failed moment for 12 tested groups subjected to two different schedules was 1.60 with a coefficient of variation (COV) of 10 percent. In other words, the average OSB gusset-plate joint failed under a load level of 63 percent of its static moment capacity after being subjected to a series of cyclic stepped loads. Respectively, the average ratio of static to fatigue passed moment was 2.06 with a COV of 12 percent.

Previously, the average static to fatigue passed moment capacity ratio of 2.2 with a COV of 13 percent was reported for two-pin dowel joints (Zhang et al. 2003) and 2.5 with a COV of 11 percent for metal-plate connected joints in furniture grade pine plywood (Zhang and Quin 2006). Comparison with these studies shows that the average ratio of static to fatigue moment capacity of upholstered furniture frame joints varies from one fastening system to another. Based on this information, it is suggested that to pass the fatigue test, an average ratio of 2.2 for wood dowels, 2.5 for the metal-plate connectors, and 2.1 for OSB gusset-plates could be used for upholstered furniture design.

Conclusions

Cyclic load fatigue tests oil stapled and glued-stapled OSB joints with gusset-plates of three different sizes were performed and compared with their static moment resistance to determine the influence of the gusset-plate size, material and fastening system on the static-to-fatigue moment capacity ratio and on failure modes of the joints. Results showed that despite differences in failure modes, both stapled and glued-stapled joints had similar static-to-fatigue moment capacity ratios. In the stapled joints, the higher ratios were associated with the staple withdrawal as dominating failure mode. In the glued-stapled joints lower ratios were associated with in-plane shear, and the higher ratios with the rupture of the OSB panels. Statistical analysis and comparison with previous studies showed that the average value of 2.1 can be used as the passing static-to-fatigue ratio for design of upholstered furniture frames with OSB gusset-plate joints. In other words, it is advised to design gusset-plate joints so that they will not be loaded to more than 48 percent of their static moment capacity.

Literature cited

Eckelman, C.A. 1988b. Performance testing of furniture. Part II. A multipurpose universal structural performance test method. Forest Prod. J. 38(4):13-18.

-- and J. Zhang. 1995. Uses of the General Serv. Administration performance test method for upholstered furniture in the engineering of upholstered furniture frames. Holz als Roh- und Werkstoff 53: 261-267.

General Serv. Administration (GSA). 1998. Upholstered furniture test method. FNAE-80-214A. Furniture Commodity Center, Federal Supply Services, Washington, D.C.

USDA Forest Serv. Forest Products Lab. (USDA). 1999. Wood Handbook: Wood as an engineering Material. Forest Prod. Soc., Madison, Wisconsin.

Wang, X., A. Salenikovich, M. Mohammad, C. Echavarria, and J. Zhang. 2007. Moment capacity of oriented strandboard gusset-plate joints for upholstered furniture. Part 1. Static load. Forest Prod. J. (In press). Zhang, J. and F. Quin. 2006. Bending fatigue life of metal-plate-connected joints in furniture grade pine plywood. Forest Prod. J. 53(9): 33-39. --, G. Li, and T. Sellers, Jr. 2003. Bending fatigue life of two-pin dowel joints in furniture grade pine plywood. Forest Prod. J. 53(6): 1-7.

-- D. Lyon, F. Quin, and B. Tackett. 2001. Bending strength of gusset-plate joints constructed of wood composites. Forest Prod. J. 51(5):40-44.

The authors are, respectively: PhD Candidate, Assistant Professor, Dept. of Wood and Forest Sciences, Laval Univ., Quebec, Canada (xiao-dong.wang.1@ulaval.ca, Alexander.Salenikovich@ sbf.ulaval.ca); Research Scientist/Group Leader, FPInnovations-Forintek Div., Ste-Foy, Quebec, Canada (mohammad.mohammad@qc.forintek.ca); and Associate Professor, Forest Products Lab., Mississippi State Univ., Mississippi State, Mississippi (jzhang@cfr.msstate.edu). The authors wish to thank the technicians from the Dept. of Wood and Forest Sciences at Laval Univ. for their technical support. Acknowledgement for financial support is also extended to the Industrial Chair on Engineered Wood Products. This paper was received for publication in December 2006. Article No. 10282.

Xiaodong Wang * Alexander Salenikovich * Mohammad Mohammad Jilei Zhang *

* Forest Products Society Member. [C] Forest Products Society 2007. Forest Prod. J. 57(7/8):46-50.
Table 1.--Test specimen configurations and results for GSA backrest
frame schedule.

Type of Gusset-plate Staple No. of No. of Static peak
joint length length staples specimens load (avg)

 (in.) (Kip)
1 -2 3 4 5 6

Stapled 6 1.5 20 9 0.86 (8.1) (b)

 8 1.5 20 9 1.01 (5.3)

 10 1.5 20 9 1.04 (4.7)

Glued 6 1 8 9 0.92 (6.9)

 8 1 8 9 1.15 (8.7)

 10 1 8 9 1.28 (11.2)

 Static Fatigue
Type of Gusset-plate moment
joint length (avg) Passed moment Failed moment

 (in.) (Kip-in.)
1 -2 7 8 9

Stapled 6 12.1 6.77 A (c) (8) 7.82 A (7)

 8 14.1 6.42 A (5) 7.47 A (5)

 10 14.5 7.23 A, B (5) 8.28 A, B (4)

Glued 6 12.9 7.82 B (10) 8.87 B (9)

 8 16.1 9.10 C (12) 10.2 C (10)

 10 17.9 8.98 C (8) 10.0 C (8)

 Fatigue Ratio of
 static/fatigue
Type of Gusset-plate Cumulative no. of
joint length cycles to failure Passed Failed

 (in.)
1 -2 10 11 12

Stapled 6 100,000 + 19,479 A (10) 1.78 1.54

 8 100,000 + 14,145 (8) 2.20 1.89

 10 125,000 + 7,897 B (5) 2.00 1.75

Glued 6 150,000 + 237 C (10) 1.65 1.45

 8 150,000 + 23,864 D (14) 1.77 1.59

 10 150,000 + 23,193 D (9) 1.99 1.78

 Ratio of static/fatigue

Type of Gusset-plate
joint length Mode of failure (a)

 (in.)
1 -2 13

Stapled 6 WR+GR 2/9
 WR+GR+S 3/9,
 WR+S 3/9,
 WR 1/9

 8 WR 5/9,
 WR+GR+S 4/9

 10 WR 4/9,
 WR+GR 3/9,
 WR+GR+S 2/9

Glued 6 M R 5/9,
 S4/9

 8 MR 4/9,
 GR+S 4/9,
 S 1/9

 10 MR 6/9, GR 3/9

(a) Mode of failure: WR = staple withdrawal and rupture;
S = in-plane shear failure of OSB; MR = member rupture;
GR -gusset-plate rupture.

(b) Values in parentheses are the COV (%).

(c) Values with the same capital letter are not statistically
different at 95 percent significance level.

Table 2.--Test specimen configurations and results for GSA seat load
foundation schedule.

Type of Gusset-plate Staple No. of No. of Static peak
joint length length staples specimens load (avg)

 (in) (Kip)
l 2 3 4 5 6

Stapled 6 1.5 20 9 0.86 (8.1) (b)

 8 1.5 20 9 1.01 (5.3)

 10 1.5 20 9 1.04 (4.7)

Glued 6 1 8 9 0.92 (6.9)

 8 1 8 9 1.15 (8.7)

 10 1 8 9 1.28 (11.2)

 Static Fatigue
Type of Gusset-plate moment
joint length (avg) Passed moment Failed moment

 (in) (Kip-in)
l 2 7 8 9

Stapled 6 12.1 6.00 A (c) (0) 9.00 A (0)

 8 14.1 6.00 A (0) 9.00 A (0)

 10 14.5 6.00 A (0) 9.00 A (0)

Glued 6 12.9 6.00 A (0) 9.00 A (0)

 8 16.1 7.33 A, B (22) 1.03 A, B (15)

 10 17.9 8.00 B (19) 1.10 B (14)

 Fatigue Ratio of
 static/fatigue
Type of Gusset-plate Cumulative no. of
joint length cycles to failure Passed Failed

 (in)
l 2 10 11 12

Stapled 6 50,000 + 402 A (1) 2.01 1.34

 8 50,000 + 365 A (0) 2.36 1.57

 10 50,000 + 2,624 C (2) 2.42 1.61

Glued 6 50,000 + 1,604 B (4) 2.15 1.43

 8 50,000 + 21,577 D (8) 2.19 1.56

 10 50,000 + 22,467 D (11) 2.24 1.63

 Ratio of static/fatigue
Type of Gusset-plate
joint length Mode of failure (a)

 (in)
l 2 13

Stapled 6 W 6/9,
 W+GR 2/9,
 W+S 1/9
 8 W 6/9, W+S 3/9

 10 W 9/9

Glued 6 GR+S 5/9, S 4/9

 8 MR 4/9,
 GR+S 4/9,
 S 1/9

 10 MR 6/9,
 GR 2/9,
 GR+S 1/9

(a) Mode of failure: W = staple withdrawal; S = shear in plane of OSB;
MR= member rupture; GR= gusset-plate rupture.

(b) Values in parentheses arc the COV (%).

(c) Values with the same capital letter are not statistically
different at 95 percent significance level.

Table 3.--Cyclic stepped load schedule using GSA backrest frame
testing schedule.

Backrest frame test (a) Joint test (b)

 Reaction Applied Cumulative
Rail loads forces moments Loads cycles

 (lbf) (Kip-in) (lbf)

3 x 75 112.5 3.15 150 25,000
3 x 100 150 4.20 200 50,000
3 x 125 187.5 5.25 250 75,000
3 x 150 225 6.30 300 100,000

 Extended test

3 x 175 262.5 7.35 350 125,000
3 x 200 300 8.40 400 150,000
3 x 225 337.5 9.45 450 175,000
3 x 250 375 10.50 500 200,000

(a) Moment arm = 28 in.

(b) Moment arm = 21 in.

Table 4.--Cyclic stepped load schedule for testing using
GSA seat load foundation testing schedule.

 Joint test (a)

Seat load Applied Cumulative
foundation test loads moments Loads cycles

(lbf) (Kip-in) (lbf)

3 x 100 3.00 150 25,000
3 x 200 6.00 300 50,000
3 x 300 9.00 450 75,000
3 x 400 12.00 600 100,000
3 x 500 15.00 750 125,000

(a) Moment arm = 20 in.
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Author:Wang, Xiaodong; Salenikovich, Alexander; Mohammad, Mohammad; Zhang, Jilei
Publication:Forest Products Journal
Geographic Code:1USA
Date:Jul 1, 2007
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