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I was reading "Illustrated Cabinetmaking" and came across this: "Where a horizontal board tops an upright, the biscuits should be offset". It does not explain why. Does anyone know?

The next sentence says: "Likewise, biscuits used in joining a shelf to an upright should be located below center to increase the shelf's resistance to toploading". Does anyone know what toploading is?

Here's the illustration in the book for both of the above:

enter image description here

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    I have a guess about what the second one is (toploading). It adds more wood sitting on top of the biscuit, thus increasing the shelf's ability to support weight. Of course this assumes the biscuit is stronger than the thickness of the wood. – Adirondack Jim Jun 21 at 16:13
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    I'd love to be proven wrong, but this feels a bit like voodoo to me. – Aloysius Defenestrate Jun 21 at 18:01
  • In the shelf the off-centre position makes some sense, not sure about on top. I can't help but feel positioning centrally gives the best likelihood of resisting all the possible forces the cabinet might experience in use. I very much doubt the author did any testing on this, just a theory they came up with (or exrapolated more widely from a specific case). – Graphus Jun 22 at 6:12
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    Maybe the offset is rooted in the possibility that it's better to know when you've accidentally flipped a shelf before cutting biscuits on the other end... (A biscuit that looks centered probably isn't exactly centered, and this could lead to micro misalignments.) – Aloysius Defenestrate Jun 23 at 3:07
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    @AloysiusDefenestrate I suppose that could be it, but every guide to biscuit joinery I've read was clear that you are to centre the mortise for the biscuit. This can't be harder than machining or carving a mortise and matching tenon. Even if you are a 1/16th off, this is wood; fix it in the finish! – jdv Jun 23 at 18:36
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I have never encountered any rules of thumb about using biscuits, but the diagram makes sense. In the top connection the biscuit will transfer stress resulting from any side to side movement of the cabinet. All that stress is resisted by the small width of the remaining shelf between the biscuit and the outside edge to the left of it. The more wood there the better. Similarly, any weight placed on the lower shelf can only be resisted by the shelf wood above the biscuit.

of course, both connection details could be improved by creating a rabbet joint at the top and inserting the lower shelf into a dado in the vertical member. In this way the entire thickness of the board is being used to address the stresses.

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  • By "side to side", I think you mean racking. If so, then wouldn't forces be on both sides of the biscuit & thus it would be better to put it in the center? – Adirondack Jim Jun 21 at 18:35
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    One side of the connection is compressing and the other side is tension. Think of the joint as a hinge connection. The joint must resist rotating forces which will affect the outside on one side or the other which is trying to sheer off the small width of wood. . – Ashlar Jun 22 at 0:21
  • Adirondack - you are viewing it from the picture on one side (right). On the top biscuit, the wide part is on the outside. Now picture the top and side on the right side of the cabinet. The offset would be with the wide part on the outside. The two top biscuit are acting as a pair, each biscuit preventing racking from different direction. – Programmer66 Jun 23 at 5:02
  • Ashlar, I'm afraid I don't understand what you're saying. I still think you're still referring to racking, but it's the tension/compression part that I don't get. If you think of the biscuit/joint as a hinge, then won't both forces be compression (one side compressing at the top and the other compressing the bottom). So, you have forces on both sides of the wood, so why not center the biscuit? – Adirondack Jim Jun 25 at 15:30
  • On one side the side wall will want to push the the biscuit into the wood top shelf, and on the other it will push the small wood profile out away from the assembly. The only thing resisting this force is the small piece of wood outside of the biscuit on the top shelf. The layers of wood will have a tendency to shear off on the small wood section that remains on the top panel. The more wood that is on the outside, the better it will resist the force. There is plenty of wood on the inside face of the biscuit, not so on the outside. – Ashlar Jun 26 at 23:37

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