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Friendly Competition Between Engineering Firms
3

Friendly Competition Between Engineering Firms

Friendly Competition Between Engineering Firms

(OP)
Hi ladies and gents

Have a wood bridge building competition between a few structures firms in my area through MdQi (http://mdqi.org/), and I am looking to load test the wood pieces somehow. We were given these packs of very small (maybe 4 times the cross section area of a toothpick) pieces of wood. I checked the manufacturer, and all I could determine is that they were made in China...they are probably 10% wood 90% asbestos or some other weird material.

Anyways, would anybody have any suggestions as to how I could quantitatively perform tensile, compression, and maybe bending tests on these little sticks?

Or - would it be better suited to assume a wood with poor mechanical properties and directly calculate?

*My* gut feeling is that applying assumed mechanical properties is going to be way off how they actually behave.

RE: Friendly Competition Between Engineering Firms

No.

1. What you need to know "exactly" how much the "average" wood stick performs: tension, rigidity (which comes from all three of straightness and tolerances in size over the length AND material properties varying over the length as well!), strain, etc. all combine.

2. But the "average" Chinese-made "wood stick" doesn't matter either: You're going to be making a structure up from several dozen (hundred?) of the d*mn things, and you need to either test several hundred (not to failure!) and select the best several dozen (which isn't quite cheating, merely "prudently gathering materials") for the actual product.

Alternately, you could test several dozen to destruction, then determine what you will accept as the mean of the structural and physical properties from that batch as a "good enough" average for the entire potential group of "wood sticks" to determine adequate design criteria.

If you do decide to use the "average" properties, then you're penalizing yourself from the start because you HAVE to accept that some 5% percent will be "below spec" by more than 2x standard deviations. So, do you overbuild so those failing 5% of the sticks won't crash your project? Or test the entire provided lot so you eliminate the worst 5%, but can assume the remaining 95% that survived the "test-just-short-of-failure" test are at nominal strength (or above)?

Avoid ANY chance of your test results getting to the competition! Avoid even the hint that you are testing your sticks statistical properties (material OR physical) ahead of time. It will give the competition too many ideas.

Consider a quick go/no-go gage if you are given your sticks only at the time of competition: Throw out the skinny or "wavy" sticks or bent sticks, accept for use only those "above average" in thickness and depth. Warning: If your project "winning" is determined by weight-of-bridge and not just capacity-of-bridge, then watch also the weight of tape, glue and fasteners as well as weight of the sticks.

RE: Friendly Competition Between Engineering Firms

(OP)
I like the idea of testing to failure and determining a statistically reasonable value. We already have the sticks, so I can purchase as many in kind as I need to test - hopefully, they won't vary unbelievably between batches.

The problem I am running into how to contrive/build or find testing devices. Finding devices to borrow or rent seems improbable. Building testing devices seems complex as they have to deliver constant and concentric forces, while having the ability to measure the force. That is were I need the most suggestions! :)

The main criteria is load-to-weight efficiency, and we are limited to 400 grams total. I have already looked into finding a reasonable adhesive with the lowest specific gravity possible to limit weight.

RE: Friendly Competition Between Engineering Firms

I suspect - after seeing many high school and a few college "bridge competitions" fail during load testing - that your assembly will fail at either a joint (where the shear stress of the beam-to-beam adhesive fails under either pure shear stress (tension or compression direction), or a combination of shear and twisting forces at a glued joint, or at a single column failing under compression when it buckles.

So, try a simpler pull-to-failure test using weights attached to a member to determine the limiting condition: Joint strength in tension or member strength. fasten three or four members together in a line using various glues/epoxies/or wood-cynoacrylics (superglues - wood superglues are at many wood hobby shops and on-line) or cyno-acrylics plus a pin to increase shear strength. Increase the weight until either a joint fails or the wood fails. Try various double-sided joints against single-sided joints: The double side increases glue area (net joint strength) and creates an even pull on both sides of the wood stick, but requires twice as much weight.

Typical twisting resistance (S) of a member? Use a very, very small torque wrench with the other end of the member clamped in a vise.

RE: Friendly Competition Between Engineering Firms

(OP)
I like the idea for the joint testing.

I imagined placing a piece in a vice and torquing it until it fails - however, how to measure the clamping force? It seems like the testing device needs to measure it directly, since I don't have constitutive properties to get back to the force from the deformation.

RE: Friendly Competition Between Engineering Firms

(OP)
Nice. Thanks, MJ.

RE: Friendly Competition Between Engineering Firms

It might basswood? Focus on the glue and joint design. Dehydrate your bridge to reduce weight, especially if you use a water based glue.

Think of every way you can cheat, then check the rules to see if it is actually cheating, or if the rules are silent on the topic.

Tie helium balloons to your bridge right before the weigh in and competition.

RE: Friendly Competition Between Engineering Firms

(OP)
Who knows what it is. There is a U.S. Gov lab you can send samples to for them to identify, and it is free for the first so many samples - the problem is that it can take 2 months to get the results back (if there are results).

You must be a fan of steroids in baseball too! bigsmile

RE: Friendly Competition Between Engineering Firms

No. You can't measure (or "add") clamping forces to the two pieces when measuring them. You can only and simply glue them together (and insert whatever little straight pin/needle/thumb tack) you can find to the joint, let it dry, then pull it (straight down) and twist it (with the little torque wrench you've bought at Sears or an auto parts store).

After drying, what ever the glue/epoxy/cyanoacrylate itself provides is all the "clamping" force you can use.

Now, let's pretend you've developed a "perfect design" that is extremely lightweight but has two "bad joints" where you need more clamping force, or a thicker diameter pin that also provides a clamping force, or something like that. Then, at that failure point, you add a nbr 4 or nbr 6 or a nbr 8 machine screw and clamp the two or three or four wood sticks together.

RE: Friendly Competition Between Engineering Firms

(OP)
Not quite sure I understand the first part. I was only referencing the measurement of force during a compression test.

RE: Friendly Competition Between Engineering Firms

Outline of rules

Each bridge shall be built using craft sticks (supplied in your kit) and
glue (supplied by participant). The bridges will have a Span Length of 20
Inches and a maximum bridge weight of 400 grams.• Each Bridge will be loaded to failure as determined by the Judges.
• The overall winner will be based upon an efficiency index determined
by dividing the weight the bridge holds in pounds by the weight of the
bridge in grams. The highest efficiency rating wins.

Details here

http://mdqi.org/images/stories/mdqi_documents/2014...

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Friendly Competition Between Engineering Firms

While a universal test machine is very nice to have, almost any kind of test can be done pretty accurately using dead weight techniques. A bucket and string, with pulleys or levers if necessary, can apply load to just about anything. Just fill the bucket with water or sand and weigh it on a appropriate scale. To determine ultimate strength simply keep filling the bucket until something breaks, then weigh the bucket.

RE: Friendly Competition Between Engineering Firms

(OP)
Composite: I agree. The tensile test I think can be relatively simple to construct with some of those pin vises Mint mentioned earlier. Trying to apply a concentrated compression load (using those ordinary means you mention) onto a stick that is .0937"x.0937" in area is where it seems tricky.

Greg: Yeah, the rules are pretty simple and open for all sorts of things - no glue rules and such.

RE: Friendly Competition Between Engineering Firms

Quote (Rules)

... Each bridge shall be built using craft sticks (supplied in your kit) and
glue (supplied by participant). ...

Define "glue" - LOL.

How about "glue" = resin reinforced with carbon fiber? Otherwise known as carbon fiber.

Of course you'll need to include a smattering of wood. Convert a couple of wood bits to veneer on a wee tiny lathe, and use a few milligrams of veneer to decorate a few small sections of the carbon fiber... sorry, "wood and glue" bridge.

RE: Friendly Competition Between Engineering Firms

(OP)
I spoke to the organizer and the glue can be whatever type of adhesive you want it to be. It is going to have to be a bit more than a 'smattering' of wood to be under 400 grams! bigsmileI like the carbon fiber-glue idea, but I have a feeling buckling of compression members is going to be my biggest problem.

RE: Friendly Competition Between Engineering Firms

About five years ago the SAMPE bridge building contest was won by a team that used a helium balloon to make the bridge weight close to zero. A weighless bridge has an infinite efficiency. The rules were changed after that.

RE: Friendly Competition Between Engineering Firms

(OP)
HA! That is why the rules have to be explicit and exhaustively comprehensive!

RE: Friendly Competition Between Engineering Firms

Compositepro, I wasn't aware that anyone had done that before. Isn't there some saying about great minds :) ?

I always like to say "rules are rules, and not rules are not rules."

RE: Friendly Competition Between Engineering Firms

"What are rules about using a wooden "pin" at the joints?

Can you use a toothpick (a shiskabob-type round skewer would be about the right diameter) or only wooden craft (Popsicle) sticks?

RE: Friendly Competition Between Engineering Firms

(OP)
It doesn't say anything about that, but that would be tough because the craft sticks are only .0937" -/+ square. I had a friend recommend drilling a small hole in the ends of the sticks and epoxying in welding wire. That sounded cool, except for when the bridge does finally break the judges will clearly see embedded bits of steel wire...

I also thought about slicing the sticks in half and cross-laminating them into a sheet with epoxy and cut out gusset plates from the sheets.

RE: Friendly Competition Between Engineering Firms

Hmmmmn.

Now, remember that you don't need a 'pin" at every joint, only that 20% most highly loaded. (Even in simple Wrren truss, not every beam and span is equally loaded).

So, if you laminated 4x popsicle sticks together, your original 0.0937 flat stick is now a 3/8 x .375 thick square beam. So cut that beam into 4x sq pins, and insert those pins into the most threatened joints. If a sq hole is difficult, and it will be hard to do either automatically or manually, put the sq pins on a lathe and turn them down to a 3/16 diameter round shape. From a single 3/8 x 3/8 x 4 inch beam, you get 4x 4 inch long 3/16 pin rolls, for 20-30 pins. Then use a drill to drill the holes.

Remeber, rules said glue and wood sticks. Didn't say anything about "how" you're gluing the sticks together.

RE: Friendly Competition Between Engineering Firms

test the max. compressive force of those sticks using a vice, a cylinder, a pressure gauge and a calculator.

RE: Friendly Competition Between Engineering Firms

I'm sorry, that should have been vise (like a machinist's vise, not miami vice) (unless google translate is letting me down again ? )

RE: Friendly Competition Between Engineering Firms

OK, so here is a theoretical question for the structural readers;

Assume you need a long tension member.

Which would be more efficient (greatest tension force sustained at failure)/(total weight of the tension member)?

A string of 4 inch long x 0.0937 thick x 3/8 wide popsicle sticks glued at each end (so they overlap by 3/8 inch)?

Or a double-thick string of popsicle sticks but with each popsicle stick overlapped with its neighbor for gluing surface 2 inches long at end? Note that, since the popsicle sticks can be narrowed down (from 3/8 thick to perhaps 3/16 thick), the net extra strength would be useful since the 'ply" effect of two layers of wood add up?

RE: Friendly Competition Between Engineering Firms

@ racookpe1978 : could you sketch that please?

RE: Friendly Competition Between Engineering Firms

(OP)
So...update: I have my compression testing apparatus built - should be breaking sticks in the next week.

I have been thinking about the topology of the truss bridge, and wondering if it would be worth it to try to get a demo of a optimization program. My quick Google search landed me on Solidworks and Genesis by VR&D.

I did find this seemingly primative portable optimization program (http://www.upct.es/~deyc/software/tto.php), but it is rather clunky to use and there isn't much for options - it doesn't seem to give a rats rear-end about member length or provide an output to .dxf. Check it out though - I have never seen anything like it.

The load is essentially a point load at the center, so I would think that the general shape should be a triangle to mimic the moment diagram, but am curious if an optimization software could provide me a more detailed topology where I can constrain member lengths (to keep as many as I can to the length of the wood sticks), and maybe control a few other variables too.

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