If you have followed some of my earlier blog posts, you know I am passionate about testing. In my post, Testing – Then and Now, I said, “There simply is no substitute for a physical test.” Something I haven’t discussed in much detail, however, are some of the complexities involved in a good test.
For wood connector testing, we follow ASTM D7147-11 Testing and Establishing Allowable Loads of Joist Hangers. The actual testing is relatively straight forward – build at least three setups, test them, measure the deflection and ultimate loads.
It actually was that simple years ago, but modern test standards have more requirements than just breaking the part. First off, the steel used in the connector is important. To prevent overestimating a connector’s performance, ASTM D7147 has limits on the strength and thickness of steel used for testing relative to the specified material.
The test standard acknowledges that actual steel strength will exceed the specified strength and the 3/2.5 term in the reduction factor gives you 20% leeway to exceed the specified minimums and not take reductions in allowable loads. Locating production parts made with the right strength steel requires a little bit of searching and we need to do a base metal thickness test on production parts. Prototype parts are somewhat easier because we can hand pick the steel used for making them.
Once we have the parts made with the correct steel in the test lab, we need the right wood. Two properties that affect hanger performance are specific gravity and moisture content of the wood. Moisture content is simply a measure of how wet the lumber is, and the test standards require load reductions if testing is done on wood at less than 11% moisture content. We test the moisture content and specific gravity of every board we receive prior to building test setups. All the good wood is stored in the conditioning room.
Specific gravity is a measure of how dense the wood is – denser wood usually means better test performance. Similar to steel, the test standard requires reductions if your tested specific gravity exceeds the specified. Unlike steel, there is no 20% fudge factor. Once a test is run, specific gravity samples are taken, numbered, measured, and then put in the oven to dry. With testing complete, we can finish the test report and do the calculations to load rate the product. It is a lot of work to create one allowable load for a table in a catalog or a flier – and I still love testing!
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