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I want to build drawers for my kitchen. On Youtube I found this Channel, where a guy does woodworking. In a video about making kitchen drawers, he says that the bottom surface of the drawer will be stronger if the joint connecting the bottom to the sides is positioned lower. https://youtu.be/wLMQ_6Y29pY?t=5m15s (5:15 to 6:03).

To me, it should make pretty much no difference. Am I wrong?

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Using a tie-and-strut model makes it easier to explain the difference here. A tie-and-strut model is a simplification of the structure which describes where the structure is under tension, and where it is under compression. The model gets effectively simplified into a truss. Below we have two examples of the axial load on members of two different trusses with identical loading (positive values are tension, negative are compression):

enter image description here

Looking at the values, you'll see that the results are mirror images of each other: same values, but different signs. Importantly, notice that the first truss has a negative (compressive) force of 634.9 between the supports and the upper chord, while the second one has a positive (tensile) force of 634.7 between the supports and the lower chord.

Now, looking at the video, we basically have two possible layouts, which are converted into the overlaid trusses:

enter image description here

enter image description here

Comparing with the truss results shown above, it is clear that the first diagonal emanating from the support will suffer compression in the first format and tension in the second layout.

Now, what will resist the forces represented by that diagonal? The combined plywood layers, of course. However, that force needs to be transmitted from layer to layer. In the case of compression, that transfer is done by contact. However, in tension, that transfer occurs via the glue. If the glue were to fail, then the plywood layers will separate and the structure will progressively collapse:

enter image description here

Meanwhile, with the other orientation, that force is compressive, so such a failure is impossible (other failure states will certainly happen before the "compressive strut" collapses).

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  • $\begingroup$ Thank you for this superb answer! It makes perfectly sense now. $\endgroup$ – ty. Aug 14 '17 at 16:32
  • $\begingroup$ @Wasabi why in your one strut & tie model is there a -100 force in the middle vertical and 0 in the other? is this because of balancing the forces at the nodes, and there was a 100 load on the top middle node? $\endgroup$ – Forward Ed Aug 14 '17 at 19:17
  • $\begingroup$ @ForwardEd: Exactly. Both models have loads of 100 at each node along the upper chord. At midspan, that creates a difference: the first truss absorbs that load via the diagonals, while the second requires the vertical to transport the load to the bottom chord for it to then be resisted by the diagonals. $\endgroup$ – Wasabi Aug 14 '17 at 21:12

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