# Design of multiple wood layers

I am currently working on designing a teeterboard for the circus (a seesaw that people jump up and down on the ends), and need some advice on my methodology for wood selection. The primary wood will be Vic ash (a.k.a Tasmanian Oak), with a possible second layer of pine underneath. Currently i plan to do 3 point bend tests on each wood standalone, and with various thicknessses as a double or triple layer (ash/pine, pine/ash, ash/pine/ash, pine/ash/pine etc). This will be used to get material data.

My primary concerns are with scalability, i.e can the results stress values obtained be applied to the full size modeldue to the somewhat unpredictable and irregular nature of wood. My other concern is incorporating the glue into analysis. I do not know how to get a reasonable theoretical calculation for the multi-layer board, unlike the single board for which the calculations are extremely simple.

• You should link to your original question : engineering.stackexchange.com/q/19754/10902 – Solar Mike Mar 15 '18 at 8:40
• The original questions is not overly relevant, as we have shifted away from using FEA, as well as adopted a multi layered approach. – Jonathan Carver Mar 15 '18 at 9:33
• I would say scalable within reason. I don't know if i'd trust much a tiny model made from wood veneer for example. For the mixed wood analysis I would recommend you go straight to FEM, unless you are already familiar with composite laminate theory. – agentp Mar 15 '18 at 13:45
• Some of our oldest buildings are made of wood and they did not have FEA etc etc – Solar Mike Mar 15 '18 at 14:38
• Theoretical calculations aside, I think you should make the board out of marine grade plywood laminated with some fiberglass cloth with some type of resin. These materials are much more uniform , stronger and more predictable than what you are proposing in your question. You will wind up with a composite laminate that as agentp says, you can apply laminate theory to. – William Hird Jul 14 '18 at 10:31

The results will be scalable, but you will be assuming the quality of the wood to be uniform throughout.

In the Building trade there are standards that specify the size, number and location of defects such as knots etc - these reduce the carrying capacities etc.

You need to either make sure the wood has no or few defects or oversize the material to cover the effects...

Laminated would properties have to be calculated and then analyzed by conventional methods.

For a substitute I, second moment of area you can substitute $\sum da. dy^2$

With $\sum da_1dy^2+ da_2dy^2 + da_ndy^2..$ But with the difference that you multiply the areas of different laminates by their Young's modules proportions. So if a layer's E is 1.3 times the other layer it's area is multiplied by 1.3 but with the same center of geometry, Dy is the same.

For shear you can use composit beam shear equations. All these are easily doable on a spreadsheet that you save for alternative materials and dimensions. So questions of scalability is mute.

• Will that beam - teeterboard be in shear? Or bending... – Solar Mike Mar 16 '18 at 19:55
• The beam would be in bending under dynamic loading if approximately 20 times the weight of jumper, assuming he jumps 20 times more than the play of the board. It will also be under large shear specially over the support. All the loads should be multiplied by appropriate multiplication factor. – kamran Mar 16 '18 at 21:35
• surely that is bending over the support? – Solar Mike Mar 16 '18 at 21:37
• Yes, but wiplash at the ends is large too. – kamran Mar 16 '18 at 21:41