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Structural Testing for Chairs
Question
Forum Responses
Plan B: compare your design and joinery methods to chairs that you know have survived years of service - look in a thrift shop or library. Light construction can be risky. If you posted a picture of the chair then we could comment on likely weak points of the design.
From the original questioner: I’m using 6mm floating tenons in backsplat about 2" long. Also 1" into both pieces. From contributor J: Chairs don't often fail catastrophically at the back splat. An inch of tenon in each side will make it difficult to come out. You should be ok, provided the leg to rail joints are strong. Don't underestimate corner blocks either. From the original questioner: It's the backsplats that are the question. The leg to seat rails are beefy enough with corner blocks rub glued and screwed. From contributor P: We've made a chair with back splats tenoned into the rails for many years. I wouldn't do it as a floating tenon, rather mill it out of the end of the splat. Re-infoce the rail it's going into. A picture of our chair is below.  Click here for higher quality, full size image From contributor P: I think you should be loading your chair as a user does and looking at the sequence of failure. What you want to avoid is a sudden catastrophic collapse that leaves the user on the floor with no warning. That's why the chairs I design have two sets of structural elements: the piece-to-piece joints and the corner blocks. The joints are going to fail first, and then the corner blocks are going to hold the chair together long enough that it will be obvious to the user that something is wrong before they fall to the floor. If you want to avoid lawsuits, don't injure your clients. If they have plenty of warning that something has broken and they can still use the chair, then any further problems are on them.
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