Research provides solutions for upholstered seating frame failures
By Jilei Zhang, Ph.D.
This article is the first in a series written by Jilei Zhang, Ph.D. Dr. Zhang is an assistant professor at the Forest Products Laboratory at Mississippi State University. Look for the second installment in this series on upholstered furniture frame testing in our September issue.
Solutions to upholstered seating frame failure problems have proven elusive for most furniture manufacturers... until now. Thanks to current research being conducted at Mississippi State University’s Furniture Research Unit (FRU), new answers to questions about failure are being provided. FRU currently is reviewing data accumulated over the past ten years from 39 manufacturers involving 322 upholstered frames. The information collected during the ten-year period includes:
- the number of frames that passed a heavy-service acceptance level
- the number of frames that failed the test,
- the types of joints most commonly used
- frame material usage trends
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| Seat Load Foundation tests had the highest failure rate at 24 percent. |
Frame Testing Results Preliminary analysis of the data is reflected in Table 1. It shows that Seat Load Foundation tests had the highest failure rate at 24 percent, followed by Backrest Foundation tests at 19 percent, and Sidethrust Load on Arms-Outward at only 2 percent based on the testing requirement of satisfying the Light-Service acceptance level.
Based on the testing requirements for passing heavy-duty service, arms-outward tests showed a 93 percent failure because of joints, and a five percent failure due to member material breakage. The furniture performance test results implied that current furniture strength design practice is mostly based on a mixture of experience, and trial and error. The main reason could be that design engineers lack design information such as loads on joints and members, and joint and member strength properties, especially fatigue strength property data.
As more plywood and other engineered materials are being used in upholstered frame construction, these results can aid furniture manufacturers as they continue to re-engineer products and seek new frame materials. Composite manufacturers, likewise, can use the data to develop new products for the furniture industry.
Most service failures of upholstered furniture frames are fatigue related. Furniture performance tests such as GSA FNAE-80-214A are based on the cyclic stepped load model, i.e., tested frame members and joints are subjected to the cyclic stepped loading rather than a constant fatigue loading.
Optimum design of upholstered furniture frames to satisfy GSA tests requires information concerning fatigue strength properties of joints and member materials subjected to fatigue loadings. This is especially true now as more plywood and engineered composite materials are being used as frame structural materials. The strength properties currently used to design upholstered furniture frames mainly have been determined by static load tests. Tests to determine the fatigue properties of joints and members subjected to cyclic fatigue loading are very limited.
Current research projects in the Forest Products Laboratory focus on analyzing design loads for key structural members and joints, understanding fatigue behaviors of joints and materials commonly being used in upholstered furniture frame construction, and predicting their fatigue life. Current ongoing projects are summarized as follows: Frame Design Loads Of three major frame systems:
- the seat foundation frame system,
- the back foundation frame system, and the
- side frame system in an upholstered sofa frame,
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| Each spring has a transducer at each end, connecting to the frame. Each transducer is monitored separately, and each produces two readings: the vertical load component and the horizontal load component transmitted through the respective spring to the frame. |
the seat foundation frame is perhaps the most difficult to design. This is not only because of its complex structure and load conditions, but also unknown design loads on critical members such as the front rail and stretchers, critical joints such as the front rail to front post joints, and metal components such as sinuous springs.
As a first step toward obtaining design loads for the seat foundation of an upholstered sofa frame equipped with the sinusoidal type of springs, a bi-axial force transducer data acquisition system has been designed. When the project is finished, the magnitude and distribution pattern of spring loads on front and back spring rails will be determined. Then design loads for members and joints can be established based on GSA performance test load conditions. Joint and Material Fatigue Strengths
Joint fatigue strength property studies include joint types of staple, staple-glue-block, dowel, screw, gusset-plate and metal plate. Material studies include plywood, Timberstrand™, Engineered Strand Lumber, oriented strand board, particleboard, oak and yellow poplar. Initial fatigue testing results of two-pin moment-resisting dowel joints showed larger variations among joint fatigue strengths in terms of cycles to failure. The coefficients of variation from fatigue tests ranged from 37 to 132 percent, but joint static tests showed only 7 to 18 percent. The joint fatigue life can be predicted based on S-N curves (stress versus the log number of cycles-to-failure). Staple Withdrawal Strength
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| Jilei Zhang, Ph.D., says the FUR research will cut manufacturers’ material costs. |
Staples are increasingly being used by the furniture industry in the construction of joints. Wood, wood composites, or a combination of the two, can easily be joined with staples. Staple direct withdrawal is the most common failure mode. Design data related to direct withdrawal strength of staples is limited.
The staple withdrawal strength project examines the influence of specific gravity, shear strength, board orientation, staple gage, and depth of staple penetration on the staple direct withdrawal strength. When the project is concluded, a general design formula in terms of wood material properties and staple parameters will be developed.
Research outcomes will be incorporated into furniture design software packages and will make computerized optimum furniture frame design possible. This effort will help furniture manufacturers not only cut material costs, but also significantly reduce the product engineering design cycle. This will allow products to be sent to the market earlier and will enhance the competitiveness of manufacturers. Approved for publication as Journal Article No. fp194 of the Forest and Wildlife Research Center, Mississippi State University.
Test |
Number of Tests
Performed |
Failed |
Light Service |
Medium Service |
Heavy Service |
| Seat Load Foundation |
111 |
24% |
32% |
11% |
33% |
| Backrest Foundation |
58 |
19% |
12% |
19% |
50% |
| Sidethrust Load on Arms-Outward |
172 |
2% |
34% |
29% |
35% |
| Performance test results of upholstered furniture frames subjected to heavy-duty service loads |
