An Investigation of the Mechanical Characteristics of Truss Plates on Fire-Retardant Treated Wood

Systematic comparison has been made of the influence of a fire-retardant treatment on stiffness and ultimate strength of metal plate connectors on nail-glued plywood plates. Matched, treated, and untreated tensile specimens were made of. Douglas-fix and southern yellow pine using long-tooth and short-tooth type connectors and plywood nail-glued plates. All tests were repeated at each of two wood moisture content levels. One hundred specimens were included in the study of tension joints. A chart record of load vs. slip was determined and includes the linear or design range of the fastener plus a part of the plastic flow range. “t” test values indicate a significant difference between treated and untreated specimens at the 20 percent moisture content level. Rarely will wood stay at this level in service; therefore, load-slip values for untreated lumber could be used in design for FRT lumber connections. Due to the varying modes of failure, averaging the results of maximum loads would be unrealistic. Lower “t” tests are run for averages of similar failures. The plywood was adversely affected by the treatment as indicated by the significant “t” values for both species. The figures generally support the National Design Specification reducing 10 percent for FRT lumber. However, the conclusion is drawn from a relatively small set of data. The long-tooth plates tended to pull out, while the short-tooth plates failed through tension failure of the metal. This would suggest further examination into the effect of the salts on the cut edges and corners of the metal plates. The tests were done on the specimens only 3 to 4 months after treatment and fabrication. Electrolytic action should be investigated and its long-term effect on the plates.

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