Effect of Shock Loading from Series Testing on Tensile Strength of Lumber and Connector Systems

Knowing the time-dependent strength characteristics of lumber will help promote more efficient use of our timber resource. A very important characteristic is duration of load in tension. Unfortunately, insufficient information exists on this property because duration of load tensile testing is expensive. One way that might reduce testing costs is to test several specimens end-to-end in a series instead of one at a time. This study aims to solve two problems that arise in series testing: 1) How does the sudden failure (shock loading) of one specimen affect the static strength of the surviving specimens? (If the strength of survivors is reduced by the shock, then considerable loss in duration of load can be expected.) 2) What kind of connector system can effectively transfer the load from the testing machine to the specimens? Both problems were evaluated using 140 No. 2 Douglas-fir 2 by 4’s and 140 Douglas-fir Select Structural (SS) 2 by 4’s. The sample was restricted to specimens with near maximum knot sizes allowed in the two grades. Sixty specimens of each grade were tested singly for tensile strength; the remainder were tested four to a series. Only 8 percent of the No. 2 series specimens and 22 percent of the SS series specimens had residual strength that fell below a previously attained strength in series testing. The reduced strength could not be solely attributed to shock induced by the sudden failure of a weaker specimen, as some of the reduction may be attributed to damage from prior loading. Two different connector systems were developed for this study–one for No. 2 specimens and a stronger one for SS specimens. Many of the specimens failed in the connector system (19% of No. 2 and 52% of SS) rather than in the specimen. This showed that stronger connectors are needed.

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