Stiff Rubber Bands 4

[Lion06]

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?

 

[SlideRuleEra]

Lion06 - Try calling an auto tire shop. They may have an old inner tube that could be cut to make bands just the size you need. Very stiff, as a kid we would use those for all sorts of projects.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Ingenuity]

I have done similar demonstrations and the Lego blocks were too stiff vs the props of the elastic, so I used self-made small polystyrene blocks/bricks with dress-makers elastic. The dress-makers elastic moles is various cross sectional sizes and shapes, and comes in long lengths (typically yards).

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.

 

[Lion06]

Sliderule - Great idea! I will put in a few calls today.

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.

 

[Ingenuity]

Lion06
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.

 

[JStephen]

If you can locate rubber O-rings of that size, they might work. (Smaller O-rings are used as rubber bands to attach speedometers, lights, etc. to bicycle frames or handlebars).

 

[dhengr]

Lion06:
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.

 

[KootK]

I was thinking along similar lines to dhengr. If it's an audience that can appreciate it, you can't beat those before and after strain lines for getting the idea across. Unless it's important that you demonstrate the effect of drape and/or cable elongation, I think that you might be better off just using two small diameter threaded rods for your post-tensioning. Run them along the sides of your foam block and show the students what happens when you dial up the post tension. You could even do multiple runs with the eccentricity varied. If you wanted to demonstrate the effect of drape, you might be able to rig up something similar where your use two rods on each side of the block that meet up at the underside of the block at midspan. Kind of like how one might post tension a timber beam.

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.

 

[KootK]

Personally, I think that external PT may well be more instructive. However, if you wanted to do it by the book, you could buy two foam blocks and glue them together stacked with a threaded rod in a garden hose installed at the interface between blocks.

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.

 

[Lion06]

I like the idea of showing the strain, but I like the idea of external post tensioning for the demonstration for two reasons. 1 - It allows a drape instead of a straight tendon and you can easily see the drape and 2 - Having the drape allows for a multispan beam instead of a single simple beam.

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.

 

[manstrom]

Bungee cords? Surgical tubing? They do make rubber bands that you can use for working out. Maybe Home Depot, or a place that sells fitness equipment.

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

 

[JAE]

Surgical tubing?

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[Archie264]

Years ago I did something similar using the cheapest playing cards I could find. I bought 10 decks, cut them in half and punched holes in each of the cards (I did this while watching tv). It was helpful in *sort of* illustrating the principles involved. I used chord for the reinforcing since, like you, I didn't have strong enough rubber bands. One thing about that set up: there was no doubt that plane sections remained plane.

 

[Signious]

I have attached a PDF of the rig I used in demonstrations for PT structures. It isn't my rig, they used 2x4 on the narrow and I used 6x6 blocking so I could put in 2 PT rods.

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)

 

[BUGGAR]

I tried something similar. It didn't work. So I let the kids make slingshots with the elastic bands and taught them first aid instead.

 

[KootK]

That's a pretty sweet setup Signious. I can't resist taking a swing at this myself.

@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.

 

[Lion06]

Nice setup, koot.

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!

 

[KootK]

I really like the wing nuts too. Trouble is, the rods are too stiff to drape I would think. I wonder if you could have multiple rods pin connected at the deviators and allow the deviators to slide around a bit in horizontally slotted holes. That way you could drape and still tension only from the ends.

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.

 

[Archie264]

Yeah, Signious, another kudos from here. I'd like to try to build that setup as well, some day.