Bogdan began to question me about just how much stress these plastic beam modules could deal with. When he caught on that I was post-tensioning the beams he had no trouble accepting that they'd likely do the job. Just for fun, though, I went ahead and printed up four modules and assembled them into a 400 mm beam, post tensioned them and ran them through their paces.
I was going to print special end plates, but it's getting late and my day job will be rather intense tomorrow, so I used one of the end plates from the last post-tensioning exercise with the herringbone rack column. For the other end plate I used a strongly designed washer that I printed for another purpose some time ago.
First, I did the same loading as last time, but this time the beam is 30 mm deep instead of 20 but much less heavily built.
No deflection to speak of. No problem. That Maglite weighs two pounds and is point loaded at the centre of the beam.
That was nice, but then I decided to get serious about loading, so I broke out one of my ten pound bar bell weights and partially distributed loaded the beam.
A couple of millimetres deflection, but again not much. I then threw another ten pound bar bell weight on for good measure.
Now you start to see some serious deflection. Interestingly, the deflection is not because the beam is failing but rather because the lightly designed end plate on the right is beginning to fail. I replaced that plate with a heavily designed herringbone gear and repeated the exercise.
The deflection is better, but I began to worry that the shear stress at the ends of the beam, which I had planned to handle with purpose designed end plates is ill-applied to the light frame beam itself.
For a final exercise, I point loaded one of the 10 lb weights at the centre of the beam by blocking it with a couple of pieces of scrap ABS.
Guys, post-tensioning is a wonderful technology and very well suited to the scale that we are working at. Several people have suggested using ABS filament for the tension member. I think the thing to remember is that conventional wisdom in designing post-tensioned beams is that the tension cable or rod needs to be a LOT stronger in tension that the material it is paired with. In real life, you see post tensioning done with steel rod or cable and concrete. Concrete has effectively NO tensile strength. I personally don't think using ABS filament is going to give satisfactory results as a tension element in such beams. I use thin studding, lock washers and wing nuts because that is easy to work with and beams can easily be put in compression by hand.
Anyhow, I think that if we want to we can get a lot of the structural steel out of our next generation Reprap machines and greatly improve the printed fraction using this approach.