Performance tests of school chairs constructed with round mortise and tenon joints

Tests were conducted to compare the performance characteristics of school chairs constructed with pinned but unglued round mortise and tenon joints with those of chairs with glued but unpinned joints. Chairs with 0.125-inch cross pins in 0.723-inch diameter tenons developed over 90 percent of the strength of the chairs with glued tenons, whereas chairs with 0.313-inch diameter pins developed only 80 percent. Thus, results of the tests tend to indicate that chairs constructed with round mortise and tenon joints with small cross pins should provide nearly the strength and durability of comparable chairs constructed with glued joints, and therefore cross-pinning could be considered as an alternative method of joint construction in areas of the world where adhesives are in short supply. Throughout many regions of the world, there is a persistent need for durable, attractive, low-cost school furniture that can be produced by local industry using local wood resources. Initial research indicates that furniture constructed with round mortise and tenon joints satisfies these criteria. Specifically, only low-cost/low-technology machining and joinery processes are required to construct the furniture. Furthermore, the furniture can be manufactured from wood residues or plantation thinnings. Several studies of school chairs constructed with round mortise and tenon joints have been published (Eckelman et al. 2000, 2001, 2003; Haviarova et al. 2001a, 2001b), and there is ongoing research addressing the design characteristics of the joints themselves (Eckelman et al. 2002). Research has also been conducted concerning the commercial production of school chairs constructed with round mortise and tenon joints (Quesada 2001, 2003). Additionally, one small prototype production line was set up and operated in Costa Rica that produced chairs for six elementary schools, and chairs for another seven schools were produced on a line set up in the Institute of Technology of Costa Rica. Initial performance tests indicated that properly designed chairs constructed with round mortise and tenon joints were both strong and durable; importantly, they were highly resistant to cyclic loading that would be associated with high levels of usage. Additional performance data were needed, however, to provide a broader perspective of the expected performance of such school chairs. In addition, tests were needed to determine whether cross-pinning of tenons is feasible, either as a back-up measure to enhance structural integrity in case of inadequate gluing or glue bond failure, or as a substitute for gluing in areas where adhesives may be unduly costly and in short supply. The purpose of this study was to provide further insights into the expected performance characteristics of these chairs, including chairs with cross-pinned tenons, and to demonstrate their performance in terms of relevant standard performance tests.