Stiff Rubber Bands
Stiff Rubber Bands
(OP)
I am giving a presentation to a group of high school students and I want to demonstrate how post tensioning works with concrete. I'll be using legos and rubber bands. I bought a box of large rubber bands, but they're very soft - too low of an E for my taste for this demonstration. Does anyone know where I can get some long, preferably 12" - 18", that are very stiff?






RE: Stiff Rubber Bands
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RE: Stiff Rubber Bands
I used a heated paper clip to create the holes within the small styrene "bricks" and then threaded the elastic into the holes of the styrene and used match sticks tied to the ends to simulate anchorages.
I threw in a few puzzles too...creating sloped faced 'bricks' that when assembled created curved "beams" to demonstrate the effect of curved concentric prestress on curvilinear elements.
Good luck.
RE: Stiff Rubber Bands
Ingenuity - You went a step above what I'm doing. I was just going to get the rubber band completely around the bottom of the leggos. A little unrealistic from a non-external post-tensioning standpoint, but I'm just trying to get across the difference in load carrying capacity with the post-tensioning in place.
RE: Stiff Rubber Bands
Enjoy the presentation. And keep the samples after the demo - you will be surprised how many times you will use it to explain the concepts to fellow engineers...or architects...and clients too.
RE: Stiff Rubber Bands
RE: Stiff Rubber Bands
I think the problem with lego blocks will be that they fit together (frictionally) too tightly. This will mask the prestressing, until they finally (suddenly) break free of each other and possible just come apart, in kind of an explosion. Why not take a 4"x4" or 6"x6" piece of foam rubber 24 or 36" long, think in terms of craft store or upholstery supply store. Mark this beam with a .5x.5 or 1"x1" grid work across each face (depth and width) and also lengthwise, with a fairly fine permanent marker. Then, cut a .5" deep slit down the middle of the bottom face, lengthwise, and press the rubber band up into that slit, and anchor the ends. I’ve never done this to show prestressing, but rather just to discuss beam bending and torsion. The inked grid work shows +/- strain due to plane bending and also shows the strain parallelograms from torsion. This should also show the camber from the prestressing. Look for small dia. rubber surgical tubing or material akin to an o-ring.
RE: Stiff Rubber Bands
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Stiff Rubber Bands
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Stiff Rubber Bands
I may use a combination of the two. I want to show a failure of the beam without post-tensioning and compare two identical beams - one without post-tensioning and one without - to demonstrate the difference in deflection and load carrying ability.
RE: Stiff Rubber Bands
When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.
-R. Buckminster Fuller
RE: Stiff Rubber Bands
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RE: Stiff Rubber Bands
RE: Stiff Rubber Bands
Materials:
-36" 6x6 Lumber
-(2)38" threaded steel rod
-(4)Butterfly nuts with threading to match rod
-.125" - .25" Thick rubber matting
Build:
-Cut 6x6 Lumber in equal lengths (8" cuts would be fine, but I think 4" cuts gives a better demo of deflection)
-Use rubber cement to glue 5.5" x 5.5" chunks of rubber matting to each side of all the 6x6 lumber stubs
-drill 2 holes twice as large as the PT threaded rod at whatever your embedment depth is
-Assemble
Voila! It is a little more work intensive to build, but I think it gives a good visual on PT systems and lasts forever. Now, start your debate on weather to pack
(image taking from Post Tensioning Institute Slideshow)
RE: Stiff Rubber Bands
RE: Stiff Rubber Bands
@Lion06: If you're going to do multi-span, maybe you should go for gold and try to demonstrate hyper-static forces!
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Stiff Rubber Bands
Signious - I like this setup a lot. The wing nuts allow you to easily demonstrate the effects of increasing the post-tension force. And you can have one setup with the wing nuts locked off with no pretension to compare to a mild reinforced setup.
Now I'm wondering if there is a hybrid scenario of these two. I'm thinking the wood block setup with the pins running through the blocks as deviators (I would install enough deviator to ensure a minimum of two per 6" block and to approximate the parabolic drape) and a cables or wires with threaded ends to accept the wing nuts.
Thanks for all the awesome ideas, everyone!
RE: Stiff Rubber Bands
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Stiff Rubber Bands